writes:
>Who ever the dill was, that said we were the sunburnt country,
>obviously
>didn't own a telescope.
>Whilst my 216XT is on holidays back at Meade (the subject of another
>story) I've been refining my polar alignment in between clouds. That
>can
>be pretty frustrating, as one night I can track for 10 minutes
>accurately, and the next available night, 3 or four minutes only. No
>amount of adjusting the mount overcame this to any great extent. Last
>night (almost clear) I remembered discussions on mirror slop,
>something
>I hadn't come across before, so I tried the following:-
>Centre an east or west horizon star, give the OTA four or five sharp
>raps with the fingers and watch in the reticle to see the star move.
>Centre it and do the drift bit with mount adjustments. Centre a
>meridian
>star and
>rap the OTA again, the star moves up to 30 arc seconds. Centre and
>align
>again. Seems to improve alignment.
>I'm wondering if any-one else has any thoughts as to whether the
>mirror
>would move in a predictable way on say, my 8" LX200. If it does, then
>it
>may be necessary to rap the OTA before imaging, when the 'scope was
>moved to another part of the sky.
>Comments would be appreciated.
>
>Regards,
>Coffs Harbour N.S.W. Australia

Your post is very interesting and I will give it a try, its at least
worth that, that is, if I can remember. Maybe an automatic thumbper.
The way to determine mirror flop (without thumbping) is to focus on a
star with a reticle in the East or West with high power, slew to the
opposite side of the sky than back to the original star. If the star is
exactly aligns with the reticle you don't have any mirror flop. You can
also do this test with a guide scope and the OTA by focusing with a
reticle in each scope on the same star, this will show if you have any
mirror flop and flexures between the OTA and guider.

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