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Curious artifact on the Horsehead


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A few days ago I captured 90mins (18x5mins) of subs of the Horsehead and Flame nebulae. I took flats (albeit only 9) but my first attempt at calibration/stacking had horrendous vignetting. So I reprocessed without flats and present this here. I'm curious as to what the 'barley-corn' shaped artifact is by the nose of the Horse and how it can be removed without spoiling the image.

Some background... Last year I removed 30mm from the length of my Skywatcher 130P so that I could achieve focus with my Canon 1100D (without having to use a barlow). I was happy with the wider field of view except for coma. I purchased a coma corrector earlier this year but had to remove a further 10mm to achieve focus again! This image is the first taken with this much-abused telescope.

I think I have resolved the flats issue so will reprocess it again. Software used: captured with APT; calibrated/stacked in Nebulosity; post-processed with PixInsight.

John

post-34867-0-56878900-1416259136_thumb.j

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Thanks Ivo. I guessed it was probably a reflection of some sort. The scope was fairly well protected from stray light by the observatory dome, but the target was quite low. It was the first time I have had to open the lower shutter so maybe I did pick up some stray light from a house. 

John

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  • 2 weeks later...

Don't laugh, but it resembles Jupiter.

Sure a sub from a Jupiter session hasn't accidently been stacked?

Putting my flak jacket on now....

Michael

haha I have done this a few times myself along with the classic ,forgetting to take off the bahtinov mask on an imaging run

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I'm sure the cause could be a few things as suggested above. But i would guess that what you are more interested in at the moment is how to get rid of it.

If it doesnt to be in any of your subs (so you cant re-stack without it) the you could have a gentle play in Photoshop with the clone stamp tool, and i mean gentle! Zoom right in and see if you can get rid of it without introducing any sign it was ever there, may be a bit of a challenge, may not.

Also, it looks like you havent used any flats judging by the dust bunnies in the image, you could try a stack with flats to see if it solves the issue.

Good luck!

Callum

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Hi John,

If it is a reflection from ground object maybe it will move because the scope is tracking??? I would try different framings in order to see how it behaves :)

Thanks again Ivo. I've not had an opportunity to image with that scope since but I will certainly try framing the target differently to see whether it changes position.

John

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Hi, 

Are you using a Skywatcher coma corrector? They are known to cause reflections when very bright stars are in the field:

http://www.pleiades.hu/tesztek_csillagaszat/korrektorok_2.html

Cheers

Hi,

It's a Celestron coma corrector. The scope is a budget Skywatcher Newtonian designed for visual observing. I've butchered it for photography and thought that if the coma corrector gave me a decent field then I wouldn't have to buy another scope. At a focal length of 650mm it fills a gap between my William Optics (419mm) and Meade SCT(2032mm). I see from the link the sort of effects with the Skywatcher corrector. It could be. I'll need to run some more tests.

Thanks.

John

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haha I have done this a few times myself along with the classic ,forgetting to take off the bahtinov mask on an imaging run

I've not managed to do that - yet, he says! last year I made or more fundamental error though... I left the scope unattended for a couple of hours imaging M33. When I returned to see what I'd got, I thought I'd captured the disappearance of a galaxy! The first frames were good but then there was a a frame where M33 seemed to be fading and then gone from the next 3 frames. Took me a little to realise the mount had meridian flipped! (And I want to taken seriously, some chance!)

John

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This region is notorious for internal reflections which vary with spacings etc. I'm pretty sure it will be from Alnitak or a neighbour.

Olly

Thanks Olly. I'm wondering whether if it could be caused by some metal plates on the lip of my observatory dome? When fitting bearings to improve the opening/closing of the main shutter, I had to use some metal to strengthen the construction. Having imaged with my other scopes for a couple of months, I'd not noticed any issue so had dismissed it. However, I may have been unlucky to get the scope pointing in just the wrong position and - with that very bright star - got the reflections. 

John

post-34867-0-00712400-1417724745_thumb.j

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I'm sure the cause could be a few things as suggested above. But i would guess that what you are more interested in at the moment is how to get rid of it.

If it doesnt to be in any of your subs (so you cant re-stack without it) the you could have a gentle play in Photoshop with the clone stamp tool, and i mean gentle! Zoom right in and see if you can get rid of it without introducing any sign it was ever there, may be a bit of a challenge, may not.

Also, it looks like you havent used any flats judging by the dust bunnies in the image, you could try a stack with flats to see if it solves the issue.

Good luck!

Callum

Hi Callum,

Thanks for your guidance on the clean-up operation! I did take flats but something must've gone wrong somewhere because the resulting stack was almost white. I'll take another look and see if I can get the flats incorporated. The defect is in all subs (a single frame below).

I will give it a little gentle cloning and post it back.

John

post-34867-0-17120900-1417725356_thumb.j

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Hi John,

If it is a reflection from ground object maybe it will move because the scope is tracking??? I would try different framings in order to see how it behaves :)

Hi again Ivo. When I went through the subframes, I noticed the reflection is in the same place on all subs. The target moves in the field, so could it be the coma corrector as starfox suggested? Below are frames from the start and end the the session.

John

post-34867-0-21573000-1417726415_thumb.j

post-34867-0-10564700-1417726417_thumb.j

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If it's the same in all subs it does suggest an internal reflection somewhere in the optics. Over 90 minutes of imaging the artifact would move if it was reflecting off something outside the scope.

Hi, I've examined the subframes a bit further. While the horsehead 'drifts' slightly to the left in the field of view, the reflection moves to the right, i.e., it isn't stationery, like I first thought.

So, does that conclude an external source causing the reflection?

John

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Re the metal plates around the shutter:

It would be a simple matter to paint them black with some heat resistant barbeque paint just to eliminate them.

Heat resistant paint because it uses a pigment rather than a dye for the colour and is a lot less reflective at IR wavelengths.

Cheers

Andrew

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Re the metal plates around the shutter:

It would be a simple matter to paint them black with some heat resistant barbeque paint just to eliminate them.

Heat resistant paint because it uses a pigment rather than a dye for the colour and is a lot less reflective at IR wavelengths.

Cheers

Andrew

Cheers Andrew. I used bitumen-based paint to waterproof - and blacken - the interior of the dome and had considered this for the plates but on a non-porous surface it would be glossy. Hadn't thought about heat-resistant paint. I had some once - wood-burner stove paint - but that was years ago! Thanks for the tip.

John

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  • 1 year later...

Thought I'd post an update, having promised to show a re-processed image. I'd taken flats at different exposures and found a set that worked properly. I now calibrate in Nebulosity and stack/post-process in PixInsight.  I've perhaps not been as gentle (with PixInsight's Clone Stamp) on removing the artifact as I could - I think it is clear where it was.

Original image and reprocessed:

IC434_ISO800_90mins_LRGB1.jpg

intdrz_IC434_ISO800_90mins_2x2_median_Crop_Fix_DBE_CC_NR_MaskStr_NR_Enh (Large).png

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