Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://www.atnf.csiro.au/observers/docs/ca_obs_guide/ca_obs_guide.pdf Äàòà èçìåíåíèÿ: Fri Nov 21 11:31:20 2008 Äàòà èíäåêñèðîâàíèÿ: Thu Jan 15 01:16:43 2009 Êîäèðîâêà: Ïîèñêîâûå ñëîâà: guide 8.0

AUSTRALIA TELESCOPE NATIONAL FACILITY

GUIDE TO OBSERVATIONS WITH THE COMPACT ARRAY

Version 8.0 (Novemb er 2008)

www.atnf.csiro.au/observers/docs/ca obs guide


CONTENTS

2

Contents
1 Introduction 2 Centimetre Observations (20­3 cm bands) 2. 1 2. 2 Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2. 2. 1 2. 2. 2 2. 2. 3 2. 3 Primary Amplitude Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . Secondary Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bandpass Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 4 4 5 5 6 6 6 7 7 7 7 8 8 8 8 8 9 9 9 11 11 11 14 14 15 17 17 17 17 18

Sensitivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3 Millimetre-wave observations (12mm­3mm) 3. 1 3. 2 3. 3 3. 4 12mm Observations ......................................

7mm Observations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3mm Observations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3. 4. 1 Warnings for mm observations . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3. 5

Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3. 5. 1 3. 5. 2 3. 5. 3 3. 5. 4 3. 5. 5 Flux calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bandpass calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Phase calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pointing calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Paddle (vane) calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4 Choosing an Observing Frequency 4. 1 4. 2 Available Frequency Range . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Interference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

5 Choosing Angular and Frequency Resolution 5. 1 5. 2 5. 3 Image Complexity, Angular Resolution and Observing Time . . . . . . . . . . . . . . . . Array Configurations and Baselines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bandwidths and correlator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

6 Additional Observing Notes and Techniques 6. 1 6. 2 6. 3 Short Observations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Polarimetry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mosaicing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .


CONTENTS 6. 4 6. 5 Multi-frequency Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Reference Pointing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3 18 19 19 19 19 19 20 20 20 20 20 21 21 21 21 22 22 22 23

7 High Time Resolution, Pulsars, Planets and VLBI 7. 1 7. 2 7. 3 High Time Resolution and Pulsar Observing . . . . . . . . . . . . . . . . . . . . . . . . . Solar System Ob jects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Tied Array Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

8 Other Things to Consider 8. 1 8. 2 8. 3 8. 4 Bandwidth Smearing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Confusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Weather . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Artefacts ............................................

9 When the Observations are Finished 9. 1 9. 2 Data Reduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Publications Advice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

10 References and Further Reading 11 Observing Prop osals 11.1 Deadlines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Further Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A Using the Web-based Scheduler


1 INTRODUCTION

4

1

Intro duction

This document is intended to give astronomers enough information to prepare proposals for observing with the Compact Array near Narrabri. It has been traditionally updated every semester, and this tradition may continue for the first few semesters of the Compact Array Broadband Backend (CABB) era, however it is envisaged that the document will become more static, with links to regularly updated webpages. The complementary ATNF User Guide http://www.narrabri.atnf.csiro.au/observing/users guide/users guide.html contains information needed for observing, and a guide exists for the analysis of Compact Array data using the recommended MIRIAD http://www.atnf.csiro.au/computing/software/miriad software package. (A somewhat outdated AIPS guide http://www.atnf.csiro.au/computing/software/atca aips/atcal html.html is also available.) The Compact Array is a 6 km east-west array of six 22 m antennas located at latitude 30 degrees south. There is a 214 m northern spur. The smallest synthesized beamwidths in Right Ascension for each observing wavelength are shown in Table 1, but bear in mind that, for east-west arrays, the beamwidth in Declination is greater by a factor cosec(Dec). At high angular resolution, the telescope is only useful for observing southern ob jects. North of Declination -24 , full (u, v)-coverage is unobtainable; near Declination zero the beam is highly elongated north-south; and north of Declination +48 sources are below the telescope's horizon and inaccessible. For lower angular resolution, the northsouth and hybrid arrays improve the (u,v)-coverage for equatorial sources, but only up to a maximum north-south baseline of 214 m. The Compact Array usually operates under a semester system with two application deadlines each year: June 15 for observations from October 1 to March 31; and December 15 for observations from April 1 to September 30. The 2009APRS semester is an exception: For the ATCA only, the 2009 APRS semester wil l run for a three-month period from 15 April 2009 until 14 July 2009. Proposals submitted for the 2009 APRS will be considered for this period only. For this semester, CABB will be available with 2-GHz bandwidths with 1-MHz spectral channels. A separate call for ATCA proposals will be announced on 15 April 2009, with a deadline of 15 May 2009 for a '2009 JULS' semester. The 2009 JULS will run from 15 July 2009 until end-September 2009. ATCA proposals not scheduled in the 2009 APRS semester should be resubmitted for consideration in the 2009JUL semester. It is expected that one or more CABB zoom modes will be available for this semester. See http://www.atnf.csiro.au/observers/apply/avail.html for more details. Based on previous exp Detection experiments servations of bright co the year. Observations proposal. erience, observing at 3mm usual at 7mm and 12mm usually end mpact sources, for which self-ca at times other than those indica ly ends by October 15, restarting in late April. by October 31, restarting in early March. Oblibration is possible, may be made throughout ted above require an explicit justification in the

An archive of previous Compact Array proposals and observations can be found at http://atoa.atnf.csiro.au/. There is also pro jects database for proposals submitted between 1990 Quarter2 and the 2005 OCTS observing semester at http://www.atnf.csiro.au/observers/search pro j.html All ATNF Telescope Applications must be submitted using OPAL http://opal.atnf.csiro.au/.


2 CENTIMETRE OBSERVATIONS (20­3 CM BANDS)

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BAND NAME () Frequency range (GHz) Fractional frequency range Numb er of antennas Numb er of baselines Primary b eama Synthesized b eam (arcsec)b System temp erature (K)c System sensitivity S (Jy)d Strongest confusing source (mJy)

20 cm 1 .3 ­ 1 .8 32% 6 15 33 6 32 350 140 0 .5 6km 1550 0 .0 9 0 .4
f

13 cm 2 .2 ­ 2 .7 20% 6 15 22 4 34 470 24 0 .5 6km 2450 0 .1 3 0 .6 0 .0 1 5 0 .0 7

6 cm 4 .5 ­ 6 .7 39% 6 15 10 2 33 340 2 .3 2 6km 5500 0 .0 5 0 .2 0 .0 0 5 0 .0 2

3 cm 8 .0 ­ 10 22% 6 15 5 1 41 470 0 .4 2 6km 9000 0 .0 6 0 .3 0 .0 0 7 0 .0 3

1 cm 16 ­ 25 44% 6 15 2 0.5 37 420 -- 2 6km 17000 0 .0 6 0 .3 0 .0 0 7 0 .0 3

7mm 30 ­ 50 50% 6 15 2 0.2 75 900 -- 2 6km 40000 0 .1 2 0 .6 0 .0 1 4 0 .0 7

3mm 83 ­ 105 24% 5 10 70 2 270 7200 -- 2 H214 95000 1 .1 0 .0 3 0 .1 2 5 0 .0 0 3

e

CABB bandwidth assumed b elow (GHz) Array assumed b elow Centre frequency assumed b elow (MHz) Flux sensitivity (mJy/b eam)f

