Focus position in the FITS header for doing field curvature analysis

I have a Moonlite focuser and it puts FOCPOS in absolute position (not mm) into the FITS header. The field curvature analysis function is reading 0 for focus on all files with different FOCPOS numbers. What is PRISM looking for in the header to get the number it wants? (I tried inserting FOC as a mm offset to focus, but after “saving” the header, it wasn’t recorded in the file.

Any help would be appreciated.


welcome to the forum Greg. have the images themselves been taken in PRISM?

No, I just started looking at Prism to check out the analysis. Is Prism required to register the FITS data?

in a way yes since different devs do things differently. If you took the images with prism, I believe this would ne a nonissue. I’d give it a try to see

It turns out if you simply open the FITS file in Prism and after saving the file in Prism and then edit the FITS header, it seems to be able to properly analyze it.

So my next question on this topic is whether this result is good or bad (see the resulting tilt & curvature images).

For tilt, is +/- .01 mm in one diagonal & +/- .06 mm in the other something that I should try to tweak?
For curvature, it looks like one diagonal is flat @ .006 mm & the other is - .05 mm in the corners. Again - is it worth chasing a fix?


Position X=1 Y=1 0.059 mm
Position X=4656 Y=1 -0.012 mm
Position X=4656 Y=3520 -0.060 mm
Position X=1 Y=3520 0.011 mm
Position X=2328 Y=1760 0.000 mm
Slope angle (°)= 0.61

Position X=1 Y=1 -0.050 mm
Position X=4656 Y=1 0.009 mm
Position X=4656 Y=3520 -0.044 mm
Position X=1 Y=3520 0.006 mm
Position X=2328 Y=1760 0.006 mm

well done Greg. Personally, I do think that “The enemy of good, is better”. I believe those numbers are very good.
This shows what level of accuracy PRISM can provide the user.