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Combining WEIGHT images
11-14-2007, 20:11 (This post was last modified: 11-14-2007 20:11 by Jan Kohnert.)
Post: #1
Combining WEIGHT images
Hi all,

I just figured out a bug in on of my scripts, which I did not see before, as I normally use a different camera, which does not have that problem.

I'm combining WFI images right now, which have the chip numbering such as:

-------------------
| 1 | 2 | 3 | 4 |
-------------------
| 8 | 7 | 6 | 5 |
-------------------

This is just fine in combing science frames, because the have WCS headers. But I'm running into problems when combining the WEIGHTS. The unfortunately are created from the single chips and have (of course) no WCS information. So they are combined to:

-------------------
| 1 | 2 | 3 | 4 |
-------------------
| 5 | 6 | 7 | 8 |
-------------------

Is there a way to get around this while keeping our current file naming scheme, which is ${NAME}_${CHIP}${EXT}.fits (${CHIP}${EXT}.fits is given as -SPLIT_SUFFIX)?

I'm pretty sure I do need some "global" geometry information into the fits files to let that work, if so, how could that be generated? Maybe by using the global header file which can be used for scamp, too?

I really don't want to rename all the files prior combining them...

Thanks and best regards Jan
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11-14-2007, 20:17
Post: #2
RE: Combining WEIGHT images
BTW: I just saw in the "missfits -dd" output the config option -FIX_WFI. Could that be the solution?

Best regards Jan
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11-15-2007, 11:21
Post: #3
RE: Combining WEIGHT images
Hello Jankoh,
the option -FIX_WFI cannot really help you: it is a fix for obsolete WCS keywords in very old WFI images.

If the presence of WCS in the headers solve the problem, you can try to build an external ASCII header for every chip containing rough WCS information ${NAME}_${CHIP}${EXT}.head and missfits will write it in the header of the MEF file.

Chiara.
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11-15-2007, 14:27
Post: #4
RE: Combining WEIGHT images
Hi Chiara,

Thanks for the answer, but I must admit, I was a bit too fast in typing yesterday. Rolleyes

This is what came into my mind after sending and while driving home:
The thing is, missfits combines the images step by step, creating new extensions for every additional image it finds. This is the same for the science and for the weight frames. And so there is actually no problem, as long as chip #X is in the same extension for weight and science frames (regardsless of the actual position on the sky). SWarp can put that extensions together and then just do the astrometry according to the WCS headers.

I was not aware of this yesterday. Sad

But anyway, if I'm understanding this correctly now, I think the only solution to get the chips in the "right" position, would be to change the naming scheme, wouldn't it? Just in the case somebody else stumbles over similar things... Smile

Thanks again, Jan
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11-15-2007, 15:41
Post: #5
RE: Combining WEIGHT images
Hi Jankoh,
MissFITS does not care about EXTNAME keyword.
You choose the order of extensions using indexes in the file name.

When you split a MEF WFI image with MissFITS the <name>_05.fits is the im8 extension.
When you rebuild the MEF <name>_05.fits is appended in the fifth position.

Regards,
Chiara.
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