(10 min, 128MHz)
Brightness sensitivity (K)
g

(10 mins, Dec -45 )
Flux sensitivity (mJy/b eam)

0 .0 1 1 0 .0 5

(12 hours, 128MHz)
Brightness sensitivity (K)
g

(12 hrs, Dec -45 )
Field of view (full width at half p ower). HPBW in RA for the 6 km array for all bands except 3mm, for which the H214 array is assumed. No tap er applied. In Declination, for the 6 km array (and other pure east-west arrays), the HPBW is larger by cosec(Dec). Longer arrays (up to 3 km) are p ossible at 3 mm but only with self-calibration and under favourable weather conditions. c The system temp erature at high elevation under reasonable weather conditions. These values, particularly at high frequency, are weather-dep endent. d The signal which doubles the system temp erature. e Within FWHM primary b eam -- see A.H. Bridle, reference 1, p.471. f Theoretical rms noise; one frequency; dual orthogonal p olarisation; natural weighting. The effect of confusing sources can substantially degrade this numb er. g For the array listed in the same column: see following table for shorter arrays.
b a

Table 1: Observing Parameters for the Compact Array. See also the sensitivity calculator at http://www.atnf.csiro.au/observers/docs/at sens/ .

2
2.1

Centimetre Observations (20­3 cm bands)
Pro cedure

In general, at least one member of the observing team should be present at Narrabri. Overseas observers, or others who find it difficult to travel to Narrabri may need to find a local collaborator. The `friend' implied by the `Help required?' question on the application form is usually at Epping and will assist in setting up the schedule file and in the initial analysis of the data. A duty astronomer http://www.narrabri.atnf.csiro.au/observing/support/da support.html is provided at Narrabri who, in addition to helping with schedules and initial analysis, will help in the initial setting up of the telescope and with any problems which arise in the course of the observations. The duty astronomer has no obligation to participate in the observing. Remote observing is available for experienced Compact Array observers. Use the form at http://www.narrabri.atnf.csiro.au/observing/remote form.html or email rem obs@atnf.csiro.au at least two weeks before the observations, providing dates and times. Remote observers must have their own unix account at Epping and Narrabri. See http://www.narrabri.atnf.csiro.au/observing/remote conditions.html for remote observing conditions and


2 CENTIMETRE OBSERVATIONS (20­3 CM BANDS)

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Wavelength 20 cm 13 cm 6 cm 3 cm 1 cm 7 mm 3 mm

6 km 0 0 0 0 0 0 . . . . . . 0 0 0 0 0 0 5 7 2 3 3 7 K K K K K K

3 km 0 0 0 0 0 0 . . . . . . 0 0 0 0 0 0 3 4 1 2 2 4 K K K K K K

1 . 5 km 0 0 0 0 0 0 . . . . . . 0 0 0 0 0 0 0 1 0 0 0 1 7 0 4 5 4 0 K K K K K K

ARRAY 750 m EW352/367 2 3 0 1 1 2 . . . . . . 0 0 9 0 0 0 m m m m m m K K K K K K 0 0 0 0 0 0 4 . . . . . . . 3 5 2 2 2 4 0 m m m m m m m K K K K K K K

H214 0 0 0 0 0 0 0 . . . . . . . 4 5 2 2 2 4 3 m m m m m m m K K K K K K K

H75 0. 04 0. 06 0. 02 0. 03 0. 02 0. 05 0. 4 mK mK mK mK mK mK mK

Table 2: Continuum brightness temperature sensitivity (12 h, bandwidth as specified in Table 1, dual orthogonal polarisations). See also the sensitivity calculator at http://www.atnf.csiro.au/observers/docs/at sens/ .

http://www.narrabri.atnf.csiro.au/observing/rem obs.html for details on how to conduct observations remotely. Target of Opportunity (ToO) requests for observations of extremely important transient or non-predicted events can be made at any time, see http://www.atnf.csiro.au/observers/apply/too apply.html for details. Observations are controlled automatically with an observing schedule. This observing schedule should be prepared beforehand using the command-line or web-based version of ATCASCHED http://www.narrabri.atnf.csiro.au/observing/sched. Documentation for ATCASCHED is available, an extract of which is given for the web-based version in Appendix A. During observations the observer can intervene and repeat or skip parts of the schedule. You can ask the control computer to cycle repeatedly through a schedule. Continuous on-line monitoring of the visibility data is provided during observations; you have the choice of viewing the instantaneous correlation function or its transform, or of viewing time plots of a number of quantities including phase, amplitude, and delay.

2.2

Calibration

Through your schedule, you should make sure that observations of the target source or sources are interspersed with observations of calibrators. Calibrator sources are of three types: primary, secondary and, for spectral-line observations, bandpass. Note that no specific polarisation calibrators are required. All observations should contain a primary and secondary calibrator. 2.2.1 Primary Amplitude Calibration

All observations must be ultimately related to a compact source in the southern sky whose flux is constant, unpolarized, and known. For frequencies below 30 GHz, PKS B1934-638 is used; this source should be observed at least once a day. The values (following a revision of the flux density scale in August 1994) are 14.9, 11.6, 5.8 and 2.8 at 1384, 2368, 4800 and 8640 MHz, respectively (Reynolds 1994: http://www.atnf.csiro.au/observers/memos/d96783.pdf ) and 1.06 Jy at 17728.5 MHz (Sault 2003: http://www.narrabri.atnf.csiro.au/calibrators/data/1934-638/1934 12mm.pdf ). The flux densities are incorporated in the on-line calibration software (CACAL) and MIRIAD calibration software. For frequencies above 30 GHz, observations of a planet (Uranus is recommended, Mars may be an alternative in the most compact arrays), will be required for primary amplitude calibration (on


2 CENTIMETRE OBSERVATIONS (20­3 CM BANDS)

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suitably compact configurations). Suitable software exists in MIRIAD for using planetary calibration data. 2.2.2 Secondary Calibration

To correct for changes in gain and phase caused by receiver, local oscillator, and atmospheric instabilities, you need to look periodically at a known compact source. For a number of reasons, including possible baseline solution errors arising from the frequent Compact Array configuration changes, this secondary calibrator should be as close as possible to the target source. Observers concerned with accurate positions will also need to choose a secondary calibrator whose J2000 position is accurately known. It is preferable to make 5 min calibrator observations once every 30 min. Less-frequent calibration is possible at 20 cm and more frequent calibration may be required at 3 cm. At 1 cm and shorter wavelengths, it is preferable to calibrate as frequently as possible (consistent with not spending too much time off source), unless the array is very compact. In summer, or during the day, phase stability can be poor. Phase stability can be monitored for bright compact sources with vis, or with the ATCA seeing monitor: The seeing monitor is described at http://www.atnf.csiro.au/observers/docs/7mm/seeing.pdf (though in 2008 the a new satellite beacon was adopted, resulting in a change in the frequency from 30 GHz to 21 GHz). Observations requiring maximum phase stability (e.g., at wavelengths less than 6 cm) should therefore be made in winter, or at night. Most compact sources are variable, but they can be calibrated for the observation against PKS B1934-638. The most up-to-date calibrator list is available by position or flux-limited online search: http://www.narrabri.atnf.csiro.au/calibrators/. 2.2.3 Bandpass Calibration

A spectral-line observation will normally require a bandpass calibrator. At low frequencies, the primary calibrator PKS B1934-638 is usually enough, but at wavelengths of 3 cm and shorter a source such as 0537-441, 1253-4055, or 1921-293 (usually >5 Jy) may be needed. The Compact Array bandpasses are stable, and so a single bandpass calibration is normally sufficient, unless you need high dynamic range.

