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Old 28-06-2019, 09:57 PM
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The_bluester (Paul)
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OSC cams and filters

I have been using a ZWO ASI294 for some six months as my first toe in the water for AP, firstly with my existing SCT (A non Edge HD Celestron C925 that I have been using visually for almost ten years)

A few months ago I bought a Skywatcher Evostar 72mm to gain some widefield experience and have something easier to guide than a 2200mm focal length SCT. Results in fainter objects were pretty pleasing at my experience level but bright stars in frame were giving me some spectacular reflection problems.


I had read that cameras like the ASI295 are still quite sensitive into the infra red and that this can play hob with focusing with programs like SGP as the poorly focused IR or NIR component will impact HFR measurement and even manually focusing using HFR you can find yourself focusing in part on information that you do not want, and even if you get the focus right, you may find the poorly focused (In the case of the refractor in particular, obviously) IR/NIR data producing bloated stars. For that reason there are recommendations out there to use an IR cut filter with OSC cameras. Background sky brightness is also supposed to benefit due to atmospheric heat being re radiated in the infra red.

Long story short, I was buying a focus motor to enable automated focus runs on the 72mm and grabbed an IR filter while I was at it. And the difference is gobsmacking.

The two images here are basic integrations, 2 subs each, 120 second subs without the IR filter, 300 seconds with. Stacked, background neutralised and star colour calibrated and stretched using the same settings in APP. Photoshop used only to reduce them down to a size I can post here. The star at the bottom left is Gacrux, I got a similar improvement on Antares. Aside from the background brightness reduction I have to assume that the AR window coating is not effective in the infra red due to the other obvious reflections. The reflection blobs calculate out by diameter in pixels and F ratio to be multiples of the distance between the sensor and the window at the front of the camera. I am reasonably sure that the diffraction like pattern around Gacrux is reflections from micro lenses adjacent to the pixels that should be lit. The "Ghost" diagonally opposite is a similar reflection and stays opposite in the frame from wherever the star in question is.

TL;DR? If you are using an OSC astro cam, particularly with a frac, seriously consider using an IR filter!
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Last edited by The_bluester; 29-06-2019 at 09:46 AM.
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  #2  
Old 29-06-2019, 05:09 PM
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LewisM
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Been using a UV/IR on all my refractors for years. Makes a MAJOR difference.

I bought a 67mm Marumi UV/IR for my FSQ-106N - it sits at the front of the drawtube step-down adaptor, 60mm away from the sensor of my SXVR-M26C APS-C sized OSC CCD. No ghosting.
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Old 29-06-2019, 06:36 PM
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The_bluester (Paul)
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I got an adapter made to mount the ED80 focal reducer on my 72mm (For some reason Skywatcher use an internally threaded drawtube on the 72mm instead of an externally threaded one on the 80mm and some other larger ones)

Josh Bunn on these forums made it for me. I have just packed it off back to him to get an M48 filter thread turned into the clear aperture of it so I can hide the filter up inside the drawtube, in front of the reducer/corrector. Pity I did not think of that when I got him to make it to begin with. If the female thread end of the filter was longer I would probably have just left it between the reducer/corrector and my OAG, but it had not quite one full turn of thread in it so it was a little nerve wracking in that location with the guilder and camera hanging off 0.75mm of thread.

The filter I have is the ZWO IR cut only. I did wonder if a UV cut would also help at the other end of the spectrum but at least for now the camera is finally working in the way I expected. Results on the 72mm were OK but there were a lot of bloaty looking stars and the insane reflections which were obviously somewhere longer wavelength than the cutoff of the filter. I was chasing odd reflections with the SCT too so it might have the same issue.

I am not sure how to deal with it for the SCT if I image with that any more, unless I can find a filter with much longer female threads it will be tough to mount one apart from shelling out for a filter wheel as a step towards LRBG/Narrowband and putting in a UV/IR cut filter which would end up used as a lum filter later on whenever I eventually go mono.
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Old 01-07-2019, 10:46 AM
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The_bluester (Paul)
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I was about to ask the question if I should consider a UV-IR cut filter rather than the "IR" cut filter I have now but digging up details it looks as though the ZWO "IR" filter band pass is 400 to 700nm, so a different filter is not likely to net me any gains at the blue end.
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