How to determine? How often to measure? Stability?
Lab Hot-Cold loads
On telescopes Hot-Cold loads
Astronomical - standard astronomical calibrators
Frequency and time dependent
Requires Nufss, Tatm, Tcal
Takes telescope time or ancillary radiometer
Accuracy? 2-4 %
Elevation, frequency, feed dependent
Stable - Once modeled
Depends upon source size
Nua - Point sources
Nubeam - Extended to first null
Nufss - Very extended
How to measure with sufficient accuracy?
Beam shape is important.
A specification on the dynamic range is needed to determine beam shape.
Doppler track a single window
Not needed with current Spectrometer - single window
How to apply vector tu, Nu, Tcal?
Use GBTIDL model (in development)?
Doppler track in software after the fact.
Current LO knows nothing about the antenna position.
Discussion on polarimetry
Calibration signal to go to both polarization to determine phase changes.
Calculate a Mu-Matrix for each element.
Off axis beams will have worse asymmetry than the center beams.
Cross polarization terms could get as high as 10%.
Proposal: Drop one of the polarizations to improve Tsys?
If the continuum performance would improve significantly, yes.
Will removing the OMT help Tsys at K-band?
VLBI experiments need dual polarization.
Lost polarimetry in Ka-Band.
The only science case for dual polarization is polarimetry.
Better frequency coverage without the OMT.
There is a bandwidth limitation that is not due to the OMT.
Polarization is needed to identify RFI.
Correlated signals across feeds could be used instead.
Will the prototype be thrown away?
Not a prototype, but phase A.
If dual polarization is dropped, could you have 16 elements?
No, more like 8 - 9 because current backend limitations.
More integrations per pixel.
2 more samplers.
Polarization vs Number of feeds.
The KFPA and future GBT instrumentation
Tuesday, November 27th
Recap of previous day's science discussions
Frequency coverage should be pushed to its highest possible limit. 27.5 - 28 GHz
Dropping the feed rotator will compromise science (projects that require deep integrations where the array foot print has rotated on the sky). However, this might still be the way to go to avoid rotator complications for the prototype.
By dropping the OMT it might be possible to lose ~15K in Tsys.
Calibration and observing modes are well understood.