Long exposure mechanism and analysis during it - Creative Uses of CHDK - CHDK Forum
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Long exposure mechanism and analysis during it

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Offline sf

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Long exposure mechanism and analysis during it
« on: 01 / October / 2014, 16:48:49 »
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Hi all,

I have been thinking about a particular CHDK project for a while and would like to ask you for help, opinions and advice.

My goal is to be able to somehow analyse intermediate data during very long exposures (order of 10s of seconds) using CHDK. Therefore I would like to ask you whether such a thing is even possible. How does the long exposure mode work - is it just that the chip is being illuminated continuously for its entire duration? Or is some software stacking done already while taking the picture? I would like to be able to see "intermediate" photos during the long exposure to correct for things and in general improve the final picture.

Thanks for help!

-SF-
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Offline waterwingz

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Re: Long exposure mechanism and analysis during it
« Reply #1 on: 01 / October / 2014, 23:19:42 »
Quote from: sf on 01 / October / 2014, 16:48:49
I have been thinking about a particular CHDK project for a while and would like to ask you for help, opinions and advice.  My goal is to be able to somehow analyse intermediate data during very long exposures (order of 10s of seconds) using CHDK.
Certainly a challenging project (see below).  Might be possible to do with CHDK scripting (Lua shooting hooks) but it sounds more like you will need to be willing and able to hack the CHDK C & assembler code.

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Therefore I would like to ask you whether such a thing is even possible.
I'd say it's possible.  Easy?  No.  Possible?  Yes.

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How does the long exposure mode work - is it just that the chip is being illuminated continuously for its entire duration? Or is some software stacking done already while taking the picture?
The sensor chip is illuminated continuously for the entire duration and then the result is "read out" into a memory buffer.

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I would like to be able to see "intermediate" photos during the long exposure to correct for things and in general improve the final picture.
This is where is gets interesting as I think you were on the right track with your previous question.  You could configure CHDK to take a sequence of one second images and process those in the camera  (like CHDK RAW add).  There would be a short gap between images and you would only be able to do so much during that time. 

So I guess the next question is what sort of corrections do you think you want to apply?
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Offline sf

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Re: Long exposure mechanism and analysis during it
« Reply #2 on: 02 / October / 2014, 04:58:19 »
Thanks. I don't really understand the inner workings of the camere, so cannot estimate myself, but I suppose that a continuous long exposure would be superior to the addition of several shorter exposures of the same total length. I suppose the main source of error could be the process of quantizing the 'illumination' levels of each pixel into the color depth the camera uses. Do you know how to estimate that?

I suppose that during a long exposure time the 'value' of a particular pixel is more finely quantised (electron charges?) than the number of numerical levels in gets rounded to once being read out. Is it true? That would possibly make the result of the combined exposures worse than the single long one.

Problem: I do astrophography and the sky moves due to the rotation of the Earth. I do have a mount that mechanically corrects for that but working with very long exposures (30s at least) and high magnifications (40x at least) means that the mount's correction is not exactly right (it is enough to be 10px of to loose the advantage of the long exposure since the light spreads out and one looses details). Though it is possible to do several minutes without the telescope (1x magnification), applying the magnification increases the problems magnification-fold. 

Idea: In the old days of photographic plates, astronomers had another telescope attached to the big one and checked for any star movement. If they saw some, they correct it manually. Since I have only one telescope (which is quite heavy for the mount already), I wanted to be able to see whether the stars actually drift during the exposure and correct for it.

Limitations: The idea with scripting a series of shorter exposures is a good one, though I am worried that the crude quantisation one applies by doing so might smear out the fine details I am going for in the first place. I have already tested such an approach (with JPEG, not RAW though - might be an issue?) and it seems not to be that helpful.

Thanks for answers!  :)
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Offline reyalp

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Re: Long exposure mechanism and analysis during it
« Reply #3 on: 02 / October / 2014, 22:47:37 »
Quote from: sf on 02 / October / 2014, 04:58:19
Thanks. I don't really understand the inner workings of the camere, so cannot estimate myself, but I suppose that a continuous long exposure would be superior to the addition of several shorter exposures of the same total length.
I don't think this is true in general. Most modern astrophotography (and much professional astronomy) is done with stacking.

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Idea: In the old days of photographic plates, astronomers had another telescope attached to the big one and checked for any star movement. If they saw some, they correct it manually. Since I have only one telescope (which is quite heavy for the mount already), I wanted to be able to see whether the stars actually drift during the exposure and correct for it.
Analyzing raw images on the cameras would be very slow and require you to write a bunch of code that works in the limited environment of the camera. Note that you would have to do this with a series of exposures, sensor readout is a destructive process, so there's no way to look at the value without ending the exposure.

You'd probably be better off using existing astrophotography software to do it after the fact on your PC. Stacking software should also be able to align images to help you deal with the tracking errors. You should be able to find a lot of information about this on the internet.

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