A description of the sefd curves.

Jul06

Links:
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:

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.

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:

sefd.datR5 (lbw)

The format of the sefd curve files. (top)

The first column of each line tells how this line should be interpreted:

; 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 columns containing:

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 only.

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.

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 where y=(za-10)
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 azterms2
type 6: y=c0 + c1*(za-10) + c2(za-10)^2+c3(za-10)^3 (same as 2 but no az terms)

The az terms contain:

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) + c8*sin(za)*cos(3az)+c9*sin(za)sin(3az)

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:

istat=sefdget(az,za,freq,rcvrNum,sefdval,date=date). In the generic idl routine section.
istat=corhsefdget(hdr,sefdval,date=date,az=az,za=za). 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..).

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