Quote:
Originally Posted by AdamJL
Hi all
I've been trying to figure this out for a few weeks now, and I can't for the life of me understand this.
For a long time, my narrowband flats have been showing two abberations.
1. Odd partly circular reflections
2. Edge brightness in one corner.
I have taped up EVERYTHING. Anywhere light can leak, I've plugged it up. I know the issue is from the filter wheel onwards, because if I rotate the filter wheel and camera, the issue rotates as well. I have cleaned and reset my filters multiple times. I have purchased different filter masks made by a different company (first was from Testar, second was from Buckeyestargazer).
I have taped the edges of the filter masks themselves. I have taped any metal inside the filter wheel to minimise reflections.
No matter what I do, I get these abberations.
That said, I do notice that the brightness "centre" for my flats appears to not be in the centre of the frame, suggesting possible tilt in my image train. However, this was an issue before I replaced my focuser as well so I don't know if tilt is the issue that could be causing it.
Something is reflecting inside my filter wheel, and it's showing up in the narrowband flats because they reflect the most light. It's being bounced up into something.
Any ideas?
Only thing I can think of is that my flattener uses an M48 thread, which then opens up into the filter wheel extension that is M54 in size. It would seem odd that this is the cause, because it's QHY's own extensions and if I have the issue, so would many others.
Lastly, I've attached the images below in order of how they are setup in my filter wheel. As you can see, the curved reflection stays the same for the narrowband images. For SII, you can even see the beginning of another curved reflection in the bottom right hand corer.
The other issue: the "corner brightness" shifts between each frame.
Broadband flats look fine.
Any help most appreciated!
Thanks
Adam
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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