Re: [APML] Vixen VC200L for film Astrophotos

["John C. Mirtle" <spampit@shaw.ca>]

Bill,
    Most Newtonians I have seen invite flexure. Unless you have a metal or
carbon fibre tube, the tube is often the biggest culprit. The oversized
secondaries are rarely counterbalanced, and the weight of the secondary over
long exposures can cause even Novak spiders to flex. Even the changing
weight/angle of the camera and focuser can cause the tube to flex. Of all
the scopes I have seen in it's price range, the folks at Vixen have at least
made an effort to reduce flexure. All adjustments (including the mirror
cell) are in tension with push-pull screw arrangements. No springs! The
mirror is secure and the external focuser locks up solid. The
spider/secondary assembly has been described as being able to survive a
nuclear blast - nothing is going to move here. The scope requires a separate
guidescope to shoot at f6, it appears that they built it with this in mind.
    Perhaps the VC200 does shift over long exposures, I don't have a
guidescope to test this. I am using an OAG because I have one and I like
using it. Manfred seems to have good luck, but we are sure drawing on a
small pool of users! My point is that there are reasons other than primary
mirror flop that cause Newtonians to fail. For a look at the innards, Harry
Pulley ripped his apart - http://members.rogers.com/hpulley/vc200l.html .
Note that the three mirror clips are covered with a full aperture ring to
avoid diffraction.

John Mirtle
Calgary, Ab. Canada

----- Original Message ----- 
From: "William R. Mattil" <wrmattil@ix.netcom.com>
To: "Discussion of Film Astrophotography" <astro-photo@seds.org>
Sent: Wednesday, August 27, 2003 6:32 PM
Subject: Re: [APML] Vixen VC200L for film Astrophotos


Radu Corlan wrote:

>
> I seem to recall that the VC200L has fixed mirrors - so no problem with
> mirror shift there.

While fixed mirrors may indeed be an advantage, all Newtonians have them
and seperate guidescopes almost never work with a Newtonian. Probably
the biggest improvement would a symetrical secondary and the Vixen has
very stout secondary support mechanism.

Regards

Bill

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