This is a very quick page showing the photos of the scope that I am in a continuous process of building. Design started in September 1999, construction started in November, the mirror arrived at the beginning of December and I had First Light on Christmas Day!
The scope is designed as a "learning scope", I don't think it is perfect but I have learnt a lot making it and I anticipate that there will be a process of continuous improvement - or at least change.
View of the scope. It is an 8.75" F/7.2 Dobsonian. The optics are purchased, the rest is almost entirely home made. The tube is made from 2mm MDF, glued together with reinforcing down the corners, between the mirror and the bearing and by the focuser and spider.
Rocker assembly, made from veneered chipboard, the base of the rocker is a piece of kitchen worktop.
Altitude bearing. made from 12mm ply, edged with iron on plastic edging strip. Runs on Teflon furniture glides. the Azimuth bearing also uses Teflon furniture glides. These are very light, the altitude in particular. I had to polish the base with wax polish to get the azimuth to about the same smoothness.
This shows the top of the scope, with the spider and diagonal holder. The spider is made from 4 pieces of 2mm by 10mm strips, bent at the centre and pop riveted together to form a square hole that takes the 6mm bolt holding the diagonal holder.
The diagonal holder is made from two disks of 3mm aluminium, the holes for the adjusting screws were drilled through both plates together. The holes in the lower plate were tapped to take the 4mm adjusting screws. The diagonal part is made from PVC tube, glued to the lower disk, with the mirror glued to it. All this uses Aquarium sealant.
The mirror cell showing the collimation screws and the base of the mirror. The mirror cell is a disk of ply with the collimation bolts through it. The mirror is glued to the top of the collimation bolts with silicone rubber aquarium sealant.
The tube (and mirror) survived a close inspection, George is looking for the biscuits that I was rattling.
I am now adding computer control to this scope, this uses the Scope drive system developed by Mel Bartels. Features of my implementation are:
Moulded gears with JBWeld on 12mm ply disks.
The altitude motor is 200 steps, and has a resistance of 35 ohms per winding. It came from a printer where it moved the print head.
The Azimuth Motor is also 200 steps but is marked 1.1A the windings are about 4 ohms each.
I am using the pin driver PCB from the printer to drive the motos. This has 9 power Darlington transistors with a current limiter circuit on each that seems to limit the current to about 1.1A This is driven by a 74LS06 hex inverter with a 1k pull up to 5V.
I am using skateboard bearings, ABEC3 type. The azimuth bearings are mounted on two pieces of aluminium angle that is let into the circular base board. Another bearing is let into the rocker base to locate the rocker laterally.
The azimuth bearings are bolted to 3mm plates that are bolted to the rocker so that the bearing sits just above the existing Teflon bearings. I should be able to remove the bearings, move to a different base plate and continue as a hand pushed scope.
This shows the azimuth bearings, one is fitted, the other is upside down to show how it is made.
This shows the Azimuth drive and the moulded gear on the azimuth base plate. The metal spring holds the threaded rod drive against the moulded gear.
The Azimuth motor, attached to the rocker with an aluminium angle. A flywheel is made of 2" diameter washers bolted to the threaded rod. The rod is attached to the motor drive with a short piece of plastic tubing. This will probably need upgrading.
This shows one of the Altitude bearings. It is bolted to the plate and in theory can be removed to return the scope to the Teflon bearing seen just above. there is only one plate so there is a risk that the bearing support will bend slightly. I think that the distortion will not change as the scope moves and in any case I didn't have room to fit a second plate on the inside of the rocker.
This shows the altitude drive. The JBWeld worm can be seen round the edge (not very even) with the motor and plastic rod drive below. The rod is held against the worm by a bent piece of steel strip. In the photo this is held with double sided tape but now it is screwed on. The tension on the spring is quite critical, too light and the rod comes out of the worm and too much and the friction increases. the motor has now acquired a flywheel, I think it helps but the drive speed is restricted by the azimuth drive.
I set the step size in altitude by setting the
scope tube horizontal with a level, reset the altitude to 0, then
slewed the tube so that it was vertical using a level and noting
the altitude shown on the scope display. The stet size can be
adjusted using the formula:
New Step Size = measured slew angle * Old Step Size / Slew Angle from Scope
A domestic quality level seems to be able to measure the angle to about 0.05 degrees, try the level round both ways to check for errors and repeat the experiment a number of times to check for repeatability.
For Azimuth I attached a laser pointer to the rocker box, made a mark on a piece of paper taped to the wall about 12 ft away, rotated the scope through 360 degrees so the laser pointer spot was over the same mark and recorded the azimuth as shown by Scope. the step size can then be adjusted in the same way as for the altitude.
I have got as far as doing a three star init and finding that the scope position is within less than a degree of reality. The main problem is remembering to set a different star for each init. I am thinking of a way of causing scope to check for valid positions and prompting for the start position if necessary.
The main problem is avoiding stalling the motors or having the gears slip. There are two reasons, the control cables trail over the ground and can restrict the scope in azimuth and the tension on the rod retaining springs is critical. I will investigate a hollow azimuth shaft that I can thread the cable through, a better way of setting the spring tension and lubricating the threaded rod.