What to do when Photometric Color Calibration Doesn't...

Juan Conejero

PTeam Member
Sep 2, 2004
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Valencia, Spain
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This image poses a very difficult problem for gradient correction. For example, where are you going to put samples on the third quadrant (bottom left)? I don't see any obvious gradients in the image you have uploaded, although judging them is also quite difficult. The multiscale gradient correction method requires another image acquired with a wider field instrument. In this case you'd need something like a 50 or 35 mm wide-angle lens.
 

wrichards

Well-known member
Jun 22, 2020
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This image poses a very difficult problem for gradient correction. For example, where are you going to put samples on the third quadrant (bottom left)? I don't see any obvious gradients in the image you have uploaded, although judging them is also quite difficult. The multiscale gradient correction method requires another image acquired with a wider field instrument. In this case you'd need something like a 50 or 35 mm wide-angle lens.
Yeah - I just read the reference article you cited in your previous post and realized that I don't have a larger image to use for the Multiscale Gradient Correction so that's not going to work here.

So I guess my choices are to do DBE, then PCC, then DBE again, or just skip DBE altogether. What do you think?
 

pfile

PTeam Member
Nov 23, 2009
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doing DBE after PCC can upset the color balance unless you tick "normalize" in the target image selection area of DBE. why do you need to do DBE twice?
 

wrichards

Well-known member
Jun 22, 2020
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doing DBE after PCC can upset the color balance unless you tick "normalize" in the target image selection area of DBE. why do you need to do DBE twice?
Well, it's either that or SCNR, but if I do DBE prior to PCC then I get the "Hulk green" result that has to be re-corrected.
 

RodrigoQ

Member
Jul 11, 2020
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These images can be calibrated without problems with our PhotometricColorCalibration tool. However, default parameters are suboptimal for these wide-field images because they lead to poor sampling of photometry data, so they must be changed to achieve good results.

Take a look at the following screenshots:

Lagoon/Trifid:

Rho Oph:

Default photometry parameters usually work well for most regions of the sky. However, wide field regions covering dense milky way areas pose particularly difficult problems, mainly because of the huge amount of stars and the complexity of the background.

As you can see, I have tweaked photometry parameters in order to ensure good sampling of the APASS catalog. For the Lagoon/Trifid image I have forced a limit magnitude of 12 and a photometric aperture of 8 pixels. With these parameters PCC has performed an absolutely perfect white balance calibration with about 3000 stars from the APASS catalog. The result is excellent, as expected from the white balance function graphs.

For the Rho Oph image I have used a limit magnitude of 10 and a photometric aperture of 6 pixels. These values are more appropriate for a wider field. The result is also very good, although APASS data for the r' filter shows much more dispersion. Despite that, the achieved white balance is excellent IMO.

As for background references, they are not very critical, as you can see in the examples above. Just try to select a relatively free area, such as a dark nebula, and avoid selecting background references too far from the center of the image if you have significant gradients. Use the Statistics tool to define a reasonable upper limit for background neutralization.
Hi @Juan Conejero ! thank you so much for share with me the correct way to perform the PCC. To be honest with you, I didn't expect this photo be so tricky, I learned a lot reading you and all the persons in this Forum. I already perform the PCC on my image and obtain the same result and right now I'm experimenting with other parameters just to understand the process. Thank you everybody! Looks like there is a long way to learn and master PixInsight!

Best Regards,

Rodrigo Quiroga
 

ecoles

Active member
Sep 9, 2014
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PCC works for me 95% of the time. But there are some images it just refuses to calibrate. I have read this and several other posts and followed all the reasonable suggestions, and still no results. In the end, I calibrate it the old fashion way.

The image is the starfield around OU4. It's bright with lots of stars. The message I get from PCC is the following.

*** Error: Failure to process image file 'C:/Users/Eric/AppData/Local/Temp/PCC_B_JUUJKMJXL5IU.xisf': Could not locate any stars in the image

Of course, this is probably pilot error and I am always open to suggestions. Also, if anyone is interested in trying to solve this image, please let me know and I will upload it.

Thanks in advance for your help.

Eric Coles
 

pfile

PTeam Member
Nov 23, 2009
5,298
63
this happens from time to time and the error message is less than revealing - the problem is that there were no, or not enough stars in the catalog at that RA/DEC to do the photometric analysis. i guess there are some pretty big holes. i think this is the 2nd or 3rd time i've seen someone report that problem.
 

ecoles

Active member
Sep 9, 2014
31
0
Thanks Rob. That's odd considering the number of stars in the image. Is there any way to match the RA/Dec (21:11:20/59:56:12) to a particular catalog and overcome this issue? I hate to give up.

Eric
 

pfile

PTeam Member
Nov 23, 2009
5,298
63
i think it's just more that there are areas which, for whatever reason, the APASS catalog has poor coverage. check the coverage map here: https://www.aavso.org/apass - you'll see that most of the sky is not covered well and the emphasis is on the milky way. still there are places with holes even in the MW.

i don't know if there are other catalogs that are appropriate for photometry. maybe there are but they would have to be added to PCC...