Re: [APML] Guide scope advice

["Jeff Crilly" <jlcapml@sbcglobal.net>]

> From: "Stuart Heggie" <stuart.j.heggie@sympatico.ca>
>
> Emmanuele, I don't qualify as a guru but here is the advice I've received over the years:
> 
> 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.

Also, the 90mm is ~1000mm FL; I'd be worried about the guider "chasing the seeing".

> 2. Guiding should be done near or above the focal length of the imaging scope. However, people are
> all over the map on this and you will find some people successfully using 200mm camera lenses to
> guide much longer focal length rigs. Anyway, you could go with the short-tube refractor and put a
> barlow in it and you'd be fine I think. Many fine imagers use 60mm guidescopes - try that before
> buying anything.
> 

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.)

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.

And you'll need rings to mount the OTA.  (maybe get rings
sized for an 80mm OTA in case you want to "upgrade".)

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.

jeff

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