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Originally Posted by Harpspitfire
i was looking at that french link here on PE with the different mounts and graphs, with no mounts being perfect and the chinese being the worst- what would you consider the 'acceptable' error in arc seconds under these conditions- using a DSLR, f/l around 1000-1500, 2 minute max exposures?
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Based purely on experience and not any maths (sure you could work it out somehow using maths):
At 1500mm focal length and the small pixel size of DSLR chips, I'd expect you to notice PE on any mount with a PE of about 10 arc sec or greater. About 5 arc sec (takahashi's etc) I'd expect wouldn't be a problem.
A mount like the Meade LX200 or Losmandy GM-8 with their PE correction enabled I would not expect to have any problem, but then you have an effective PE of under 5 arc seconds anyway.
It will of course depend where you are pointing in the sky as to how noticable the error is in images.
All the 2 minutes (short) exposure means is that some will be good and some bad (assuming > 10 arc sec error). What will happen is that (unless the PEC is really bad - like the chinese mounts) you will hit good and bad spots in the PE resulting in some good and some bad images. I notice this on my LX200 when I do not have it's PEC engaged - some 3 minute exposures @ 2160mm will be fine, others will be trailed. Of course with PEC enabled all are fine.
PEC is more effective on higher quality mounts, where the PE is 'smoother' when it exists.
Roger.
