[AdamJL]

Thank you all for the comments

Originally Posted by Joshua Bunn

Hi Adam.
Are you able to just rotate the camera While keeping the filter wheel still? Might be able to isolate something further.

Hi there Josh.
Good point. I can indeed rotate the camera independently, but it requires me taking it apart from the filter wheel (it's held together with screws). I'll try this step next!

Originally Posted by gregbradley

How are you taking your flats?

Are your adapters blackened. no shiny threads?

Also note anodised black surfaces still reflect in infrared.

Greg.

Hey Greg. I'm using an A3 tracing panel currently. I've got a Pegasus Flatmaster on order as well which will only be here at the end of the month unfortunately. However I've tried sky flats and the same issue occurs.

My adapters are "black" but not sure about blackened. Good point about shiny threads! There is a bit of shiny thread to my flattener adapter. I think it's consumed by the female part, but I can't tell when it's connected.
Is there a way to cover up these shiny parts?

Originally Posted by JA

Hi Adam,

Sorry to hear of your woes. The best evidence you have for trying to help solve the circular arc reflection problem is that, presumably in an identical setup from image to image as shown in your posted images, your narrowband filters exhibit the circular arc reflection whereas the LRGB filters do not.

Why is this? Your signature indicates you use : "Camera: ASI2600MC, QHY268M + CFW3 (Antlia Pro 3nm LRGB+Narrowband)", but doesn't say which size, mounted or unmounted, etc; however:

Looking at information and images for the filters from the Antila info on line already suggests a reason for the difference of why your narrowband filters exhibit the circular arc reflection whereas the LRGB filters do not: Antila's narrowband filters, like most narrowband filters, are reflection coated, whereas their LRGB are not reflection coated and are translucent coloured optical filter glass. (BTW; there are some broadband filters that are reflection coated)

Referring to their LRGB filters, Antila further states: "Each filter is fully multi-coated to minimise internal reflections, halos around stars and to assure a high transmission which is above 95%. To even further enhance the contrast, reduce the glare and internal reflections, Antlia applies edge-blackening coating on each filter." That, for me, begs the question of whether they do the same for their narrowband filters: hopefully they do. If they don't and if you use unmounted filters then perhaps you could experiment with blackening the filter edge (paint, texta or tape). i would try black tape or perhaps even a 2 or 3mm wide sliver of black paper around the circumference of the filter trying it first on just one filter to see if there is any difference. If there is no difference then the reflection is elsewhere, caused by the reflective coating bouncing light inside your optical train rather than reflection from the filter edge. Also your problem is not a light leak as if it were the circular arc would also be seen on the broadband filters. Somewhere in there there is something shiny or reflective. Even a black anodised surface if glossy/satin will reflect light. Try also, as an experiment to help find the location of the problem lining (selectively or completely) the inside of the optical train with black matt paper (it should hold itself in when properly cut and fitted)

Good luck in your travels.

Best
JA

Hi JA, thank you very much for your detailed response.
I'm using 36mm unmounted filters currently with two different brands of filter masks (3d printed).

I've tried the tape method on other filters without success. But good idea to shine light around. I'll do that next as well and see what happens.

With regards to black matte paper, can I use any type? I can pick some up today from a store and test as well.

Regards
Adam