Re: Celestron 8 troubles

From: Roger Hamlett (rogerspamignored_at_ttelmah.demon.co.uk)
Date: 07/08/04 

Date: Thu, 08 Jul 2004 17:05:43 GMT

"Jon Kickerston" <none@nospam.com> wrote in message
news:xNcHc.6474$sD4.3908@newsread3.news.atl.earthlink.net...
> Thanks, guys, for the elaborate discussion. I read each post and the
> information about corrector/ secondary orientation was useful. In my
> friend's C8 case, we still haven't figured out the problem. Way back
when,
> a few weeks ago, I had a thread about my scope- a 12" Meade SCT- that is
> showing a complete lack of contrast and, at one time, the secondary
could be
> turned by hand. I realigned it to some tick marks I found on the inside
of
> the corrector and secondary housing. Whether these marks indicate the
> proper position, I don't know, but the scope's performance hasn't
improved.
> My next test will be to find out whether or not spherical aberration is
> present.
>
> Thanks again for this discussion.
>
> Jon
Remember that SA, is _only_ corrected in a SCT, for one seperation of the
primary/secondary (normally corresponding to about 4" of back focus on the
scope). With any optical configuration that is either longer or shorter
than this, there will be SA, getting worse the further you are from this
point. This is why a lot of the time people will say that their scope
displays a lot of SA, not realising, it is the configuration that they are
using, rather than the scope itself, that is making this worse.

Best Wishes

> "Jon Kickerston" <none@nospam.com> wrote in message
> news:OjzGc.3722$sD4.1566@newsread3.news.atl.earthlink.net...
> > I was star testing a friend's C8 last night and found something very
odd
> > which I can't quite pinpoint the cause of: the star diffraction
patterns
> > appear somewhat elongated, not round as they should be. The rings all
> seem
> > to be dead on, but just aren't round. Any idea what's causing this?
> > Solution?
> >
> > Thanks,
> > Jon

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