Re: [APML] Guide scope advice

["William Schwittek" <syzygy@vnet.net>]

I agree.  I guide a 155mm f/7 refractor with an 80mm f/6.25 guidescope
(500mm fl) and an ST4 all the time.  Although a longer guide scope
wouldn't hurt it is not needed.  I tried the guidescope with a barlow
and noticed no improvement.

Flexure is a tough problem and you must mount your guidescope as
securely as you know how to your imaging scope.  It can be hard to
track down where things are flexing.  For a while I had my Traveler
mounted side-by-side with my guidescope and just never got nice round
stars.  I finally mounted the guidescope piggyback and the problem
went away instantly.  I've heard from others that had the same
experience.  I'm not sure what was flexing but clearly changing the
mounting scheme cleared the problem up "over night."

- Bill

----- Original Message ----- 
From: <howard@howardedin.com>
To: <astro-photo@seds.org>
Sent: Sunday, August 29, 2004 1:54 PM
Subject: Re: [APML] Guide scope advice


| Emmanuele,
|
| Save yourself the headache and avoid any moving mirror guidescope.
|
| A 80mm Short Tube (ST), typically f5 or 400mm fl, will work file to
guide your lens with a ST4. I used a ST80 w/ST4 to guide my Televue
500mm scope for several years. The big advantage to the ST80's are
they are light.
|
| As has been discussed on APML before, the ST4 will guide with sub
pixel accuracy. While not as accurate or as sensitive as the STV it is
a great guider. The consensus has been that a ST4 can guide accurately
with 1/2 the fl of your imaging device.
|
| For manual guiding you want a longer focal length. It is possible to
use a barlow with a ST80 which should provide good guiding if the
eyepiece is 10mm - 12mm fl.
|
| In either case, the main issue is flexture. If you get a cheap
guidescope it may have a sloppy focuser assembly. It is possible to
tune them up (consider adding a second eyepiece thumbscrew).
|
| Adding a barlow may  be problematic with a ST4 attached to it; this
increases the load on the focuser. It may not be a big issue if you
are using an eyepiece to guide.
|
| The 60/700 you have might work if it is solid (really wonder about
that .965 focuser + adapter combination) but you will certainly run
into situations where a good guide star is difficult to find with the
limited aperature. I would try it first since it is the cheapest
route!
|
| I am assuming you will use a good solid mounting method for the
guidescope (like Losmandy rings).
|
| Howard.
|


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