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Silly question about syntax
05-03-2014, 03:51
Post: #1
Silly question about syntax
Hi everyone!

first time user of Sextractor, Scamp & Swarp.
I've been able to install everything without too much pain, and make it work on CFHT-Megacam data.
I wonder however if I use Swarp properly.

Basically, I do:
> swarp fitsfile1 fitsfile2 fitsfile3 ... fitsfile20

where all the fitsfiles and the raw data, that, I believe, have not been touched by sextractor or scamp
those last two softwares created catalogs and headers, that I did not use in my call with swarp

so, what's the point of using sextractor/scamp if swarp does not use their outputs?
what is it that I'm missing?

thanks a lot for those tools, very useful and powerful!!!

Nicolas
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05-03-2014, 16:26
Post: #2
RE: Silly question about syntax
Hey

Basically you misunderstood the typical workflow when you want to coadd some images. Usually images from telescopes only come with an inaccurate preliminary astrometric solution. Just load one of your images (with e.g. Aladin, ds9, etc.) and try to overlay coordinates of stars to see if they match. Some data flow implementations include an astrometric calibration, but from most telescopes you will only get a rough solution. Data from image services (like 2MASS, WISE) are already calibrated.

If your data is not calibrated you have to first find your astrometric solution and this is where you start using these software packages.

First you start with SExtractor to build a source catalog, then you calculate an astrometric solution with Scamp and in the end you use this calculated solution to resample (and maybe coadd) them with Swarp. So just be sure that your data is astrometrically calibrated before you swarp them.

Hope this helps
cheers
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05-05-2014, 19:31
Post: #3
RE: Silly question about syntax
Thanks Stefan, this is how I see it.
But then, how does Swarp knows about what I did with Sextractor and Scamp???
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05-06-2014, 18:45
Post: #4
RE: Silly question about syntax
So from the SExtractor catalogs you calculate your astrometric solution with Scamp which in turn produces the correct WCS fits header keywords (in a simple output text header file). You can then chose an option in the SWarp configuration file (HEADER_SUFFIX) to read in those external headers produced by SWarp. Smile
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05-06-2014, 19:35
Post: #5
RE: Silly question about syntax
Ok, this is the option I did not find, thanks Stefan!

So far, I've been running Swarp without that option, and it does the trick very well ... that's why I thought I missed a point like, e.g. Scamp writes in the original FITS files or something like that
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02-27-2015, 20:43 (This post was last modified: 03-13-2015 06:40 by lucylee378.)
Post: #6
RE: Silly question about syntax
Your data is not calibrated you have to first find your astrometric solution and this is where you start using these software packages.




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