A description of the gain curves.


When are new gain curves made
Where are the gain curves located and links to the files
The format of the gain curve files
The fit types used
Code to evaluate the fits

    The gain curves provide the point source gain of the telescope in Kelvins/Jy. They are fit to the average of polA and polB (Stokes  I/2) and are a function of azimuth and zenith angle. The  data for the fits is taken with the heiles calibration scans (spider scans).

    The gain curves are used to compute the flux of objects measured by users. This method relies on the cal deflection to transfer the flux of the known calibrators to the flux of the unknown source observed by the user. We say that the gain curves are in Kelvins/Jansky since we think we know what the cal temperature is in Kelvins. A better way would would have been to say that the gain curves are in calUnits/Jy. It doesn't really matter if we have the correct cal Temperature. We only need to guarantee  that the cal temperature does not change from the time the gain curves were made , to when the user used applied the gain curve to their source.

When are new gain curves made.  (top)

 New gain curves are computed when the gain of the telescope changes (e.g.: moving a horn, reshimming the elevation rails, resetting the primary/secondary/tertiary surface, etc..) or when the cal values change (e.g.: new diodes or some connection from the diode to the dewar changes). Spider scans are taken starting at the change until enough data is acquired to do the fit (this may take several months). When the new gain curve is installed it contains two dates:
  1. The starting date for this gain curve. Data taken after this date should use this curve.
  2. The installation data of the gain curve. Data taken between 1. and 2. should be reanalyzed using the new curve.

Where are the gain curves located.  (top)

    The gain curve information is kept in ascii disc files (one file per receiver). The filename format is gain.datRn where n is the receiver number (1-327, 2=430, 3-610, 5-lbw, 7-sbw, 8-sbh, 9-cb, 10=cbh, 11=xb, 12=sbn). So the gain curve file for lband wide (lbw) is gain.datR5. These files are located in the idl directory /pkg/rsi/local/libao/phil/data at the observatory. Anyone who downloads the ao idl distribution will have these files as part of the distribution (under ./data). The links below point at the gain curve files:

The format of the gain curve files.  (top)

    The first column of each line tells how this line should be interpreted: There is a set of column labels above each data set (as a comment line).

The fit types used: (top)

    There are currently 6 types of fits that can be used. They are identified by the 2nd (type) column in the ascii file. The az terms contain: Note 1: The (za-14) terms are only included when the za is greater than or equal to 14 degrees za. Below za=14 the fit is linear.

Code to evaluate the fits  (top)

    There are idl routine that evaluate the gain curves. Given az, za, rcvr, frequency, and date they will find the correct file,  input the coef's for the correct date range, evaluate the function interpolating to the requested frequency. The routines will not extrapolate. If the requested frequency is outside the measured frequencies, then the value at the last frequency measured is returned. The routines are: home_phil