Light scatter

[Spidy]

My galaxy captures have an annoying ring around the outside of the frame. Nebula captures don't show, or at least it's very hard to discern this ring. I've attached a shot trying to show what I mean but you can see it better in PI .xisf files (which I will attach if anyone wants them).


My guess is this ring is caused by light scatter because I'm imaging from my backyard and neighbor's lights, street lights and even moonlight can contribute.


I can eliminate my calibration frames because the ring is in individual subs as well. What do you guys think, light scatter, or am I barking up the wrong tree?

[Imme]

What telescope?

It's not reflections between lens cells is it?

Is the inside of the tube blackened?

[Spidy]

Celestron Edge HD 9.25, ZWO OSC on OAG with guide cam.

[Spidy]

Here's a better example of what I mean.

[Imme]

Looks like an internal reflection to me.....however I’m sure someone with more experience with those scopes will have a better idea

[xelasnave]

I dont know if it will help but I use ( usually) baffled dew tubes some 18 inches to 24 inches long on my scopes to reduce light that as humans we dont notice entering the OTA...now if you think I am crazy take a look at the Hubble Space Telescope and take note how far the baffles extend past the secondary unit...I am sure when I was imaging in Sydney with a street light across the road that there was a noticeable improvement although I never did any before and after tests to have a meaningful comparison...in any event the long dew tube I am sure improves contrast...I like to add baffles as they must work better than just black paint...anyways look at a tech drawing showing the layout of Hubble ..I figure if they do it they may think similar to me on the matter.

If your problem remains past my suggestion I can not offer any other input.

Alex

[Spidy]

Alex, your suggestion is sound and makes perfect sense. It was one of the troubleshooting steps I was going to undertake in my next imaging session anyway as I have bought a Dew shield but yet to use it.


I also suspect that my Flat frames will need to be updated which might help mitigate this further in post production.

[Dave882]

Definitely update those flats. I reckon the circular scatter is probably a slight shift in focus since your last calibration flats. Even with eaa I do flats for every time I touch the focus. Doesn’t take too long once you got a system...

The side-to-side scatter could well be stray light. Good luck!

[Spidy]

Hi David.



Yeah, I reckon fresh flats will help.



Just to clarify, those images are not calibrate, just debayered, background extracted and stretched. They are single images.


I hope a Dew shield will help.

[multiweb]

All the inverse vignetting and the bright corners is caused by noise. The SNR is higher in your central field so when you calibrate the frame you bring up all the edge noise. Simple fix get more data and it'll go away.

[Spidy]

Thanks Marc. That's definitely a possibility.


I'll use a Dew shield, take fresh flats and take more subs next time I'm out.


It's surprising though as I have my gain set to 90 and they're 2 minute subs.

[multiweb]

Higher gain can introduce noise at times. Another thing I just thought of that might be a contributing factor is dew on glass surfaces or poor transparency combined with light pollution.

[Spidy]

Yeah, I think this is a combination of factors really. I will be cleaning my corrector plate with Eclipse solution and pec-pads, I will be doing a manual focus with a bahtinov mask, I will then use a dew shield to capture lights and then, I will finally take fresh flats.


If, after all of that I still get this halo, it's gotta be light pollution and I'll be selling my scope :confused2:

[multiweb]

If you have a corrector plate then it's the likely culprit. Apply gentle heat to it early evening before night time and keep it dry. A dew shield will make a world of difference for this too. Also try to bubblewrap your scope tube to keep it warm from sky exposure. All these will help greatly.

[Spidy]

Thanks Marc.



It's a relief to see nobody responded with " your camera is leaking light" or "your OTA is broken".



Seems to be a simple "common" problem. I'm also in the process of buying a Pegasus Power box and a dew strap, so with all the activities I listed earlier and the dew strip, I'm hoping I'll eliminate this glow entirely.

[multiweb]

The sky sucks heat. Anything exposed will soon end up with a lower temperature than ambient and will dew up more or less quicker depending on material and air humidity. That's why we wear beanies. Once you're dewed up you can't do much about it. But you can delay it by mitigating early evening into the night. Via insulation, applying heat, ventilation. Depends on your power situation, telescope design, and imaging site. Find something that works for you that is repeatable and stick to it. Make it part of your pre-imaging routine. Then you can cross that one out of the list and move on to the next looming issue. :)

[Spidy]

:rofl::rofl::rofl:

[Tulloch]

I had the same issue due to my imaging train and a focal reducer.
https://www.cloudynights.com/topic/696611-meade-f63-focal-reducer-on-c925-gives-very-uneven-lighting/?p=10035346

Moving to a full diameter T-adapter helped, but it's still not eliminated...

[Spidy]

Thanks Andrew. One of your images in that post look very close to what I get. I also considered the visual back, but not the reducer. I was looking at buying one of these (https://www.bhphotovideo.com/c/product/1574468-REG/celestron_93666_large_sct_edgehd.ht ml) to see if it makes a difference.



So many things to consider.:rolleyes:

[Tulloch]

Yes, I think if you can keep the opening as wide as possible it will help... I think the spacer I used was too long (as my reducer was acting at f/5.1) so a shorter extension tube will help.

[Constant]

Gentlemen, great thread, thought provoking with liberal lashings of problem solving. Spidy, this is the "stuff" that ignites my love of Astro. Cheers All

[Spidy]

Haha, thanks David. Hopefully all this discussion will lead to a solution.:shrug: