Re: [APML]: barndoor tracker question

["Richard Keppler" <rkeppler@telusplanet.net>]

On Thu, 26 Feb 1998 13:29:54 CST, Tom Krajci wrote:

>
>
>> From:          "Richard Keppler" <rkeppler@telusplanet.net>
>
>>...  I figure that a 200mm focal length is
>> really starting to push the design limitations of the device, although
>> a nice lightweight 135mm will go for at least 10 minutes.  A single
>> hinge barndoor will be good for about 15 to 20 minutes at most with a
>> standard 50mm focal length.  A double hinge barn door can be made,
>> which tracks even more accurately, allowing longer shots to be taken -
>> 20 to 50 minutes depending on the focal length of the lens used.
>
>I've made and used a single arm/hinge/whatever (the simplest kind 
>there is) barndoor and was surprised at how good this will track if 
>you are careful.  Some hints:

 Yes, some very good pointers there.  In my initial response, I was
speaking from the viewpoint of a first-time builder/user, in the
traditional method with fixed drive rate and soda-straw 'eyeball' polar
alignment.  With the addition of the significant modifications you have
made, especially the addition of a good polar alignment device such as
the rifle scope, the platform will definitely perform better, although
a lot of user experience will be required to allow such long tracking
at high precision, even with these modifications.  The mount would have
to be quite beefy as well, with some sort of very heavy duty tripod to
eliminate flexure and vibrations due to wind.  As an aside, how do you
manage to support such a long focal length lense, such as the 300mm
without camera flexure.  A friend has a 280mm f/2.8 which requires a
platform specifically designed to support the lense and camera in order
to eliminate the camera body flexure due to the mass of this huge
beast.  He uses it on a 3" ball mount attached to the side of his
German Equatorial scope platform though.  I think the tripod I use with
a small barndoor would collapse under the weight.

Rick.