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Large halo around image


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Hi all,

I'm currently working on the Horsehead nebula in an HaRGB composition using an ASI 1600MM-C, with Gain 300 Offset 50. Had no problems with Ha or R components so far, but the G component is causing issues (have not got all of B yet). Attached is a quick uncalibrated stack of around 30 x 20s images. There is an obvious halo towards the left of the image. I've already tried calibrating with darks + cosmetic correction, have used the full stack of images (180 x 20s), and have tried Linear Fit instead of WS, with the same results. I don't usually use flats as I have 2 inch filters and a pretty clean optical train; have attached the R channel for comparison (100 x 30s). Not sure if this is an issue which can be solved by flats, or whether this could be the filter itself? I've taken an image of M31 with the G filter, and did not see this kind of issue there. I suppose the only points to note are that there are white LED streetlights right next to my garden, and the moon was out (but it was well past culmination), but surely that would affect all RGB channels? Would appreciate any insight.

Cheers

G.png

R.png

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10 minutes ago, steppenwolf said:

Have you checked that there isn't a blob of something on one of the optical surfaces? This isn't a vignette but it could be an over-corrected dust mote. You have a second one (smaller at top tight edge of frame).

Haven't checked that, good idea. Could I calibrate this out with flats in that case? Just that I'm imaging tonight and don't want to take the gear apart in case I mess up the alignment from last night.

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59 minutes ago, steppenwolf said:

Have you checked that there isn't a blob of something on one of the optical surfaces? This isn't a vignette but it could be an over-corrected dust mote. You have a second one (smaller at top tight edge of frame).

 

39 minutes ago, PhotoGav said:

Uh oh, welcome to the bright world of Alnitak... what scope are you using?

WO GT81 with 0.8x reducer (382mm FL, f4.7 with reducer). I knew I was going to have trouble with Alnitak! I think you're on to something Gav. Just did a stack of B images 40 x 20s, and getting the exact same effect (also a bit of flare, courtesy of the moon). I didn't realise Alnitak would throw off a halo this large, considering the star itself isn't too bad in the final image. Interestingly, I'm noting that the dust mote Steve picked up at the top right of the image 'moved' when seen in the B image (attached); that may also be the moon causing reflections.

Any ideas on how to counter this? I'd really like to salvage this!

B.png

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This is definitely Alnitak causing reflections. I had exactly the same issue when I imaged it last year. What filters are you using? Unfortunately the only way I dealt with the artefacts generated was to get rid of them in Photoshop. I'd love to know what it is that causes this and whether all telescopes suffer from it. Is it a telescope thing or a filter thing? I don't think it has anything to do with the Moon, by the way.

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9 minutes ago, steppenwolf said:

Having seen the second image (Blue filter) which shows an even stronger halo at the top right, I'd agree with Gav - the lack of any artefact from the red channel is a little puzzling but hey ho, there we go!

 

3 minutes ago, PhotoGav said:

This is definitely Alnitak causing reflections. I had exactly the same issue when I imaged it last year. What filters are you using? Unfortunately the only way I dealt with the artefacts generated was to get rid of them in Photoshop. I'd love to know what it is that causes this and whether all telescopes suffer from it. Is it a telescope thing or a filter thing? I don't think it has anything to do with the Moon, by the way.

Thanks both, appreciate your input. I guess it'll have to be a case of post-processing it out. Frustrating as I've been wanting to get after the HH for a long time, and just managed to get the rig working properly. I'm using Astronomik 2" Type 2c LRGB filters. I'm just puzzled about why the R filter isn't demonstrating any reflections, as you have mentioned Steve.

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