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

["Frank Schwartz" <fschwartz@rcn.com>]

Wow- You pretty-much answered all my questions.  The thing about the 400
speed films being as fast or better than the 3200 is really good news.  I
don't much understand emulsion speeds and graininess, but that's ok, I don't
really need to.  The rest of the info is really helpful as well.
Thanks a bunch!

Frank

----- Original Message -----
From: Mike Rogers <mrogers@idcomm.com>
To: <astro-photo@seds.org>
Sent: Sunday, January 02, 2000 1:47 AM
Subject: Re: [APML] 80mm refractor as a guide scope? (was: ETX as
Guide-scope)


> 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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