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Astrophotography - IS based Tracking for long exposures

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Astrophotography - IS based Tracking for long exposures
« on: 23 / January / 2009, 15:18:21 »
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To photograph faint objects in the night sky you often need long exposures. But even a 15 sec exposure will make the stars / objects streak because of the earth's rotation. People have been able to get great results using stacking but I'm wondering if another method is possible using CHDK.

First, its not clear to me from DataGhost's incredible thread if full IS control has been achieved. Anyways, *if* we can control the IS mechanism by code, I would like to counter the apparent motion of the stars due to earth's rotation.

Method:
A script could take a long enough exposure shot and then detect the star trail path to get a sense of the velocity (and path taken) by the stars. Or take two shots after an interval and detect the motion. To take the actual shot, the IS would be used to appropriately compensate this known motion over the exposure time for the shot. => Much longer exposures possible with low ISO which I hope could more easily give nice pictures.

Problems:
Will not work the parts of the sky near the pole star since the stars near a pole star will appear to move around it very slow and the ones away will move very fast.

Should work decently enough with enough zoom and for stars away from the from the pole star.

Possible?

(and I hope SX10 will soon be supported by CHDK :D)

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Offline fbonomi

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Re: Astrophotography - IS based Tracking for long exposures
« Reply #1 on: 24 / January / 2009, 02:58:42 »

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Online reyalp

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Re: Astrophotography - IS based Tracking for long exposures
« Reply #2 on: 24 / January / 2009, 03:49:43 »
I have worked within the camera accessing the RAW data and it's painfully slow. All I needed was an aproximate histogram, so I was considering only one 1/1000th of the pixels and disregarding the difference in RGB (ignoring the bayer pattern in the RAW data). With all these simplifications, it takes about a second to build the histogram. If I were to process all the pixels it would take about 1.000 seconds, about a quarter of an hour.
You should be able to do much better. Reversing the order of bytes in the entire raw buffer in my a540 takes ~1/3 sec (with cache enabled and hand written asm) Applying curves to the entire raw buffer, uncached, with sub-optimal C code only takes a few seconds. Raw sum, subtract take tens of seconds, despite reading and writing the SD card.

You don't have to process the whole buffer either, you just have to distinguish one star from the noise. One way to do this would be to take a single, long exposure, and then measure the length and direction of a handful of trails. This should be doable with processing only a tiny fraction raw buffer using relatively simple code.

I do agree that tracking the stars is probably the more difficult approach. Just calculating the required shift based on user entered parameters (zoom, declination, camera orientation) would be far more straightforward.

The stuff on IS control was in http://chdk.setepontos.com/index.php/topic,1365.0.html

Depending on zoom, shifting tens of pixels would be enough to get the maximum 64 sec exposure without trails. OTOH, just taking shorter exposures and using software on your PC to register and stack them will probably give better results. Even at ISO 80, noise is pretty horrendous with a 64 sec exposure. Still, for the 15-64 second range, I think the idea has some merit. The IS movement doesn't actually have to be terribly smooth, because the it will still be small compared to the length of the exposure. I bet you could do one discrete step per second over a 64 sec exposure without much visible impact.
Don't forget what the H stands for.

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Offline fbonomi

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Re: Astrophotography - IS based Tracking for long exposures
« Reply #3 on: 24 / January / 2009, 04:07:35 »
Quote
You should be able to do much better.
Yes, i know my code is .... sub-optimal :-)
Anyway, I have worked on applying a transform function on images (for de-skewing) and they are really  processor-intensive

Quote
The stuff on IS control was in http://chdk.setepontos.com/index.php/topic,1365.0.html
ooh, there's so much in that thread that I forgot the IS part..

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Depending on zoom, shifting tens of pixels would be enough to get the maximum 64 sec exposure without trails.
w/o zoom, the camera (for example, the SD1100) has roughly 70 pixels / degree, so at most (at the equator, 65 seconds) we would have just below 20 pixels of displacement.

Zoomin in, this value would increase by the zoom factor (i.e. 4x = 80 pixel)



Re: Astrophotography - IS based Tracking for long exposures
« Reply #4 on: 24 / January / 2009, 09:47:48 »
You raise good points. I code for a living and I work on "embedded systems". The idea with the star tracking was to try to automate it as much as possible. We need to find just a couple of stars and check their trails. And if it doesn't work out now, we could use some manual input of the trail (not just the direction, but also the magnitude which might be a bit hard for the user to guess).

I thought it would be an interesting thing to explore. I didn't know that we were limited to 65 seconds of exposure (what's limiting us?) And I really don't know how noise is going to affect this.

Perhaps manual IS control might end up being helpful for something else altogether.

Re: Astrophotography - IS based Tracking for long exposures
« Reply #5 on: 24 / January / 2009, 09:59:58 »
Near the pole the motion is anyway so small that stars motion it's usually not a problem.
Actually, you have to work hard to get star trails around the pole...

I was thinking of wide angles like this:


Though its a very long exposure, even for short exposures the stars away from pole star might show some streaking.

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Offline fbonomi

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Re: Astrophotography - IS based Tracking for long exposures
« Reply #6 on: 24 / January / 2009, 10:17:08 »
I know, but I am afraid IS won't be able to spin your camera :-)


Re: Astrophotography - IS based Tracking for long exposures
« Reply #7 on: 24 / January / 2009, 11:22:56 »
Which is why I said these are problems which are beyond the method I suggested lest anyone think otherwise :D.


 

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