Blocky Artefacts Around Stars

[gnomus]

I am imaging at 3.2" per pixel resolution (a Williams Optics Star 71 and a Moravian G2-8300 camera).  I have no issues as far as I can see with tracking/guiding.  My individual stacks are good.  However, when I combine RGB I often get litle black 'blocks' at the edges of stars.  Only certain size stars seem affected.  I've shown these to some folks.  Someone thought it might be aliasing.  I also remember somebody once saying something about 'clamping' but for the life of me I cannot find any reference to that.  In any event, any thoughts on how to eliminate this issue?

[pfile]

those are likely to be ringing artifacts introduced at the registration step. you can adjust the clamping factor on the default interpolation method (which is lanczos-3 IIRC) to see if they go away. if not you can try another interpolation method, like Bicubic Spline. hover your mouse over the Interpolation method label in StarAlignment to see an explanation of the different interpolation methods.

rob

[gnomus]

OK.  Thanks.  I'll try some different settings.  Any starting suggestions?

[pfile]

maybe go down by halves in a binary search? so 0.15 to start?

rob

[gnomus]

OK.  That was definitely the issue.  I chose 'Bicubic B-Spline' (which has a fixed clamping threshold of 0.1).  Problem has gone. 

Thanks for your help.

Steve

[pfile]

no problem!

this happens with dim stars when the image scale is large. another way to combat it would be to use drizzle. you'd be amazed at how much detail can be recovered. you need to have dithered during aquisition though.

rob