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Re: [APML]: barndoor tracker question
> 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:
1. A 9x rifle scope helps with accurate polar axis alignment - xerox
the north pole chart from Uranometria and laminate it so it's dew
proof. You'll use it to put the polar axis right where you want it.
..after you memorize the handful of faint stars near the pole.
2. Calibrate your barn door's drive rate with a laser pointer
shining on a wall about 75 feet away. You can detect errors of about
15 arcsecond magnitude. . .enough for most 300mm lenses and color
film. I've got a spreadsheet that helps lay out the location of the
projected spot depending on the distance from hinge axis to wall,
thread pitch, etc. If you want a copy I'll be happy to e-mail you
this Excel spreadsheet.
3. If you have some type of rate control on the barndoor's motor you
can compensate for the inherent slowing down as the barndoor runs
longer and longer. I was able to do this manually for up to 25
minutes.
4. If you have a laptop control the motor you can automate this
correction process I mention in #3. I wrote simple QBASIC code to do
this that may help. Ask and I'll send you the code.
My results? Unattended astrophotography with a plywood/hardware
store-quality-hinge barndoor that allowed:
300mm lens shots for 25 minutes.
20mm lens shots for 60 minutes.
I got to take naps during exposures! ;-)
Tom Krajci
Capt Tom Krajci
B-52 Intelligence Officer
"There are only two possible outcomes in any war.
Operational success or intelligence failure!"
http://spur.barksdale.af.mil