Re: [APML]: barndoor tracker question

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

On Thu, 26 Feb 1998 11:39:47 -0500, Jim Fedina wrote:

>polar align these drives before using them. After polar alignment, how
>does the drive get turned into another direction? In other words, I
>polar align the drive with the north celestial pole. Then I want to take
>a 10 minute exposure of Orion. the NCP is to the north while Orion is to
>the south. How do you mantain polar alignment while turning the barndoor
>drive? Is it that the barndoor drive stays pointing at the NCP and you
>turn the camera only? I'm trying to think of how that theory would work
>as I am writing this.

 Hi Jim.

 Yes, you are correct, the hinge on the barndoor points at Polaris.  A
ball head camera mount allows the camera to point at whatever portion
of the sky you want to photograph.  For long focal length (>80mm) shots
in excess of 5 minutes, you will want the barndoor to be aligned very
closely to the celestial pole - just slightly off of Polaris as you may
already know.  Maximum shot lengths will vary on just how close you
align the hinge-line, the focal length you are using (heavy telephoto's
may cause camera body flexure), the accuracy to which the barndoor has
been made, the accuracy of how it is driven, and the rigidity of the
tripod it is mounted upon.  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.

Rick.