Re: Vixen GP, or CG5-AS GT?
From: Roger Hamlett (rogerspamignored_at_ttelmah.demon.co.uk)
Date: 12/14/04
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Date: Tue, 14 Dec 2004 21:03:51 GMT
"Stephen Paul" <spaul219@hotmail.com> wrote in message
news:cpl5pd$5ql$1@transfer.stratus.com...
> If you had your choice (for free) between a Vixen GP, with RA drive and
wood
> legs, and a Celestron CG5-AS GT, which would you choose, and why?
>
> Primarily, my concern is the difference in machining. I percieve the
Vixen
> GP to be the better quality mount, and I fancy using it for imaging one
day,
> not to far off.
>
> Would the CG5 be as adequate for imaging?
>
> For me, the GoTo only has value, if I have an 8" SCT or something
similar in
> aperture mounted on it. In fact, that possibility is the only real
reason
> that I even consider the CG5-AS GT as a viable replacement for the GP
(well,
> that and that massive 2" steel leg tripod).
>
> Generally speaking, I don't like GoTo as it takes away my freedom to
slew
> the mount manually, hither and yon. Although there _are_ times when I
just
> want to punch in a number, and get an object in the eyepiece.
>
> Man, why is every decision so hard? (And don't say, "money".)
Money!...
The Celestron mount, is 'not bad', and with some 'tweaking', can become
quite good. In the past, I very carefully lapped the gears and worms in
one of their slightly older variants, with a micro abrasive, cleaned it,
relubricated, re-adjusted (changed the way the end float on the worms was
handled), and found that it was performing as well (lightly loaded), as a
Vixen GP. Giving small, and very smooth PE curves, and happily handling a
light scope, and taking many quite acceptable CCD images. A lot will
depend on 'luck'. The best examples, are quite good 'out of the box',
especially if used with a light scope. However the largest scopes offered
on the Celestron mount, are uncomfortably large/heavy for the mount, and
most are too large/heavy to image well. The latter mounts, seem to be
generally 'better', than the older versions, with smoother bearings, and
better PE.
The key 'point' about the Vixen mounts, is that the overall finish on the
machining, is 'good in the class', giving quite smooth PE curves (often as
much PE as cheaper mounts, but with a nature that makes it easier to
correct).
Wood legs are good, in that the material has excellent vibration damping
characteristics. This is why 'surveyors' tripods, use generally either
wood, or one of the plastic/fibre composite materials. Metal legs _need_
to be heavy to match wood in this regard.
For imaging, using longer exposures, and focal lengths, you really need
drives on both axes, and an ability to accept guide corrections. A simple
'RA drive', wold probably not be adequate.
A lot depends on the focal length involved. You can accept a lot more
mount errors at 600mm focal length, than you can at 2.5m focal length.
You can still slew a GoTo mount, but you have to do it with the motors,
unless you add optical encoders (money again...), or are prepared to
re-synchronise after the slew.
Though you want to say that money doesn't matter, it presumably does,
otherwise you would simply buy a Vixen GP-DX, with a SkySensor 2000, which
is one of the best GoTo controllers around, and has an autoguide input,
and is fully capable of taking guided images, out of the box. A good
example of the CG5-AS GT, gives nearly the same capability, at a much
lower price, _but_ there is more variation between the units, and some
features (like PEC), are still not working, or working as they should....
So I'm afraid it does come down to money. If working on a limited budget,
and wanting a complete 'package', to move a reasonably light scope, and
allow guided imaging, then the CG5-AS GT would win. If instead, I was
confident that more funds would become available in the future, I'd
probably get the GP-DX (rather than the GP), and use it with one motor,
till the money allowed purchase of motors with an autoguider input. I
might even consider a real 'bargain basement' solution, like trying to
source two of the Meade DS motors, and attaching these with an Autostar
controller to the mount.
Best Wishes
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