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Old 06-02-2012, 02:41 PM
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allan gould
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allan gould is offline
Join Date: May 2005
Location: Brisbane
Posts: 4,482
An interesting booboo - autoguiding at 5000mm FL

I was attempting to look at the characteristics of a Lumicon 2" screw in focal reducer that I had not used for quite a while and was attempting to determine the optimum distabce between the chip and the rear element of the FR with my 10"SCT. I can normally guide up to 40 min with the 10" at 2500mm but this time I kept the exposures to 5min as I thought i was going to have quite a few images at each separation to evaluate.
The first photo is a greyscale shot from QHY10 camera, 5min at 2500mm while the second is with the FR placed ~40mm from the chip. I the tried 60 and 90 mm but it was getting worse at the edges of the field so I thought what a crappy FR and left it at that. However while putting everything away after the imaging run I realised that I had screwed in a 2x 2" barlow that I had and not the FR from Lumicon. So the second shot is actually a 5 min exposure at ~5000mm. At the time I wondered why the histogram was showing less light gathered and why it looked a little dim (just like me).
Anyway I was pleased that at the extreme fl the stars are actually pinpoints in the centre of the image with no tracking errors - dont look at the abberations at the edges. So IF i wanted to get shots of small galaxies now I have a way to do it.
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