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Weird Rainbow reflection

Sky-Watcher Explorer 200PDS Equipment Artifacts Help Needed
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Max Kuster avatar
Max Kuster avatar
Got the first light with my new camera yesterday and noticed a weird reflection in the lower part of the image:


The equipment is as follows:
  • Zwo 2600 MC DUO
  • M54 Tilt adapter
  • 5x2" EFW
  • ZWO M54 to M48 (2mm Thick)
  • 1mm Spacer and 3 M48 Extension rings (6, 5 and 4 mm), attached to a Baader MPCC Mkiii coma corrector.
  • Scope is a Skywatcher 200PDS with an Aperture Mask and CNC secondary spider (Slightly misaligned to the focuser).

Collimation was pretty good, shot in B3 new moon, no streetlights or similar. This is 3 hours of 180s subs @gain 100 with a Baader UV/IR cut 2" filter, stretched in PI after ABE. 
There still is some tilting in the upper left corner and the backfocus is slightly off. 

Can this be a reflection from some of the bright stars outside of the FOV?

Or is this caused by an internal reflection / light leak? Any help would be appreciated. 

CS 
Max
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Oskari Nikkinen avatar
Oskari Nikkinen avatar
Have seen this a few times with my 8" newt and the 0.95x Maxfield corrector. Like in this example i had a bright star a few degrees away from the edge of the sensor (NGC4236 for example, and a comet near Pollux as another).

Tube was fully flocked at the time so the only option that was left is an internal reflection within the coma corrector, since i upgraded to the Televue Paracorr i have not seen a drastic rainbow reflection like this although I have seen a very slight version of it, but it was maybe 1% of the problem in the Maxfield.

I think there might not be anything you can do about this issue if its a reflection inside the corrector. You can try flocking your tube and spider vanes and see if the problem goes away or at least gets better, but it might not help. You should flock anyway as it has other benefits so not a waste of time in the end.
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andrea tasselli avatar
andrea tasselli avatar
It's an off-field reflection of a bright star. The only cure is to shift the field so it doesn't happen.
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Christian Koll avatar
Christian Koll avatar
andrea tasselli:
It's an off-field reflection of a bright star. The only cure is to shift the field so it doesn't happen.

I agree, this is an off-field bright star reflecting in your OTA.

You may want to try to find the reflecting part inside your OTA by pointing your telescope against a bright white wall during daylight (indoors), remove the camera and peek into your OTA from the camera's view.
Most likely you will find a reflecting surface, that can be blackened with some special paint or black velvet.

I had the same issue with some extension tubes on my Esprit 100. Got rid of that by painting the inside of these extension tubes with Noctutec anti-reflection lacquer.

All the best!
Chris
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John Hayes avatar
John Hayes avatar
I agree that it’s stray light from an off-axis star—and judging from the arc shape, it might be from a reflection from a smooth surface inside of an adapter tube.  The rainbow colors are possibly from diffraction from the turning marks inside of the tube.  I encountered what appeared to be this exact same thing many years ago and I’m not sure if I can find the image but if I can, I’ll post it. 

John
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Max Kuster avatar
Max Kuster avatar
@Oskari Nikkinen @andrea tasselli @Christian Koll @John Hayes 
Thanks for the answers, this is reassuring. I changed target yesterday night and the reflection did not appear. I will check the inside of the tube and the adapters and eventually flock/paint them. 

CS
Max