Re: [APML] 80mm refractor as a guide scope? (was: ETX as Guide-scope)

["Mike Rogers" <mrogers@idcomm.com>]

Frank,
I think the rule of thumb is at least half the focal length as the primary
OTA.
The 8" SCT is 2000mm so your guide scope should be at least 1000mm.
You can try a barlow but this adds some instabilities to the setup that may
cause some new problems.

It sounds like you know about all of the "difficulties" involved in what you
are trying to do. The Meade 8" LX50 is what I started with (and almost quit
with).  The problems with guiding with the OAG are probably the "seagull"
shaped stars. The ST4 does pretty well guiding on these stars but previous
posts suggest that the 201 doesn't work to well with these deformed stars. I
started with a ST4 and Lumicon Cassigrain OAG but was never able to get my
8" LX50 to track correctly due to the PE. I have since moved to a Mountain
Instruments MI-250 GEM. With my re-mounted 8" SCT and Lumicon Cassigrain
OAG, guiding works fine (although I still have problems finding a suitable
guide star).

I don't want to discourage you because I was where you are a year ago but
there are a lot of problems with using the Meade LX50 for Astrophotography.
Chris Vedeler has probably tried the hardest of anyone on this list to use a
guidescope with a SCT so he may be able to add more info on this topic.

Good luck with this endeavor,
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Mike Rogers
<mrogers@idcomm.com>
<http://www.idcomm.com/personal/mrogers/>

"That which we persist in doing becomes easier - not that
the task has changed, but our ability to do has increased."
                                             Ralph Waldo Emerson
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

----- Original Message -----


> I've been kinda out of it for quite some time, but caught the tail end of
> this discussion.  I have an 8" LX50 (2000mm focal length), and yes, I know
> all the PEC problems, etc, so I don't need a lecture about that, but I do
> know that OAG guiding is horrible and difficult.
>
> I was considering using an 80mm short tube refractor I have for guiding.
I
> have a 201xt, and yes, I know I should have an ST4, but again, this is
what
> I have- it works great through the OTA, but sucks through an OAG.  The
> problem is, the 80mm refractor is a 400mm focal length, and I remember
> reading something about that being a bad thing, and I don't remember why.
> The refractor obviously has no mirror flop, even has a nice focus lock,
and
> if I use Chris Veddeler's mirror stabilizer gizmo, and mount the refractor
> down tightly, there should be very little slop.  My question, I guess is,
is
> the 400mm focal length unusable?  What if I used a 2x or 3x barlow to make
> it 800mm or 1200mm?
>
> I have a 4.5" mak that I tried once, and although it seemed to work ok,
it's
> heavy, and anything much away from the zenith, even with proper weight
makes
> for difficult balancing.  This 80mm is light in comparrison to the mak,
and
> is a lot easier to mount on the OTA.
>
> For the record, an LX-50 with the Jordan Blessing DEC fix kit, a concrete,
> permanent pier (in my observatory), latitude adjuster, and a few other
mods
> guides just fine with a 201xt through the OTA with a camera mounted
> piggy-back.  I have some rather nice shots thru my Nikon F2 w/ 300mm f/4.5
> telephoto.  It just gets difficult with the limited magnitude the 201xt
has,
> the distortion through an OAG, and the fact that you're working with a
> fraction of the usable view to find a guide star.
>
> Suggestions?  Any specific articles in the archives about guidescope focal
> lenghts?
>
> Thanks
> Frank Schwartz
> (in between web sites)



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