--- David Samuels <david.samuels@No-Spam> wrote:
> Are you saying that a SCT is bad for the job or bad for newbies?
I'm pretty sure Pete meant newbies. See below...
> Isn't the lens speed resolved with f/6.3 or f/3.3 reducers/field
> flatteners?
Yes... and no. Using reducers is not trivial as spacing requirements are
pretty exacting, or else you get aberrations that can result in
seagull-shaped stars. In fact, even when properly spaced, you can still
get these. <g>
> The LX200 has a primary mirror lock and microfocuser. Would that be
> sufficient to solve the focus shift problems.
The microfocuser will resolve the focus shift problem when fine focusing
(which should pretty much be all the time when taking CCD images). The
mirror lock may resolve the problem with mirror flop, but this is not as
big an issue for CCD imaging. Focus shift can be killer, so your
microfocuser will be a godsend in this regard.
> As far as tracking and guiding goes, if it works within spec, are there
> still tracking and guiding issues? What issues are those?
If you are imaging with a CCD camera that has small- to medium-size
pixels, then your resulting system resolution with an SCT at f/10 or even
f/6.3 will be quite high. At high resolutions, the guiding tolerances are
very small, since even a tiny guiding error will be "seen" by the camera
and recorded. Imaging at f/3.3 can resolve this, but see my comments
above regarding reducers. Using a camera with large pixels is a better
solution IMO.
This is not really an issue specific to SCTs, it applies equally to any
instrument with a relatively long focal length. Also, the heavier the
instrument, which usually results from larger aperture or longer focal
length or both, then the heavier duty the mount and drive system must be
to track accurately.
However, SCTs make great platforms for imaging with smaller scopes or
cameras with lenses mounted on top of them!
Cheers,
Paul Sterngold
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