Barndoor Mount

This is the setup that got me started in Astrophotography.  It's a Type 4 Two-arm Tangent Drive, usually called a barndoor mount.  I built it after reading an article on the various advantages and disadvantages of one-arm and two-arm barndoor mounts.  Supposedly the Type 4 variation is good to a couple of arc-minutes after over an hour or tracking, but that's way beyond my limit, since I have to drive mine manually. That's right, I turn the round knob at the end of the threaded rod at one RPM, which causes the arms to separate at an angular rate that's very close to the rotational rate of the Earth.  Mine's made of Walnut, but plywood would work just as well.

Barndoor_frontview.jpg (38352 bytes)Two later upgrades are visible in the photos.  The first is the addition of a watch with a second hand mounted on top of the threaded rod.  I used to just wait 15 seconds and then rotate the knob a quarter turn, but this only works well at short focal lengths.  With a watch on top of the rod, I can rotate the rod (and watch) constantly, paying attention to keep the second hand more or less stationary with respect to the rest of the mount.  This should improve tracking considerably.

Barndoor_rearview.jpg (48549 bytes)The second improvement concerns polar alignment.  I used to simply sight along the hinge on the end of the mount, attempting to line it up with Polaris.  This works pretty good, but the addition of a finder scope will allow much more precise alignment, and allow using the mount in Australia where there is no pole star.  Australia? Yup, both of these enhancements were made so that a friend of mine could take this mount with him on vacation down under. 

And here's a shot I made of Comet Hale-Bopp using this device.  This was the photo that really got me hooked on astrophotography, and motivated me to acquire all of the equipment I use today.

HaleBopp_85mm_RG1000.jpg (96834 bytes)