Many years ago work friends invited my children and I to join them at a Telescope evening. Amongst all the very expensive & impressive telescopes, Peter had also brought along an old 10” Dobsonian for us to have a look through. His vast knowledge of the night sky and the ease of using the Dobsonian left a very lasting impression indeed.
Dob’s are a mirror based reflecting telescope with a basic (usually timber made) ground-level mount that moves in the vertical plane (Altitude) and horizontal plane (Azimuth) by simply pushing/pulling by hand to change position and held in place by simple friction.
When it comes to telescopes, "Big IS Better", it’s all about gathering light and not so much about magnification. In fact the maximum useful magnification of my 200mm Dob is probably no stronger than a 70mm Refractor from a toy shop, but I can see far more detail due to the extra light collected.
At another telescope evening I had the good fortune to meet an enthusiast who had built an “Equatorial table” so a Dob could track an object for long exposure photography. The idea is to rotate the telescope in the opposite direction to the earth’s rotation so objects in the sky appear stationary.
Up to this point everything are bits and pieces that were at hand, (yes – even the over-the-top main pivot bearing came out of the junk box!) but it was soon realized the DC motor wasn’t producing the speed consistency needed (speed would vary at even the slightest drop in power or load change), so a new reduction utilizing a ‘Stepper’ motor was fabricated.
The next problem to address was the position of the eyepiece on the telescope. With the mount now inclined, the eye piece was getting into some awkward positions to view through.
To overcome it, a ‘saddle clamp’ was fabricated – clamping lightly on the tube (with felt again) so it could easily be rotated and held in place by friction (with a couple small wheels to stop it sliding down). It also included a fine adjustment rod for a bit better altitude control.
With all these extra bits came new complications – the DC motors really needed a bit more voltage to prevent stalling, but the tiny stepper on the ‘Eq’ runs quite hot even at 6Volts. The cables to the ‘hand’ controller would easily move the scope off position. The obvious solution was to use stepper motors everywhere, and route the cables thru the center of the ‘Eq” pivot. They would then connect to the hand controller at the base so it would have less effect on the scope (the amount of rotation during an evening’s use would be minimal, so cable windup would not be an issue).
It was also time to look closely at the ‘Elephant in the room’.
The Open Fork
Layers of ‘marine grade’ ply is the base material of choice as condensation can be quite significant on some nights. Once laminated to sufficient thickness to achieve the desired rigidity, it is machined in a lathe so the surfaces are true (and a large center hole for the cables to pass through the Eq bearing - which was always hollow but not utilized).
Adding a running rail around the outside for the support wheels so they could run on the outer edge, means the base can be a smaller diameter.
Even though the new base is smaller in diameter, the ‘Eq’ frame was built for the original Saxon mount, so it has plenty of room for the alloy frames to over-hang without fouling. (wider angles = more rigidity).
Tilt travel is now unlimited and exceeds the abilities of the previous simple rod arrangement, so provision for a new system is also required.
The frame width is increased slightly to accommodate a ‘Worm & Wheel’ with felt friction (same as the ‘Eq' drive arrangement). The wheel is later trimmed back to a ‘part sector’ as full rotation is not required.
As one Stepper was already at the base driving the ‘Eq’, its driver & micro controller were proven. The program for it is quite basic using a 10 turn potentiometer to finely adjust the speed, (plus a remote knob to go faster or slower as needed) - but it IS speed critical and can’t go off doing other stuff willy-nilly.
It will now become the Master, (the hand controller still needs to control the Eq speed, even when the others aren’t connected) so the program is modified to send commands to a Slave controller without impacting its ‘time-critical’ cycle.
As the other two stepper motors are mounted up on the scope – running 10 or 12 wires up to them is problematic, so the second (Slave) micro Controller and the Driver modules are housed in a separate enclosure and positioned up on the fork near the tilt motor. Only 3 wires are now required to connect to the base unit (2 for power and 1 for command data).
The tilt motor is permanently connected to the module – so cunning positioning of the grommet for the motor cable also allows access to the controller’s USB socket in case some program changes are needed. The focus motor connects via a 5 pin microphone jack (top left). The base lead could have been hard wired but I used a 4 pin microphone jack instead (bottom left) - so it doesn’t accidently get mixed up in the dark.
The hand controller has to be easy to operate in the dark, capable of a few bumps & knocks, something I could program the micro controller to read from, and preferably something I already had (or cheaply acquire).
Left-to-right: PS2 controller socket, Battery socket, 5Volt regulator & heat sink, ‘Eq’ stepper driver (attached to center wall), Master micro controller (bottom), Socket for the lead to the Slave controller (top), Eq ‘base’ speed control pot (top right), Aux power jack (bottom right).
In conclusion the end result is most pleasing – The proof is in the pudding! The cordless PS2 controller is fantastic, as it allows fine trimming away from both scope and laptops, which helps to keep better night vision to enjoy the beautiful night sky views.
Now if it would just stop raining!
The simplicity and construction used intrigued me greatly and I soon began developing something for mine. Instead of adding a new axis (something else to cart about) I decided to simply tilt the ‘Azi” plane of the existing unit to the Equatorial angle (the earth’s axis) and drive it instead. Minimal latitude adjustment is needed as at 5 feet tall, an 8” ‘Dob’ is not the sort of thing you cart about as ‘carry on’ luggage!
After mocking up a timber mount to assess balance & stability, a steel unit was fabricated (more mass for stability) and a simple drive system created using a length of threaded rod meshing into a plastic gear (teeth cut with a matching threading tap). The telescope is rotated via a felt friction pad to retain that great Dob feature of simple push/pull to look in different directions. A couple of wheels are positioned to run in the same tracks as the original Azi pads for stability.
As my interests in imaging through the scope increased, so did the limitations of my ‘handiwork’, mostly with shake when trying to adjust the altitude (tilt) or the focus. Motorizing both was the obvious solution and as the speed was not critical, the DC motors previously tried on the ‘Eq’ drive were rebirthed.