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).
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:
When are new sefd curves
The starting date for this sefd curve. Data taken after this date should
use this curve.
The installation data of the sefd curve. Data taken between 1. and 2. should
be reanalyzed using the new curve.
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:
Where are the sefd
curves located. (top)
The first column of each line tells how this line should
The format of the sefd curve
There is a set of column labels above each data set (as a comment line).
; the line is a comment
! yyyy ddd This is the start of a sefd curve set of data. yyyy ddd
is the start date for the sefd curve. yyyy is a 4 digit year (2004)
and ddd is the day number of the year counting from 1.
all other lines are sefd curve data. These lines are white space separated
col 1 the frequency in Mhz for this measurement
col 2 The type of fit used for this sefd curve (see below for the codes).
col 3 -numcoefs. The coef's for the sefd curve fit
colLast-3 the sigma for the fit in (K/Jy).
colLast-2 the polUsed for the fit (this is always I for stokes I).
col last 2. the cal values used (pola,polB). These values are for reference
There are currently 6 types of fits that can be used.
They are identified by the 2nd (type) column in the ascii file.
The fit types used: (top)
The az terms contain:
type 1: y=c0 + c1*za + c2*(za-14)^2 + c3*(za-14)^3 + azterms1
. See Note1.
type 2: y=c0 + c1*(za-10) + c2(za-10)^2+c3(za-10)^3 + azterms1
type 3: y=c0 +c1*(y)+ c2*(2*y*y-1.) + c3*(4*y^3-3*y) + azterms1
type 4: y=c0 +c1*za +c2*(za-14)^2 +c3*(za-14)^3 (same
as 1 but no az terms). See Note1
type 5: y=c0 + c1*(za-10) + c2(za-10)^2+c3*(za-10)^3 with
type 6: y=c0 + c1*(za-10) + c2(za-10)^2+c3(za-10)^3 (same
as 2 but no az terms)
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.
azterms1: c4*cos(az) +c5*sin(az) + c6*cos(2az) + c7*sin(2az)
+ c8*cos(3az) +c9*sin(3az)
azterms2: c4*cos(az) +c5*sin(az) + c6*cos(3az) + c7*sin(3az)
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:
Code to evaluate the fits
In the generic idl routine section.
In the correlator idl section. The header is an interim correlator header.
It takes the receiver number, az, za, date and frequency from the hdr.
This will also work for wapp data (although alfa may not work..).