Tuesday, July 27, 2010

Dark Side of the Moon

I wanted to try and make good use of the time before the moon gets out of the way.  I figured I would start in on my dark frame library.  Now dark frames are used to pound down the noise inherent in any long duration exposure at high ISO.  The noise increases with temperature as well.  So a dark frame library has to involve separate sets for each exposure/ISO/temperature combination.  Yikes.

This can all be done in the field right after the exposure run but that doubles the time (each dark must be the same duration as the "lights").  Not to mention that you gotta take your flats and flat darks as well.  So a library is appealing.

I've read that a common practice is to bin your darks in 10deg F increments so that any given run of light frames is within 5deg F.  Next would be to decide what your most common exposure durations will be.  For this trip I expect to be shooting 30sec, 60sec, 120sec for sure.  Next ISO.  Most likely 800 and 1600. See how this rapidly gets "hair shirty"?  Not to mention keeping the storage business alive and kicking.

I am limited by whatever the ambient temperature is on a given night and that has had me come to grips with a looming specter.  It's hot here.  Like glorious shorts and tee shirt in the middle of the night hot.  A fertile hothouse for noise.  It is just a limitation of DSLR imaging.  I suspect that this is the main thing that drives people to a dedicated cooled CCD camera.

Undaunted I proceed.  Marek and I discussed just how many dark frames to take for a library master dark.  The noise decreases as the inverse sqrt of the number of exposures so at some point taking more reaches diminished returns.  We agreed that "many tens" would be a good target.   Last night I shot 32 60sec ISO800 images and tonight I'm shooting 64 30s ISO800 images (why 64?  because I can.  because I can while I am writing this blog entry).

Last night it was 68F.  Tonight ... 74F!  And of course when and if I get to actually take some lights it will be 55F and will have to take my darks in the field anyway.  No mind ... I will have some hot darks to take home as souvenirs.  I only hope that they are not the only astro images I take while here.  Very very hair-shirt.
No, not stars.  Noise.  A hot dark frame.

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