Help With an Image Artifact

You are right, it’s most likely a stray that’s coming from Alnitak.  Anti-reflection coatings on optical surfaces aren’t ever perfect so it could be coming from a multiple bounce within the optics.  However, based on the narrow annular form, it may be due to a glancing stray from a small portion of a connector tube or filter edge.  Regardless, these things are always hard to deal with.  Here are some things that you might try:

1) Try imaging the same field but move the center of the image around a little bit.  It may be there is just one very specific pointing angle relative to Alnitak that produces the stray.  Changing the pointing angle of the scope a small amount might be enough to eliminate the problem.  That might work if it’s due to a direct reflection from a flat surface, but it might not do much for glancing stray due to scattered light.  This might be your only option with the Hyperstar adapter.

2) Imaging a field containing such a bright star is the acid test for determining how immune your system is to stray light.  When it fails, the best thing to do is to do some simple diagnostics to look for the source.  Point your scope at a bright scenes during the daytime, pull the camera, and look backwards through the system.  Put your eye near the focal plane and move it around looking for any bright edges in your system.  This discussion and image shows the analysis that I had to do on my 20” system and it gives you some idea of what to look for:  https://www.astrobin.com/xfedon/F/?nc=user.  If you find bright edges (and you probably will), you’ll have to get them painted black or cover them with black flock paper.  The page that I just referenced also talks about options for super black paint.  Of course all of this work only works for light that might be coming from internal reflections but that’s often the biggest source of problems.  The big problem with Hyperstar is that you have to use a mechanics mirror to look backward through the system so that your head doesn’t block light in the entrance pupil.  The other thing is that you don’t have many connector tubes that can cause the problem...so this approach may not be of much value in this particular case (with Hyperstar.)

Good luck with it.   It may be a challenge to fix with Hyperstar but it’s worth a shot.  Otherwise, you might be faced with a processing challenge that can be solved with a clone-stamping tool.

John

You mention a circle of light in the centre of each frame. I recommend also to take a look at the rear end of the telescope, as there could be stray light coming into the baffle tupe. The visual back should be sealed tightly to not let light in, as it goes straight into the Hyperstar and the camera.

Good luck!
-Martin

D. Jung:
What you see there is nothing unusual.
Nothing in your imaging train is 100% reflective, transmissive or opaque; that is true for your mirrors, lenses and filters, even your sensor.
In your case, what is likely happening is that light passes your optolong filter (which has about >90% transmission or in other words 10% reflectivity) and part of that light reflects of the sensor and back to the optolong filter. From there, part reflects back to the sensor.
Assuming your sensor reflects about 1% you would get 0.9*0.01*0.1=0.001 the light back as double reflection.
This doesn't sound like much, especially considering that this double reflection will be out of focus, but when you image something very bright like Alnitak, it can become visible.
This would explain the doughnut shape around Alnitak, which is very similar to an out of focus star.

If your filter is tilted against the optical axis, you can also get ghosts like you have in your image.

Thank you. Very helpful info.

-Kelly