2.3

Sensitivity

The general expressions for the flux and brightness sensitivities are given in the document AT/01.17/025, http://www.narrabri.atnf.csiro.au/observing/AT-01.17-025.pdf and these general expressions have been used in Table 1. The observer has control of the integration time (t), bandwidth (B), observing wavelength (), number of baselines (N) and synthesized beam size () only, and with these variables and the system sensitivity (Ssys ), the expressions reduce approximately to: rms Flux Sensitivity = S = 7.55 â 10
-5

Ssys (N tB )-

1 /2

(Jy),

and

rms Brightness Sensitivity = T = 1.36 â 103 S 2 /( ) (K), where Ssys is in Jy, t in min, B in MHz, in cm, and is in arcsec. For the full 6 km array, N=15. There is a convenient ATCA sensitivity calculator at http://www.atnf.csiro.au/observers/docs/at sens which takes into account system temperature, observing frequency, array, correlator configuration and (u, v)-weighting. It has been updated for CABB observing.


3 MILLIMETRE-WAVE OBSERVATIONS (12MM­3MM)

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3
3.1

Millimetre-wave observations (12mm­3mm)
12mm Observations

All six antennas of the Compact Array are equipped with 12mm receivers covering the frequency range 16­26 GHz. The 12mm system shares a common dewar with the 7mm and 3mm receivers, the separate feed horns at the top of the dewar are moved to the Cassegrain focus of the Compact Array antennas using the rotator positioning system. Because the 12, 7 and 3 mm receivers have separate feed horns, observations cannot be simultaneous in the 12, 7 and 3 mm bands. For more information about the 12mm system, refer to the 12mm web pages http://www.atnf.csiro.au/observers/docs/12mm.

3.2

7mm Observations

The ATCA can observe within the frequency range 30­50GHz. Feeds and receiver were installed into the existing mm-wave receiver packages on all 6 antennas in 2007, with the 7mm upgrade jointly funded by the ATNF and NASA's Deep Space Network to enable the ATCA to participate in occasional spacecraft tracking. Changing the observing band to or from 7mm requires first rotating the turret to the 12mm position, and then translating the feed package to bring the 7mm feed on axis. The latter stage takes about 2 minutes, and so changes to or from 7mm are much slower than any other band change. The wide 7mm band presents difficulties in signal down-conversion, as the aliased signal from the first down conversion stage can also be within the 7mm band. Although image rejection filters are used, variations in receiver gain across the band, combined with the frequency-dependent performance of the filters themselves, can result in appreciable signal levels being added to the observing band. Generally, this aliased signal does not cross-correlate and is present as a contribution to the noise level. However, in some circumstances, notably as the delay rate drops to zero (e.g., around source transit on north-south baselines), the aliased component can cross-correlate and be visible as "beating" on some baselines. This data must be flagged during the data processing. For more information, refer to the 7mm web pages http://www.atnf.csiro.au/observers/docs/7mm. A swap-program may, if possible, be scheduled for 7mm pro jects -- see the following section for details.

3.3

3mm Observations

The inner five ATCA telescopes (i.e., excluding CA06) are outfitted with a 3 mm receiver and can observe in the range 83.5 to 106 GHz. A noise diode has been added to the 3mm system on CA02 (only) to aid with 3mm polarization calibration. Aliasing effects similar to those described at 7mm can also arise in the 3mm band. A limited form of flexible scheduling is operational for observations at millimetre wavelengths. Consult the document `Flexible Scheduling at ATCA' http://www.narrabri.atnf.csiro.au/observing/flexsched.html for details. For more information, refer to the 3mm web pages http://www.atnf.csiro.au/observers/docs/3mm.


3 MILLIMETRE-WAVE OBSERVATIONS (12MM­3MM)

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3.4

Pro cedure

Observations in the mm band are run from a schedule file in the same way as cm-band observations. However, due to the effects of atmosphere stability, mm observations require bandpass and phasecalibration checks at a much more frequent rate. Note that as ATCA of the Doppler shift to the 3mm system. http://www. does not Doppler track, for the line of interest. The sky frequency can narrabri.atnf.csiro.au/o it is important to have a good estimation of the magnitude This is particularly so for the higher frequencies available be calculated using an online calculator at bserving/obstools/velo.html.

A simple phase reference observation takes about an hour to run, including calibration. The following procedure is typical: Pointing calibration Bandpass calibration Paddle calibration Phase calibrator Target source Phase calibration Paddle calibration Target observation. Either the command line of ATCASCHED, or the web version http://www.narrabri.atnf.csiro.au/observing/sched/ can be used to construct the observing schedule. 3.4.1 Warnings for mm observations

Elevation angles close to the array horizon should be avoided because of the increased opacity and poorer atmospheric stability. Therefore, observations of ob jects with a declination further north of -50 are best made using hybrid configurations, or one using the N-S spur in order to achieve sufficient (u, v)-coverage. Observations using very compact telescope configurations should be made with caution, as telescope shadowing can be a significant problem, especially for sources at declinations which do not achieve elevations high above the horizon. See http://www.narrabri.atnf.csiro.au/observing/shadowing/ for details.

3.5

Calibration

For observations using a narrow bandwidth, a delay calibration is best made using a coarser channel resolution. Note that using a different correlator configuration will not affect your delay calibration as long as the frequencies and bandwidths match those of your target observations. 3.5.1 Flux calibration

Generally, planets are the most reliable mm primary flux calibrators, the best being Uranus. Details of planet rise and set times can be determined at http://www.parkes.atnf.csiro.au/cgi-bin/utilities/planets.cgi. 3.5.2 Bandpass calibration

Bright extragalactic continuum sources (such as 0537-441, 1253-4055, or 1921-293) are useful for bandpass and delay calibration as well as pointing. A 15 minute integration will usually provide an adequate bandpass, except at the narrowest bandwidths where extra time should be allowed. Refer to the ATNF calibrator catalogue http://www.narrabri.atnf.csiro.au/calibrators and use the form to select bright sources (by specifying a lower limit to the flux density) near your


4 CHOOSING AN OBSERVING FREQUENCY

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target source ­ but be sure to avoid Cen A, i.e., 1322-427, sources with large "defects" (see catalogue webpages for details) at your frequency in your array, and resolved planets. 3.5.3 Phase calibration

