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General Introduction and Consortium Guidelines

Next: Pulsar Surveys Up: Minutes of the 1st Previous: Ly-alpha Survey

Minutes of the talk on:
General Introduction and Consortium Guidelines

Don Campbell (NAIC)

History of the Arecibo Observatory and ALFA

Nov 1959	Construction contract signed. Telescope has limited steerability and thus needs to be close to equator to see ecliptic. Spherical reflector optics.
1960	Construction begun.
1962	Hole in ground saved a lot of work but still huge amount of excavation. Rim road in place, towers in place, raised triangle & attached to towers by cables.
Nov 1962	Triangle in place, feed arm construction at bottom of bowl.
Feb 1963	Feed arm raised into place (about 100m long).
Jul 1963	Installation of first generation wire mesh reflector (1/2 inch mesh). Arecibo is an astronomical telescope but not actually conceived by radio astronomers!
1963	Construction complete. Right away it was realized that you could do a lot more if you could drive telescope to higher frequency.
1967	Pulsars discovered.
early 1970's	Development of idea to replace wire mesh with 38,000 1m X 2m panels.
1974	Surface partly covered; surface accuracy 3mm but still a problem with line feeds. PSR1913+16 discovered with AO which lead to the 1993 Nobel prize going to Hulse & Taylor.

Limitations of Line Feeds

While extremely successful at turning Arecibo into a premier instrument for radio astronomy notably for the study of pulsars and HI line work, the line feeds nonetheless had distinct disadvantages:

Line feeds are inherently lossy and narrow band, requireing separate feeds for each frequency band.
Line feeds are tough to make so that propagation velocity up circular guide is maintained. In particular, they are really tough/impossible to make for the highest frequency bands (too many holes!), and thus Arecibo cannot take advantage of its surface accuracy.

During the '80's, design work was done on a Gregorian feed system. The idea was to change the optical path to a Gregorian system allowing wideband operations with lower losses. The upgrade was first planned by Sebastian von Hoerner (NRAO) and Tor Hagfors (NAIC), then Per Simon Kildal and Lynn Baker (NAIC).

Another limitation to the sensitivity of the system arises because, as you move out in zenith angle, the illuminated area on the reflector spills over. Hence another improvement would come from the construction of ground screen around the main dish to reduce spillover noise. The ground screen reflects cold sky instead of ground pickup.

2nd Arecibo Upgrade

ground screen
dual reflector Gregorian
new pointing system
active cable/tiedown control
new S band radar
improvements to 430 MHz system

The upgrading project also required infrastructure improvement because of increased load, in particular the additiona of new cables and new cement archor blocks. The ground screen is 15m high and 1 km long. A jacking system controls the altitude (height) and attitude of platform which otherwise shifts, among other things, when the radome moves up/down the azimuth track.

On May 16, 1996, the Gregorian dome raised was raised from a position at the bottom of the bowl. Inside the dome is a 22m secondary reflector and a 8m by 9m tertiary. The ALFA system will sit on the rotary floor, and a tertiary skirt (mentioned in German Cortes' talk) will be installed around the top of the tertiary reflector to reduce spillover.

The reflector surface needs to be reset periodically to maintain its surface accuracy and shape. There are reflecting targets on all 38,000 panels and we use photogrammetry techniques to set each panel to 0.67mm accuracy in X,Y,Z. Using this method, the 11mm surface accuracy immediately after the upgrade has reduced to about 3mm. We hope to be able to get to 2mm, providing sufficient accuracy to allow opertation potentially up to 15 GHz.

Current performance

11 K/Jy at 21 cm, 10 K/Jy at 12 cm and about 7 K/Jy at 6 cm
about 6 arcsec pointing
hopefully tertiary skirt will reduce Tsys by 5K
new L-wide has extra noise below 1300 MHz which we need to work on reducing

The Tertiary skirt under design by German will be fabricated and installed in 2003 to reduce spillover. It will benefit all receivers.

