a telescope for visual and ccd astrophotography use

The telescope pictured here is a 12.5" f/4.5 Newtonian on a split ring mounting. It was built over a period of about 18 months in my garage, the most sophisticated tools I used were a bandsaw and drill press. The primary mirror is from Nova Optical, I built the rest. It won the first place award for craftsmanship at the 1994 Stellafane meet.

Overview

The mount has stepper motor drives on both axes, controlled by electronics mounted in the base. A 12V gel cell battery, also in the base, provides self-contained power for 20+ hours. Digital encoders mounted on the RA and DEC shafts feed to the guts of a commercial digital setting circle unit mounted in the hand paddle. The hand paddle also includes controls for setting the drive rate (Guide at 2x, Set at 8x and Slew at 32x), moving the scope E, W, N, and S, controlling the electric focuser, a red LED map light, and electric collimation.

The mirror is mounted in an 18 point flotation cell. Each of the three sets of 6 points connects to a screw that protrudes though the bottom of the mirror box. A worm wheel on each screw is driven by the worm on the collimation motor (3 of them total). The controls on the hand paddle provide up and down for each of the motors. The scope is easily collimated on a star in 30-60 seconds, without taking your eye from the eyepiece.

The mounting ring is 30" in diameter made from two sheets of 3/4" oak plywood glued together. The edge is a 1/16" thick aluminum strip which rests on two stainless steel 1/4" shafts, providing a 120:1 final drive ratio. The main RA bearing, at the rear, and both DEC bearings are nylon blocks, lined with Teflon, and bearing on turned aluminum shafts 1-1/4" in diameter.

The cradle assembly allows adjustment for any latitude from 18 to 54 degrees. A jumper on the main circuit board provides RA drive reversal for the southern hemisphere, though the farthest south I've taken it so far is the Florida Keys for the Winter Star Party.

Mounting

The ring itself is 30" in diameter. It is made of two sheets of 3/4" oak veneer plywood glued together. The circle was cut with a router before the split was cut out, and then it was smoothed by rotating it against a belt sander. The edge was filled with epoxy, then a 1/16" thick aluminum strip was screwed in place. Angle irons help reinforce the thinest section of the ring. The yoke is also a double thickness of plywood and there is a single thickness support attaching the bottom of the ring to the yoke (barely visible in the photo). The stepper motor for the declination tangent arm is also visible in this photo. The wires for the dec drive, dec encoder, cooling fan, and focuser run inside the yoke and ring. The wires were layed into routed grooves before the two plywood layers were glued together.
The cradle holds the ring assembly and provides the R.A. bearing. It is made from double thickness plywood, like the ring, except for the drive end which is single thickness bolted to a 1/4" aluminum plate. The two aluminum plates are 2-1/2" apart and hold the drive shaft bearings. The drive assembly and R.A. stepper motor are located between the plates. The stepper motor drives a worm and the worm wheel is clutched to a ladder chain sprocket. The ladder chain connects the driven sprocket to the two 1/4" drive rollers, two idlers, and a tension adjustment sprocket. The wires for the motor run between the plywood layers, inside the cradle.
The cradle rests inside the base assembly, with the curved bottom of the cradle resting on the two black pipes running across the base. The cradle can slide on the pipes, allowing for latitide adjustment. Two latitude clamps press the sides of the base to the cradle to lock the latitude in place. The remainder of the base is the electronics bay where the gel cell and drive electronics reside.

Bearings

There are 6 bearing blocks used in the 12.5" Split Ring mount - 2 on each of the two DEC shafts and 2 on the RA shaft. The bearings are made from Nylon blocks with Teflon linings. Nylon was chosen because the original idea was to use it for the bearing surface. Well, it turns out the Nylon had too much friction, so I drilled the holes out a little larger and epoxied on some Teflon strips. You could use metal or wood blocks, but the Nylon is easier to machine than metal and stronger than wood so it turned out to be a good choice after all.

