star spikes with filter

[alacant]

Hi everyone

Scenario with gso 203/1000 Newtonian reflector.

•  No filter: all ok.
•  east of meridian + filter; pronounced spikes [1]
•  west of meridian + filter; less pronounced spikes [2]

Here are examples directly from the camera with the stars in question at top right (1) and bottom left (2). Debayer and stack lessens the effect but... 

My theory is that the filter on the end of the cc protrudes into the tube; it is not shaded by the focuser barrel. It is worse with the uhc. Reflective surface? Dunno.

Has anyone seen this effect before? Any fixes? Any or all comments most gratefully received.

TIA and have a great weekend.

 

[1] 

[2]

 

Edited December 18, 2020 by alacant

[vlaiv]

There is one possibility that you can test easily.

I think it has to do with filter sticking out of focuser tube and blocking the light.

Light coming at an angle will be affected, while one coming straight down or from the opposite angle will not. This means that only stars to one side will be affected - like in your image. Here is diagram:

Not the best diagram, but you get the idea - if focuser is protruding into the light path - it will create optical artifact.

This is known thing with 130PDS for example, have a look here:

https://stargazerslounge.com/topic/210593-imaging-with-the-130pds/page/75/

People cut of end of their focuser tube.

In any case - this is something you can easily test - just put a piece of cardboard to extend focuser tube a bit instead of filter - does not need to be much thicker than filter cell - if effect is there - it is not due to filter but rather from light blockage.

[alacant]

5 hours ago, vlaiv said:

known thing with 130PDS

Hi Vlaiv. Thanks, yes of course. Exactly the same effect as on an unmodified 130pds. I should have thought of that.

The solution In this case I suppose -as it's the cc and filter innermost rather than the focuser barrel- would be to move the mirrior up the tube.

Cheers