NAIC

NAIC is supported by National Center for research in Astronomy and Atmospheric Science. NAIC is a multidisciplinary Research Center used by scientists on an open and competitive basis for research in space and atmospheric science, solar system radar studies, and radio astronomy. There is much synergy between these rather diverse science areas.

The distribution of telescope time among the science areas in PY2002:

Radio astronomy used 75% of the available telescope science time
Solar system used 8%
Space and Atmospheric Science used 17%

Current AO Scientific Staff Breakdown for 2003 was shown. The scientific staff is small and split over the 3 science areas.

ALFA Concerns

ALFA needs to be combined effort between NAIC and consortia.
ALFA footprint: uniformly spaced feedhorns in focal plane do not translate into uniformly spaced beams on the sky.
ALFA beams: non uniform performance, no uniform footprint on sky.
Coma lobe: off axis horns have signficant sidelobes and coma lobes, each different. Will need to be dealt with.

ALFA spectrometers

EALFA	100 MHz, 4K channels minimum
GALFA	8 MHz, 8K channels (1 kHz BW/chan)
PALFA	300 MHz, 1K channels
SETI	provided by SETI folks

The WAPPs, as discussed in Jeff Hagen's talk can handle the EALFA minimum requirement within a couple of months. If EALFA agrees to this, then NAIC will work on PALFA spectrometer; this latter instrument has not yet been started and is very important. The GALFA spectrometer might be done with commercial hardware. Decisions on the path to provide all the backends need to be made soon so that they are all ready at the end of 2004 when ALFA is ready.

Guidelines for the Consortia

Among the issues that we must jointly consider are:

Relative responsibilities of NAIC and the Consortia for planning and implementing the surveys and producing basic products and archiving
Timing for submission of proposals
Proprietary rights

Proposed Guidelines:

NAIC has the ultimate responsibility for the success of the survey, so NAIC intends to provide oversight of the program via NAIC/consortia committees that are chaired by e.g. the NAIC ALFA project manager or project scientist
The consortia will submit proposals in reponse to a request by NAIC before the start of next year (end of 2003).
Proprietary time should be as small as possible, with "0" as a goal after the production of each block of data

The tasks that we can identify that require both Consortium and NAIC participation include:

Specification of the technical requirements based on the science requirements
Data taking hardware/software requirements and implementation
Observation planning
Data analysis software specification and development
Software process and pipeline from data analysis to the basic archival product
Data archiving

Possible Grant Support for US based Participants

Additional support might be available for:

Production of data taking hardware and software
Planning and assisting with observations
Data analysis
Support for graduate students resident at Arecibo.

Discussion/Questions:

Karen O.	Bill Sisk and Jeff Hagen have the impression that they could make WAPPS expanded to 200 MHz.
Don	Yes, but that requires lots of work and so the work on the pulsar spectrometer could not take place at the same time. There will have to be tradeoffs here. If 100 MHz is adequate, to start with at least, then we'd propose to start with 100 MHz for EALFA and work on the pulsar spectrometer, then perhaps later/if time, expand the WAPPS also. We currently do not have the technical capability to do both.
Karen O.	If this group says we need 200 MHz, then is NAIC just going to say tough?
Don	I hope that everyone is willing to be realistic. A number of people say that while not ideal, 100 MHz would be enough initially. If a really strong statement is made, we will take a look at it. But I am hoping that 100 MHz will be adequate initially so that NAIC could focus on delivering the pulsar spectometer at the end of 2004.
DJ P.	The way you propose grant support seems to be different from the way national observatories function. Usually the observatory provides both the instrument and the software to support it. In this case, NAIC is not producing software the way, say, NOAO does by developing the needed tasks in IRAF.
Don	NAIC anticipates being very strongly involved in production of basic analysis software, at least through the basic product, calibrated spectra, maybe with RFI excision at each position. But the consortia will be involved in development of software to produce this basic archival product. But after that, we intend to leave it up to others.