Orwell Astronomical Society (Ipswich)

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Equatorial Mount For A 60 mm Refractor

I use a 60 mm Prinz refractor for general astronomy and in particular for projecting images of the Sun, several of which have appeared in editions of the OASI Newsletter. I bought the telescope from another member of OASI, who had put it to good use. The telescope originally came with a standard altazimuth tripod and I had the idea of tilting the normally vertical azimuth axis and fitting a drive so as to create effectively an equatorial mount. I decided to keep the tripod and head low for better rigidity and viewing while laying down: it is possible to use a zenith prism (one was provided with the telescope) to provide very comfortable viewing in this configuration.

At first, finding the vertical axis somewhat stiff, I thought that the drive would need a lot of power and even then there was likely to be some jumping and wobbling in the head of the mount. I therefore decided to dismantle the entire mechanism to determine whether I could fit some roller bearings. In doing so, I found that the main bush had a hairline crack almost separating the casting into two. I cut out the cracked part of the bush, and sorting through my stock of "special" parts (odd bearings, washers, bushes, etc) I found two ball races and a cage together with some ball bearings, originally from a thrust bearing, which suited requirements. The original rubber compression thrust and take-up washer also needed to be replaced as it did not give enough compression to keep the head from wobbling more than I liked. I found a suitable strong compression spring and fitted it together with a new bolt to make the head free from any wobble, side- or end-play yet very easy to turn.

At this stage I was in a position to add a drive to the head. I wanted something that could run for several hours at a stretch, had no hanging weights and would be driven electrically at low voltage so that the telescope could operate in its new equatorial mode without mains or power from a car battery. Instead of taking the trouble to construct an entire motor myself, I chose instead to use a small electronic drive with integral gearhead, with an output shaft speed nominally one revolution every 30 minutes. (I believe that the heart of the motor is a pair of solenoids working at about eight impulses per second. However, I'm not going to risk taking it apart to verify this!) My previous tests of the motor had shown that its speed could be adjusted reliably, to at least 10% faster or slower than nominal, by inserting a tiny screwdriver through the casing and into a slotted preset variable carbon track resistor on the printed circuit component board. Further, I had tested the motor for accuracy of speed at different temperatures and at different voltages, finding that although rated at 12 V DC it would still start at 7.5 V and was within reasonable tolerance at up to 14 V. I decided to use a 10 V ex-GPO rechargeable lead-acid battery that I had in my possession. At 10 V, the motor consumed only about one watt and the battery held enough charge for at least 24 hours' continuous use. I mounted the battery in a box matching that of the charger and control unit - I made the battery box oversize but with location blocks, so that air could circulate freely but the battery could not slide about. The battery could thus easily be removed, so that for transportation in a small vehicle the mount, tripod and boxes could be put on their sides and the battery carried separately, the right way up.

I constructed the charger from a second-hand mains transformer with two intermediate taps on the secondary, using a rotary switch and variable resistor to control the charge rate. When charging, the motor is automatically switched out of the circuit to prevent impairment of its circuitry. I fitted a meter which can be switched to indicate charging current, motor current, charging voltage, motor voltage or battery voltage. The meter is second-hand, but I chose it because it had the correct calibration line intervals and a luminous dial (though I later found that this in fact was no longer very luminous!) All that is necessary to charge the battery is to unwind the cable and plug it into the mains. The charger unit needs to be ventilated while charging, and for this there is row of vent holes in the box; I use a fresh piece of sticky tape to mask them in order to keep the dust out of the unit at other times. About three hours charging will last for at least a week of running and with my time restrictions this equates to at least a month's observing! I arranged for the battery to operate a pilot light indicating that the motor was running.

The weight of the battery, charger, control box and meter help to hold down the southern apex of the tripod base. The tripod leg pointing south is very considerably extended, while the other two legs are kept at their shortest length. This puts extra strain on the clamps holding the legs in position: if these slipped, the result could be strain on the motor or drive mechanism, and a loss of equatorial alignment. I therefore fitted a small, metal strip-plate to strengthen the clamps. This does, however, compromise my ability to dismantle the tripod and head quickly. Nonetheless, the telescope tube itself, together with any attachments, can easily be removed from the forked "U" casting of the head - the declination axis can be undone by turning only one screw.

