View Full Version here: : Sqaure stars in images
meade expert
05-10-2021, 02:24 PM
HI
I would like to ask the group what things can cause square stars in an image.
The image was taken through a Vixen like scope 200mm f4 with thick spider vanes.
The camera was a asi533MC colour OSC with a Baader coma corrector.
bojan
05-10-2021, 02:37 PM
This is diffraction on spider vanes.
By.Jove
05-10-2021, 02:38 PM
Square stars results from strong four-sided symmetry anywhere in the path.
If you consider that that you actually have a circular objective cut into four smaller quadrants, each one with two straight sides and a curve, hypothetically you can re-arrange them as a square aperture with the curved bits overlapping in the middle. This will give square stars just as surely as shooting through a square objective does.
Only way to get rid of them is either:
- use a scope with no vanes, or
- if you must have vanes, make them curved with a radius equal to the semidiameter of the objective
As Bojan said, the diffraction spikes could certainly play in to the issue, especially with THICK vanes and maybe binning the image or cropping in too far /too hard etc... many possibilities.
You need more information to get to the root cause.
Best
JA
LewisM
05-10-2021, 03:53 PM
Ah, the Vixen Blocky Stars - I bet the scope was either a VC-200L or an R200SS. :)
Well known issue, usually resolved by thinning the over-thick spider vanes, making sure the primary mirror clips aren't stressing the mirror and making sure not to under-sample images.
meade expert
05-10-2021, 04:01 PM
Thanks all for your input.
Yes you are correct its a VC-200L clone scope.
How thin would the vanes have to be ground down to eliminate this effect?
LewisM
05-10-2021, 05:23 PM
I seem to recall thinning to 3mm was considered safe
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