View Full Version : A problem with longer exposures
12th of December 2008 (Fri), 13:50
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.
12th of December 2008 (Fri), 14:19
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:
12th of December 2008 (Fri), 14:55
You have alot of detail there if you solve this problem you'll have a very nice image .
13th of December 2008 (Sat), 05:31
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.
13th of December 2008 (Sat), 09:20
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.
13th of December 2008 (Sat), 09:47
If you'd like to work with the original RAW file it's here: (7.1mb)
I'd love to see the results.
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)
13th of December 2008 (Sat), 10:27
Here's what I've managed to do with the image posted here with just a few tweaks.
13th of December 2008 (Sat), 11:43
It definitely looks better. A little bluish cast though?
13th of December 2008 (Sat), 11:45
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.
13th of December 2008 (Sat), 16:58
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.
Excellent image though. Surprising detail for a short exposure.
13th of December 2008 (Sat), 19:06
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.
13th of December 2008 (Sat), 22:41
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.
13th of December 2008 (Sat), 23:38
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?
14th of December 2008 (Sun), 17:40
Wow, looks much better I'd say!
16th of December 2008 (Tue), 23:05
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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