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

[Wil Milan <wmilan@airdigital.com>]

Frank,

Since you're asking for opinions, I think you're going to put a lot of effort
into something that will be very frustrating and never accomplish what you
want (which I assume are sharp, well-guided deep-sky photos).  

It's like starting across a bridge that you know only goes halfway across the
river: You'd be better off putting your money and effort into something that
eventually *will* get you where you want to go. In this case, a short guide
scope atop an SCT is *not* going to get you well-guided photos, the 201XT
is never going to guide as accurately as an ST4, and it will never guide on any
but the very brightest stars (particulary with the LX50, which needs very short
autoguiding cycle times). 

I've done exactly what you're doing, buying a 201XT to save money, then
expending a lot of effort and frustration into making it work. It was all a
waste of time because in the end I got rid of the 201XT and eventually got an
ST4 anyway. Had I done that from the beginning, or even opted to wait until I
could afford a used ST4 (which is what I eventually did) I would have expended
less money, less effort, and less frustration and had a good autoguider much
sooner.

Wil Milan / Astrophotographer.com
http://www.astrophotographer.com/
"The heavens praise your wonders,
O Lord..." Psalm 89:5


Frank Schwartz wrote:
> 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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