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Filter creating additional diffraction spikes?

Carl M

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Hello there,

A while ago I posted a topic with the same problem, and initially I thought something was up with collimation or mechanically. Previous thread:

However, now I have found the problem to only exist when my Astronomik CLS-CCD filter is installed into my Canon 1000D camera. The only thing I can seem to do that changes the effect of these spikes is move the bright star into corners of the image. When I remove the filter, the issue is no longer present. Obviously I could just image without the filter, but I don't think I will be able to expose for as long without LP saturating the image.


Any ideas as to what else could cause this? I'm struggling to think of anything the filter is possibly doing to the light path to cause this artefact, other than possibility of some sort of internal reflection. I've even intentionally made collimation bad to see if the spikes changed shape compared to collimating properly, and nothing changed. It does seem as though something could be intruding the light path, but I can't figure out what it would be and why adding a filter would create the effect?

I appreciate any suggestions!

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You could be right there, it's very difficult to tell... should have taken an image in the top left without filter and the lines may have been a bit more apparent if they are there.

If that would be the case then I'd assume that the diffraction spike is coming from the secondary not being centred and the light cone clipping a bit of the focuser. I did have a single diffraction spike problem before without the filter, perhaps since then I've recentred the secondary and made the problem worse or better which has changed the pattern of the spike.

I hadn't thought of that, thank you - time to recollimate!

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