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Max. exposure time with DSLR and ALT-AZ mount

Astrophotography
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#1 ahmedelmorsy

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Posted 26 February 2021 - 03:55 PM

I shot with no-tracking DSLR and 135mm lens with 2-Sec exposure time to have sharp stars. My question is that; how long I can extend my    exposure time if I use iOptron AZ Pro GoTo Mount with same DSLR and same Lens without having star trial.



#2 kathyastro

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Posted 26 February 2021 - 04:03 PM

If your mount is tracking well, you should be able to get 30s exposures without noticeable field rotation.  That is an average, and varies across the sky, with the maximum being at the zenith.

 

For a non-tracking mount, the normal rule of thumb is a "rule of 400" (sometimes 500, sometimes 300, depending on your source).  Your maximum exposure before star trails become objectionable is 400 divided by the focal length.  Using your example, 400 / 135 = 3 seconds (roughly), which fits well with your experience with 2s exposures.  The actual limit will depend on the declination (high declination is better) and with your tolerance for short trails.


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#3 ButterFly

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Posted 26 February 2021 - 04:32 PM

Your question has no answer yet because there is not enough information.

 

Field rotation is a rotation.  Objects twice as far away from the rotation axis move twice as fast.  Without knowing the object size, chip size, how much space you want around the object ..., there is no answer.  Without knowing how far away from the rotation axis the desired edge is, one can say nothing about how long it will take for a star to move across one whole pixel.  When tracking unguided, the rotation axis is near the center of the chip, but when using a guide star for tracking, the star is the rotation axis.

 

The rotation rate also varies by latitude (the polar alignment error) and position of the object on the sky (its declination).  At any latitude other that +/- 90, the rotation rate through the zenith is infinity.  When at the North pole (pole alignment error of zero) and the pole is not on the chip, the field rotation is just the difference between the angular velocities of the North and South side of the chip.  That can be hours or days during winter depending on chip size and declination.

 

For good background reading, see:

 

An Analysis of Field Rotation Associated with Altitude-Azimuth Mounted Telescopes: the Potential Effect on Position Angle Measurements of Double Stars.

 

RASC Calgary Centre - Field Rotation with an Alt-Az Telescope Mount

 

Field Rotation & its Impact on imaging

 

Field Rotation, Polar Alignment, and Declination Drift


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#4 ahmedelmorsy

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Posted 27 February 2021 - 03:20 AM

WOW, I'm very happy with the two answers I got, the one from Mercury-Atlas is a very good rule of thumb to start with, and the one I got from Surveyor-1 is very detailed and precise. Thanks for the help.




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