Dave wrote:
> Are you saying that a SCT is bad for the job or bad for newbies?
> I am learning that a german eq mount doesn't have a problem pointing
> at 90 degrees up from the base while cameras and things are
> attached - the fork-mounted SCT can't.
Well...most SCTs have a hard time with pointing straight up while in alt-az
mode...but you'll have it on a wedge to do photography/imaging, and it's not
at all a problem then :)
> You say that an SCT is a bad beginner CCD OTA - due to accuracy,
> tracking, guiding, focus shift, small fields of view. Isn't
> the lens speed resolved with f/6.3 or f/3.3 reducers/field
> flatteners?
The add-on reducer/correctors are a compromise. The f/6.3 version does a
pretty good job, but WILL introduce vignetting (light falloff at the outer
edges of the film/CCD chip).
The f/3.3 reducer neither flattens the field nor corrects it, and the
vignetting is severe with even fairly small CCD chips. A native f/4.5 small
refractor is designed to be operated at that focal ratio, and does a much
better job. You buy shorter exposure times at a cost.
> The LX200 has a primary mirror lock and microfocuser. Would that
> be sufficient to solve the focus shift problems. As far as
> tracking and guiding goes, if it works within spec, are there
> still tracking and guiding issues? What issues are those? How
> do SCT owners resolve them?
The mirror lock works, and the back-end focuser does what it's supposed to
do. Those two things were the best add-ons Meade made for the GPS series.
As far as "works within spec," do you know what the "spec" is for tracking
accuracy for an LX200 (or a NexStar, or any other commercial fork-mounted
SCT)? I thought not. That's because the manufacturers don't publish
tracking accuracy specs, only *pointing* specs, and they're entirely
different. With a typical CCD camera, operating your 12" at f/10, you'll
need to track within 1 arc-sec to avoid trailing stars. I can guarantee that
your mount will not track that well -- even very expensive, hand-tuned GEM
mounts usually only guarantee 5 arc-secs or so. Typical performance for the
Meade/Celestron forks is 15-30 arcsecs. It can get somewhat better with PEC
trained, but not good enough. You WILL have to guide to get good results.
Toss in gear backlash, quick-swing tracking errors, and other mechanical
oddities that are typical in mass-produced fork mounts, and guiding can be a
real challenge. It *can* be done, but to get good results you usually have
to tune the mechanicals yourself (lap the gears, put in replacement gear
parts, adjust all the tolerances, etc.). Most folks getting good imaging
results with a fork SCT have done all of those things. I did on my 10"
LX200, and got the tracking (without PEC) down to about 11 arcsecs. I
finally sold it and got a Losmandy G-11, which got it down to 8 and was much
more solid and stable. Then I got an AP900GTO, which is "under 6" (haven't
really accurately measured yet) without PEC, and about 2 with PEC -- and I
still guide :)
Your 12" can do the job, but at those long focal lengths (even with a
reducer), you're in for some real challenges. Best of luck.
Paul