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Old 15-08-2018, 08:51 PM
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Why halos on bright stars with green and OIII only?

As per the title, I seem to be getting halos around bright stars with the Baader 36mm unmounted green filter and the 12nm QHY OIII filter in the 36 unmounted. None of the others show it. Also, I never seemed to get any halos with the QHY LRGB filters that I just replaced.

I have heard and read on the Baader site to turn the filters against the arrows that are on the edge to combat halos and was wondering if anyone has had any experience with this?
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Old 15-08-2018, 09:23 PM
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Atmos (Colin)
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Not sure if it would be the same for this but I have read that some filters and directional and that it can cause issues if they're placed in the wrong direction.
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Old 15-08-2018, 09:56 PM
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Joshua Bunn (Joshua)
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I get green Halos in my RGB captures, they are off putting. I'm currently using Custom Scientific 52mm unmounted filters, so I'm going to watch this thread. Ive also heard the direction they are facing has an effect... but it made no difference to my filters.
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Old 16-08-2018, 06:19 AM
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What telescope are you using Bart?

The reason I'm asking is because OIII signal is usually significantly weaker so it often gets stretched more than Ha consequently making halos more visible. A telescope which has noticeable chromatic aberration (achromat, ED doublet) will produce halos around bright stars due to the out of band signal slowly leaking though the filter, even with the best narrowband filters.

Last edited by Slawomir; 16-08-2018 at 07:28 AM.
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Old 16-08-2018, 07:33 AM
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I'm using a WO FLT132. I can post some pics tonight if it helps. It is not from false colour, it definitely looks like a reflection issue and is slightly bigger than the star itself.

Last edited by Bart; 16-08-2018 at 08:43 AM.
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Old 16-08-2018, 06:02 PM
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Images would certainly help in identifying the cause.

Towards the end of this short review there is a comparison of halos around a bright star from a 12nm OIII and 3nm OIII.
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Old 16-08-2018, 08:36 PM
ericwbenson (Eric)
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Hi Bart,
The filter direction has no effect whatsoever on haloes. I have demonstrated the math on Cloudynights a few times. The intensity of the haloes is linked to the quality of the AR (antireflection) and passband coatings on either side of your filter, and/or the presence of other uncoated/badly coated surfaces in the optical train (e.g. Panasonic 16Mpix CMOS sensor). Unfortunately there is nothing you can do about it except exchange the filter (sorry for the bad news), but maybe it is acceptable?



The direction can impact light leakage from around the edge depending on which side the filter the aperture stop is resting, but that shows up in flats and as bright streaks from stars slightly out of the FOV, different problem from what you are reporting.


Regards,

EB
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Old 17-08-2018, 08:51 PM
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Thanks for all your replies and very interesting post, Eric.

From my image cropping, I can see that the halos are on more images than I thought so it is likely that it is a camera of flattener issue perhaps.
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Click for full-size image (Master_SII_Crop.jpg)
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Old 17-08-2018, 09:07 PM
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The second last image with the largest halo looks a bit like there could be some fogging up happening somewhere in the optical path.

Most of the images though show stars that I would normally expect.
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Old 19-08-2018, 09:20 AM
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OK, so I'm glad you think it is normal. Maybe I'll do a few more images and then see what happens.
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