Barn Door Drive

The overall layout of the system requires the barndoor drive to ride piggyback on the main scope. This posed some problem for attaching it to a truss tube scope. So, after a brainstorming session with Andy Saulietis, I fabricated an "A" frame to attach to the top of the Primary mirror cell box, and have the apex attached to the secondary cage support ring. Since the guidescope rotator concept is to have the rotation axis of the guidescope rotation system aligned with the rotation axis of the main scope's optical axis, there needs to be a way to adjust the guidescope rotation axis's aim.

The apex has a screw mount to go up and down (pitch adjustment), and the attachment to the Primary mirror cell slides side to side (yaw adjustment). The guidescope itself rides in a saddle that has two sets of adjustment pads. One set is used to carry the guidescope in a "home" position. "Home" has the guidescope aligned with the rotation axis of the barndoor drive. Once these are set, they are locked in place and not moved. The other set of pads is used to aim the guidescope at a guidestar, and allows up to a 10 deg aiming movement in each axis. I needed that much due to shooting under a 3.5 sky, I sometimes have to move a distance to get a good guidestar.

	Apex of Barndoor attachment Truss Note the threaded "pitch" adjustment
	Rear of barndoor attachment Truss Note the sliding mount for "yaw" adjustment

The beauty of using a barndoor drive is that you can adjust the dimensions of the barndoor drive to allow the stepper motor on the barndoor drive to rotate the guidescope the same rotation per motor pulse as the image plane de-rotation rotates per pulse. This is important (since they HAVE to rotate exactly at the same rate), since it allows the barndoor drive motor to use the same pulse train as does the imageplane derotation system. The circuitry for the barndoor motor is simply 4 TIP120's, with their bases connected to the outputs going to the imageplane rotation motor. The power to the barndoor motor is sent to the common center tap winding, just as in all the other steppers on the mount, and each coil comes back to one of the TIP120's (same setup as all the other motors use). The side benefit of this is the two motors do not have to be the same type motor.

The difficulty is in doing the calculations!!! So, I built an Excel spreadsheet to allow me to enter different values for the dimensions and components to optimize the design and allow the rates to be the same. Click here to download a copy of the Excel spreadsheet.

The guidescope in these photos is a 6"f/8, and it weighs quite a bit. That required adding more counterweights under the altitude trunion box that I wanted, so I have installed a smaller saddle to carry a smaller 60mmf/11 guidescope. The smaller scope seems to still gather enough light to allow the QuickCam I am using for the autoguider to use Mag 6.5 stars under my 3.5 mag sky, and only require 1/2 second exposures. The shorter focal length will probably be the telling issue though as I use the system. For that reason I made the smaller saddle big enough to use a longer focal length guidescope should I need it.