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 Post subject: Re: Long-exposure imaging with an alt-azimuth mount?
PostPosted: Thu May 14, 2015 11:25 am 
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Joined: Mon Oct 12, 2009 6:47 pm
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Location: Bolton
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:
Attachment:
File comment: Image stack showing field-rotation and tracking errors over 40 minutes from an alt-az mount aiming at 82 degrees declination
fieldrot.jpg
fieldrot.jpg [ 276.34 KiB | Viewed 3600 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:
Attachment:
File comment: 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
lovejoy_13may.png [ 203.98 KiB | Viewed 3600 times ]


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 Post subject: Re: Long-exposure imaging with an alt-azimuth mount?
PostPosted: Fri Oct 09, 2015 10:33 am 
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Joined: Mon Oct 12, 2009 6:47 pm
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Location: Bolton
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:
Attachment:
File comment: The Andromeda galaxies, using 135mm lens at f/5.6 on HX916. 33x 75-sec on alt-az mount.
m31_135.jpg
m31_135.jpg [ 237.06 KiB | Viewed 3177 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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