Fullerscopes drives

*

I had a recent contact from France, enquiring about the drives on his MkIII. This has prompted me into publishing some details.

To follow the stars, the polar axis on an equatorial telescope mounting should be made to rotate in slightly less than 24 hours . In fact 23Hours : 56Minutes : 04Seconds is the desired period of rotation. The so-called Sidereal Day. A star will pass the same point on the night sky in this time. The polar axis is required to rotate once in 1436 minutes.

A clock running at Sidereal time runs slightly faster than a normal clock. The latter is based on 24 hours per day.  Or one revolution of the polar axis in 1440 minutes. 24 x 60 = 1440.

A 24 hour telescope drive is usually close enough. Because a variable frequency oscillator (VFO) will easily allow the motor to be sped up a little. Just enough to follow the stars more accurately.

The Fullerscopes MKIII mounting uses 144 tooth, bronze, worm wheels of about 82mm diameter. The 144:1 gear ratio requires a drive of one revolution in ten minutes on the worm. Or 1/10th rpm.

The MkIV mountings used much larger 6" worm wheels with 359 teeth. A difference of 2.5:1 compared with the MkIII mounting. The MkIV used much faster final drive speeds on their synchronous, drive motors. My own motor is rated at 1/4 rpm.  Or one final drive shaft revolution to the worm in four minutes.

The polar axis worm drive is usually provided by a geared, Crouzet, mains, synchronous motor. The motor requires 240Volts @ 50Hz mains supply in Europe.

Some basic MkIII mountings would have only a hand wheel or flexible stalk for a rather laborious hand drive to the worm. As seen in this image of a dual hand and motor drive (above). A slipping clutch in the bronze bush on the left of the worm housing allows the two drives to act independently. The clutch is a simple nylon plug pressing against the worm shaft. 

A much better drive option than manual on the MkIII was a tiny, synchronous, gearbox motor. (even without any means of variation to drive speed) This still provided all that was needed for visual observation. (see image) This tiny motor lasted for decades before the internal wiring broke away flush with the motor housing.

I have no idea if any Fullerscopes were exported to lands with alternative voltages or frequencies. They certainly called their best instruments their "Export" models.

The Crouzet motor backplates are always clearly marked with their power requirements and usually their final drive speed on the gearbox output shaft.

The declination axis was also supplied with an identical worm wheel and a reversible motor drive at extra cost when ordering.

Guy's MkIII mounting motors. The RA on the left and the reversible Dec motor on the right.

Only photography really requires greater accuracy in drive speed than a simple motor. The eye can easily forgive an object drifting slowly off-centre in the field of view. The telescope can be easily nudged to bring it back to centre again. While a camera will produce only fuzzy pictures if there is any "wandering" of the image being recorded. Long exposure photographs, particularly with long focus optics, are the most demanding of all!

Modern digital cameras can take very reasonable "snaps" of the Moon and planets. Provided, of course, that there is a drive to keep the object adequately centred in the eyepiece.

Fullerscopes could provide variable frequency oscillator drive boxes with control paddles. These VFOs changed over time. They usually consisted of a large, black box with various sockets and a hand-held, control paddle. The paddle and drive motors would be plugged into the sockets on the box via flexible leads furnished with a variety of "DIN" type plugs.

'Guy's complete VFO drive system with Hz indicator box. The paddle has a rear frequency control knob. Unlike mine which is on the front.

The Fullerscopes VFO box isolated the outputs with relays and transformers to ensure safety at the telescope. It would still be a good idea to provide an earth spike close to the telescope. To earth the pier and mounting together. A wooden tripod would need to be bypassed and an earth taken direct to the mounting.

The "Sync" light flashes at the frequency rate supplied by the VFO. Giving a handy confirmation that it is actually working. The flashing rate can change from a steady glow to rapid flashing when the VFO is set to maximum on the paddle control knob.

Reverse on the Polar Axis motor is simply an off switch. It just relies on the sky overtaking the now-stationary mounting. The VFO cannot provide enough over-speed to achieve meaningful slewing. So it does not attempt to.

Internal components of the VFO box. The two transformers isolate the output. Ensuring no heavy currents can flow in the event of a fault in the motor wiring.

My Own VFO box has 3 and 5 pin motor connection sockets. 

In the UK it is common practice to use an RCD device on the mains lead to any electrical equipment which is used outside. Some European countries have only 2-pin plugs without any earth lead at all. Talking to a qualified electrician might be advisable. Particularly when considering the often damp nature of everything when observing under the night sky. Dew and icing is almost always a problem in some places. I have tried a simple test screwdriver on my mounting but it did not "light up".

A scan of an old, 1970s, Fullerscopes catalogue describing the "Skytracker" and MkIV mounting details.

A cropped and enlarged image of the Skytracker text.

Click for an enlargement.

Fullerscopes used the name "Skytracker" for their VFO drive systems. The one illustrated is a MkIV. This has nothing to do with which particular mounting it was used with. The 9 pin socket is for the paddle lead.


The mains socket has a lift up tab concealing the fuse. Do not be tempted to uprate the fuse if it blows! It is there for your safety and that of the electronics.

My MkIV mounting RA motor.

The colour codes for the connecting wires are: Orange to Live and grey to Neutral from the Skytracker lead.The Earth wire is earthed to a motor fixing screw.

These leads are from the output sockets of the Skytracker. Not the mains lead going into the Skytracker box.

 

.

My reversible, MkIV mounting, Declination drive synchronous motor.

Here, the central Earth wire from the Skytracker box is connected to the purple motor wires.

The Live and Neutral Skytracker wires are connected to the motor's brown and white wires respectively.

The connecting lead from the Skytracker box to the Declination motor is pre-coiled. This allows the much greater movements typical of the Declination axis.

My control paddle seems less sophisticated than Guy's in some respects. 

His has lights to indicate in which direction the drive is being corrected. This gives direct confirmation that something is happening. Which is not always obvious through the eyepiece. Sometimes without a considerable delay.

The large knob on the front of my paddle adjusts the drive frequency up and down on either side of 50Hz. Judging by the flashing light on the large VFO box the top frequency is well over 100hz. It really is very rapid. The light stays steady when I have an exact 50Hz setting.

Push buttons provide a red light for use when the observer's eyes are dark adapted at the telescope. 

The other button brings the base frequency of the Skytracker back to 50hz. 

Internal view of the control paddle.

Here is Guy's superb website:

 http://astronomiedelangrola.pagesperso-orange.fr/index.html

Click on any image for an enlargement.

*