Re: LX-200 pointing trouble

Lawrence Feb 29, 2004

>Subject: Re: LX-200 pointing trouble
>
>Hi Lawrence,
>
> Hmmmm... just how much error are we looking at here? A few arcminutes?
>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