GBT Scientific Working Group Meeting of November 29, 1994 
 
                   
                AGENDA

  I.  General Project News

 II.  Spectrometer News 

III.  Spectrometer data rates (see information below)

 IV.  X band and Ku band receivers on the 140 foot (see attached information)

  V.  Use of the GBT's active surface (see attached info)



If there are any topics you would like discussed please contact me:
jlockman@nrao.edu
       



	*	*	*	*	*	*	*	*	

		SPECTROMETER DATA RATES

	The design of the spectrometer has progressed to the stage where
we need to specify the fastest rate at which data will be transferred from 
the device to a computer.  This will determine the necessary size of the
computer, among other things.  
	To get the discussion going, we propose the following maximum data
rates (minimum data transfer times ) for transfer of data from the 
correlator:

	Minimum dump interval of raw lag products ("spigot" mode):  ~1 us

	Minimum dump interval for complete frequency-time data from the 
	    Long Term Accumulator: 10 sec

	Minimum dump interval for spectra:  0.1 sec

The adequacy of these rates will be discussed in the meeting.


*	*	*	*	*	*	*	*	*	*

		USE OF THE ACTIVE SURFACE

	Discussions are just beginning on how the active surface will be used
in normal operations.  The following table shows expected aperture 
efficiencies both with and without use of the active surface.  These 
numbers were calculated assuming that the surface is operated in table 
look-up mode.


	Freq	Efficiency Off	   Efficiency On

	1.4	    0.71		0.71
	5.0	    0.67		0.71
	9.0	    0.58		0.69
	15.	    0.41		0.67
	22.	    0.20		0.61
	42.	    ----		0.38


	It seems likely that most observers will want to use the active 
surface for work at 15 GHz and above, and possibly at lower frequencies.  
Because motors and actuators wear out, we would like to use the active 
surface only when necessary.  One might adjust the surface at the beginning
of a 10 min. integration and not move it until the start of the next 
integration, for example.  An operations plan is being developed and will
be discussed by this group in a few months.  In the meantime, input on 
these matters is welcome.
						F.J. Lockman

*	*	*	*	*	*	*	*	*	*	

	GBT RECEIVERS ON THE 140 FOOT -- THE KU BAND SYSTEM

[This note is condensed from a longer report by Ron Maddalena based on 
one evening's measurements.]

My first impression of the Ku-band receiver, given the small amount of
time I had to play with it, is that we have a winner. The tippings 
I did last night gave the following total zenith system temperatures:

			RCP     LCP
			------------
	12.0 GHz	33.1 K	27.7 K
	13.6		28.3	27.2
	15.0		32.6	28.5

Although the measured opacities were low (0.022) they sometimes do 
get lower (~ 0.015) so on the very best of days, Tsys might be a
few K lower than I measured.  Thus, it is safe to say the receiver has, 
on excellent weather days, a Tsys of 26 ~ 30 K throughout the band.  
This is EXCELLENT in comparison to the maser and noticeably better than the
older HFET.  Take another several more K off because of scatter, etc. 
and you'll have the GBT numbers.  Impressive!!

I took spectra over four 80 MHz sections of the A/C, separated by 70 MHz, 
to cover a 290 MHz total bandpass.  This bandpass was moved across the 
receiver in ~ 290 MHz steps to get an almost complete sampling of the
receiver bandpass.  All frequencies produced acceptable bandpass shapes in
40 seconds of integration.

					-- Ron Maddalena