Very strange effects after stacking

Dirk (@dirkvl)

I don't claim to understand this, but I have seen almost exactly the same results under certain circumstances. I have a theory, but not enough evidence to be sure about it.

My theory is that when you are using a fast focal ratio, with a colour camera, and the light pollution is perhaps stronger than normal, that too long an exposure time will produce an image that won't calibrate correctly.

In my case I was using an F2 lens with a 2600mc in a suburban back yard with a little bit of moon. I was shooting 60s exposures (which is about the same as 240secs at F4 - so in the same ballpark as you) and I got similar results. I thought I had stuffed up my flats, but redoing them didn't help.

I could be completely wrong about this, but if this theory is correct then you need much shorter exposures when shooting with your fast F4 system under those conditions.

I hope this helps (and doesn't mislead).

JC

@connor231 

Hy John, thanks for your reply

Its worth to try

goin to test it perhaps this evening , almost full moon and they expect here some clearences

Gr Dirk

Hy,

could it be caused by the filter i screwed on the top of my corrector, it has a mettalic edge.... (see attachement)

And it looks like those effects only appear when there is some moonlight....

Dirk

I doubt that the filter is to blame - but who knows. At least that gives you something you can test. I did have a similar idea about reflections off the lens shield I was using on the F2 lens, but I dismissed that idea. Maybe I dismissed it too quickly.

JC

Hi Dirk, 

I have an Apertura 6" F4 newtonian with a ZWO 183MC pro and have never had any issues shooting at 180 seconds or with moon light. Now, I haven't used the newest versions of APP but I doubt that is the issue. A couple of thoughts. I believe a filter will change focal distances so I personally would not want anything in front of the coma corrector that might effect it's operation. I would place it in a filter drawer or filter wheel between the corrector and the camera. I think I have also read that if the focusing tube extends into the telescope tube, that can have some effects on the photo. I don't know if yours is that way when taking photos or just to show the filter on it. Also don't know if it would cause the effects you're seeing. If it is that way when taking photos, I wonder if taking flats with the shiny filter extending into the tube could cause some odd reflections. Good luck.   

Tom

Tom (@tfergu01)

I think these are good suggestions, but surely if the filter is the issue it would affect all flats taken with the filter in place. Dirk (@dirkvl) has said that he has not had this problem before. Just the same, I agree with your suggestion that the filter would be better placed behind the corrector (if Dirk's backfocus allows it).

Dirk's bortle class may be very different than yours. Another possibility is that he (and I) might have had some very light high cloud that magnified the lunar light pollution. I know - I'm clutching at straws here 😉 , but I would really like to understand this problem better.

JC

Hi John,

Oops, I thought when he wrote "I got" , he had acquired a new telescope. In that case, yes, seems odd to have an issue now. Hopefully Mabula can figure this one out.

Tom

Just did a new test last night  with no filter .....same result see attachement

Getting a little desperate....

Dirk (@dirkvl)

I am left with only two suggestions, and I'm not that confident about either.

Firstly, did you try my suggestion of much shorter exposure times ? Something like 30 secs, just to see if the problem goes away.

Secondly, I noticed that the inside of your newt's tube looks like a pale grey. I guess if the lunar separation (from your target) is low then there might be a lot of moonlight bouncing around inside your tube. I understand that a lot of newt owners recommend lining the inside of their tubes with some sort of black matt flocking material, although I thought this was mainly to improve contrast. Might be worth considering.

My question for other readers of this forum. Surely Dirk and I can't be the only people who have experienced this issue ? Is there anyone who has solved this problem, or at least been able to understand the cause with reasonable certainty ?

JC