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FORUMS Photography Talk by Genre Astronomy & Celestial Talk
Thread started 12 Dec 2008 (Friday) 13:50
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A problem with longer exposures

 
Jeff
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Dec 12, 2008 13:50 |  #1

IMAGE: http://seivertfamily.com/POTN/ASTRO_2321.jpg

This is a single 60 second image, ISO1600. See how the center becomes lighter? If I try to take images longer than 45-60 seconds this ends up ruining the image, especially on fainter objects. So how do you guys avoid this with exposures of 2-5 minutes? Do you drop the ISO down to say 200? That would make a 240sec image equal this 60 sec / ISO1600 shot.

I'm still looking at GEM's and stacking, but if I can't get longer exposures without the center overexposed, then solving my field rotation issue won't matter.

Thoughts?

Jeff
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Nighthound
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Dec 12, 2008 14:19 |  #2

This is a good example of light fall off, some will refer to it as vignetting but vignetting is caused by direct obstruction like when shooting afocal and the eyepiece cylinder becomes visible. It is similar in that the light path is being narrowed somehow. You can minimize this by getting the camera as close to the focal reducer as possible. I had a problem with my LX200 when I was using the Meade T-adapter for SCTs. It had about a 2" cylinder that attached to the focal reducer at one end and the camera T-ring at the other. Then I picked up the Orion T-adapter with the chrome cylinder and black top. The black top unthreads from the chrome cylinder(designed to slip into a 2" focus tube) and then can be threaded to the f/6.3 focal reducer and the camera T-ring on the other end. This brought the camera much closer to the reducer and greatly reduced the light fall off effect. Shooting darks and flat frames to add in the stacking will clean up this effect and help with noise as well, although I have never tried them.

Here's what it looks like:
http://www.optcorp.com ...5&kw=orion%20adapte​r&st=2external link


Steve
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Celestron
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Dec 12, 2008 14:55 |  #3

You have alot of detail there if you solve this problem you'll have a very nice image .




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Adrena1in
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Dec 13, 2008 05:31 |  #4

jseivert wrote in post #6865489external link
Thoughts?

If that's M51 and a single 60s shot then I'm VERY envious of your kit and skies...I've tried several times to get M51 and can't get anything, even with lots of images stacked.

At first, when I read this thread the other day, I thought you meant the centre of the M51 galaxy was getting too bright, but I see what you mean now, in that's it's vignetting. (I often don't notice things like that as my eyes aren't really trained to see that sort of thing straight away.)

As Nighthound says, flats are meant to help reduce this problem. I honestly reckon if you took 20 or 30 60s images and stacked them with a bunch of flats, (did I read somewhere that you need 9 or 15 flats to make it worthwhile?), then I reckon you'd have a cracking shot there.

But I do see your point, that sometimes you do need to expose for longer. However, I do believe that even if the frame vignetting gets so bad that it looks like the frame is ruined, there'll still be a host of "true" data in the frame which, when stacked and flats added, will show some lovely data.

I mean, I've taken long exposures before and thought to myself, "These are all rubbish...I can't see anything useful in there", but after processing I've often been very pleasantly surprised.


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Sorarse
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Dec 13, 2008 09:20 |  #5

Most of that can be resolved in PP. I'll have a look to see what I can do - I'm no expert but you have the makings of a very good image there and I am sure it can be improved with a bit of PP work.


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Jeff
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Dec 13, 2008 09:47 |  #6

If you'd like to work with the original RAW file it's here: (7.1mb)
http://www.seivertfami​ly.com/POTN/IMG_2321.C​R2external link

I'd love to see the results.

Have fun!

edit: evidently my server keeps choking on the transfer. I'll post when it's REALLY there.

OK It's there now. If anyone else wants to have a go at it, feel free. (and post the results)


Jeff
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Sorarse
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Dec 13, 2008 10:27 |  #7

Here's what I've managed to do with the image posted here with just a few tweaks.

IMAGE NOT FOUND IMAGE IS A REDIRECT OR MISSING!
http://www.88qv.com/ne​t/ASTRO_2321.jpg (external link)
HTTP response: 404 | MIME changed to 'text/html' | Byte size: ZERO

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Jeff
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Dec 13, 2008 11:43 |  #8

It definitely looks better. A little bluish cast though?


Jeff
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Sorarse
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Dec 13, 2008 11:45 |  #9

Sorry about that - I added that on purpose, as in some books it is perceived that a slight blue cast is perceived as more natural looking or is more pleasing to the eye. Easily removed if you don't like it.

ETA: I've tried to download your TIFF file, but it won't open in Photoshop (CS3) as it keeps coming up with the error message 'Unsupported compression method' or words to that effect.


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A.S.I.G.N. ­ Observatory
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Dec 13, 2008 16:58 as a reply to Sorarse's post |  #10

Flats and darks definately help. If you build yourself a simple and cheap light box, your lights will be the easier to take and acheive a "real" correction.

I built one a while ago with cheap materials with basic kitchen and shed tools. Looks good too.

http://www.asignobserv​atory.com ...t_yourself/light_bo​x.aspxexternal link

Excellent image though. Surprising detail for a short exposure.

Baz.


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dpastern
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Dec 13, 2008 19:06 |  #11
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I tried to download the RAW, non usable in DPP and it just causes CS2 to crash...pity, would have like to play with it. I tried d/l it twice as well, failed with the same symptoms in both issues.

Dave


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Jeff
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Dec 13, 2008 22:41 |  #12

Hmmmm, try now. I had my ftp setup for ASCII for a specific transfer. Changed back to binary so that should take care of it.....I hope.


Jeff
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dpastern
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Dec 13, 2008 23:38 |  #13
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Thanks Jeff, that did the trick. Here's my attempt (please remember that I have zero experience with processing astro images in Photoshop).

I did an adjustment layer:

-3 brightness and +6 contrast

then I did:

a run of Neat Image (defaults and auto profiling) - which surprisingly did a good job of cleaning noise up without removing too much detail. I didn't expect it to work that good to be honest.

I then resized to 1024 and did a small amount of smart sharpening (53%, 0.3 threshold) and then save for web.

What do the other guys do for processing I wonder?

Dave

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Jeff
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Dec 14, 2008 17:40 |  #14

Wow, looks much better I'd say!


Jeff
70D | Tokina 12-24 | Canon 50mm f/1.4 | Tamron 17-50 f/2.8 | Canon 28-135 IS| 430EX
Astrophotograpy: Meade 10" SCT, AT8IN, Orion EON 110mm APO, Coronado PST, Atlas EQ-G to keep it all off the ground.
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PM01
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Dec 16, 2008 23:05 as a reply to Jeff's post |  #15

MaximDL + DSLR.

Flat field it. It's not difficult. It'll correct for any mechanical or optical vignetting.

Nice skies by the way.




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A problem with longer exposures
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