AstroTrac Urban Short Exposure Widefield Images

The following are a series of widefield images taken from urban San Antonio, Texas (see map below), with a Canon 400D (Hap Griffin-modified) with various Nikon lenses mounted on an AstroTrac tracking platform. The full size images are resized to 2000 pixel images to make downloads managable.
The lens are older Nikon, from the mid-1960s, so their edge performance isn't great. In fact, by moderm DSLR standards, these lenses are lousy! But they are all i have that will work (with an adapter) on the Canon DSLR. The lens were stopped to f/5.6 to control coma and limit skyglow from San Antonio lights.
Subframe exposure were limited to 30 seconds at ISO400. Image stack size can be as high as 118 subframes (the capacity of a one-Gig memory card with RAW format) , depending on how many clouds and airplanes crossed the images. The AstroTrac was polar aligned once using the unit's own polar scope. One of the great things about the AstroTrac is since an autoguider is not used, passing clouds do not loose tracking lock. As long as there is tracking time left on the tangent arm screw (almost two hours of tracking is available before reset is required), more exposures can be taken to compensate for passing clouds without worrying about the loss of a guide star.


Map
This map shows the general location where I do backyard astrophotography in San Antonio, Texas. My location is well within the inner part of the city. At best under very clear skies, I can see 3rd maginitude stars, but usually 2nd magnitude is the best available. Limiting exposures to 30 seconds at iso400 and f/5.6 keeps the sky background levels to about 80 on a 0-255 scale. The resulting ugly pinkish skyglow can be subtracted in Photoshop. Amazingly, some of my 30-second DSLR images show stars as dim a 15th magnitude and rival images I used to take from dark sites with 35mm Ektachrome. The only difference is nebula and the Milky Way do not record with such short exposures.


Auriga

Auriga, Canon 400D, 111x30 seconds, ISO 400, 35mm F/5.6

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Cassiopeia

Cassiopeia, Canon 400D, 117X30 seconds, ISO 400, 50mm, F/5.6

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Cygnus

Cygnus, Canon 400D, ?X30 seconds, ISO 400, 35mm, F/5.6

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Hyades

Hyades Cluster, Canon 400D, ?X30 seconds, ISO 400, 135mm, F/4

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Lyra

Lyra, Canon 400D, ?X30 seconds, ISO 400, 50mm, F/5.6

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M36

M36-M38, Canon 400D, 100X30 seconds, ISO 400, 135mm, F/4

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M45

M45, Canon 400D, 113X30 seconds, ISO 400, 135mm, F/4

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Orion

Orion, Canon 400D, 112X30 seconds, ISO 400, 50mm, F/5.6

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Ursa major

Ursa Major, Canon 400D, ?X30 seconds, ISO 400, 35mm, F/5.6

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