Bolton Astronomical Society

Long-exposure imaging with an alt-azimuth mount?
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Author:  rwilkinson [ Fri May 16, 2014 7:54 pm ]
Post subject:  Long-exposure imaging with an alt-azimuth mount?

A couple of weeks ago I acquired an old Starlight Xpress MX516 peltier-cooled CCD camera to have a play with. I've fitted it on the back of a Soligor 200mm lens, through which it has a field of around 1.4 x 1 degrees:
File comment: MX516 and 200mm lens with finder-scope mounted on a DIY wooden bracket
mx516_200mm.jpg [ 311.02 KiB | Viewed 3829 times ]

Last week I had this with me down in the Wye valley, but the only driven telescope mount to hand was an old Celestron NexStar SLT (just like the one which the BAS owns:
File comment: MX516 imaging system on the NexStar SLT mount. I'd improvised a dew-shield from a pamphlet secured with an elastic-band!
sltwithmx516.jpg [ 239.91 KiB | Viewed 3829 times ]

Now this alt-azimuth mount is great for viewing and tracking planets (and the Moon and Sun), but since its axis isn't aligned to the Earth's, it will exhibit field-rotation on long-exposure images.

However, in practice I found that I could manage exposures of 60-sec without significant trailing. Then using the Global Matching algorithm, IRIS will rotate each image by the required amount to line up all the stars (but not the comet, of course):
File comment: Field-rotation over half an hour removed using Global Matching
fieldrotation.jpg [ 112.27 KiB | Viewed 3829 times ]

Then I could re-register the images on the comet itself, and rotate the field to get North at the top:
File comment: Comet PanSTARRS on 13-May: 30x 1-min on alt-azimuth mount.
2012k1_13May.jpg [ 15.96 KiB | Viewed 3829 times ]

So next I need to make myself a Bahtinov mask to help achieve optimum focus...

Author:  rwilkinson [ Sat Jul 12, 2014 7:42 am ]
Post subject:  Re: Long-exposure imaging with an alt-azimuth mount?

rwilkinson wrote:
next I need to make myself a Bahtinov mask to help achieve optimum focus...

Well, I finally got around to making one (from a sheet of cardboard), and tried it out on Vega last night:
File comment: Focusing my MX516 with Soligor 200mm lens using a home-made Bahtinov mask
bahtinov_200mm.jpg [ 294.7 KiB | Viewed 3705 times ]

And it does indeed make focusing much easier:
File comment: Using my Bahtinov mask with the StarlightXpress focus utility
focusmask_200mm.png [ 3.26 KiB | Viewed 3705 times ]
compared with the method of trying to make the image of a star as small and bright as possible.

Author:  rwilkinson [ Thu May 14, 2015 11:25 am ]
Post subject:  Re: Long-exposure imaging with an alt-azimuth mount?

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:
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 [ 276.34 KiB | Viewed 3221 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:
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 [ 203.98 KiB | Viewed 3221 times ]

Author:  rwilkinson [ Fri Oct 09, 2015 10:33 am ]
Post subject:  Re: Long-exposure imaging with an alt-azimuth mount?

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:
File comment: 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 2798 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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