Oliver Kloes Feb 29, 2004
> >Subject: Re: LX-200 pointing troublearcminutes?
> >
> >Hi Lawrence,
> >
> > Hmmmm... just how much error are we looking at here? A few
> >Dozens? Degrees? I assume the error is repeatable (i.e., go from one
> >part of the sky to another and back again, and it's still off in the
> >same direction by the same amount)? Any clues other than "it's off"?
>
> Hi Bill
>
> This has to be looked at in perspective. Everything to do with Guide is
> running fine (well, more or less; the other problems are not relevant
> here). What happens is that I start by driving the LX200 to a standard
> star at reasonable elevation - eg., Beletegeuse. Take an image,
> physically drive the scope until the image is centred, centre on the
> scope (sync) for the LX200, and then sync Guide (right-click, add star).
>
> On repositioning the scope (nearby), it is invariably off by a few
> minutes of arc (maybe 1 - 4), and remains so. I use other standard
> stars during the session, but find that centreing is always off by a few
> minutes. The importance to me is for astrometry. I try to keep all
> three successive images of the same asteroid field the same for
> blinking.
>
> > You mention that you don't get good results with the synch operation:
> >do things get worse? Better? Exactly the same? Still bad, but in
> >a different direction?
>
> I haven't noted that point. Perhaps I should do more meaningful
> measurements. Errors seem to be similar.
>
> Overall, I am fairly convinced that *I* am not following the procedure
> correctly, but because I think I am, I assume that I need to find a
> better procedure.
>
> Tests suggest to me that even after doing a local sync, the scope is
> always off by a few minutes on the next goto. I have also tried syncing
> the LX200 and then syncing Guide. Always the same result though.
>
> > One possible source of trouble is in the epoch used by LX-200 scopes.
> >I've been unable to find information about this, and it hasn't mattered
> >much in the past; we've been close enough to J2000 that the difference
> >would be less than the usual pointing error in the scopes. Guide assumes
> >that the RA/dec sent to or received from the scope ought to be in the
> >epoch of date. If I'm wrong, then you should hit Alt-J and enter
> >
> >SCOPE_EPOCH=2000
> >
> > This will adjust the coordinates sent to the scope by up to a few
> >arcminutes. I'd be _very_ interested to hear from anyone who tries this
> >out. The above capability was originally added mostly for folks with
> >home-brew LX-200 clones who needed an adjustable epoch... but it could
> >be that my assumptions about the epoch used in LX-200s have been wrong!
>
> I expect more clear skies tonight so I shall try this bit - though I
> don't believe that the LX200 is using an old epoch.
>
> best wishes
>
> Lawrence