When photometry fails

For me, very occasionally the photometry in Siril fails. I can take the same image and have it processed on astrometry.net successfully, (save a .JPG) and I use the resolution from astrometry.net to accurately compute the focal length for entry in Siril.

[Focal Length in mm] = (206.2648 x [Pixel Size in μm]) / [Arcseconds Per Pixel]

However photometry in Siril still fails. Is it possible to somehow have Siril use the photometry results from astrometry.net to color correct an image?

Since last version, it is very rare that atrometry fails.
Indeed, you can make a selection around many objects in the images and it enhances the process to succeed.
However, if it fails again, you can share your image. Indeed, as the algorithm works, siril needs to solve the image by itself.

I agree,usually it works just fine. Happy to share the file, here is the .fits file:

and here the result from astrometry.net:

Focal length is 475.74 and camera pixels are 4.29um


haven’t had time to look in details on why it fails with the full image. I suspect it may be because of some stars which are a bit elongated on the sides, which would tend to indicate the field is not completely corrected. astrometry.net includes distorsion modelling in the plate-solve process which is more tolerant (and more complex to implement :wink: )
Now back to you problem, you can still solve if you select a smaller area in the center of the image (to avoid the distorted stars on the outside), centered around NGC4438:

You can type boxselect 731 518 3810 2301 in the command line if you want to have the exact same selection as I have but it should be just fine if you draw the selection yourself. This solves and applies photometric correction to your image:

12:09:47: Solving on selected area: 731 518 3810 2301
12:09:47: Findstar: processing…
12:09:49: Catalog NOMAD size: 126 objects
12:09:49: 54 pair matches.
12:09:49: Inliers: 0.722
12:09:49: Resolution: 1.858 arcsec/px
12:09:49: Rotation: +176.83 deg
12:09:49: Focal: 476.30 mm
12:09:49: Pixel size: 4.29 µm
12:09:49: Field of view: 02d 41m 15.44s x 01d 47m 37.72s
12:09:49: Image center: alpha: 12h27m55s, delta: +13°06’11"
12:09:50: Normalizing on red channel.
12:09:50: Applying aperture photometry to 39 stars.
12:09:52: 2 stars excluded from the calculation
12:09:52: Color calibration factors:
12:09:52: K0: 1.000
12:09:52: K1: 0.775
12:09:52: K2: 1.026
12:09:52: Background reference:
12:09:52: B0: 1.00755e-02
12:09:52: B1: 1.44915e-02
12:09:52: B2: 3.72176e-03



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Yes I just discovered that myself! It’s a little odd because I just installed a field flattener, could that be a factor?

You could use the tilt command on a calibrated sub, not a stack which may be affected by other things. The off-axis abberation may give you a hint whereas your backfocus is ok (and also your tilt while you’re at it).

OK thanks, I also discovered that you don’t need to actually crop the image to have photometry work, just defining the area does the trick!