To find the nearest phase calibrator to your source, use the position and flux-limited calibrator online search engine from the ATNF calibrator catalogue http://www.narrabri.atnf.csiro.au/calibrators/. The list of calibrators interrogated by this engine includes OVRO and BIMA sources, which will be useful for observations of sources north of -30 . The spatial and time separation of source and phase calibrator measurements has a strong dependence on telescope configuration and the atmospheric conditions. The ATNF online calibrator cycle calculator http://www.narrabri.atnf.csiro.au/calibrators/calcycle.html can be used to estimate of the rate of phase calibrator measurements for a number of different phase decorrelation percentages. Daytime observations, or observations with long baselines should require much more frequent phase calibration measurements than those made during nighttime observations and with compact configurations. A technique that can be applied at frequencies and array configurations where decorrelation is severe, is to observe a relatively weak (0.5 Jy) `test' quasar, near the target source, in addition to your phase calibrator. The test quasar should show up if the phase calibration is adequate and it is possible to determine whether a non-detection is due to a weak source or just bad phases. Another reason to observe a second (not necessarily weak) quasar is to check the reliability of phase referencing. The observation would consist of a mosaic of source, phase-cal, and test quasar that is repeated every 5 minutes. 3.5.4 Pointing calibration

Regular pointing checks (every hour or so) are essential because of the small size of the ATCA primary beam at 3mm. Pointing checks can be made with continuum sources or masers. A list of SiO sources can be found on the ATNF SiO source catalogue list http://www.ls.eso.org/lasilla/Telescopes/SEST/html/telescope-calibration/point-sources/sio-sources.html. To refine your pointing during your observation, it is best to choose a pointing calibrator near your target source (within 10 if possible), not only because the pointing changes with position, but also because you will want to observe it throughout your run. Generally, any quasar with a flux of 1 Jy or more will suffice. 3.5.5 Paddle (vane) calibration

This is a standard procedure at 3mm wavelengths for placing the amplitudes on a temperature scale. You will need to include periodic paddle measurements in your ATCASCHED file -- about every half hour should be sufficient. The entire paddle scan takes about 2 minutes.


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Figure 1: Average Compact Array system temperatures for each observing band at high elevation under reasonable observing conditions. These are based on hot-cold load measurements with the noise diodes turned off. At 16­25 GHz, the dotted line is the sum of the noise from the receiver, telescope, ground and CMB. In all cases, the solid line includes the atmosphere at the time of observation, and thus represents the total system temperature. The 1­10 GHz measurements were made by G. Baines in August 1997, 16­25 GHz measurements by R. Subrahmanyan in October 2003, 30­50 GHz measurements by G. Carrad in May 2007, and 83­106 GHz measurements by T. Wong in September 2004.


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4
4.1

Cho osing an Observing Frequency
Available Frequency Range

All six antennas are fitted with wideband, continuously running, receivers. Cooled FETs are used at 20 and 13cm, HEMTs are used at 6 and 3cm, and InP MMIC devices at 1cm, 7mm and 3mm. The accessible frequency range is given in Table 1, and the average system temperatures are given in Table 1 and in Figure 1. Note that frequencies outside these nominal limits may be accessible. See also the next section on interference. The 6cm and 3cm receivers share a common feed-horn and it is possible to observe simultaneously at any two wavelengths within these bands. You can also switch automatically to other wavelengths within tens of seconds (with the exception of the 7mm band as described previously). Similarly the 20 and 13cm receivers share a common feedhorn and observations can be conducted simultaneously in these bands, although this will not be possible during the period between the full CABB installation and the L/S front-end upgrade. Observations at two simultaneous frequencies are possible in the 1cm band or the 7mm, or the 3mm bands. With CABB frequencies currently need to lie within 6 GHz of each other (and tighter constraints possibly resulting from the exact frequencies chosen). Typical observing frequencies for 128 MHz bandwidth continuum observations were 1384, 2368, 4800, 8640, 18496/19520, 34496/34524, 44096/44224, and 93504/95552 MHz (These frequency designations follow ATCA custom of stating the central frequency of the chosen observing frequency range.) For CABB, the nominal standard frequencies are 1550, 2450, 5500, 9000, 17000/19000, 33500/35500, 43000/45000, 93000/95000 MHz. See also http://www.narrabri.atnf.csiro.au/observing/recfreq.html. Switching between the 1cm/3mm bands, the 6/3 cm bands, and the 20/13 cm bands involves a change in feed horns by means of a turret rotation. This is done automatically under computer control and takes about 20 seconds. Turret rotation should been limited to once every 15 minutes unless a compelling scientific case is made for more frequent rotations. The additional overhead in changing to or from 7mm has been described earlier in this document. Observing frequencies may be set to the nearest MHz (n+0.5 MHz at 1 cm) only and no on-line Doppler tracking is done. Observations of weak H90 recombination lines may be affected by a trapped mode in the 6/3 cm horn at 8857 ± 18 MHz. There are also notches reported in the passband at 4550 ± 10, 5328 ± 10 and 8780 ± 10 MHz which may need to be flagged during data reduction.

4.2

Interference

In the lower frequency bands radio frequency interference (RFI) may be a problem. Figure 2 and Figure 3 show the worst-case interference across each band. The interference is worse at lower frequencies with the main offenders being microwave links, microwave TV, microwave ovens, navigation satellites and self-generated interference. There has also been significant interference at 1381 MHz from the GPS L3 beacon on occasions. These channels may have to be removed from the data. It is wise to watch for interference by continually displaying the spectrum of the signal received on the shortest baseline. You can then note any channels with narrowband interference for subsequent elimination. At 13 cm, the frequency range of 2300 to 2400 MHz, formerly occupied by microwave TV services, is presently clear again. Be prepared to move to an adjacent frequency if you meet unacceptable interference. The issue of sampler interference is discussed in http://www.atnf.csiro.au/computing/at bugs.html#Bug 17. To avoid solar interference, a rule-of-thumb is to observe at a time of year when your source is further than about 40 from the Sun, where possible. It is recommended to specify in your proposal dates that


4 CHOOSING AN OBSERVING FREQUENCY

13

20 cm band 1000

Radio interference

100

Flux density (Jy)

10

1

0.1

0.01 1.2 1.3 1.4 1.5 Frequency (GHz) 1.6 1.7

13 cm band 1000

Radio interference

100

Flux density (Jy)

10

1

0.1

0.01 2.25 2.3 2.35 2.4 2.45 Frequency (GHz) 2.5 2.55 2.6

Figure 2: Interference at 20 and 13 cm at the Compact Array. These observations were made on February 2007. The frequency resolution was 62.5 kHz, and the fringe rotators were switched off. The data plotted are a 1 min scalar average. Downward pointing arrows on the top axis indicate 128 MHz sampler clock harmonics. Observations by Michael Dahlem.


4 CHOOSING AN OBSERVING FREQUENCY

14

6 cm band 1000

Radio interference

100

Flux density (Jy)

10

1

0.1

0.01 4 4.5 5 5.5 Frequency (GHz) 6 6.5

3 cm band 1000

Radio interference

100

Flux density (Jy)

10

1

0.1

0.01 8 8.5 9 Frequency (GHz) 9.5 10

Figure 3: Interference at 6 and 3 cm at the Compact Array. These observations were made on February 2007. The frequency resolution was 62.5 kHz, and the fringe rotators were switched off. The data plotted are a 1 min scalar average. Downward pointing arrows on the top axis indicate 128 MHz sampler clock harmonics. Observations by Michael Dahlem.


5 CHOOSING ANGULAR AND FREQUENCY RESOLUTION

15

OBSERVING TIME 25 days 12 days 4 days 2 days 1 day 6â 1 - 10 minutesc 1 - 10 minutesd
a

LARGEST WELL-IMAGED STRUCTUREa AT 6 cm WAVELENGTHb 6 or 3 km array 1.5 km array 750 m array 6 4. 5 160 115 80 30 20 ­ 6 4 160 115 50 40 ­ ­ 8 5 230 100 80





This Table is based on Nyquist sampling for the size tabulated. However as the shortest spacing is 30 m, the largest smooth structure may need the addition of single dish data. b For other wavelengths scale sizes by /6 cm. c Distributed in hour angle. d 1-dimensional information only. Table 3: Largest well-imaged structure for different arrays

are not suitable for observations if the sun angle is too small. Low-frequency observations, particularly in spectral-line mode, should be made at greater distances. You are advised to observe at night in cases where good quality 21 cm HI data is essential on the shortest (30 m) baseline. For spectral-line observations, software exists in MIRIAD to model and subtract out solar interference. Some information about solar activity can be obtained from the Ionospheric Prediction Service http://www.ips.gov.au/. There appears to be no significant interference within the mm bands.

5
5.1

Cho osing Angular and Frequency Resolution
Image Complexity, Angular Resolution and Observing Time

A synthesis imaging telescope such as the Compact Array provides a great deal of flexibility when deciding how to image an ob ject. In addition to the well-known trade-off between observing time and sensitivity, we have trade-offs with maximum resolution and with the sampling interval in the visibility plane. The choice of angular resolution is fairly straightforward. Increased angular resolution (resulting from longer baselines) leaves the point-source sensitivity constant, but decreases brightness sensitivity in proportion to the beam area (see Table 2). The choice of (u, v)-plane coverage is more difficult. The amount of independent information needed to specify the image should not exceed the number of independent (u, v)-plane samples. But unfortunately neither of these `independents' is easily defined. At one extreme, consider making a high resolution image of a complex source filling the entire primary beam. In this case full (u, v)-coverage is obtained by using all baselines between 30 m and 6000 m in increments of 15 m. Although this was theoretically possible (it takes 25 separate array configurations), it was never attempted and, since the decommissioning of the second 6 km station, is no longer actually possible! If on the other hand the image is smaller than the primary beam then the Nyquist sampling interval is larger and the number of (u, v)-samples required is reduced. The observing time can then be reduced


5 CHOOSING ANGULAR AND FREQUENCY RESOLUTION

16

CONFIG NAME

MAXIMUM BASELINES (metres)

a

6 6 6 6 1 1 1 1 7 7 7 7

. . . . . . . .

0 0 0 0 5 5 5 5

A B C D A B C D A B C D

337 214 153 77 153 31 77 107 7 6 4 3 7 1 6 1

6 5 4 3 3 1 2 2 1 1 1 1

2 3 1 6 2 9 6 1 3 2 5 0

8 6 3 7 1 9 0 4 8 2 3 7

8 7 6 7 4 2 3 4 2 1 1 1

7 5 4 9 2 9 3 7 4 6 9 8

2 0 3 6 9 1 7 4 5 8 9 4

10 9 10 11 5 4 4 5 2 2 2 2

8 4 5 6 6 9 5 8 7 3 4 9

7 9 6 3 6 0 9 2 6 0 5 1

1 1 1 1

4 2 5 2 7 7 6 6 3 4 3 3

2 7 7 8 1 6 8 4 5 1 0 9

3 0 7 6 9 5 9 3 2 3 6 8

1 1 1 1

5 8 7 3

0 0 3 6

0 6 0 2

1 2 1 2

9 0 9 0

5 2 9 8

9 0 0 2

2 2 2 2 1 1 1 1

2 2 1 1 0 0 1 2

9 1 4 5 4 8 4 2

6 9 3 8 1 7 8 4

2 2 2 2 1 1 1 1

5 7 6 4 3 2 4 3 6 6 7 6

8 5 3 4 1 5 0 3 5 4 0 8

7 5 3 9 6 5 8 2 8 3 4 9

2 2 2 2 1 1 1 1

9 9 7 5 4 2 4 4 7 7 7 7

2 6 8 2 6 8 8 3 3 6 5 1

3 9 6 5 9 6 5 9 5 5 0 9

3 3 3 3 3 3 3 3 3 3 4 3

0 0 2 3 0 0 0 0 0 7 2 7

1 0 1 5 0 1 1 0 1 3 7 5

5 0 4 2 0 5 5 0 5 5 0 0

3 3 3 3 3 3 3 3 3 3 4 3

3 2 8 4 4 2 0 2 0 8 3 8

5 1 5 2 2 1 9 1 9 5 1 5

2 4 7 9 9 4 2 4 2 7 6 7

4 3 4 4 3 3 3 3 3 4 4 4

4 7 2 7 7 5 3 8 3 2 4 0

3 5 7 1 5 0 5 5 6 7 6 4

9 0 0 4 0 5 2 7 7 0 9 1

5 5 5 5 4 4 4 4 3 4 4 4

3 0 8 5 3 2 0 3 5 3 7 4

1 2 4 1 1 7 4 3 0 3 1 3

1 0 7 0

5 5 6 5 4 4 4 4 3 4 5 4

9 9 0 8

3 6 0 7 6 0 0 3

9 9 0 8 9 1 0 9

7 7 9 8 3 4 3 4

5 9 4 5 8 7 9 2

0 6 9 7 3 4 8 9

888 1056 1026 1117 4 5 4 5 1 3 4 8 3 6 4 2

6 0 1 2 5 2 4 9

4 3 5 4 7 5 0 4

5 5 5 5

0 0 0 0

4 5 5 6

9 9 5 1

0 7 1 2

5 0 2 6

0 0 0 9

EW367 EW352 H214 H168 H75

46 31 82 61 31

61 46 92 61 31

92 77 132 107 43

138 107 138 111 46

168 122 138 141 46

214 153 144 168 55

230 199 216 171 77

276 245 230 179 77

306 321 240 185 82

367 352 247 192 89

4041 4087 4270 4301 4332

4102 4286 4378 4379 4378

4270 4332 4383 4381 4378

4316 4408 4408 4408 4378

4408 4438 4500 4469 4408

a Antenna 6 sits p ermanently on station W392. Maximum baselines to this antenna (the last 5 columns) are shown for all configurations, but form part of a designed array only for configurations 6.0A to 6.0D.

