GBT Scientific Working Group Meeting of March 14, 1995 
 
      4 PM EST

                   
                AGENDA

  I.  General Project News (Bob Hall)
	Dynamic Analysis

 II.  Spectrometer News (Ray Escoffier)

III.  Antenna Position parameters (see information below)

 IV.  Switching techniques (see information below)

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

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Antenna Position Parameters

	At what interval will GBT observers wish to know the telescope position
during an observation, and how much information about telescope pointing
is needed?  The background to this question is as follows:  when observing
with the GBT the Monitor and Control (M&C) system is capable of logging
information about the telescope position at a great rate.  This 
information can be collated with data in offline software and relevant
information stored in the data header.  It will be possible for observers
to pick and choose the information that is in the data header, but it would
be useful to know what items will be in most demand.  For example, during
long integrations is it necessary to store the telescope position every 0.1 
seconds?
	Another issue is the kinds of information that are needed.  Do
observers want the raw elevation and azimuth encoder readings? How about
the readings as processed backwards into the commanded positions?  For 
example, if one commands in J2000, would it be sufficient to have stored
the actual J2000 positions as inferred from the actual encoder readings 
during the observations?  Do data on the exact pointing model also need to
be stored?  Do data on the atmospheric corrections need to be stored?

	We would like to spend some time at today's meeting discussing these
issues.


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Switching Techniques

	Many observing modes on the GBT will require stepping and scanning 
of the telescope or some part of it.  It would be useful if we could review 
the various kinds of switching techniques required for GBT observations.  
To focus discussion I somewhat arbitrarily choose the following requirements 
for the GBT.  If you are aware of other needs for switching, stepping or 
scanning, or have comments on rates, please speak up at the meeting.

Switching Modes:
	Frequency switching for spectroscopy to remove bandpass and gain
		variations.

	Beam switching between the two feeds of receivers above 10 GHz.
		This will be accomplished by an RF switch located after
		the first RF amplifier.  The distance between feeds is 
		fixed, but the angle can be controlled through the feed
		rotator.  Switching can be done at several hundred Hz or
		as limited by the backends.  (See the representative 
		receiver block diagrams in the GBT WWW entry under 
		"electronics").

	Position switching to move the telescope between a source and reference
		positions.  The switching rate will likely be limited by the 
		telescope settling time. 

	Scanning by moving the telescope while taking data continuously.
		This is the "on-the-fly" mapping mode in line or continuum.

	Secondary switching uses the subreflector tilt.  It is limited to
		+-0.2 degrees in azimuth and a factor of three less in
		elevation.

	Tertiary switching involves the as yet to be designed tertiary
		reflector.  

Switching is used for the following purposes:

	Removal of instrumental bandpass (spectroscopy) at rates ~1 sec for
		frequency switching; ~5 min for position switching.

	Removal of atmospheric fluctuations (can be reflected in bandpass
		shape changes) for spectroscopy: as quickly as possible,
		say ~0.1 sec.

	VLBI phase referencing -- frequent moves between an object and a
		calibrator a few degrees away to remove atmospheric phase
		changes.  Time scale ~1 min.
		
	Removal of total power fluctuations, gain and atmosphere, for 
		continuum mapping.  Scanning rate at least 2 deg/min.