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The shadow of the secondary mirror

ZWO ASI294MC Pro Ritchey-Chrétien CMOS
Ritchey Chretien telescopes GSO Family •9 replies•250 views
Michael Timm avatar
Michael Timm avatar
Hi fellow RC-Users,

and back again with the next problem. Recently i bought a ASI294MC Pro (which is good news). This camera has a way bigger FoV than my previous ASI178MC (which is also good news). The bad news: Because of the larger FoV i got aware of a problem i probably had all the time but didn't recognize: The secondary is really good visible on my lights. Here is an example: IC 5146 The red square shows my previous FoV. This image was calibrated with darks, flats and bias and then stacked. I only applied "Background Extraction", "Background Neutralization", "Color Calibration" and "SCNR" (and autostretch to show it here). The result is really, really bad.
I am aware that the RC suffers from its big secondary mirror, but this is a bit too much secondary in the image, or am i wrong? Do you have similar problems? If so, how do you deal with it? Any idea of what i am doing wrong?

Thanks for any comments,
Michael

Edit: Oh, the red area in the top comes from my OAG.
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Andrea Alessandrelli avatar
Andrea Alessandrelli avatar
I've never seen such problem so strong so i'll guess.. I'd say either a strong internal reflection or a light leak. I, sometimes, get a secondary reflection for instance on very bright stars in NB.

Also your filter might not be a photographic one, and these filters tend to cause reflections.
Michael Timm avatar
Michael Timm avatar
Tonight i will try the same setup without the UHC filter. Thanks for the hint, Andrea.
Michael Timm avatar
Michael Timm avatar
I have strong evidence that it really was the Omegon UHC filter. First i detected a small light leak in the OAG, repaired it, but the shadow of the secondary was still visible when imaging a flatfield. After i removed the UHC the shadow was completely gone. The manufacturer of the UHC states that the filter is suitable for photographic purposes, but after this experience i really doubt it.

I will know for sure after tonights imaging session, so keep your fingers crossed. Thanks again, Andrea (and Stefan for giving me hints via Threema :-) ).
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Michael Timm avatar
Michael Timm avatar
Yes, it was the UHC. This topic is solved, thanks again, guys.
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Andrea Alessandrelli avatar
Andrea Alessandrelli avatar
I am glad you found the cause smile If you are looking for a not expensive filter that doesn't give reflection you can try the Baader Neodymium.
Michael Timm avatar
Michael Timm avatar
Thank you for the hint, Andrea. I will consider buying this filter. At the moment i am still a bit disappointed of the Omegon UHC. I will first contact the manufacturer and ask, if i did something wrong. But when the problem persist, i obviously need a replacement. Thanks again!
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Michael Timm avatar
Michael Timm avatar
Quick update: The vendor took back the UHC. They stated that internal reflexions could happen with this type of filter. I decided to get a broadband filter instead, because my main problem in the sky are skyglow and contrast in general. So i decided to buy the Baader Neodymium M&SG. I know that this is not a UHC, but i think at the moment and at my experience level i am better off with a broadband. Thanks again to all of you!
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Michael Timm avatar
Michael Timm avatar
Tested the Neodymium yesterday on M 27. No reflection, way better than that Omegon UHC!
Andrea Alessandrelli avatar
Andrea Alessandrelli avatar
Tested the Neodymium yesterday on M 27. No reflection, way better than that Omegon UHC!


Glad to hear it! Baader makes great filters.