A Friction-drive Fork Mount by Dave Rowe

Nothing is more important for successful astrophotography than the telescope's mount. So, is it possible for an amateur telescope maker to construct a wooden mount suitable for long exposure astrophotography? The answer is definitely yes. The following images and text describe such a mount, the third mount of this basic type that I have made for film astrophotography. A friction drive has several advantages over a drive that uses a worm gear for the final reduction. It has very low periodic error, in fact, so low and slow that the error is hard to measure in this mount. A friction drive combined with a well made wooden fork is extremely solid and sturdy, and can carry very large loads without flexure or vibration. Most importantly, the mount can be made at home, with a modestly equipped wood shop, a few purchased items and a bit of scavenging in the surplus shops.

The mount base is a triagular torsion box 3" thick, constructed from 1/2" thick oak slats covered with 1/2" plywood. Three tapped wooden inserts hold the adjustable feet, two in the front and one in the back. The mount can be adjusted for latitudes between 32 degrees and 38 degrees. This image also shows a front view of the RA drive. A stepper motor (black rectangle) drives a surplus 84:1 worm-gear reducer through a homemade coupler . A timing pulley on the output shaft of the reducer drives a timing belt that in turn drives another timing pulley. This timing belt sub-sytem has a reduction ratio of 2. The second timing pulley is coupled via a simple clutch to a 5/16" diameter friction roller. This friction drive shaft turns a large plywood/aluminum wheel, as shown in the next image. Since the wheel is 25" in diameter, the reduction ratio of the drive shaft/wheel combination is 80. Total gear reduction from the stepper to the RA axis is, then, 84 X 2 X 80 = 13,440. The stepper is used in half-step mode and has 400 half-steps per revolution. Thus, the overall reduction is 5,376,000 half-steps per rotation, or 0.24 arcseconds per half-step.

The wheel/fork assembly has been placed on the base in this image. The forks are also a torsion box construction made in a similar manner to the base with similar materials. In the center of the wheel is a polar alignment scope that has been carefully adjusted to be parallel with the polar axis. The forks are 14" long, 3.5" thick and have a height of 8". They are glued and screwed directly to the wheel, and a triagular gusset is used between the fork and the wheel to make this assembly ultra strong. The strange little adaptor on the right fork holds a Nikon 35 mm camera lens on one side, and an ST-4 CCD auto-guider on the other side. This tiny little imaging telescope is used to perform precise polar alignment using an algorithm that I wrote in Excel. The handmade ball joint is used to roughly aim this camera at the pole.

The south bearing is a 2" pillow block with thick-walled aluminum tubing running through it. The tubing also forms the support structure for the built-in polar alignment scope. An 8-sided plywood cone is used to strongly couple the wheel to the aluminum tubing. It's a tricky bit of work making the compound miter cuts for the eight cone pieces. This view also shows the aluminum flat stock that covers the outside of the plywood wheel. It is tightly stretched around the wheel then epoxied and screwed to the plywood. One of the two screws can be seen near the top of the wheel. The wheel itself was made using a router table. The wheel was routed after the aluminum-tube shaft was glued to the wheel, and before the right (north) end of the shaft was trimmed off. A 2" diameter hole in the router table, in conjunction with the shaft end, acted as the center bearing while "turning" the wheel. The wheel has less than 0.2 mm error in radius, as proved by the CCD-based polar alignment camera and numerous interesting experiments. In other words, the polar axis moves less than 2 arcminutes as the polar axis is rotated 150 degrees. This is more than good enough for very long exposure photography.

This side view shows the two wooden support structures that hold up the wheel and the pillow block bearing. It's hard to tell from this image, but the wheel is coplanar with its wooden support, as it must be for correct engagement between the edge of the wheel and the drive shaft. Also shown is the motor/reducer mount, a simple pair of wooden wedges glued and screwed to the base. The simple friction clutch can be seen in this side view. A piece of UHMW polyethylene is sandwiched between two timing pulleys. The pulley with the timing belt is free to turn on the shaft and the other pulley is fastened to the shaft with set screws. A spring (not shown) forces the pulleys together, and causes sufficient static friction between the pulleys and the polyethylene to keep the mount in place. A few pounds of force on the forks overcomes the static friction and allows the scope to be moved to a new pointing direction. The clutch works, but it doesn't have a professional feel, and it must be kept quite tight. Suggestions for a better system are most welcome. The brown wooden knob attaches to the threaded rod holding the rear foot, and is used for fine elevation adjustment. At this point you might be wondering how the azimuth adjustments are made. The simplest way possible -- by light taps with a soft mallet. This proves much better than having a complicated azimuth adjustment mechanism. I can quickly adjust the polar axis to within 1 or 2 arcminutes with this technique, and the mount is much stronger and lighter without the azimuth stage.

In my designs, the declination axis is directly attached to the OTA, making a very solid unit. Alternatively, a cradle could be designed to fit a particular OTA. The declination axis is formed using two pillow block bearings, which are fastened to the forks with studs glued into the fork ends. Not shown in this image is the declination drive, an interesting tangent arm system that has a large travel. The screw in the tangent arm is directly driven by a stepper motor with a built-in gear reduction head, also found in a surplus store. The declination drive has roughly the same angular displacement per half-step as the RA drive system. The above image shows a part of the carbon-fiber tube assembly for a large Schmidt camera. In use, a guide scope is attached to the OTA for guiding, and an ST-4 autoguider is used to make the drive corrections automatically. The friction-drive fork mount is ideal for mounting Newtonian telescopes. I find that this mount is easier to build and stronger than a split-ring equatorial.