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Old 19-01-2013, 11:43 PM
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OzEclipse (Joe Cali)
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Join Date: Jun 2012
Location: Young Hilltops LGA, Australia
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Revised rule for DSLR's

These rules or formulae are based on a set of starting assumptions and have been kicking around for a long time.

In the old days of 35mm film where very fast films used for tripod astrophotography had very course grains and so a bigger movement could pass off as no drift. It was based on someone looking at a 8x10inch enlargement at normal reading distance perceiving trails as almost stationary. The eye can resolve about 1 arc minute so it can in fact resolve approximately 200 microns on a print which scales back to 25 microns on the film. However the number 700 corresponds to a trail length of about 50 microns. This is about 9 pixels on a modern DSLR. But today, we mostly look at screen based images and images that are highly reduced from full size.

The number is related to how much drift you find acceptable. So I've derived a revised formula, modernized for DSLRs.

max exposure time(s) = [14*N*P] / [FL*cos(d)]


where
N..........Number of pixels of drift
to work this out load any full frame image from your camera and scale it to the size you want to view on screen. Look at the scale percentage. Number of pixels drift = 100 / percentage

P..........Pixel size (microns) Most DSLR's are between 5 and 8 microns. You can look it up in your manual or just split the difference and make it 6.5

FL.........Focal length(mm)

cos(d)...cos of the declination. Use the declination of the stars in the field that are closest to the celestial equator in the camera field

Example : Using a 14mm lens, 5 pixels drift, on an APS-C sensor with 5.4µm pixels pointed at the celestial equator can take a 27 sec exposure. Pointed at the SCP the lens will see 45 degrees each side of the pole extends the time to 38 seconds.

cheers

Joe

Last edited by OzEclipse; 20-01-2013 at 12:50 PM.
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