Subject: Receiver priorities

November 9, 1993

Summary of Comments about Receivers made at the Scientific Working Group 
Meeting of October 26, 1993.


At the recent meeting of the GBT Scientific Working Group several options
were discussed for receiver development in 1995 and beyond:


Q-band:  The 40-52 GHz system was given very high priority.  In the 
original design, it was to consist of eight separate receivers
arranged in four dual-channel feeds.  One pair of feeds would cover
40-45 GHz and the other 45-52 GHz (see GBT Memo 66).  Because of the
VLA Q-band upgrade, we were able to obtain polarizers at a very favorable 
price that cover the entire 40-50 GHz band (with some performance expected 
to 52 GHz).  The current conceptual design is therefor to have only two
dual-channel feeds that cover the entire band perhaps optimized over
the 42-52 GHz range.  This leaves enough money in the budget for four 
additional channels.

     The consensus of the group seemed to be that a linear array was the
best configuration.  A configuration with two dual-channel and four
single channel feeds could easily fit into the available space.  


Tertiary Reflector:  A tertiary reflector gives a mechanical method 
of beam switching and, possibly, correcting for some pointing errors.
The secondary reflector is so large that it probably will not be available
for beam switching.  The tertiary might be incorporated into the Q-band
system as it is designed.  There was no strong support, however, for a 
tertiary among members of the Working Group.  


 
S Band Receiver:  After Q-band, the 1.73-2.60 S-band receiver was given 
the highest priority of the remaining Gregorian receivers.  This system
could be used for VLBI, and for planetary radar in conjunction with 
the upgraded Arecibo telescope transmitting at 2.38 GHz.

    Dual S/X capability is very popular on the VLBA and the GBT should be
prepared to support this observing mode.  This will require design of 
a dichroic and curved reflector for splitting the optical path. 


Very Low Frequency Receiver:  Interest was expressed in having a receiver
that could cover the 25-75 MHz band for spectroscopy of ionized atoms.  The
electronics for this would be quite inexpensive.  Most of the cost would be
in the feed.  


Multibeam receiver:  It was proposed that a 12-15.4 GHz 7-beam system
be built for continuum mapping and for microwave background studies.  The
receiver could cover large areas to find exotic radio stars or to 
map low-surface-brightness sources.  Development costs for the polarizer 
and some other parts of this system have already been paid, so it should
be straightforward to build.


We will develop cost estimates for the very low frequency and multibeam
receivers and bring them to the group for later discussion.  But there
are several other issues which should be resolved in the next few 
months:

QUESTIONS:
(1)  Given the uncertainties in the GBT performance at 50 GHz, and the 
need to use the unproven active-surface and laser pointing systems 
for operation in this band, should the Q-band array be given high 
priority as a first-generation receiver?  Even if it is desirable to 
have some Q-band capabilities on day 1, wouldn't it be best 
to test  the Q-band performance using the basic dual-feed system and
later develop an array?  Would a better idea be to build a single
dual-polarization feed with a tertiary for fast beam switching and
fast correction of pointing errors?  The expected HPBW at 50 GHz is 15".

(2) Are there objections to adding the S band receiver to the first 
generation group?  There is money in the budget to do this. How 
important is simultaneous S/X observing?  Should this be a priority
for 1996?

(3) What are the scientific requirements on a tertiary reflector?  
What is the lowest frequency at which it would be used?  What is the
desired range of beam-throw and rate?  Is it is desirable
to be able to position the reference beam at any angle with respect to
the signal beam?




NOTE: The next SWG meeting has been moved to November 30.