Photometic color calibration factors

Software Siril
1

Siril’s photometric color calibration is a wonderful tool at its best, but it can also be quite annoying when plate solving does not work (e.g. in wider field photos). When it does work it reports the calibration factors used as K1, K2 and K3 (all of them are equal to, or bigger than 1.000)

However, plate solving does usually work better if I crop the image (as advised in Siril documentation, too). Thus also photometric color calibration succeeds in these cases. I want to use these photometric calibration factors (for the cropped image) to calibrate the whole image, but I have not had any success. Either I have not figured out the correct way of doing it, or Siril does not support using these photometric calibration factors for the cropped image to calibrate the whole image using the very same factors. The color calibration dialog using evaluated background values can be switched on to manual mode, but in tihs dialog the factors used are not compatible with those obtained from the photometric console output.

Is there any way of doing it? Or, could it be added to Siril as a new feature (enhancing its usability)?

2

Hello. You don’t have to crop you image. An option do it for you by searching in a small area without changing the image.

Also, new version will allow to use picture already platesolved by another way.

So it should help you.

3

What does wider field exactly means? I am able to do the calibration lately with 50mm full frame equivalent focal length.
I am just using Stellarium in the telescope mode to have a similar framing with my picture and than just pick some random starts closer to the center of the picture and Terry to calibrate using their names. It takes a few tries sometimes butt usually it works…

4

I’ve never succeeded in it with a 50 mm lens on an APS-C body.

5

How do you limit the search area?

6

I am with you here, I had the same issues until a comment from @lock042 opened my eyes in another thread.
I do not limit it (at least not being aware of doing this). What I do, step by step, is:

  • open the stacked fit image in siril
  • open Stellarium and activate the ‘image sensor frame’ option. I have it already configured in the settings for the sensor and lenses I have
  • move the sky around in Stellarium until I get a framing similar with what I have in siril
  • click on a star that is close to the middle of this frame. Stellarium will tell you some info about it and its name in various catalogues. I usually pick the one that is like HD
  • open the ‘photometric color calibration’ in siril
  • click on get meta data from image or insert the numbers there
  • put the name of the star in the find field and search for it
  • click OK to let it do its magic.
    It can happen that sometimes it fails and I will just pick another star in Stellarium and search again. I don’t normally need more than 3 tries, but maybe I am just lucky :wink: