Strange blooming (?) issue with ZWO ASI 120MM-S

gruetter

Hi all,

I'm experiencing a strange imaging issue with my ZWO ASI 120MM-S. There's a wedge shaped shadow in all bright stars. I think I can exclude my telescope (8" F/4 Newtonian) as my DSLR does not exhibit the same behavior. Here's a representative sample:

Does anybody here have a clue as to what could cause this? Thanks in advance for your help!

Cheers,

Georg

moonprince

hi,gruetter

it looks like FOV of main mirror has been coverd by focuser , I guess.

gruetter

I did check this after you posted this, MoonPrince. However, this is not the case. Thanks for your tip, though. Any other ideas?

einari

What software did you use ? What gain ?

Are you sure you don't get it with DSLR if you magnify as much as this image ?

gruetter

Not sure about the gain. I believe it was somewhere between 20 % and 40 %. I haven't photographed with my DSLR for a while, so I'm not sure. I have another ZWO ASI 120 MM for guiding. I'll check that out.

But since you asked for gain, how do you guys go about photographing nebulous areas where you need to expose long enough and where stars get "blown out" by default? By the way, the image taken above was exposed for 5 seconds.

moonprince

I don't think this is blooming. actually, all ASI cameras have anti-blooming function, so I agree with einari, hope you can try different cameras or DSLR to check this problem.

gruetter

I was able to do more tests with my other camera today. It turns out the same problem occurs here as well. So I guess we can exclude the ZWO ASI 120 MM-S as a source of error.

I still have no idea what could cause this, though. There's nothing prodruding in the OTA (like the focuser) and the mirrors are collimated and reasonably clear.

Does anyone have a clue what's going on here?

Sam@ZWO

gruetter wrote:
I was able to do more tests with my other camera today. It turns out the same problem occurs here as well. So I guess we can exclude the ZWO ASI 120 MM-S as a source of error.

I still have no idea what could cause this, though. There's nothing prodruding in the OTA (like the focuser) and the mirrors are collimated and reasonably clear.

Does anyone have a clue what's going on here?

just take one image out of focus and show us

maybe we can help

gruetter

I will do that. Thank you very much for your offer, Sam.

gruetter

I was out in the field again and noticed something: The shadow is exactly where one of the clamps for the primary mirror is. This one clamp protrudes into the mirror a tiny bit (less than 0.5 mm I reckon) more than the other two clamps, which also cast a shadow, albeit much smaller. Is this an expected behaviour for a Newton reflector?

moonprince

gruetter, if you using a big ring as big as the prime mirror instand of claws , can solve this issue.

gruetter

Thanks for the suggestion, MoonPrince. I disassembled my Newton and found out, that there already is such a ring. Also, the clamps are almost 100% covered by this ring, except for the one that is apparently causing the shadow. Funny thing - that clamp protrudes into the primary mirror for only about 0.2 - 0.4 mm, or so. Could that cause the shadow?

moonprince

yap. even a little things against the roundness of primary mirror would cause a big troble. :)