Table 4: Compact Array Antenna Configurations

either by decreasing the number of configurations or the amount of hour angle coverage, depending on the array configuration. For east-west arrays, reducing the total number of configurations is the more practical option. If the source is large but partly empty it can be considered to have a size corresponding to its area. For ordinary observations Table 3 gives the maximum sizes of structures that can be reliably imaged for typical sets of observing configurations. This is only a rough guide since the actual coverage needed depends on details of the 2-dimensional brightness distribution, on the actual distribution of baselines, and on the type of deconvolution. Additionally, other techniques such as mosaicing (Section 6.3) and multi-frequency synthesis (Section 6.4), can be extremely effective at improving (u, v)-coverage. A further consideration is the minimum spacing available. This is never less than 30 m and for a given configuration can be much larger. This acts as a high-pass filter removing all Fourier components less than the minimum spacing. If this is a serious problem, short baseline information (e.g., from a single dish) can be added separately during processing.

5.2

Array Configurations and Baselines

The antennas can be moved to, and set up on, a limited number of fixed stations. Because of this and other physical restraints, the shortest spacing available is 30 m, the longest is 6 km, and the minimum grating increment is 15 m. Predetermined east-west configurations are offered, each with approximately uniform spacings, and thus with good single-day (u,v)-coverage. From 2006 October, the standard set of configurations has been:


5 CHOOSING ANGULAR AND FREQUENCY RESOLUTION 1 1 1 2 4 4 4 h h h e e e e y y y a a a a b b b s s s s r r r t t t t i i i d d d w w w w configuration with configuration with configuration with est configurations w est configurations w est configurations w est configurations w a a a i i i i n n n h h h h om om om a a a a inal maximum baselin inal maximum baselin inal maximum baselin maximum baseline of maximum baseline of maximum baseline of maximum baseline of e e e a a a a o o o b b b b f f f o o o o 7 16 21 ut ut ut ut 5 m (H75) 8 m (H168) 4 m (H214) 375 m 750 m 1500 m 6000 m.

17

t t t t

Snapshot observations using the Northern spur in hybrid configurations will generate a two-dimensional sampling of the (u,v)-plane. The Northern spur was installed so that good coverage of the (u,v)-plane was achievable for observations which are limited to hour-angles near transit. Observations at mm wavelengths should be limited to higher elevations to avoid large atmospheric opacities. Hybrid arrays are also useful for observations of northern sources which are similarly restricted in hour-angle range. The 6 km antenna may also be added to any of the shorter configurations, but in these cases the distribution of array spacings is bi-modal. The predetermined set of configurations offered for forthcoming observing terms will assist in planning multi-configuration proposals. See http://www.narrabri.atnf.csiro.au/observing/configs.html for details of previous configurations and those being offered in coming semesters. Note that proposals requiring two or more configurations will usually be allotted two or more widely separated times; therefore, expect to make two or more observing visits, or conduct the second observation using remote observing. If you are not allotted all the configurations requested, you should re-apply in the next term, as the proposal will not be automatically reconsidered. The overall philosophy is that, in each semester, there will be a 6, 1.5km, and 750m configuration, and for these baselines, the full set of 4 configurations will generally be covered in three semesters. Each semester will also contain at least one 375m configuration. Analysis of user preferences for the past few years show some configurations to be more generally useful (e.g., providing better single-configuration (u,v)-coverage), and it is desirable to offer these more frequently. Millimtere observing conditions are optimal in the (southern hemisphere) winter, so arrays of 214m or smaller will be offered mainly in the April semester. Besides the predetermined configurations, you can request any standard or non-standard configuration in any term. When writing your application, your scientific justification should include a very convincing argument of why you need to use a special array configuration, rather than one of those offered for the term. If you realise the need for such a `wildcard' request for a significant amount of observing time (e.g., more than 5 x 12h), you can enhance the probability of it being scheduled if you contact ATNF well in advance of the deadline (preferably even before the call for proposals announcement). The wildcard can then be advertised as a potential additional configuration for the term, which may then lead to other proposers requesting it, and making its scheduling viable. For those who wish to improve their (u,v)-coverage by re-observing on different days with different antenna configurations, specific sets of configurations combines well, e.g., 6A, 6C, 1.5B and 1.5D -- for more details see www.narrabri.atnf.csiro.au/observing/users guide/html/ATCA Array Configurations.html An interactive tool, the Virtual Radio Interferometer, http://www.narrabri.atnf.csiro.au/astronomy/vri.html, is available to assist users in exploring the (u,v)-coverage of standard (and non-standard) configurations. We strongly recommend that the proposer gives a clear indication of the maximum extent of their sources. You should also specify the maximum and minimum baselines, and the number of configurations (days) needed.


6 ADDITIONAL OBSERVING NOTES AND TECHNIQUES

18

Figure 4: The instantaneous linearly polarised response as a function of source position within the primary beam. The contours are the response to a point source with a true (not apparent) flux of 1 unit. Left panel: 20 cm response, with contours at increments of 0.001; Right panel: 13 cm response, with contours at increments of 0.01.

5.3

Bandwidths and correlator

The Compact Array Broadband Backend (CABB) installation in March/April 2009 will replace the existing correlator. Details of CABB options are given at http://www.narrabri.atnf.csiro.au/observing/CABB.html.

6
6.1

Additional Observing Notes and Techniques
Short Observations

You may not wish to make detailed high resolution images but instead, for example, survey a large number of compact or simple sources. Under these circumstances, if the sources are strong enough, short observations will suffice (see Table 3). In scheduling these, slew time becomes an important consideration; these may be calculated from the information given in Section 6.3, or more easily, by running ATCASCHED. A single short observation in an E-W array will give a 1-dimensional strip distribution; two or more short observations will give some 2-dimensional information. In such short observations consequent higher sidelobe levels will exacerbate problems caused by confusing sources in the primary beam. Because the Compact Array has poor (u,v)-coverage, in contrast to its good sensitivity, this confusion makes it difficult to reach the receiver noise limit when observing weak sources or searching for detections, particularly at lower frequencies. Current experience suggests that at 6 cm at least 8 short observations well distributed in hour angle are needed to reduce confusion to the noise level. At 13 and 20 cm, short observations are unlikely to ever reach the noise limit.

6.2

Polarimetry

Two orthogonal linear polarisations are measured simultaneously. The position angle of the polarisation splitter is stationary with respect to the alt-az­mounted antennas and so rotates on the sky. A suite of tasks in MIRIAD is available for polarimetric calibration of Compact Array data. No specific calibrators, other than the usual primary and secondary calibrators, are needed for polarisation calibration. An


6 ADDITIONAL OBSERVING NOTES AND TECHNIQUES

19

observation of B1934-638 is essential when only making short observations. Small drifts with time can be corrected for using the on-line measurements of the XY phase differences to an accuracy of 0.5 degrees at all bands. Best polarimetric calibration results when the secondary calibrator is observed at a good sampling of different parallactic angles. MIRIAD is routinely used to calibrate data in all bands, with consistent several months in the cm bands. The on-axis instrumental polarisation is calibrating for instrumental polarisation, we are currently able to reduce to 0.1%, and better with some care. Procedures for 3mm polarization established. results being achieved over typically below 2-3%. After on-axis instrumental effects observations are still being

The off-axis polarisation increases roughly as the square of the distance from the pointing centre at least up to the half-power point. At 20, 13, 6 and 3 cm, the instrumental polarisation is about 1.6%, 9%, 1.6% and 3% of the apparent total intensity at the half power point, respectively. At 20 and 6 cm this error is almost purely linearly polarised (there is no circularly polarised component), whereas at 13 and 3 cm the circularly polarised component is somewhat less than 1% at the half-power point. Fig 4 shows the off-axis linearly polarised response at 20 and 13 cm. Because the ATCA antennas have an alt-az mount, the off-axis response varies with parallactic angle, and will be smeared out by a factor of a few by a long synthesis. This smearing is a function of declination. The MIRIAD task offpol can be used to simulate off-axis polarimetric response of a long synthesis observation. Mosaicing smears out the off-axis response still further, by as much as an order of magnitude. At 20 cm, instrumental polarisation has significant frequency dependence, showing variations of several percent at 1327 and 1444 ± 5 MHz. Leakages of more than 10% occur at 4550 ± 10, 5328 ± 10 MHz and 8780 ± 10 MHz. Data near these frequencies may need to be flagged.

