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.
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
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)
How long are your flats? Try to dim the light source and aim for something over 2s 10000 adu then match dark frames of the same duration at the same temp to subtract and scale them when making the master. See how you go with that first.
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!
Quote:
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?
Quote:
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.
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
I would simply use any black non-shiny paper - try officeworks or maybe black tape or black felt or black velvet. You would need so little black paper to treat the edge of one filter as a trial. The object of the exercise being to see if that 1 narrowband edge treated filter displayed the reflective arc OR NOT in comparison with the other non- edge treated narrowband filters. That way you have a logical direction which rules in or rules out the filter edge as a possible concern.
BTW I just noticed that Antila in their literature say that they edge blacken their narrowband filters too. Are your narrowband filters blackened on the edges? It may make what I'm suggesting a moot point in that case, but it doesn't hurt to try.
In any event try to dull reflections inside your optical train completely at first and then in slective places/sections using the matt black paper in order to try to zero in on where the reflection is coming from. Then when you find it - fix it with matt paint. paper, tape or whatever.
1. I took off the extensions and rotated the filter wheel + camera. Note this isn't the same as rotating the camera only, but I will do that next.
2. Inside the extensions, I used black electrical tape to minimise any reflections. This isn't ideal because electrical tape still reflects so when I finish work tonight, I'll head to officeworks and get some black paper.
The result: Here's an SII sub.
The reflection has shifted. It also now has rotated. One thing I thought of is that the Flattener adapter has a male thread of 6mm in length. This screws into a female adapter 5mm thick.. so there's a 1mm overhang inside! I have no idea how to tape this up or black it out. Perhaps look for a different Flattener adapter with 4-5mm of thread? (Josh I might need your help again )
The corner brightness issue is still there. So I know that's an issue somewhere inside the filter wheel or camera. I will check how it appears upon the next rotation.
I feel like progress is being made.. thank you all for your help thus far.
Just found a felt roll I had at home for a woodworking project that I've yet to finish. Covered the inside of the extension, and no difference in my flats. So I can rule that out at least
Hi Adam.
I'd suggest removing the camera and looking up the image train to your objective with your eye, while the flat panel is on the end of the scope. Look to see if there are any shiny spots on the walls or any internal parts where light can be reflecting from. Make sure you place your eye a Good way off axis to be sure not to miss anything.
)
