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Samir Kharusi | all galleries >> Galleries >> Focal Lengths and Deep Sky Astrophotography > C14 Prime Focus 4125mm on APS Trapezium
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Feb 2006 Samir Kharusi

C14 Prime Focus 4125mm on APS Trapezium

Azaiba, Oman

Of course, at this extreme focal length we have to use a telescope, in this case a C14 at prime focus. But at this image scale, chasing high resolution, we run into major headaches with tracking accuracy and the problems of poor seeing. Despite mirror lock-up, even shutter shudder restricts the range of useful subexposures. Long exposures, a minute or longer, do not record shutter shudder but lead to over-exposed stars, bloated further by seeing. Exposures under one second but longer than, say, 1/30th second (depends on the solidity of the mount) add little tails to all the stars from shutter shudder. Nevertheless, it is possible, with sufficient determination, to image the brighter DSOs at very high resolution, through the use of very short exposures, or a combination of short and long. The inset shows a 1:1 crop from a stack of 4 second exposures, demonstrating that it is indeed possible to resolve the Trapezium stars from A to the H doublet (a pair of Mag 15 stars separated by a mere 1.6 arc-seconds under the glare of the main Trapezium stars blazing away at up to Mag 5).

For other stuff peruse my primary website:

Hutech Canon 20D without UV/IR Blocker,C14 at prime focus
7x8sec subs at ISO 1600. NB Visible+IR recorded to maximise effectiveness of short exposures. Inset is a stack of 4sec frames. full exif

other sizes: small medium original auto
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