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Constuction of fork mount and drive
Hi, Rod.
Here's some more detail on the fork and drive for the 400mm reflector, and a basic diagam
which you may be able to scale for your scope.
Constuction of the fork was done in 20mm thick plywood, in butt joined box section. Once all sections were cut out I glued and screwed the fork together, making sure to pre drill holes for the screws which are 90mm apart.
Although I did some basic plans, to work off, I constructed a 3D card board mock up of the fork to make sure all the clearances were right. As you may notice this is very compact mount, to keep a low centre of grvaity, so clearances are tight. Better to make it out of card board first than make mistake you can't undo.
When complete make sure you undercoat and top coat thoroughly.
Of course the base is made from metal, I used 50 x50 x6mm square tube for most of it. It needs to be rigid to stop twisting. The R.A bearing as described before is a trailer stub axle and bearings. This can take a 1/2 ton of weight, more than you probably need. But I find it's better to over engineer in this department.
The drive system is 240v A/C synchronise motor, Crouzet 3.5w with a One revolution per hour gearbox. I purchased this from ITC products Dandenong Victoria. It's a little lengthy to describe the construction, but the image I've attached will show you.
One last thing, because of the compact design, the R.A. wheel is quite small so this makes it a challenge to get drive, the R.A.wheel has stiff dimpled rubber on it. The drive motor shaft is grooved along it's length, when the two are meshed together with the use of a compression spring it drives well. The trick is to set the compression on the spring so you get drive, but not enough to break the gearbox if you accidently bump it.
It should be noted, this drive is for visual work on the 400mm, because of the torque created by a larger longer scope along the R.A. axis. I have the same setup on my 200mm observatory scope, and it works well enough for CCD imaging. This is because of shorter length, less weight which means less torque.
Hope this covers most things.
Cheers,
Justin.
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