Internal reflection artifacts?

[gasdoc]

Hi all,

I suspect this is a fairly common issue, but I haven't found the right search criteria to look it up on the internet for myself yet.

I started imaging last night and found these artifacts on my Ha images (brief stack of 8 x 300 sec subs). Darks and flats applied (no artefact on either of these that I can see).

My setup for this was:

1) Celestron Edge 8 HD (no dewshield on)

2) ZWO OAG to ASI 120 mini

3) ZWO filter wheel

4) ASI 1600

 

I use the imaging rig with my WO Z61 without any artifacts so I don't think it is due to any issues with the filter wheel/ OAG for example.

 

I wonder whether it is internal reflections in the SCT? -? secondary mirror reflection - if so, at least the collimation looks reasonable  

 

Second question: why do there seem to be regular artifacts (I can count about 16 in a 4 x 4 arrangement?

 

Any help much appreciated, and I fully expect a "you dufus, it's...…" response

 

Best wishes

 

Paul

Edited June 1, 2020 by gasdoc

[ollypenrice]

Don't beat yourself up, these things are a nightmare. I think the origin of the four obvious halos is 51 Cygni because the repeat reflections are all equally spaced. No Nobel prize for getting that far!! I may be wrong but I get the feeling that we are seeing a lot of posts like yours since the new CMOS astro cameras appeared. I don't think internal reflections were so prevalent with CCD though they did happen. The glass in the system includes corrector plate, primary, secondary, filter, chip window and (I think,) chip micro-lenses. The trouble is that it will probably be an interaction between some of these surfaces, but which? I wish I knew.

Olly

[gasdoc]

Thanks Olly for the reply!

So does that mean I'm stuck with this issue? To be honest, if there isn't a way to fix it, I'm going to return the Edge and stick with my refractor as it doesn't seem to suffer this issue.

BW

Paul

[johnoelliott]

Maybe if you post a photo of your setup, someone may spot a problem

[gasdoc]

Sure thing - see attached.

Fingers crossed for a simple-to-fix error!

 

BW

 

Paul

[Thalestris24]

Hi

I don't have one myself but I think this is a known issue which stems from the asi1600 sensor not having an anti-reflection coating. There have been other ASI1600mm problems reported which manifest as diffraction patterns linked to the microlenses. You might try and see if you can rearrange things a bit by changing the spacing to the filter wheel, and consequently the oag/guide cam also - just an idea. Perhaps someone else has a solution, if anyone is around.

Louise

[gasdoc]

Thanks Louise,

I’m going to try tonight with the dewshield on at the same target and check all my connections etc. If that doesn’t work, I’ll reconfigure my imaging setup although I haven’t had these issues with my refractor.

 

 Best wishes

 

Paul

[gasdoc]

So dewshield made no difference. I also posted on cloudy nights and they agreed with Louise & Olly that it’s due to diffraction issues probably as a result of the filters and/or camera. I will do some experimenting with removal/ repositioning of bits and report back

 

paul

[gorann]

I have no better ideas regarding the reflection rings but I know that the odd pattern around the brightest star (52 Cygni) is the well-known microlensing artifact that the microlenses on the ASI1600 chip give, and there is no way around that (you will also see it around bright stars with your refractor). A lot of people are happy imaging with Edge HD scopes (including me) so I expect that you will find a way to solve the issue with the ring-reflections. If not, these reflections are probably only going to be apparent if you have a very bright star in your image, so you may want to choose your targets accordingly. As I said, bright stars also do not go well with the ASI1600 due to the microlensing.

[Starwiz]

I have the ASI1600mm-Pro and have recently done a couple of nights imaging NGC6960, the witches broom which obviously has bright 4.2 mag star and I'm not seeing any of those reflections.

I can't say what the OP's problem is but I am using OAG, ZWO filter wheel and SW200P.  Hence, I wouldn't think the problem is to do with the camera or filter wheel.  I'm also not using a coma corrector yet.

John

[Laurin Dave]

Looks to me like there are two things going on..  a) ASI1600 microlensing around 51 Cygni which in your case isn't that bad and which you cant do anything about and possibly b) a reflection of 51 Cygni from the sensor glass back to the filter and then back again creating the large spaced out donuts.  Suggest you try turning the filter around and see if that makes a difference and also see if it is the same on all filters and also with no filter.   

Dave

[gorann]

Thinking more about this and reading Dave's comment I think it is almost certain that the reflections are going on between the chip and the filter and have nothing to do with the scope except that it grasps more light than your refractor and in this case from a very bright star. Turning your refractor towards Vega could possibly create the same ring phenomenon. It may be that you could sove the problem with turning the filters as Dave suggest but it could also be that the filters you use (ZWO?) are not the best and may vary in quality.

[Merlin66]

In solar imaging we can get annoying ghosting from the optical surfaces.

occasionally the use of a polarizing filter between the elements can suppress the ghosting.

Has this been tried with the ASI 1600 to subdue these reflections???