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Old 10-12-2020, 04:59 PM
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DiscoDuck (Paul)
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Join Date: Apr 2014
Location: Adelaide
Posts: 691
Halo issue with new filter

I have a question that I need the help of the IceInSpace astro brains trust on please.

I have just bought a couple of Chroma 31mm narrowband filters (3nm OIII and 5nm Ha) to use with my ZWO 1600 and RC8.

The OIII seems fine. But I notice a large halo when using the Ha. I realise this is a reflection probably between the filter and some surface (either the camera sensor or chamber glass or the reducer) rather than the other smaller halos often discussed, which are the result of internal reflections in filters. In fact, the size of the reflection indicates there is about 15mm between the two surfaces causing the reflections, which suggests it is the filter to the camera sensor. (I think that is about 16mm) (and I understand the 1600 does not have an AR coating on the sensor glass).

Some questions/points

- Why should it appear for Ha not OIII? I understand this star is a bit reddish (mag 3.8) and the OIII filter is of course narrower bandpass, so there will be less light bouncing around in the OIII case, but still it is a dramatic difference from not even visible with one to quite prominent with the other. Is there some dependency of such reflections on wavelength??

- I am 99.9% sure I've got both filters the same way up in the filter wheel. When I get a clear night, I will of course double check that flipping the Ha filter doesn't change things, but I am confident I have been consistent. But any advice on correct orientation appreciated - including if it even makes a difference (the filters do seem different to the eye between sides).

Any other comments/advice appreciated e.g. re any other possible tests I could do to pin it down/eliminate it.

Thanks,
Paul

P.S. The seeing was vastly different between the two nights these were captured too. So please forgive the bloated Ha in comparison to the tight OIII!
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