Old 10-12-2020, 05:59 PM
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DiscoDuck (Paul)
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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.


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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Old 10-12-2020, 09:15 PM
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xa-coupe (Jeff)
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The halo looks like a reflection of the OTA, I'm assuming you're using a reflector?

I have the L-extreme on a refractor and it bloats the stars too, I've been told that more data for stacking and the processing will iron it all out.
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Old 10-12-2020, 09:42 PM
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Looks like a reflection off the filter and back to and off the corrector lens.

As to why Ha and not O111? Ha is way more common and brighter than O111 so no surprise there really.

Do these filters have anti reflection coatings on both sides? There are threads about halo reflections on Cloudy Nights. Not sure about which filters beyond ZWO's.

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Old 12-12-2020, 03:24 PM
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DiscoDuck (Paul)
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Thanks Jeff and Greg for the replies.

I found a thread which suggested I have the filters the wrong way round according to Chroma (they are AR coated on only one side), but the right way round according to the consensus in that thread!

I've flipped them anyway and will await a clear sky to test ... may be some time!

Still not understanding why Ha showed a halo and OIII not. That star is a broadband emitter. Slightly red so a bit more in the Ha bandpass than in the OIII, but still should be plenty of light bouncing round in the OIII case.
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Old 19-12-2020, 04:06 PM
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DiscoDuck (Paul)
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I think I understand my halo issue now. Not solved it, but I am confident re what is happening. And I think there is no solution.

I flipped the Ha filter over, and the halo is still there, but a bit weaker and larger. (Images Dolphin Ha single sub orientation 1 and 2 attached).

The original halo corresponded to a distance between the two reflecting surfaces of about 15mm, about the distance from the filter to the sensor. The new one corresponds to a 17mm distance. So what I think is the case is that with the ZWO 1600 having no AR on its sensor glass cover, light is reflecting off there, and back off the filter and back down.

In the first case, the reflective side of the filter was nearer the camera. Hence the 15mm distance. In the second case, the side with the AR on was nearer, so the light was passing through the filter surface facing the camera and then reflecting back from the inside of the telescope side surface of the filter. It is 3mm thick, and with the refractive index, that accounts for about an extra effective 2mm in the light path. Hence the jump from 15mm to 17mm indicated by the halo size increase.

Bottom line ... the reflection is a bit weaker now, but still there. And really an issue caused by the camera NOT the filter. When the new orientation is stacked, the result is Dolphin Ha.jpg (attached). The halo is very prominent - but only I think because there is so little Ha in this area (Dolphin nebula - mainly OIII!) and that star is actually pretty bright (mag 3.8 I recall). Those bits of Ha at bottom left are very faint, and so it is a very stretched image. (The halo you can see is originally fairly weak - in the original sub it is much dimmer than the amp glow).

It is tame enough now to be correctable - sort of - a clumsy attempt is shown in Dolphin Ha fixed.jpg.

As a quick experiment to test it in the worst case scenario, I took 6x5min images of Horsehead + Flame, with Alnitak in the field of view. Previous attempts with the same scope (different camera and filter) had resulted in much stronger (and multiple) reflections, as in Horsehead old.jpg. The experiment from last night is stacked to give Horsehead.jpg. The weaker and single reflection is almost fixable - Horsehead fixed.jpg.

Sorry for the lengthy blurb. Just wanted to clarify that I don't think the Chroma filter has an issue, but the ZWO camera seems to be the root cause. And even then it's not ZWO's fault, I hasten to add. I believe the sensor with glass is supplied as is by the sensor manufacturer. I'm not sure if it's worth any follow up with either manufacturer then. It seems to be an inherent issue with the reflection from non-AR coated glass in the camera that I may just have to live with and try and process away the rare cases when there are stars bright enough to cause the effect.
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Click for full-size image (Dolphin Ha single sub orientation 1.jpg)
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Click for full-size image (Dolphin Ha single sub orientation 2.jpg)
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Click for full-size image (Dolphin Ha.jpg)
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Click for full-size image (Dolphin Ha fixed.jpg)
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Click for full-size image (Horsehead fixed.jpg)
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Last edited by DiscoDuck; 22-12-2020 at 01:08 PM.
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