help me in my DIY EQ-GOTO Split ring mount.

[nightcrawler]

I'm trying to a design a GOTO-Split ring mount. I've seen two types of these, in first type the OTA is mounted on forks between horseshoe and the polar bearing, (horse-shoe mount), the second is the OTA mounted directly on the ring (split ring mount). Of these two which is better?? I thought the latter, since there is no additional bending induced on the horse-shoe ( or Split ring). 

This is going to be my first DIY build, so any help is very much appreciated. I'm living in india at 10 deg north. I guess this would be an advantage (or disadvantage...i donno) for a split ring mount, since the angle required for polar inclination is less and nearly parallel to the ground. 

Im thinking of using a 0.2 deg Encoder servo motor for RA tracking by driving the split-ring using a friction drive. The gear ratios are coming out very good for tracking, but problem arises during slewing which needs to be at least 50 times faster than tracking. How do you guys solve this problem? using a reasonable speed motor and slowing it using PWM (pulse width modulation) during tracking and running the motor at its rated speed during Slewing? or using a Stepper motor by micro-stepping? Or by using a variable speed gear-box or anything of that type??

This is my first post, so please bear any mistakes. Am i being a little too ambitious for my first DIY-Project or is it ok??  I have a 10 inch Newtonian that im going to use.

[DragosN]

Hello Nightcrawler.

In my opinion, the second solution, mounting the scope exactly on the ring is the way to go. First, is more safe, you would not mind about flexure in the horse-shoe. And if you need to reenforce some parts, you only need to reenforce the ring, not every part. Second, you scope is not very big, and the center of gravity is not very low, close to primary mirror position. So you need more space between the connection point, and the base of the mount (RA axis).

About the other question, I'm not a specialist in engines, so, I stop here.

[Ajohn]

I take it you mean this type of split ring mount?

http://www.saao.ac.za/~wpk/scope/scope.html

John

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[DragosN]

Nightcrawler, thinking again, especially that you live at 10 deg North, I think that if you put the scope exactly un the split ring, as I said before, this ring would have to be very big, to allow a complet move to zenith. So, a more economical solution is to put the Dec axis on the horse-shoe. Anyway, you must see first where is the center of gravity of your telescope with everything you can put on it. Eyepiece, barlow, finder, camera, filter wheel, guider, everything, and design the mount after you make this measurements. 

[Ajohn]

That is one of the reasons I posted a link. It's not a bad implementation of it but notice how thin the lower part of the ring is. That  part will twist in use unless more bearings are added to the front side rather than just at the rear. Or maybe the ring could be made a lot more substantial - maybe fabricate it from 2 spaced discs. There is a commercial one that costs a lot and uses a circular bent tube but I would wonder just how accurate the bend is especially as it forms the drive. You could probably find pictures of this type of mount used in observatory instruments - the ring is massive

The other problem is the name - this arrangement is usually called the horse shoe mount but not here. The split ring of the other sort is sometimes called a horse shoe mount as well.

http://www.dalekeller.net/ATM/newtonians/keller10/keller10b.htm

That one is vice free but and English mount is far easier to make - more or less the same thing but rather than having a ring on one end it just has a bearing.

So really it would be best to post some links to what you have been looking at.

Friction drive is likely to have problems with fast slewing. One way round that may be to attach a length of toothed belt to it to convert to a geared drive. The bearings would then need to be gears as well.

Faster slewing with steppers is usually obtained by a combination of micro stepping and the speed that the motors are switched at. You would need to look at the changes in torque characteristics when micro stepping is used. You might say 1/2 step every sec for tracking and full step as fast as the motors can manage for slewing. The output from the steppers still need gearing down to obtain torque and drive accuracy.

 DC motors are switch mode (pulse) driven at some fraction of their max speed for tracking and driven continuously for max slew rate. The encoder is usually on the motor and the drive from it is reduced via gearing so that the encoder's pulse outputs come often enough to obtain a steady speed. In the past the encoders have been driven by the actual drive output but via  gears again to up the speed. People have even used optical encoders out of mice. Direct shaft encoding mounts are available commercially but they use encoders that output fantastic pulse rates per turn and are also very accurate.  In the case of your 0.2 degree servo motors gearing would be needed on the output to make that accurate in drive terms, so for say 1 arc second it would be 60^2 * 0.2 to 1.

There is another style of mount that I feel could be used  more - an example here, fork mounting

http://strock.pi.r2.3.14159.free.fr/Ast/Art/Ouranos.html

Same problem - massive parts but that could be overcome by fabricating them. It doesn't show any details of the main bearing. It will be a steel bar with a point on the end. That rests in a hole in a metal plate and the other end will be supported with 2 metal squares with a V in them, the V's making contact with the bar. Replace the plate with a hole with a plate holding a spherical  bearing and it would be fairly easy to add adjustment to it.

John

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[Ajohn]

May as well add another aspect to home made mounts. Sometimes aluminium is a better option than wood. It can be routed in the same fashion as wood even using the same cutters or milling cutters such as end mills and slot drills.   Some what lighter cuts  are needed though but it can be done. Really the speeds are too fast for milling cutters but if all else fails...........

John

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