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Author Topic: Dark Frame Subtraction - how it works  (Read 16862 times)

Sussex Landscapes

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Re: Dark Frame Subtraction - how it works
« Reply #20 on: December 12, 2010, 12:06:11 pm »

Why wait so long before starting the next exposure?
exactly, if your doing either star trails and or single frames nothing will work with that sort of gap :)
s
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RFPhotography

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Re: Dark Frame Subtraction - how it works
« Reply #21 on: December 12, 2010, 12:48:59 pm »

Because I didn't go out with the idea of shooting star trails.  I was shooting for another purpose and decided to try this when I got back just to try out the process for future reference.  Had I actually been shooting for star trails I wouldn't have left such a gap between exposures.
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ternst

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Re: Dark Frame Subtraction - how it works
« Reply #22 on: December 12, 2010, 10:22:25 pm »

Just as a note of reference, I did a number of long exposures with a Nikon D3s last year - eight hour exposures - with very little noise or hot pixels, and that is with the dark frame exposure turned off. I used a simple setup using a small motorcycle battery to power the camera in the field via a converter/adapter and it worked great. The temp was always below freezing which helped keep the noise down. So it is possible to do really long star-trail photos with a digital camera, you just need a cold night, a good battery, and the right camera.
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jmb

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Re: Dark Frame Subtraction - how it works
« Reply #23 on: December 13, 2010, 12:39:50 am »

If you are interested in eliminating the gaps from your star trails, check out Max Lyons's program Star Tracer.

http://www.tawbaware.com/startracer.htm

Here are some of the images he's created with it.

http://tawbaware.com/forum2/viewtopic.php?t=5934
http://tawbaware.com/forum2/viewtopic.php?t=5941
http://tawbaware.com/forum2/viewtopic.php?t=5940

JMB
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RFPhotography

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Re: Dark Frame Subtraction - how it works
« Reply #24 on: December 13, 2010, 12:35:32 pm »

Just as a note of reference, I did a number of long exposures with a Nikon D3s last year - eight hour exposures - with very little noise or hot pixels, and that is with the dark frame exposure turned off. I used a simple setup using a small motorcycle battery to power the camera in the field via a converter/adapter and it worked great. The temp was always below freezing which helped keep the noise down. So it is possible to do really long star-trail photos with a digital camera, you just need a cold night, a good battery, and the right camera.

I posted another topic about keeping a camera warm in the cold here and on another forum and the idea of a car/motorcycle battery combined with a power inverter and AC adapter came up a couple times.  That seems like a nice, relatively inexpensive solution.

JMB, nice.  Cool link.
« Last Edit: December 13, 2010, 12:41:46 pm by BobFisher »
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hjulenissen

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Re: Dark Frame Subtraction - how it works
« Reply #25 on: December 14, 2010, 07:51:05 am »

If I made a database of dark frames shot at various exposure times/iso and temperatures... How close would that be to doing in-camera NR? And how large should the 2-d matrix be?

Could my image raw/database application (in principle) analyze all of my images taken with a given camera to extract this info blindly?

-h
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PierreVandevenne

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Re: Dark Frame Subtraction - how it works
« Reply #26 on: December 14, 2010, 04:33:16 pm »

That's the ideal scenario. Unfortunately, if you are looking for the best quality possible and at very faint signals, there's a practical "but". The problem is sensor temperature. For longer exposures, dark noise becomes the dominant factor and, as a rule of thumb, is expected to double for every 6 C.

The typical astronomy workflow includes flats, bias and dark frame subtraction (calibration frames are actually averages of multiple darks, bias and flats because they also suffer from noise, noise in the noise if you want) but the sensors are thermo regulated and the temperature information is stored in the FITS header (FITS are raw files for astronomy purposes). Software such as Maxim keep libraries of dark frames and can even scale them for temperature variations if you don't have the correct calibration frames, with variable results. 

For photography, as long as the temperature doesn't change drastically, a single dark frame can probably provide good results, equivalent to what the in camera single dark frame substraction provides, with significant time savings compared to taking a dark after each shot.

 
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