Pictured above is one of the DEC shafts. You can see the Teflon strips - the bright white next to the shaft. As you can see, there are two blocks separated by about 3/8". On the shaft between the two blocks is an aluminum collar that fits snugly between the blocks, this keeps the shaft from sliding in either direction. You can also see the DEC Encoder which is geared to the shaft. On the other DEC shaft is the tangent arm drive.

Here you see the bearings on the Right Ascension shaft. The collar mentioned above is clear here. The RA encoder is on the right. The rectangular block of aluminum on the left is how the RA shaft attaches to the yoke - the shaft portion was turned from a block this size and the end was left square.

To make blocks like these, first drill the hole for the shaft. I had 1-1/4" shafts, so I used a 1-3/8" forstner bit - the two pieces of 1/16" Teflon make up the remaining 1/8" of the diameter. Use plenty of water as a lubricant/coolant as you drill or the Nylon will melt instead of cut cleanly. Next drill the holes that will mount and hold the blocks together. I used 1/4" socket head cap screws for this, so I drilled 5/16" holes. The cap screws go into T-nuts in the wood beneath the bearings. Plastic handles snap onto the top of the screws to make hand assembly possible (see photo below). The last step is to cut the piece in two to make the two block halves. The bottom half of each block is then screwed to the wood with a countersunk screw in the center where the shaft will sit. This just holds the lower block loosely in place when the scope is disassembled, it's the cap screws pictured below that really hold the bearing.

Working in this order will ensure that all the holes line up properly, but only if you put the proper mating halves together and in the same orientation when you setup the scope. To make this possible, even in the dark, I cut a series of grooves into the sides of each block set. There is a different number of grooves in each set, so mating them up is easy.

DEC Tangent Arm Drive

Since only minor photo-guiding corrections are generally needed in the DEC axis, a tangent arm drive works just as well as full worm wheel and worm.

The driven rod (a 1/4"-28 threaded rod) is connected to a stepper motor with a bellows coupling not visible inside the wooden split ring. The rod rests in ball bearing assemblies at the bottom and the top. The angular metal structure around the threaded rod exists just to hold the top bearing.

Riding on the threaded rod is an aluminum block that has been drilled and tapped for the rod. This block can swivel around the rod - this is essential to allow the curved path of the arm end to follow the straight rod (see below).

Connected to the end of the tangent arm is a rod-end assembly that has a swivelling ball joint. This joint, combined with the threaded aluminum block provides the play needed for the path length difference between the tangent arm's curved path and the threaded rod, which is straight. A simple knurled screw attaches the arm to the block.

On the other end of the tangent arm, the DEC shaft has been turned down to 1/2" diameter and threaded. A round plate of 1/4" thick aluminum is screwed to the shaft so that it can not slip. Next, the tangent arm itself is placed on the shaft. A second, identical round plate, removed in the photo below, is placed over the arm. A pin driven through the shaft and resting in a slot in the round plate keeps the outer plate from slipping. A thin sheet of leather is glued to each round plate to provide additional friction.

Also visible in the above photo is the hole in the shaft that the outer plate locking pin rests in.

The tangent arm assembly and the outer, slotted plate is shown below.

This final photo shows the entire DEC tangent arm assembly from above. The large wing nut tightens the tangent arm between the two plates to lock the DEC axis to the drive.

Focuser

Plans for the Crayford focuser used on this telescope are available here.

First Light Images

As the primary use for this scope is CCD Astrophotography, the first light consisted of a number of short test shots taken with a Cookbook 245 CCD camera. These were from my driveway with a high pressure sodium streetlight about 45 feet away.

All except M42 are raw images, they have not been processed or enhanced in any way.

M57 - The Ring Nebula in Lyra
Note the central star - magnitude 14.8 in an 8 second exposure!

M42 - The Orion Nebula
4 second exposure. Some processing on this one - it was gamma scaled and I edited out some blooming from the trapezium stars.

M1 - The Crab Nebula in Taurus
8 second exposure, lots of noisy pixels in this one.

NGC891 - Egde-on Spiral Galaxy in Andromeda
8 second exposure, really needs a longer exposure to get a good tonal range.