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Hi Jay Qu,
Very nice composition. Love the image.
With respect to your comment about exposure times, trails and pixel peeping...
These rules or formulae like the 700, 600, 500 rule are based on a set of starting assumptions and have been kicking around since the days of film.
In the old days of 35mm film 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 on an 8x enlargement.
Using the number 700 corresponds to a trail length of about 50 microns on the 8x10 print, 600 to about 42 microns and 500 to about 35 microns. These in turn represent about 9, 7, and 5 pixels on a modern DSLR. But today, we mostly look at screen based images and images that are highly reduced from full size.
A few years ago, I've derived a formula, modernized for DSLR's, that takes, pixel size, and final viewing size into account.
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 3 and 6.5 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
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