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Ccb CMB Foreground Point Source Observing Campaign

This is an area to organize our CCB effort for Spring 2006, program codes

Here is an index to what follows

At various points in this writeup I refer to existing data, and here is where you can find it

Observing Program

Observing Strategy

Occasionally some component of the system will fail and your pointing will fail. Some things I've had trouble with are the dynamic (temperature-based) corrections software; the online pointing analysis software (GFM); and the subreflector being in a wacky state (say a slew rate was left in the z-axis from the previous observer somehow). Usually this will be reflected in trouble with your initial peak and focus; however to guard against this we should probably monitor all our observations. Things to watch are: peak/focus solutions are obtained and updated; solutions are reasonable (<10" at night, perhaps up to 20 or 30" during the day); subreflector slew rates are zero; and that nod amplitudes are consistent on the pointing sources.

A big worry BSM had initially was the scan-start ringing. This happens when a bug in the antenna manager or servo code caused an abrupt scan-start motion that excited vibrations in the GBT feedarm. However this appears to be much less problematic than earlier experience indicated. Just looking at the bright-source data from 21 and 22dec, I would say that no scans have their fitted flux densities affected by more than 10%-- and most by much less-- by being off-source or vibrating initially. For these observations 20 sec was spent in each NOD phase, and a 20 sec dwell was inserted at the start to let ringing die off. The cleanness may be due to the observing strategy: before the scan starts we have already done a slew to the beam we're going to start with, and perhaps this antenna problem just isn't invoked that way. I suggest that we cut the starting dwell to 10 sec (matching the 10 sec per NOD phase we have planned anyway) and watch this like a hawk... if the behavior continues to be good, we can probably eliminate even that 10 sec dwell on our real sources-- probably keeping it on the calibrators as a check.

There are two low-level, known servo issue that are visible in our data but won't significantly affect our results. One is a 1 Hz oscillation in the servo, often visible on bright sources. This will impose a few percent multiplicative RMS to the signal. THe other is that we consistently "overshoot" when slewing in one direction of the NOD. BSM thinks this is due to a (known) stiction problem in the servo-- in one direction you are slewing w/the sky, in the other against, and go through zero velocity. Under low-wind conditions the antenna indicated positions capture this overshoot quite well (see the agreement between model and data), and it is a small fraction of the time in the nod, so it won't bias our results.

GBT staff have characterized long-term deviations from the pointing model in great detail. A writeup of recommended procedures for pointing (and the active surface as well) is here. In summary, at 30 GHz we should rederive peak and focus offsets every 30 minutes during the day; in my experience we can expect 45 minutes to be safe at night.

Observing Details

[GfmDataProcessingDefaults]
BeamSwitched: Raw
Here is some GFM documentation.

Source Lists/Logistics

Nearly all the files referenced in my template scheduling block live in the world read-writable directory ~bmason/gbt-obs/gbtcbiruns/ The exception is that the python source code for my custom-defined scanning procedure, and for autopeakfocus2, live elsewhere, but my SBs add those to the path. Larry and Brian agreed to put SBs for individual runs there directly or in subdirectories of it as seems appropriate. These will be ASCII text files that will need then to be imported and validated into ASTRID.

Data Analysis

A useful link: our Ka/CCB channel identifications summary

Structure Description: CcbCmbIDLGuide

Discussion for June 9: CcbCmbJune9

Results

Current to do list

-- BrianMason - 27 Jan 2006

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