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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.
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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.
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ALFA footprint: uniformly spaced feedhorns in focal plane do not translate into
uniformly spaced beams on the sky.
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ALFA beams: non uniform performance, no uniform footprint on sky.
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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
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GALFA | 8 MHz, 8K channels (1 kHz BW/chan)
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PALFA | 300 MHz, 1K channels
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SETI | provided by SETI folks
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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.
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Last modified: Thu Apr 24 11:20:44 EDT 2003