A description of the sefd curves.


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

    The sefd (System Equivalent Flux Density) curves provide the point source sefd of the telescope in Jy. They are fit to Tsys/Gain using the average of polA and polB (Stokes  I/2). The fits can be a function of azimuth and zenith angle. The data for the fits is taken with the heiles calibration scans (spider scans).

    To use the fits you first divide the source deflection by Tsys giving source flux in Tsys units. Since sefd is Janskies/Tsys you then multiply by sefd to get source flux in janksies.

    The gain curves assume that the cal value remains constant from curve making to curve use. Trouble can occur if the cal values change because the cal diode drifts or the cal diode cable becomes loose. The sefd curves assume that Tsys remains constant between the curve generation and the use of the curves. This is normally very stable. Since Tsys includes the sky contribution, it could change if the sky contribution varied (eg. you are looking at a region of the sky that has continuum radiation).

When are new sefd curves made.  (top)

 New sefd curves are computed when the sefd of the telescope changes (e.g.: moving a horn, reshimming the elevation rails, resetting the primary/secondary/tertiary surface, etc..). They do not need to be redone when the cal diode value 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 sefd curve is installed it contains two dates:
  1. The starting date for this sefd curve. Data taken after this date should use this curve.
  2. The installation data of the sefd curve. Data taken between 1. and 2. should be reanalyzed using the new curve.

Where are the sefd curves located.  (top)

    The sefd curve information is kept in ascii disc files (one file per receiver). The filename format is sefd.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 sefd curve file for lband wide (lbw) is sefd.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 sefd curve files:

The format of the sefd 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 sefd 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