Star Stretching to avoid bloated stars

[plawrence1]

I've been using pixinsight for a few years as my image processing software and I have always struggled with keeping star sizes small. I've tried numerous star reduction methods but I've never been happy with the dimmed fuzzy stars they give me. Some methods I can think of:

Stretching
MaskedStretch - Copying stars into HistogramTransformation strecthed image (Following Harry's tutorial)
HistogramTransformation - Using Lightness Mask

Post-Sretch Reduction
MorphologicalTransformation - Morphological Selection using starmask
MorphologicalTransformation - Erosion using starmask

Typically I have found that blending the MaskStretch stars in 50-50 with the HistTrans image gives me the best results, but I'm still not happy with the attached.

I have wondered if my equipment is the source of my problems, I have a 200mm f/4 Newtonian & an Atik 428 Mono CCD. This gives me a fast scope with a small field of view, pixel scale is 1.17 arcsecs. I have attached an image of my auto-STF of my Lum stack.

My question would be, can anyone suggest other methods to try to allow me to produce nice tight stars without fuzzy halos?

Thanks in advance,
Pete

[plawrence1]

The attached image is my blended stretch.

[RickS]

Hi Pete,

Do you use deconvolution?  It can help tighten up the stars.  I tend to use MT Selection for star reduction.  If you find that the stars get too dim and fuzzy you can liven them up again with some careful sharpening (e.g. increase bias values for small scales in MultiscaleLinearTransformation.)

Cheers,
Rick.

[plawrence1]

Yes I use deconv, it works well for reducing stars.

I feel like there must be some form of iterative stretching technique that will work better for what I'm trying to do.

I spent a couple of nights fiddling with donut star masks (masks to cover halo with centre removed) but it was much too harsh and ended fairly unnatural looking. I find that all of the techniques online seem to dim the outer edges of the stars rather than actually making the stars smaller.

[RickS]

Some people have had success with the Starmask contour mask option - it's supposed to generate a donut type mask, at least for larger stars.

[plawrence1]

I was able to create the mask manually without much trouble, it just didn't do quite what I was after.

Perhaps my stars are just something I have to live with. As I said earlier, I do get the feeling this is something to do with having a fast scope.

[RickS]

If you can build a mask that targets the halo and outer edges of larger stars then you should find that CurvesTransformation will let you shrink them.  I do this with especially bloated stars and/or large halos.  You want a curve with an initial "ledge" to match the background brightness level.  You probably won't be able to do all the stars at once unless the background level is constant across the image, so this can be a time consuming procedure.

Cheers,
Rick.

[plawrence1]

That sounds interesting. Can you explain the 'ledge' part? Maybe a screenshot?

[RickS]

Something like the attached pic...  The "ledge" will be higher or lower depending on the background brightness.  It's easy to adjust with a real time preview.

[plawrence1]

That's great, I'll have a play with that later