Re: [APML] Guide scope advice

[George Anderson <georgea@cam.org>]

Emmanuele

You might want to look into changing the focuser on your 60/700. That
way you could benefit from a sturdier focuser and 1.25" tube diameter.

George Anderson
Montreal Quebec

Clear skies and good health  

Emmanuele Sordini wrote:
> 
> At 09.49 29/08/2004 -0700, you wrote:
> > > From: "Stuart Heggie" <stuart.j.heggie@sympatico.ca>
> > >
> > > 1. Never use a guidescope with a movable primary so the Mak-Cass route
> > should be abandoned.
> >
> >Let me echo this.. I couldnt agree more.  I used a 4" mak (orion
> >apex/skywatcher) to
> >experiment with webcam guiding.  It had really minimal flop, but any flop
> >was too much.
> >Stars were oblong.
> 
> I already suspected this problem, since I had mentioned in the "Cons"
> department :-)) But I wanted to know other fellow AP'ers opinion, which
> match mine.
> 
> >Also, the 90mm is ~1000mm FL; I'd be worried about the guider "chasing the
> >seeing".
> 
> Good point. I supspect I won't ever use anything with a FL longer than
> 500-600 mm, so 1000 mm might be overkill and cause some problem with
> seeing. But the ST4 has some setting to deal with this, doesn't it?
> (Scintillation Adjust?)
> If I ever happen to shoot at FL > 600 mm, Ill be using a SCT with OAG.
> 
> >I guide 530mm with 200mm.  However, I use a ST7E / CCDOps which
> >supports centroiding.  My understanding is that the ST4 does
> >not support centroiding, so I would look for a better match
> >of FL to the imaging OTA.  (Fwiw, The STV/Efinder is a ST237
> >w/ 100mm FL OTA and it works.)
> 
> Well, but the STV is in another league :-))
> 
> >Since you have the 60/700 refractor, I would also suggest using that.
> >The ST4 chip is sooo tiny; .965 should not be an issue.
> >I suspect its enough aperture to find guidestars in 2 or 3
> >second exposures, though the fov is not great.
> >The only gotcha would be the focuser -- it needs to be
> >somewhat rigid.
> 
> To my feel, it's rigid enough, i.e. I have to put some effort into racking
> the tube back and forth; also, there doesn't seem to be any noticeable wobbles.
> 
> >And you'll need rings to mount the OTA.  (maybe get rings
> >sized for an 80mm OTA in case you want to "upgrade".)
> 
> I already have got rings. I built them myself a while ago, because I had
> thought to go the guidescope route. Their inner diameter is 90 mm, so that
> I think every refractor less than or equal to 80 mm in diameter should fit
> them OK.
> 
> >I dunno about the ST80 w/ the ST4.  Personally, I suspect
> >it will work ok.  I've used the longer 80mm f/11 orion
> >chinese refractor.  Its fairly nice, but the focuser does
> >leave something to be desired.  There is only a single
> >lock screw, but it will probably support the weight of the st4.
> >The st7 is a bit heavy.  Instead I use an extension tube
> >and have the focuser racked in a fair bit.
> 
> The adapter I have has only one lock screw, but I could always drill one
> more; the refractor's drawtube has two.
> 
> Thanks,
> Emmanuele
> 
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