I struggled to decide whether to use a worm and spur gear or a pinion and spur gear for the equatorial drive. Many years ago, I constructed a Meccano telescope mount with carefully curved rack strips for the main gears, their radius being 414 mm, together with a 10 mm radius pinion. This arrangement had worked well enough, so I decided to use a similar arrangement for the Prinz telescope. I had a couple of metal disks left over from a previous project, and used the larger of them with some Meccano racks to make a 81 mm radius gear meshed with a 10 mm pinion (from a Meccano step-down gearbox). The pinion allows quick adjustment with a clutch interposed between the motor and the final drive which allows slippage if the telescope is accidentally knocked. The gear, being a pinion, also allows slippage of the set-screw if the telescope is knocked, whereas a worm drive would not. Variations in drive due to changes in the angle of attack of the pinion teeth are very small, and certainly acceptable for the Prinz telescope. The gearbox itself provides a 3:1 reduction - the pinion turns about one revolution in 1.5 hours. 15 teeth on the 10 mm pinion and 240 teeth on the 81 mm main drive gear gives the required one revolution per 24 hours. For convenience, I marked easily visible hourly intervals (10 teeth) on the main gear wheel.

I initially constructed the clutch from two 1" diameter rubber-tyred Meccano pulleys, but this did not provide adequate friction. Slipping in two discs of industrial sandpaper, back to back, stapled together, between the pulleys provided an easy solution. To adjust the pointing of the telescope one either adjusts the original clamp or pulls the clutch shaft. The first approach is the quickest and simplest. The latter is for finding objects by co-ordinates; it needs accurate baselining and sidereal time calculations, but is, however, a useful facility. The declination setting is fixed by an original sliding rod and I was unable to improve upon it. However, I did add a 100 mm indicator disc, with white markings on a black background, similar to the polar indicator.

The change to equatorial mode tended to make the fork for the declination axis a little back-heavy and, to offset this, I added a fixed "U"-shaped counterweight bar. I also found it necessary to add a lenshood counterweight when using a camera with the telescope: a weight on a single clamp ring provided an ideal solution, being readily adjustable.

I marked an angle of 38° on a sheet of A4 card. (This corresponds to the latitude of Ipswich, approximately 52°.) Using this together with a horizontal reference line behind the head was sufficient to perform preliminarily adjustment of the tripod legs. The tripod legs measured for splay at 34° to the triangular base. The remaining 4° was provided by a longer foot-adjustment screw at the southern apex. I used planed timber, 47x21 mm to construct a base for the telescope. I painted all woodwork with two coats of exterior grade white paint. I fitted end lids to the charger/control and battery boxes, then primed them and fitted them to the base.

To enable changing the speed of the electronic drive, I took the cover off the motor circuitry and carefully bridged the manufacturer's product with a pair of thin leads. Not caring too much for soldering wire joints with the attendant possibility (or inevitability?) of damaging the preset or neighbouring components, I used very small machine screws and washers. I drilled a 1 mm diameter hole in the printed circuit board to give the screws purchase and then twisted a lead round each, held in place with a washer and nut, just small enough to allow replacement of the proper cover. I fed the leads through the cover, keeping them long enough to allow stretching to any observing position, and terminated them on a variable resistor and fixed resistor in series fitted inside an empty 35 mm film canister, operated with a knob. The film canister acted as a convenient hand-held unit. I then adjusted the preset resistor inside the motor cover to bring the range of adjustment of the hand controller into the appropriate range, from slow lunar rate to slightly faster than siderial rate, within the 280° rotation of the variable resistor.

I have found that the above somewhat Heath Robinson arrangement has worked perfectly well during the last year. I usually use a 15 mm focal length eyepiece, allowing fair eye relief, and assuming that the base in set reasonably, objects remain in view without adjustment for at least half an hour even at the celestial equator. I have fitted a solar projection attachment on occasion, finding it be be well balanced by the lens-counterweight.

I still hope to lay a concrete slab base on my back lawn with three positioning pieces in it for the foot-screws of the tripod. I find it convenient, though, to use several different observing positions about the garden and even from the house. Unfortunately, the terrace blocks run N-S and are fairly close together - making such movement necessary. I am lucky, however, in having an uninterrupted meridian zone, widened somewhat by changing the position used for observing.

The 60 mm telescope mounted as described above can do much useful work, has excellent portability and setting up takes barely five minutes at most; one minute to carry outside, already rigged with eyepiece and other equipment is often enough. Keeping the telescope indoors when not in use is best for the electrics!

Roy Adams