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 Post subject: I have blown the core
PostPosted: Thu Dec 29, 2011 9:20 pm 
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of the Orion nebula.
How can I avoid that ?
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orion3.jpg by Radu Coman, on Flickr
Sky Watcher 750/150 with the 50D in prime focus, 28 exposures of 45 seconds with darks flats and bias frames (I think that 45 seconds are too much for the EQ3 mount) stacked in DSS curves and levels adjusted in PS.
Bob how do you get those pinpoint stars ?

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 Post subject:
PostPosted: Thu Dec 29, 2011 9:58 pm 
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Bob had previously posted a technique for separating the stars from the rest, allowing you to manipulate them independently for recombination. I still haven't given that a serious try yet... in this case, I don't mind that the stars have a size to them.

I love the detail you got here... is this with an unmodified 50D? Any extra filters? Are you in an area of low light pollution?

As for the blown core... my only thought is to do some lesser exposures for that region, then recombine them later e.g. by HDR like methods.

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 Post subject:
PostPosted: Thu Dec 29, 2011 11:05 pm 
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Hi popo,
Yes it’s the unmodified 50D. No filters. I drove 1 ½ hours to the location, on the heights near the Dead sea. I’m doing this with folks from the astronomy association once a month, obviously when is a new moon. The visibility was good, but not so good as other times, and the only major light pollution is from Jerusalem, about 30 km away. North east and it’s felt up to about 15 to 20 degrees above the horizon. So it was good but not great. The Milky wai is visible to the naked eye. The wind was more problematic , about 11 km/h, and that’s too much for the EQ3 mount.
About the blown core. If I make some frames with let’s say 1 ½ stops less ISO and stack them in DSS with the higher sensitivity ones, would solve the problem ?

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Canon PowerShot S100
Canon 50D , SIGMA 10-20 f3.5 ,Canon EF 24-105 L IS USM, Canon EF 100/2.8 macro Canon EF 50/1.4 ,Canon EF 85 f1.8,Canon EF-S17-85 4-5.6 Old Tamron 28-300 inherited from my Canon Rebel G film camera
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 Post subject:
PostPosted: Thu Dec 29, 2011 11:50 pm 
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I don't know how DSS handles mixed exposures, but worst case treat the exposure separately and manually combine later.

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 Post subject:
PostPosted: Fri Dec 30, 2011 12:59 am 
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Hi Radu,

Still a very nice shot though. 8)

In case you haven't found it yet the thread popo referred to is Star Removal - A simple PhotoShop operation.

The procedure needs practice and a willingness to experiment but I'm already at the stage where I'd be lost without it. Yes, you can separate the stars from an image and allow separate workflows for stars and nebulosity but, provided you have already aligned the frames, it is just as easy to combine stars from a totally different set of subs to those used to capture the nebulosity. I've yet to tackle the Orion nebula as I'm still waiting for my TEC 140 'scope (4 months late now!) but I do know that you will have to take subs with a very wide range of exposures and combine them during post-processing. I imagine that there will be plenty of tutorials out there on how to use PhotoShop layers to do the job. At the end of the day if you can remove the stars from all the nebulosity frames you are left with essentially a standard HDR situation with bracketed frames. Once the nebula image is to your liking then add back the stars that look most appropriate.

All theory on my part however, even though it is how I intend to work when I get the chance, so please take the above with a large pinch of salt.

Bob.

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 Post subject:
PostPosted: Sat Dec 31, 2011 2:34 pm 
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Hi Bob and popo,
Thanks for the explanations and the tips. I’ll surely have another go at it in about three weeks. I wanted to go the last Thursday but, alas, clouds came in unexpectedly (in spite of the forecast) so everybody stayed home.
My stars are egg shaped. Is this coma aberration or tracking error? I think that coma should be evident towards the edge of the frame ( what you see is not cropped) and not in the centre of it. But I’m not sure. Investing now in a better mount is not possible, so I wonder if a coma corrector will improve the situation

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 Post subject:
PostPosted: Sat Dec 31, 2011 3:48 pm 
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If all the stars are elongated in the same single direction, that's a tracking problem. If the elongation is worse away from the middle of the image (usually) going outwards that's optical.

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 Post subject:
PostPosted: Sat Dec 31, 2011 4:35 pm 
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Hi Radu,

Your image responds well to the star removal technique. The stars can be sharpened up and, when working with the "no stars" part PhotoShop's Shadows/Highlights adjustment can be applied to recover more of the faint stuff - even from the JPEG there's more there than is being easily seen. A little judicious noise removal in the fainter parts of the nebulosity helps mitigate the effect of the Shadows/Highlights stretch (Topaz Labs DeNoise PhotoShop plug-in allows you to tune noise reduction in that way) and the stars can then be added back. Nothing to be done about the blown core, of course. If you aren't imaging for a little while give it a try.

Bob.

P.S. I'm assuming you have PhotoShop, of course. Apologies if you haven't.

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Leica D Vario-Elmar 14mm-150mm f/3.5 - f/5.6 ASPH.
OM-D E-M5, H-PS14042E, Gitzo GT1541T, Arca-Swiss Z1 DP ball-head.
Astrophotography: TEC 140 'scope, FLI ML16803 camera, ASA DDM60 Pro mount.


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 Post subject:
PostPosted: Sun Jan 01, 2012 1:51 pm 
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Hi popo and Bob,
I'll give it a try and show you the results for inspection :D
And, yes, I have photoshop .

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 Post subject:
PostPosted: Thu Feb 02, 2012 5:25 am 
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Tried out PixInsight ond the same data. Almost. No flats, darks nor bias, I don't know how to do that yet. The Trapezium is stil blown out but the core is much better. Rain and clouds prevented me to go out and grab some more on the new moon.
PixInsight rawks :-)
Image
M42_V3.jpg by Radu Coman, on Flickr

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 Post subject:
PostPosted: Thu Feb 02, 2012 5:33 am 
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Wow, very nice shot!

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PostPosted: Thu Feb 02, 2012 7:36 pm 
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Hi mbrandste, thanks for the compliment. :-)
I know it’s still a far cry from what it should be, but you know , It’s great fun.
All the time effort, time, and money invested in one shot gives you lots of satisfaction combined with lots of frustration. Reminds me what I felt the first times, as a 10 years old, when I was developing my first photographic papers and saw in the dim red light the image magically revealing itself.

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Canon PowerShot S100
Canon 50D , SIGMA 10-20 f3.5 ,Canon EF 24-105 L IS USM, Canon EF 100/2.8 macro Canon EF 50/1.4 ,Canon EF 85 f1.8,Canon EF-S17-85 4-5.6 Old Tamron 28-300 inherited from my Canon Rebel G film camera
Canon580EXII
http://www.errre.net


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PostPosted: Sat Feb 04, 2012 7:59 pm 
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Looks stunning, much improved.

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PostPosted: Mon Feb 20, 2012 4:30 pm 
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Thanks guys :D

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Canon PowerShot S100
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Canon580EXII
http://www.errre.net


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