Barndoor Mounts for Astrophotography
There must be hundreds of variations on the theme of simple star-tracking barndoor camera mounts for astrophotography. Mine is about as dirt simple as any. It incorporates a few refinements which may make a difference and which are explained below. It goes through periodic revisions which I've described and illustrated on these pages.
The basic barndoor mount uses a straight thrust bolt in the fixed lower arm. Turning the bolt forces the hinge open at a slow, precise rate. The rubber band keeps the hinge from flopping open if the camera is accidentally overbalanced and increases the pressure between the bearing surfaces to reduce chatter and vibration when you turn the bolt. The cap nut which rides against the bottom of the top half of the hinge acts as both a bearing surface and as a cam to help compensate for tangent error.
I've provided a link to a computer program below which helps balance out all these dimensions and considerations.
Aim the barrel of the hinge at Polaris with the open "mouth" of the hinge facing east, open the shutter, and turn the bolt through a fraction of a turn every so many seconds. The interval between twists determines the amount of star trailing due to latency (the time the camera just sits still allowing stars to drift). The pitch of the drive bolt and its distance from the axis of the hinge determine how much of a twist is required to keep the lens aimed at the same bit of the sky. (Latency is the overwhelming source of star trailing out to half an hour or a little more after which design and fabrication errors become dominant. Misalignment on the pole is the second most serious problem. An 8x50 finder telescope fitted with a bracket to sight along the axis of the hinge helps.)
The barndoor mount shown here is built around a 32-pitch drive bolt. If the spacing from the axis of the hinge to the bolt were 7.14 inches, the bolt would need to be turned at 1 rpm to track the turning sky. That would be a quarter turn every quarter of a minute. Easy, no? But I built this particular mount around a shorter hinge, only 5.36 inches. The barndoor mount fits in a camera case like any other piece of gear. Portability costs something in concentration because the shorter arm requires one revolution in 1.33 minutes rather than 1 RPM. That's not too bad: a quarter turn every 20 seconds. Giving the bolt a quarter of a turn while watching a sweep second-hand move 20 seconds is a little like rubbing your head and patting your belly at the same time.
Even so, it works pretty well, eh?
PC-Based Design Aid Software (c. 1990):
I wrote the code available at those links for my own use so it's not as user-friendly as it might be. Hit "?" for Help and just play with it some. (You might start with an unspecified base, a 32 pitch bolt, a 60 second rotation period, and a .2 inch radius of the cap screw. The program will calculate the ideal base length for you, display the predicted performance of the mount, and you can take it from there.) The ticks on the horizontal axis of the graph are five minutes of time, and the vertical scale is the equivalent star trailing you'd get with a stationary camera expressed in seconds of time (so look at the sixth tick mark from the left to see design error at 30 minutes, etc). On today's fast computers, the program is nicely interactive as you use hot keys to adjust various design parameters (hit "?" for a list and plow thru some arcane remarks). It will write a log file of all your trials (to "C:@barn.dat" by default), so you don't need to take notes.
(Survival guide: "X" or "x" to exit. If you run the program from within your browser, avoid hitting "r" to rerun or be ready to use the Program Manager to terminate the application if things lock up. All in all, it's probably better to save the executable to a convenient location and run it from a DOS prompt. Better yet, load the source code into an old BASIC interpreter and run it from there.)
The venerable Nikon F shown in the pictures has been retired in favor of a Canon 20D. The ball & socket has been replaced by a more robust model. The hex nut holding the whole affair to the tripod has been replaced by bevels on the lower arm which fit Arca/Kirk/RRS quick release clamps. The rubber bands worked fine, so they just get replaced by new ones from time to time.
The small pixels in the Canon 20D show the 20-second latency period when using any but the shortest focal length lenses. So it's time to add a motor drive to remove that source of error entirely. I mounted a motor on this barndoor mount years ago but it was a clunky and cumbersome affair. Since that time, I'd like to think I've learned a few things. The next time I put a motor on one of these mounts, we'll see if I can do it right. Or right-er at least. A gravity powered clock mechanism would be especially elegant, but you make photos using the mount you have, not using the mount you wish you had. A geared DC motor is on the way.