6.3

Mosaicing

Ob jects which approach, or are larger than the primary beam will need to be mosaiced. In such cases, the recommended spacing of pointing centres is half of the primary beamwidth. The Compact Array antennas and control system allow for rapid switching between pointing centres -- as frequently as once per integration cycle. This enables a source to be rapidly mosaiced without necessarily losing (u,v)-coverage. In mosaicing mode, data is not recorded when antennas are driving between fields. The antenna acceleration limit is 800 deg/min/min, and the slew limit is 38 deg/min in azimuth and 19 deg/min in elevation. Mosaicing mode may also be useful for observing large numbers of nearby sources, as the observing overheads are reduced. The `The Australia Telescope Compact Array Users Guide' http://www.narrabri.atnf.csiro.au/observing/users guide/html/atug.html) explains how to set up mosaic files. The `Miriad Users Guide' http://www.atnf.csiro.au/computing/software/miriad/userguide/userhtml.html describes how to reduce a mosaic data set.

6.4

Multi-frequency Synthesis

As (u,v)-distance is proportional to frequency as well as baseline length, different (u,v)-spacings can be obtained, not only by varying the antenna configuration, but also by varying the frequency. Additional (u,v)-coverage can be obtained in the mm bands by observing at multiple frequencies. Observing at two frequencies has the added advantage of increasing sensitivity, as they can be observed simultaneously. Observing more than two frequencies requires time sharing. While this will not improve the sensitivity further, it can significantly improve the (u,v)-coverage. However, there is a trade off between gaps in the tangential and radial directions in the (u,v)-plane. Typically two or three pairs of frequencies, observing each setting for 10 minutes, is a good compromise.


7 HIGH TIME RESOLUTION, PULSARS, PLANETS AND VLBI

20

When you use both bandwidth synthesis and two or three configurations, and require (u,v)-coverage to 6km, the best choice of configurations is not two or three 6km arrays, but a combination of 6km with 1.5km and 750m arrays (all arrays using the 6km antenna). A program (mfplan) is available in MIRIAD to help select configurations and determine optimum observing frequencies. The flux density will often vary significantly between different frequencies and, furthermore, this variation itself (i.e. the spectral index) will vary across the source. This complicates the task of combining data from the different frequencies when you want high-dynamic-range images. However software is available in MIRIAD to account for the spectral variations in the imaging, deconvolution and selfcalibration steps. These algorithms solve for, or use, both a basic flux-density image and a spectralindex image. For typical spectral indices, they are appropriate for frequency ratios less than about 1. 25.

6.5

Reference Pointing

After each reconfiguration a pointing solution is determined at night when thermal effects are least; these typically show rms errors of 10 arcsec. These solutions degrade with thermal effects, especially in summer where an rms of about 30 to 60 arcsec is more likely. A reference pointing mode is available. In this mode, a 1 Jy calibrator, about 5 to 10 away from the target will hold the pointing to 10 arcsec rms. A bright (say 5 Jy) calibrator at 2 to 3 from the target will reduce the errors to about 2 to 5 arcsec rms. Each reference pointing pattern takes typically 18 integration cycles. Reference pointing should be reserved for wavelengths of 12mm and below. See the Reference Pointing Guide http://www.narrabri.atnf.csiro.au/observing/pointing for details.

7
7.1

High Time Resolution, Pulsars, Planets and VLBI
High Time Resolution and Pulsar Observing

CABB is expected, in time, to offer high time resolution and pulsar binning modes. Details will be made available at http://www.narrabri.atnf.csiro.au/observing/CABB.html.

7.2

Solar System Ob jects

The Compact Array can track sources with non-sidereal rates, such as planets or comets. In this case delay tracking is adjusted continuously to account for source proper motion. However the pointing tracks a fixed celestial position during a scan. Thus scans must be short enough that there is not significant proper motion across the primary beam in the course of a scan. This is rarely a problem. JPL ephemerides of the planets are built into the observing program, and a simple mechanism exists to import current JPL ephemerides of other solar system ob jects (e.g. new comets).

7.3

Tied Array Mo de

A tied array capability is available. It provides tying of the array at two frequencies and, for CABB initially, at a bandwidth of 64 MHz. The tied array adder is controlled via a process called CATIE which runs within CAOBS. This allows the choice of which antennas are included and whether the adder produces linear or circular polarisation


8 OTHER THINGS TO CONSIDER

21

outputs. CACAL is used to phase up the array and has an option to allow the insertion of a 90 phase offset between the A and B linear polarisations at each antenna, thereby forming circular polarisation at the tied array output. The tied array adder feeds into the DAS which provides outputs for the VLBI disk-based recorders (at bandwidths between 62.5 kHz and 64 MHz) and for the correlator. Simultaneous Compact Array and tied array operation is possible.

8
8.1

Other Things to Consider
Bandwidth Smearing

Bandwidth smearing (chromatic aberration) can be reduced by analysing each spectral channel independently. Nonetheless at 20 cm, with the 6 km array, radial smearing of the images may still be a problem. See Appendix D (averaging in frequency) in Killeen (1993) http://www.atnf.csiro.au/computing/software/atca aips/atcal html.html for the functional form.

8.2

Confusion

The presence of field sources in the primary antenna beam can limit continuum image quality. These sources produce unwanted sidelobes in the images and can lead to dynamic range and aliasing problems. The brightest source expected on average in the half-power primary antenna beam (away from the Galactic plane) is listed in Table 1. Note that confusion becomes much worse as you go to lower frequencies, and that this can be particularly serious for snapshot observations (see Section 6.1). The SUMSS catalog and database http://www.atnf.csiro.au/computing/software/atca aips/atcal html.html and Molonglo Galactic Plane Survey (MGPS-2) catalogue and database http://www.physics.usyd.edu.au/ioa/Main/MGPS2 are useful references to search for potential confusing sources in the 20cm band, where the effects of confusion are greatest.

