Sqaure stars in images

[meade expert]

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]

This is diffraction on spider vanes.

[By.Jove]

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

[JA]

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]

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]

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]

I seem to recall thinning to 3mm was considered safe