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Strange "noise" in long exposures

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Offline art

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Strange "noise" in long exposures
« on: 11 / May / 2009, 17:32:05 »
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Hello,
First of all I'l like to thank everyone who contributed to CHDK for this very, very useful software. It extended functionality of my A620 quite a bit. Until I buy a more advanced camera, I'll rely on CHDK a lot. :)

I've been playing with astrophotography recently and I noticed a strange blue coloration on my photos with long exposures (see attached dark frame). Where is it coming from? I have the dark frame subtraction turned off under raw. The blue halo disappears when I have it enabled. It is most visible on 64 second exposures but it also shows on 15 second ones. The halos are always in the same spot. I took several dark frames covering the lens entirely to make sure it is not some light seeping through. No luck. Any ideas? Is it possible to get rid of it? I do not want to use built in dark frame subtraction because it disrupts timing etc., especially for long exposures and star trails.
Any help or insight would be greatly appreciated.




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Offline fbonomi

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Re: Strange "noise" in long exposures
« Reply #1 on: 11 / May / 2009, 18:22:24 »
That's normal: there are some other components near the image sensor. These components heat up the reanrer parts of the sensor and that's what appears.

Dark frame subtraction is made onpurpose to remove that effect: the camera takes an identical "dark" photo just after the real one and subtracts it.

If you disable dark frame subtraction, you'll have to do the subtraction later yourself: take another shot (with identical parameters) while covering the lens and do the subtraction yourself.




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Offline art

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Re: Strange "noise" in long exposures
« Reply #2 on: 11 / May / 2009, 19:01:50 »
I was suspecting warm sensor or somethig radiating but wasn't quite sure. At first I thought those were glow from city lights until they started appearing on every photo :)
I tried manually removing it but the results were not quite good. In place of halos I got dark areas, unlike the canon's built in dark frame removal which was clear even for 64 second exposures. I am not sure about ISO, but I think it was the same. Everything else was the same for the dark frame exposure. I used Mediachance's free BlackFrame NR software. Is there anything else I could try? Is it known what algorithm canon uses in my line of cameras? The picture I attached would me considered the "dark photo", correct?

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Offline hotvedt

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Re: Strange "noise" in long exposures
« Reply #3 on: 11 / May / 2009, 20:53:30 »
Maybe you want to try dcraw's dark frame subtract. It works very well for this purpose.
Of course only possible if you shoot in either raw or dng-format.


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Offline fbonomi

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Re: Strange "noise" in long exposures
« Reply #4 on: 12 / May / 2009, 02:51:58 »
Dark frame subtraction is so simple that I don't think the program can be wrong, especially if it's made just for that.

I think the problem lies in your photos, i.e. they aren't actually shot in the same conditions. Comparing a "real" photo with the dark frame should show roughly (actually, exactly) the same noise "halo".

If the halo is stronger in one photo than in the other, then the parameters don't match.

In order to have almost the same conditions, I try to shoot my dark frame immediately after the real ones. I often take many many long-exposure shots (for example, for stacking or for doing star trails) and what I do is
1) setup parameters (for example, 15 sec ISO 100)
2) put the camera in continuous mode (so that it keeps taking photos as long as the button is pressed)
3) use a script to "press the button forever"
4) start taking photos
5) wait that enough photos are taken (e.g. 30 of them)
6) cover the lens and take other 3-4 dark frame shots

Of the 30 "real photos" I discard the first ones, when probably the camera was colder.

Of the dark frames I only keep one, a "really dark" one. Keep in mind that there will be always a "partially dark frame" at the beginning of the "dark" sequence, because you probably cover the lens while the camera is exposing. Also, at the end of the "dark" sequence there might be other similar stuff
Doing this, I hope that other parametes are as close as possible, such as external temperature, time since the camera was turned on (with time, the camera heats up and the noise changes) etc.

« Last Edit: 12 / May / 2009, 03:13:33 by fbonomi »

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Offline art

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Re: Strange "noise" in long exposures
« Reply #5 on: 12 / May / 2009, 09:36:59 »
Thanks for nice explanation! I'll follow your steps exactly next time and see what comes out of it :)

I'll try dcraw. At least it can be batched.

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Offline art

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Re: Strange "noise" in long exposures
« Reply #6 on: 12 / May / 2009, 09:50:58 »
fbonomi, one more question. It was probably answered elsewhere but I'll ask here since you already have experience with star trails.

How exactly do you stack multiple star ptotos? I tried it in photoshop with stacking. When I use average, I get rid of the noise but the trails almost disappear as they get averaged too. I used "maximum" to get the bright trails, but with this mode the noise remains. I also tried a free program called startrails and it gave me results similar to maximum mode in photoshop. What is the best way to merge multiple star photos?


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Offline art

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Re: Strange "noise" in long exposures
« Reply #7 on: 13 / May / 2009, 09:18:53 »
of course any one else who can help, please chime in   :)


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Offline hotvedt

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Re: Strange "noise" in long exposures
« Reply #8 on: 13 / May / 2009, 10:51:04 »
You can try to stack them in photoshop, and set blend mode to "lighten" on the layers.
It will not take away the noise, for this use the dark-frame on individual images maybe before stack.

Also look at http://www.tawbaware.com/imgstack.htm (the brighten - use brightest pixel -mode)

Re: Strange "noise" in long exposures
« Reply #9 on: 19 / May / 2009, 01:32:13 »
I think this thread has some relevant infomation.

 

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