In my last M57 image taken with the 130PDS, there is a strong ghost reflection diffraction spike in the lower right area. There are a few more, but they are mainly ultra faint and only visible in 100% resolution. So far I could not explain what causes them.
Yesterday I did M57 again, but with the DSLR 90° rotated to see any difference in the reflections. I did only 12 exposures. No flats, darks or bias.
At first glance no reflections were visible in yesterday’s image. But there is also a reflection in the lower mid left area.
If I check the field around my field of view in Stellarium I see a bright star on the reflection opposite side.
Both stars (Solafat and Sheliak) are around mag 3.5. So I think these reflections must be caused by bright stars outside the field of view.
So far, I’ve no other explanation for it.
The big question is how to prevent these reflections… maybe darken the inner tube and the spider vanes…
Michael explains the reason for these reflections in his comment below. Thanks a lot for this!
The reflections are caused by bright stars that are outside of the DSLR’s field of view (red), but within the telescope’s field of view (green):
the reason for the “ghost spike” is the secondary spider! What you normally see as bright spikes on stars is only the first diffraction order! What you see as ghost spike is the second diffraction order. In between (between the first and second spike) is a minimum that can be seen as a dark region. This is physics and with your telescope everything is fine! The brighter the star is, the more clearly these “ghost spikes” become visible. This effect occurs with all secondary spiders, regardless of the design! On sheet metal spiders this effect is often not as clearly visible as on milled spiders, because the alignment of the spider arms is not perfect and therefore the diffraction is not as constructive in the second order compared to milled spiders.
This can be remedied by a strip of velour attached to the side of the struts of the spider. This disperses the light more and tends to make the spikes smaller! But keep in mind that the light from diffraction ist not gone away, it will spread around the star.
In your case, the origin of the ghost spike is a star outside the imaging field, but inside the field of view of your telescope 😉
Hello Michael, many thanks for the clarification. What I didn’t take into account is that my 130PDS has a bigger FOV than the DSLR imaging field. So it becomes perfectly logical that bright stars outside of the DSLR imaging field, but still inside the FOV of my 130PDS, creates these ghost reflections. Will update this posting soon 🙂
BTW: Your website is a great resource for Newton owners!