"Barn Door" Mount for Astrophotography
By Dean Ketelsen, Member
|
|
 While
being able to track stars accurately may seem a daunting challenge, it
is really quite easy, as long as we are talking about wide angle to
slight telephoto camera lenses. More powerful lenses (greater than
180mm) are probably best left for equatorial mounts, real worm gears,
and likely a guide scope. The barn door mount I built took only a few
hours of effort and cost only a few dollars over the tripod and camera
ball mount I already had.
There are several designs out there - the one I made is called a
curved sector drive, because the threaded rod it uses is curved. You
can use a straight tangent drive, but the drive screw should be turned
at different rates depending on where it is along the tangent drive.
The curved sector drive uses a constant drive rate so is simpler from
that aspect. The only complication is that the curve needs to be
pretty well matched to the distance from the hinge.
Initial Calculations
To get started, I needed to figure out the distance from hinge to
sector drive. I wanted to use threaded rod - 1/4"X20 threads per inch
(tpi). Two feet of it at Ace hardware was only $.75! I also wanted to
turn the drive nut once per minute, so I needed to calculate how big
would a worm gear be that had 20 tpi and a drive that rotated once per
minute. In a sidereal day (length of time it takes the stars to circle
the sky once) there are 23h56m, 1436 minutes in all, or 1436 teeth in
our imaginary gear. At 20 tpi, that would make a circle of 71.8"
circumference, or 22.855" diameter. If our barn door was half that, or
11.43" between our 1/4"X20 drive and the hinge and the drive nut was
turned once per minute, it would precisely track the stars! You can
make a similar calculation if you chose a threaded rod with more or
less teeth. If you chose a 1/4"X28 threaded rod (standard fine thread
for 1/4 inch size), you could make a more compact barn door mount as
you would only need 8.16" between hinge and drive. In any case, I used
1/4"X20 and got great results.
Tracking Issues
The next task was to bend the threaded rod to match the 11.43" radius
so that the mount would track correctly and there would not be a lot
of drag as the barn door mount was rotated about it's hinge. Because
the rod is so springy, you need to bend it around a form that is
significantly smaller than 11.43" radius. I happened to have a piece
of aluminum irrigation tubing scrap from a telescope tube that was 12"
diameter and it turns out that was just about perfect for bending to
the radius I needed. I first used a compass to make a circle that was
the desired 11.43" radius, then took several tries to match the rod to
it. Eventually, with a little touchup bending, it fit as perfectly as
I could make it without any tools.
 Check
out the photos of my barn door mount. I happened to have some
lightweight aluminum structural material which is easy to work, but I
could have used some 1/2" plywood with good effect as well. You need
to get a hinge that has very little play and the best I've found are
the piano hinges that you can get up to several feet long. They are
quite rigid and yet rotate freely. I bought a 12" version and cut it
down to about 3" wide for my mount, but in hindsight, I should have
gone wider to at least 6" for increased rigidity. The 3" section of
hinge is barely adequate for my purpose here. In addition, if you make
a wider barn door you could mount multiple cameras on it for meteor
showers! The pictures show how I made the mount with the hinge on one
side and holes for the curved drive on the other, with that spacing of
11.43" held carefully. You have the option of lowering the upper
movable part of the mount if you put the sector drive on the west
side, or like I did, raise the upper part of the mount with it on the
east side. You need to have some way to hold the mount to the angle to
match your latitude, in our case about 32 degrees. I chose to keep the
tripod head level for maximum stability and built in a tilted bracket
to get my 32 degree adjustment. Fasten it to your tripod, adapt your
camera to the upper plate and you are about ready to go. I already had
the ball camera bracket, but you can similarly go to a camera store
(the Jones Photo Pro Division on Country Club has a huge variety of
camera mounts and tripod heads) and get an equivalent. Similarly, if
you have TWO tripods, you can take the head off one of them to use as
the camera mount on the barn door.
For the drive, I glued a long narrow piece of plastic to a 1/4X20 nut
so that I could turn the nut accurately during the long exposures. In
practice, imagine that it is the sweep second hand of a clock and that
is the speed that you need to turn it. I chose not to complicate
things by trying to motorize it, but it could easily be done. If you
have a timer of some sort and a scale mounted to the bottom plate near
the drive nut, you can easily move it accurately enough for a 135mm
lens. A small red flashlight helps to see what you are doing. Think of
it this way - in my "rules of thumb" I told you about the longest
exposure you could take with a camera lens before it showed trailing
was 650/lens focal length. So for a 50mm camera lens, you can go for a
13 second exposure before it shows trails. Now use this as a guide to
how accurately you need to turn your drive nut on the barn door mount.
At 13 seconds before trailing shows, you could make an adjustment
every 10 seconds and no trailing would result. Similarly, for a 135
lens, your "window" is about 5 seconds, so you must be on the ball
when tracking the longer focal lengths. I went with a 135mm lens for
10 minutes, so it can be done even manually turning the drive nut with
good results.
As for alignment on the pole, you can go to a lot of work to put in
 some
sort of pointer, but any error in installing it relative to the
rotational axis of the hinge adds to the overall pointing error. I
just sight up along the hinge to Polaris, and even with my short 3"
long hinge, it seems accurate enough. A longer hinge as I suggested
would make a more accurate sight.
That's about it - check out the photos to see some pretty good results
(click on the image for a larger version). E-mail me if you
have any questions at
ketelsen@as.arizona.edu.
To the
Top
|
