Sometimes when I get data from the archive, it appears that the observers have not observed a flux calibrator. There is a phase calibrator. Is it possible to use this data? Is the data really only good for imaging and getting the morphology of an object, but you can not trust it for absolute fluxes? I feel like I should be able to do something clever to get fluxes...
Tuesday, June 17, 2008
Calibrating Data without a Flux Calibrator
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If you have a question about AIPS that CANNOT be answered by reading the
6 comments:
I've run into this once. I decided not to bother with the data, but I do wonder how the person tried to calibrate the data. (In this case, the data was never published, so I don't know what they did.)
The best you can do is bootstrapping. This is legit; I've done it when eg observations of the flux calibrator were messed up because of weather. And when someone didn't observe a flux cal, it's all you can do.
First thing to check is whether there's a bandpass calibrator.
Then check whether the phase calibrator (or the bandpass calibrator if there is one) is in:
http://www.aoc.nrao.edu/~smyers/calibration/2008/
and/or (for older data):
http://www.vla.nrao.edu/astro/calib/flux/
Depending on your observing frequency, you may want to calculate a spectral index from the information in the database and interpolate to your observing frequency. However, if you're observing in a standard VLA band/with a standard continuum setup, may be no need.
Then, in SETJY, you use the zerosp parameter to enter the value from the monitoring database:
eg
>inp
AIPS 1: SETJY Task to enter source info into source (SU) table.
AIPS 1: Adverbs Values Comments
AIPS 1: ----------------------------------------------------------------
AIPS 1: INNAME 'AC904_FEB17' Input image name (name)
AIPS 1: INCLASS 'CH.0' Input image name (class)
AIPS 1: INSEQ 2 Input image name (seq. #)
AIPS 1: INDISK 3 Input image disk unit #
AIPS 1: SOURCES '2136+006' Sources to modify.
AIPS 1: *rest ' '
AIPS 1: QUAL -1 Source qualifier -1=>all
AIPS 1: BIF 1 Low IF # for flux density
AIPS 1: EIF 1 High IF # for flux density
AIPS 1: ZEROSP 9.0247 *rest 0 I,Q,U,V flux density (Jy)
AIPS 1: OPTYPE ' ' ' ' => use other adverbs
AIPS 1: for required operation
AIPS 1: 'CALC' => determine
AIPS 1: 3C286/3C48/1934 fluxes from
AIPS 1: standard formulae
AIPS 1: 'REJY' => reset source
AIPS 1: fluxes to zero.
AIPS 1: 'REVL' => reset velocity
AIPS 1: to zero
AIPS 1: 'RESE' => reset fluxes &
AIPS 1: velocities to zero.
AIPS 1: CALCODE ' ' New calibrator code:
AIPS 1: '----' => change to blank
AIPS 1: SYSVEL 0 Velocity of source (km/s)
AIPS 1: RESTFREQ 0 0 Line rest frequency (Hz)
AIPS 1: VELTYP ' ' Velocity type 'LSR,'HELIO'
AIPS 1: VELDEF ' ' Velocity definition 'RADIO',
AIPS 1: 'OPTICAL'
AIPS 1: FREQID 1 FQ table entry to use for
AIPS 1: velocity information and
AIPS 1: 'CALC' option
AIPS 1: APARM *all 0 (1): Pixel to which SYSVEL
AIPS 1: refers ( 0=>1)
AIPS 1: (2): Only for 'CALC' option:
AIPS 1: <= 0 => use latest VLA
AIPS 1: values (1999.2) or,
AIPS 1: for 1934-638, the
AIPS 1: ATCA value of 30Jul94.
AIPS 1: 1 => use Baars values
AIPS 1: or old ATCA/PKS values
AIPS 1: for 1934-638
AIPS 1: 2 => use VLA 1995.2
AIPS 1: values or for 1934-638
AIPS 1: the ATCA value of
AIPS 1: 30Jul94.
AIPS 1: >= 3 => use oldest VLA
AIPS 1: values (1990) or,
AIPS 1: for 1934-638, the
AIPS 1: ATCA value of 30Jul94.
