The "Pantene" Project
Below is a project my dad completed recently on remote-controlling his Dobsonian telescope, which I’m rehosting for anyone interested!
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.
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.
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.
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.
In keeping with the ‘simple’ theme, friction drive was chosen (belt this time), this means manual adjustment is still an option if something goes wrong out in the middle of nowhere in the pitch dark of night.
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 original mount had served me well, but always had some compromises,
It’s big (especially the base board), which makes it difficult to transport, and it’s a bit heavy in the wrong places;
Has flex when side on – the tilt friction brake arrangement is only on one side, so that side board is effectively carrying most of the weight in certain positions – the design is really only suited to compression loading in the upright position – a tube brace down each side helped a bit, but the base board also flex’s somewhat;
Scope swing limited to about 120° due to the stiffening panel – makes looking in the southerly direction difficult at times when on the ‘Eq’ mount.
With this in mind a new ‘Open fork’ idea evolved, and as with other projects of unpredictable outcomes, each component is developed and assessed before moving onto the next bit – that way if it is a flop, or needs a total rethink I haven’t wasted money on bits I no longer need.
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.
Aluminum is the other material chosen because of the weight savings and stiffness that can be achieved.
Not having the equipment to weld alloy, I had to resort to epoxy and riveting for assembly. Initially I wanted to be able to ‘Flat Pack’ it for easier transporting, but side flex soon dictated otherwise.
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.
The Tilt pivots are also revisited with a pair of modified “Brewer’s” clamps to hold small self-aligning bearings on each side of the saddle, this is mainly to ensure the tilt drive gears stay engaged, but also now both sides can share the side load for better stability, and assembling the scope is quick and easy in the dark.
The next phase is building the control for the stepper motors.
The Stepper motors used in this project are small, cheap, low voltage and produce good torque at low speed, (used in air conditioning units to move the louvres etc) but unlike a DC motor, each of the coils in the motor have to be energized in correct sequence to get the right direction and speed . For this to happen they need a ‘Driver’ to feed each coil and a ‘Controller’ to say which one & when. These particular motors have a short 5 wire lead to connect to its Driver. The Driver needs 4 signal wires to connect to a Controller, plus another 2 more for supply power.
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).
Enter the Sony PlayStation 2 controller – overkill for sure, only the 2 thumb sticks are used as they are easy to find in the dark (plus a button for a safety lock for the focus stick). A big bonus is they are now available as cordless.
The battery chosen is a 7.2V from a battery drill as it has a 1 hour recharger, I also had another old charger to gut out for the scope electrics to live in (it has the correct receptacle shape for the battery). A 5Volt regulator is added for the Eq stepper motor (the others use the 7.2 volts as they only operate momentarily and the Micro Controllers have their own onboard regulators). Once the micro controllers and the PS2 controller are proven on a development ‘bread board’, the components are transferred or copied into the new ‘base box’.
The ‘Base box’ position is a compromise for easy access to plug in the battery (it’s the on/off switch – battery in for On, out for Off), but also not placing the PS2 receiver or battery where they could accidentally get kicked in the dark.
Final assembly involves checking that the cables reach/clear properly through all the various positions. As the telescope tube can rotate in the saddle, it is difficult to get perfect balance. With usage to date, it appears the slight imbalance has an advantage in keeping the scope more stable in light breezes by keeping the gear trains slightly loaded.
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!
Ps: I wonder if the PS2 controller could also operate the Laptops at the same time?