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Photometric Calibration using SDSS frames
12-01-2014, 11:24
Post: #1
Photometric Calibration using SDSS frames
As novice astromatic user I've had the last few weeks to enjoy the simplicity of creating an image stack from dithered images using Astromatic software (Sextractor, Scamp, Swarp).

Now this stack is finished I would like to do photometric calibration on it and from what I gathered I can use Scamp for this. Because I'm imaging SDSS sources, I can include a photometrically calibrated SDSS frame in the scamp run and have the relative photometry of Scamp calibrate the photometry of the stack.

At least this is what I gathered from the scamp documentation and this forum.

However, it seems the trick is not working and I would like to request your help to solve this issue (or tell me it's fundamentally impossible).

My procedure:
* download SDSS frame and use python to add the following keywords to the header: PHOTFLAG=T, PHOT_C=22.5, PHOT_K=0.0, AIRMASS=1.0
* Run Sextractor on SDSS frame and stacked frame to produce FITS_LDAC catalogs
* scamp coadd.cat sdss-r.cat -c BLERG.scamp

So far I've identified 2 potential problems, but I'm unsure how to check them:
1) the SDSS frame is not recognized as photometrically calibrated
2) there are no overlapping sources detected (while there should be, it the same part of the sky and both catalogs contain at least few hundred sources).

I've added the scamp config file and xml for further info.

Thanks in advance for any help.


Attached File(s)
.xml  scamp.xml (Size: 24.19 KB / Downloads: 460)
.txt  BLERG.scamp.txt (Size: 8.76 KB / Downloads: 416)
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12-01-2014, 11:32 (This post was last modified: 12-01-2014 11:33 by hbouy.)
Post: #2
RE: Photometric Calibration using SDSS frames
Hi RJanssen,

We should try to solve your issue, but am just wondering: why don't you calibrate the stack image a-posteriori?
Just extract the sources from the deep stack, cross-match with SDSS, and derive the zero point.

It might be a quick workaround...

Hervé

ps: if you still try to scamp your coadd.cat sdss-r.cat, make sure both have the same PHOTINSTRU_KEY! otherwise they will be considered as two different photometric instrument.
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12-01-2014, 14:55
Post: #3
RE: Photometric Calibration using SDSS frames
The workaround is a good one. (Cleary got to focussed on getting this to work).

(12-01-2014 11:32)hbouy Wrote:  ps: if you still try to scamp your coadd.cat sdss-r.cat, make sure both have the same PHOTINSTRU_KEY! otherwise they will be considered as two different photometric instrument.

The PHOTINSTRU_KEY was the first issue. Using python I edited some header into, such that the FILTER matched between the stack and SDSS, and I now have 1 photometric instrument.

However, in the .head files the following is still off:
PHOTLINK = F (for both SDSS and Coadd)
FLXSCALE =/= 1 for the SDSS frame (even if MAGZEROP set to 22.5)

If I understand correctly from the manual:
mag = MAGZEROP - 2.5log10(Flux / [FLXSCALE*EXPTIME])
If yes, the even the SDSS exptime does not seem to be able to explain the FLXSCALE = 1.087e8 for MAGZEROP = 22.5.

Thanks,
Reinier

ps. In the scamp manual, eq 27, page 34, I think the second log should also be log_{10}.
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04-23-2018, 16:20
Post: #4
RE: Photometric Calibration using SDSS frames
Following up on this thread - does this mean that MAGZEROP and FLXSCALE are redundant keywords (minus the EXPTIME factor) ? I ask because I am calibrating photometrically NIR images obtained over several nights and am unclear of what values should be put into the SWARP .head files. Naively, I was assuming that MAGZEROP would be the average zero-point shift for a night (i.e., an average over N images) and that FLXSCALE is the offset of individual images from MAGZEROP, but it seems this is not correct.

Thanks !

Christopher
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