StarAlignment - distortion/coma correction?

[mikey2000]

Hi All,

My coma corrector is very sensitive and if I don't get it sub-millimeter accurate, I get stretched stars in the corners of my images (stars become slightly elongated, pointing towards the center of the image)  Only slightly, like long eggs, not great long streaks.  It's not too bad, just annoying.


I was looking at the StarAlignment process online resource and I notice the newest version of PI seems to have more options for this process than the documentation suggests.  In particular, I'm interested in the Distortion options (right at the top of the StarAlignment options).


Will these distortion options help me correct this sort of optical aberration?  Or should I be looking elsewhere in PI?

Thanks from a newbie!

[pfile]

i don't think that the distortion correction will fix these optical aberrations. it could help a little, since it does 'morph' the registered frame a little bit to get the stars to line up properly against the reference. but if the star is oval, it will probably still be mostly oval after the distortion correction. it really depends on the nature of the optical aberration - for instance, if the star's centroid is not moved from where it's supposed to be by the optical problems, then the distortion correction won't do anything.

the only way i can think of to work on the corner star shapes would be to do a motion blur deconvolution against only the corners, but PI will always apply the deconvolution to the whole image. plus the PSF angle would have to be different for each corner. you'd either have to make 4 masks that expose only the corners, or draw previews and extract the corners into their own images, deconvolve, and then put them back (i think SubstituteWithPreview can do this but not 100% sure.)

rob

[mikey2000]

Hmm.  Maybe I'm barking up the wrong tree.

My position is that I'm currently a Startools user but evaluating PI.   Startools has a module that performs this kind of correction rather well (especially if the problem is only slight to start with)   Here's a link http://www.startools.org/modules/lens

Is there any way to do this easily in PI?

[pfile]

well the distrortion function in PI is intended to overcome lens distortions like pincushion and barrel distortions, but any star shape improvements are going to be coincidental (i think). i guess you will just have to try it. what you need to do is solve your image using ImageSolver, then use the solution as input to StarGenerator to make an undistorted reference. then run StarAlignment with that generated star field as the reference, ticking undistorted reference and selecting 2-d surface splines as the correction method and see what happens.

rob

[TinySpeck]

GIMP does a really nice job of correcting the type of lens distortions that StarTools Lens does, using Filter / Distortions / Lens Distortions.  The adjustments GIMP provides are actually better than StarTools and easier to use.  If you don't mind jumping out of PI to do your distortion corrections in GIMP, it's a good alternative.