8.3

Weather

At short wavelengths and/or long baselines, atmospheric refraction can cause serious phase errors. The problem becomes progressively less serious at longer wavelengths and shorter baselines. Atmospheric conditions are most favourable during winter and at night. A seeing monitor at Narrabri continuously monitors the phase stability at a frequency of 21 GHz and over a baseline of 200 m. See http://www.atnf.csiro.au/observers/docs/7mm/seeing.pdf for details (noting that this paper describes the initial system. In 2008 the a new satellite beacon was adopted, resulting in a change in the frequency from 30 GHz to 21 GHz).

8.4

Artefacts

As with other synthesis instruments, system errors such as DC offsets and sampler harmonics can lead to artefacts at the centre of the field. It is therefore advisable to displace the source positions a few (synthesized) beamwidths from the field centre.


9 WHEN THE OBSERVATIONS ARE FINISHED

22

9
9.1

When the Observations are Finished
Data Reduction

Data are calibrated to the extent possible on-line and stored in a modified FITS format known as RPFITS. You should copy it onto a DVD, or your own hard disk, at Narrabri, and you can be reduce it, at both Narrabri and Epping using MIRIAD (or AIPS). Observers intending to reduce data at Epping after an observing run should see http://www.atnf.csiro.au/observers/accomm concerning accommodation, and http://www.atnf.csiro.au/computing/bookings concerning allocation of workstations and disk space. Narrabri currently has public workstations running solaris and/or Linux. Both sites have DHCP laptop connections for visitors. The Epping site also operates a wireless network, for which a WEP key is required.

9.2

Publications Advice

Observers are requested to acknowledge the ATNF in any publications resulting from the use of the ATNF as follows: `The Australia Telescope is funded by the Commonwealth of Australia for operation as a National Facility managed by CSIRO.' Where possible, authors are requested to include one of the terms, `ATNF' or Australia Telescope', in the ABSTRACT of their papers. This is to facilitate electronic searches for publications that include ATNF data. Please inform Christine van der Leeuw (Christine.VanderLeeuw[at]csiro.au) of any publications which include ATNF data.

10

References and Further Reading

Before observing, you should read the more detailed `Australia Telescope Compact Array Users Guide'. This contains, among other useful information, complete lists of commands for the scheduling program (SCHED), and the observing program (CAOBS). A general list is: 1. Perley R.A., Schwab F.A. & Bridle A.H. (1989) `Synthesis Imaging in Radio Astronomy' Astronomical Society of the Pacific Conference Series, 6. 2. Taylor G.B., Carilli, C.L. & Perley R.A. (1999) `Synthesis Imaging in Radio Astronomy II' Astronomical Society of the Pacific Conference Series, 180. 3. `The Australia Telescope' (1992) J. Electr. Electron. Eng. Aust., 12, No. 2. 4. `The Australia Telescope Compact Array Users Guide' (2006) http://www.narrabri.atnf.csiro.au/observing/users guide/users guide.html 5. Killeen, N. (1993) `Analysis of Australia Telescope Compact Array Data with AIPS' http://www.atnf.csiro.au/computing/software/atca aips/atcal html.html 6. Sault, R.J. & Killeen, N. (1998) `Miriad Users Guide' http://www.atnf.csiro.au/computing/software/miriad/userguide/userhtml.html 7. ATCA Data Acquisition Problems and information http://www.atnf.csiro.au/computing/at bugs.html


11 OBSERVING PROPOSALS

23

8. Reynolds, J. (1994) `A Revised Flux Scale for the AT Compact Array', ATNF Internal Report, AT/39.3/040 http://www.atnf.csiro.au/observers/memos/d96783 1.pdf 9. Sault, R.J. (2003) `ATCA flux density scale at 12mm'. http://www.narrabri.atnf.csiro.au/calibrators/data/1934-638/1934 12mm.pdf.

11
11.1

Observing Proposals
Deadlines

The ATNF usually offers two semesters each year. The October semester runs from October 1 to March 31, with a proposal deadline of June 15, and the April semester runs from April 1 to September 30, with a proposal deadline of December 15. See http://www.atnf.csiro.au/observers/apply/avail.html All ATNF Telescope Applications must be submitted using OPAL: http://opal.atnf.csiro.au/

11.2

Further Information

Requests can be addressed to Dr Jessica Chapman (Jessica.Chapman[at]csiro.au). General Enquiries Observing information Enquiries about Parkes Enquiries about Narrabri and Mopra Enquiries about VLBI Accommodation Remote observing requests atnf-enquiries[at]csiro.au observing[at]atnf.csiro.au parkes[at]atnf.csiro.au narrabri[at]atnf.csiro.au vlbi[at]atnf.csiro.au accommodation[at]atnf.csiro.au rem obs[at]atnf.csiro.au

You can contact any staff member of the ATNF by E-mail. The general address is:firstname.surname@csiro.au. Many documents for the AT, including user guides and proposal application forms are accessible on the ATNF World Wide Web server:ATNF home page Guides and application forms Compact Array schedule SCHED (telescope scheduler) ATNF On-line Archive ATNF Pro jects Database ATNF Positions Database Visitor information http: http: http: http: http: http: http: http: / / / / / / / / / / / / / / / / www.atnf.csiro.au/ www.atnf.csiro.au/ www.atnf.csiro.au/ www.narrabri.atnf. atoa.atnf.csiro.au/ www.atnf.csiro.au/ www.atnf.csiro.au/ www.atnf.csiro.au/ observers/manuals.html observers/sched.html csiro.au/observing/sched observers/search pro j.html observers/search pos.html observers/visit/

Original by Lister Staveley-Smith Maintained by Erik Mul ler Coaxed toward the CABB era by Phil Edwards


A

USING THE WEB-BASED SCHEDULER

24

A

Using the Web-based Scheduler

To run the java interface to the atcasched program you will need to enable java in your browser. Due to security restrictions on applets printing the schedule listing is not straightforward. The Web based scheduler offers several advantages over the terminal (command-line) version and is particularly suitable for new users. We encourage users to try it out and give us feedback. A brief users' guide to get you started: · Make sure you're running the correct browser version and select the page http://www.narrabri.atnf.csiro.au/observing/sched/atcasched.JNLP · First select the correlator configuration file and type in your pro ject code (C123 or similar). · Now enter the details of your program source, including RA, Dec, the frequencies and observing modes. · Press ADD to add the source to the schedule · Now press SEARCH to search for a nearby calibrator, specify the search parameters and press the SEARCH bar. · Pick a suitable calibrator from the list, the distance to your source is given at the very end of the search line (scroll right). · A double click on the calibrator will add it to your schedule · Close the search window. · Now fine-tune your schedule by copying and pasting scans and/or adding more sources. · An on-screen listing is obtained with the LIST button. Use the PRINT bar for a hardcopy. · Enter the schedule name and press WRITE to write it to the schedule area on xbones. Note that you cannot overwrite schedules created with the terminal version of ATCASCHED so if the write fails, try another name. · If you come back later and want to reload your schedule, press the LOAD button and then enter a few characters of the schedule name (any part will do) in the "filter" box. Hit return and pick the file you want. · Expert users can enter atcasched commands in the command line box. The applet downloads the catalog of ATCA calibrators by default. You access it by pressing the source button. A number of other catalogs are available from there as well. Fuller documentation is available online (http://www.narrabri.atnf.csiro.au/observing/sched/).