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Creating .ahead files
12-06-2006, 18:19
Post: #1
Shy Creating .ahead files
Hello,

as I'm getting deeper and deeper involved in this piece of software, the question comes to my mind, how to create those ahead files for different cameras.

I'm using the software on FORS2 images, we will adopt it also on the WFI.

The thing is, I believe SCAMP will be much more reliable also with not the best values set for MATCHING if there is such an ahead file.

So: How the generate them? Cool

TIA!

Best regards Jan
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12-07-2006, 14:09 (This post was last modified: 02-17-2010 17:11 by Emmanuel Bertin.)
Post: #2
RE: Creating .ahead files
Hi Jan,
Actually once you have in hand a clean SExtractor catalogue (say, "mycamera.cat") of an exposure for which each and every individual chip of the mosaic can be properly positioned on the sky with SCAMP, obtaining a good .ahead file is easy. Just run:
scamp -DISTORT_DEGREES 1 -MOSAIC_TYPE LOOSE -HEADER_TYPE FOCAL_PLANE mycamera.cat
A "mycamera.head" will be written, which you can rename for instance "mycamera.ahead".
mycamera.ahead contains the relative positioning of chips in the focal plane in the form of CRPIXi and CDi_j parameters.
Now when you want to use it with a set of new exposures, you only have to run:
scamp -MOSAIC_TYPE SAME_CRVAL -AHEADER_GLOBAL mycamera.ahead *.cat
Of course this is mostly for mosaics, but it may also be useful for single-chip cameras.
- Emmanuel.
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12-07-2006, 14:42
Post: #3
RE: Creating .ahead files
Thanks for the answer,

but do I really just need one good catalogue? I mean, would it not be better to get mean values for different good observations? And if so, cat one just take several good header made by the options you suggest and than determine means for every value?

Thanks again, Jan
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12-07-2006, 15:05 (This post was last modified: 12-07-2006 15:10 by Jan Kohnert.)
Post: #4
RE: Creating .ahead files
One last question come in mind:

The FORS2 camera has two chips but unfortunately they are stored in different files and so I get two different catalogs per single exposure. And so, if I use the options above, I obviously get two header files.

Do I have to merge the with scamp before the ahead-Procedure, maybe with MERGEDOUTCAT_NAME and MERGEDOUTCAT_TYPE settings in the scamp.conf? I tried but got an empty merged catalogue...

Best regards Jan
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12-11-2006, 16:42 (This post was last modified: 12-11-2006 16:43 by Jan Kohnert.)
Post: #5
RE: Creating .ahead files
Hmmm,

I tried to make those ahead files out of a pair of good catalogues as mentioned above and put them as external headers for every file. Unfortunately this destroys the astrometric solution completely (it just put all field onto each other, instead of putting chip 1 up and chip2 down for all the pics, respectively). Sad

Then I tried to concatenate the files for chip1 and 2 together and put it as "global header" to the catalogues. Same effect. Sad

I also tried to calulate mean values of of the values of all good calibrated catalogs, but this does not change anything.

Sad, dunno why, regards Jan
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12-11-2006, 18:37
Post: #6
RE: Creating .ahead files
I have now used MOSAIC_TYPE UNCHANGED and then calculated mean values for all the parameters of the good fields and then created one "ahead"-file per chip out of them.

That seems to work quite well. I will test it ons some other images and then report again here.

If somebody is interested, I can post the files here (if they're working, of course. Wink )

Regards Jan
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05-09-2007, 22:39
Post: #7
RE: Creating .ahead files
Hi,

I am trying to generate a global .ahead file for the FLAMINGOS near-IR imager so that I can use SCAMP in combination with SWARP to generate a large mosaic, but I'm running into some problems. The main issue is that FLAMINGOS exhibits pretty severe focal-plane distortions, and there typically aren't enough astrometric standards in a given frame to model the distortions (which are worse in the corners) from scratch. Fortunately, the distortions are ~constant between exposures, and are well-modeled by a 6th-order polynomial in each dimension (derived by the instrument team using pinhole images).

If DRIZZLE were being used, the coefficients of the distortion model would be inputted as an text file; it seems that the analogous technique in SCAMP is the global .ahead file.

So my question is whether anybody can help me convert the known polynomial coefficients for FLAMINGOS into the appropriate PV_xx_xx FITS keywords used by SCAMP? The distortion model is parameterized in the same way as for the HST instruments, as described here:

http://www.stsci.edu/hst/acs/documents/isrs/isr0011.pdf

Unfortunately I have no examples to go from, since all the existing global .ahead files (for WFC, MEGACAM, etc., in the config subdirectory) only describe a 1st-order focal-plane model.

I would be happy to provide the necessary data and files if that would be useful.

Thanks very much,
John
NYU
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04-01-2009, 23:28
Post: #8
RE: Creating .ahead files
Hello,

I'm using the THELI pipeline on some data from the SOAR Optical Imager (SOI). It's a 2 CCD mosaic with 2 amps per chip, so 4 "images" in each exposure. It has a 5'x5' field of view with 0.154" pixels.

Could anyone walk me through creating a default .ahead file for this instrument. I've managed to get one field astrometrically calibrated, so I have a stacked image with good astrometry in it. This is my first foray into scamp, so I'm a complete idiot. To what image should I try to create the .ahead file, the final combined image, one that was used in the combine, etc? Tried the command referenced above, got a error listed below. . .



scamp -MOSAIC_TYPE LOOSE -HEADER_TYPE FOCAL_PLANE N1.192_scamp.cat

> WARNING: scamp.conf not found, using internal defaults

----- SCAMP 1.4.6-MP started on 2009-04-01 at 18:25:15 with 4 threads

----- 1 inputs:
> Examining Catalog N1.192_scamp.cat
> *Error*: XWIN_IMAGE parameter not found in catalog N1.192_scamp.cat [1/4]


Any help appreciated!

Cheers,
AstroRoyale
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04-07-2009, 11:39
Post: #9
RE: Creating .ahead files
Simply use a catalog you used to create the astrometric solution within your pipeline. Be sure to choose the catalog, which give the best result (you may use the xml output of scamp in your pipeline to find out, which is the best catalog).

Then, use "scamp -c your_configfile.conf -MOSAIC_TYPE LOOSE -HEADER_TYPE FOCAL_PLANE best_cat.ldac" and rename the produced header to whatever name you like. :-)

Best regards Jan
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05-18-2009, 14:36
Post: #10
RE: Creating .ahead files
Hi AstroRoyale,
the SExtractor parameters that must be present in the catalogues for SCAMP to work are listed on page 3 of the SCAMP user's manual.
- Emmanuel..
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