Going for a Drive is "simpler" then you thought

( The concept that is! )

The 40 Pounder Project is now equipped with a "visual assist drive system" mainly for viewing planets.

Driving an Alt,Az scope for planetary observing can be accomplished with very low technology and simplicity so before I start I would like to list what you will not find on this particular drive unit.

1. No equatorial platform

2. No computer

3. No stepper motors

4. No large power supplies

5. No setup time

At first the movement in two axis at two different rates of speed that are continuously changing sounded like a job for a computer but looking closer I soon realized that it would be possible to have periods of time where the two drive motors could run at a constant rate giving me a chance to do some high magnification planetary observing. Without any further thought and on a whim I set out to drive the scope first in Azimuth figuring it would be the easiest of the initial experiment. Having the left over 16" by .080" cut out disk from the secondary cage gave me a head start, I soon had it strapped to a jig and was introducing it to a 1/2" x 20 tpi spiral tap turning at 400 rpm ,within 10 minutes I had made a 1020 tooth gear (2) on my little bench lathe. Using a piece of 1/2" stainless steel fine thread ready rod(3) for a worm and a 1 rpm 3v DC gear reduction motor (Hankscraft hobbie motor) to power it I was ready for the first trial run. Waiting for Mars to reach the Meridian I could feel my anxiety building would this low tech idea work! Well for 30 minutes looking due South I observed Mars at 500 power with almost perfect tracking I new at that point the plan was a go ahead, it was time to build the Altitude drive.

The decision for this drive was simple, the basic tangent arm but with one little trick which at the time I did not fully realize. As the scope climbs in altitude the arm would be slowing down as it extends to meet the ready rod so therefore it would work to my advantage instead of reducing the motors speed during this time it would be happening automatically. The arm is based on the same design you have seen for years on amateur scopes, a piece of ready rod, DC motor and a lock up clutch assembly the only change I made was the quick release mount (5) from the altitude bearing and the spring loaded split nut set back adjuster (6). At this point I completed the drive box (4) consisting of 8 AAA (12v, dc) batteries for the power source, 2 LM317 variable voltage regulators and 4 slew function buttons, operating time 45 hrs. Because the two motors are tracking from 1.2v to 6v depending where you are in the sky I have left over voltage to do a direct 12v slew function or a total power kill to slew in the other direction, no drive, objects have to drift the opposite direction, therefore it's a type of slew especially at high power. It turns out to work beautifully when centering planets after the direct drive clutches are induced. I should mention there is a safety factor to look out for in this simple design you see I manually lock up both drives when I am viewing therefore if someone were to come along and give the scope a good shove I am afraid you would have to say bye to one or two of the gear assemblies. So far there has been no problem but I am looking at safety clutch designs.

Final result is 15 to 20 minutes of tracking without touching any speed controls, after that I slew back to the center of the field deciding which motor to adjust, make the change and I'm good for another time period. One of the nice things about this setup is that you maintain the rigidity of your alt,az mount in other words you are not sitting up an equatorial platform that from my experience always takes away from the mechanical performance (vibrations during focus) of the original scope.

I would like to say even though my drive system is well machined and nice to look at the same principals can be applied to any dob.

Scope in deep space
Close up, Azimuth drive
Cose up, Altitude drive

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