Science Advisory Committee
Resolutions to Unresolved Issues:(March 2008)
- Exact frequency coverage of the front end. Is this optimal for the science? What are the technical options? Technology dictates 18-26.5 GHz. (limitations are primarily waveguide modes and polarization degradation caused by phase shifter).
- What are the desired polarization characteristics of the array? Is the choice of circular polarization for all feeds the best? Circular.
- Noise injection: is there a need for various intensity cals? No
- Noise injection: One per pixel: What phase balance is needed between polarizations? All cals fired simultaneously sufficient? Yes
- Feed rotator: is it necessary? What should its specs be? (This needs to be considered in the contect of the performance of the off-axis feeds, which may not be symmetric around the central pixel). No Rotator
- Continuum mapping capabilities: should this be any consideration? If so, what does it imply? No detectors (outside the GBT IF system) or pseudo correlation provisions.
- A baseline stability requirement for determining isolation and other system specifications. Specified.
DJ Pisano's Observing Modes Memo: Spectral-line Observations.pdf
The 7 feed receiver may eventually replace the current GBT K band receiver which has two pairs of two beams for upper (18 to 23 GHz) and lower K (22 to 26.5 GHz) band frequencies. Some observations with the 7 feed receiver will use only a sub-set of the beams (as noted by DJ's memo above). Further notes on observing modes are given here:
- 1 Beam: Primarily for VLBI and frequency switched observations of point sources. Also possible for high spectral resolution position switched observations
- 2 Beam: Current normal K band observing mode for position switched, NOD, observations, allows up to 4 spectral windows.
- 4 Beam: Possible observing mode for observing two spectral bands while mapping.
- 7 Beam: Normal mapping mode, allowing dual polarization, single spectral window observations
We note, for completeness, that simultaneous observations of multiple spectral windows will only be possible within the 1.8 GHz spectra window of the downconverters. This is a slight change from the current K band system which allowed multiple spectral windowed observations with 3.6 GHz spectral window of the IFRACK. Note that the normal mode of the IF Rack will be all pass, so that the IF Rack will always pass the full pair of 1.8 GHz wide spectral windows (for the two polarizations).
Table of observing modes with the GBT spectrometer: pdf
A memo authored by J. Condon discussing observing modes and comparisons with the EVLA. Condon Memo