AIPS 1: (3): Only for 'CALC' option:
AIPS 1: multiply the calculated
AIPS 1: fluxes by APARM(3) with
AIPS 1: 0 -> 1
and for eg 3C286
AIPS 1: SETJY Task to enter source info into source (SU) table.
AIPS 1: Adverbs Values Comments
AIPS 1: ----------------------------------------------------------------
AIPS 1: INNAME 'AC904_FEB9' Input image name (name)
AIPS 1: INCLASS 'CH.0' Input image name (class)
AIPS 1: INSEQ 1 Input image name (seq. #)
AIPS 1: INDISK 2 Input image disk unit #
AIPS 1: SOURCES '1331+305' Sources to modify.
AIPS 1: *rest ' '
AIPS 1: QUAL -1 Source qualifier -1=>all
AIPS 1: BIF 1 Low IF # for flux density
AIPS 1: EIF 1 High IF # for flux density
AIPS 1: ZEROSP *all 0 I,Q,U,V flux density (Jy)
AIPS 1: OPTYPE 'CALC' ' ' => use other adverbs
AIPS 1: for required operation
AIPS 1: 'CALC' => determine
AIPS 1: 3C286/3C48/1934 fluxes from
AIPS 1: standard formulae
AIPS 1: 'REJY' => reset source
AIPS 1: fluxes to zero.
AIPS 1: 'REVL' => reset velocity
AIPS 1: to zero
AIPS 1: 'RESE' => reset fluxes &
AIPS 1: velocities to zero.
AIPS 1: CALCODE ' ' New calibrator code:
AIPS 1: '----' => change to blank
AIPS 1: SYSVEL 0 Velocity of source (km/s)
AIPS 1: RESTFREQ 0 0 Line rest frequency (Hz)
AIPS 1: VELTYP ' ' Velocity type 'LSR,'HELIO'
AIPS 1: VELDEF ' ' Velocity definition 'RADIO',
AIPS 1: 'OPTICAL'
AIPS 1: FREQID 1 FQ table entry to use for
AIPS 1: velocity information and
AIPS 1: 'CALC' option
AIPS 1: APARM *all 0 (1): Pixel to which SYSVEL
AIPS 1: refers ( 0=>1)
AIPS 1: (2): Only for 'CALC' option:
AIPS 1: <= 0 => use latest VLA
AIPS 1: values (1999.2) or,
AIPS 1: for 1934-638, the
AIPS 1: ATCA value of 30Jul94.
AIPS 1: 1 => use Baars values
AIPS 1: or old ATCA/PKS values
AIPS 1: for 1934-638
AIPS 1: 2 => use VLA 1995.2
AIPS 1: values or for 1934-638
AIPS 1: the ATCA value of
AIPS 1: 30Jul94.
AIPS 1: >= 3 => use oldest VLA
AIPS 1: values (1990) or,
AIPS 1: for 1934-638, the
AIPS 1: ATCA value of 30Jul94.
AIPS 1: (3): Only for 'CALC' option:
AIPS 1: multiply the calculated
AIPS 1: fluxes by APARM(3) with
AIPS 1: 0 -> 1
From here on, calibrator normally (obviously excluding the source you've done SETJY on from your GETJY inputs). If you're lucky, there will be two phase calibrators, or a bandpass and a phase calibrator, or something that will let you estimate your errors...
Oops, I didn't mean to include the 3C286 part in there (just the 2236 set of inputs)...
This often happens when the object is being regularly monitored as part of a long-term program. If it is always observed with the same phase cal, chances are that the phase cal changes in flux only fairly slowly with time. In which case you can take a look in the archive for observations within a few days which contain both the phase cal of interest and a flux cal, at the observing frequency you want. Reduce that dataset up to the point where you have run GETJY on the phase cal, and then use that flux with Claudia's ZEROSP trick on the dataset you're interested in. If you're wanting to be even more clever, do this twice, with data either side of your observation date, and linearly interpolate the phase cal fluxes between the two observations.
I have a data with no phase calibrator. I could use a NVSS map to self calibrate it, but then the flux calibration is not done. I can get 1/3 rd of the peak flux expected at the filed (central part). Is there a way to get the flux calibration done ?
Regards.
Hi Anonymous,
I would contact the NRAO helpdesk with your question. Logon to my.nrao.edu and click on the helpdesk tag.
Amanda
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