Long-exposure imaging with an alt-azimuth mount?

Blog the previous nights viewing. Link to images on the gallery or simply describe what you observed the night before. Leave the technical and howto in another forum.
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rwilkinson
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Re: Long-exposure imaging with an alt-azimuth mount?

Post by rwilkinson » Thu May 14, 2015 11:25 am

Last night I managed to capture a series of 1-min images of Comet Lovejoy, using a 70mm refractor on a Celestron NexStar alt-az mount.

The comet is quite close to the pole-star at the moment (declination 82 degrees), so I was hoping that it would be easy to track, but once I'd centred it on my CCD sensor I could see trailing even on 15-sec exposures. However after a few minutes it settled to a stable position, and I was able to use 60-sec exposures without trailing.

But having my imaging sensor aligned with the terrestrial horizon rather than the celestial axis meant that I'd have to deal with field rotation as well as the tracking errors:
fieldrot.jpg
Image stack showing field-rotation and tracking errors over 40 minutes from an alt-az mount aiming at 82 degrees declination
fieldrot.jpg (276.34 KiB) Viewed 4921 times
Happily these effects are easily dealt with using the global matching stellar registration in IRIS, and then using its TRANS2 process I could align my image-stack on the comet:
lovejoy_13may.png
Comet Lovejoy on 13-May. 37x 1-min using Atik 16IC on 70ED refractor operating at f/4.4. The star-trails show how far the comet moved over 40 minutes.
lovejoy_13may.png (203.98 KiB) Viewed 4921 times

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rwilkinson
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Re: Long-exposure imaging with an alt-azimuth mount?

Post by rwilkinson » Fri Oct 09, 2015 10:33 am

Last night I fixed a Pentax-M 135mm lens on the front of my HX916 camera and fitted this on the little Celestron NexStar alt-az mount.
I could see a little frame-rotation on each of the individual 75-sec exposures, but after running them through IRIS's Global matching Stellar registration routine, this effect has been mostly cancelled out:
m31_135.jpg
The Andromeda galaxies, using 135mm lens at f/5.6 on HX916. 33x 75-sec on alt-az mount.
m31_135.jpg (237.06 KiB) Viewed 4498 times
The top-left and bottom-right corners of the stack were lost due to the frame-rotation
Down here in dark skies of the Wye Valley I could run without any light-pollution filter, but I still needed to do a little Gradient removal on the final stack.

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