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Modifying a Losmandy 492 Digital Drive Controller

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#1 TheBigEye

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Posted 30 September 2012 - 10:54 AM

Hello - I'm working on an experimental project to convert an older Schaefer mount to use Hurst stepper motors, identical to those found on the original Losmandy G11 mount. I have pair of those motors and they work fine. I just need to reprogram the 492 controller so that the base sidereal rate runs at 2X. Any info/advice would be appreciated! - Chuck

#2 jrcrilly

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Posted 30 September 2012 - 11:02 AM

The prom from a 492 rigged for a GM-8 or a CI-700 should work; those have a 180 tooth worm wheel.

#3 Lynnblac

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Posted 01 October 2012 - 12:13 AM

My Schaefer mount required just the opposite. My 180 tooth worm wheel needed a 8 min worm cycle. The 492 drive box ran a 4 min (G11) cycle. So I used 2 to 1 spur gear reduction at the worm. You will need spur gears to adapt the stepper to the Schaefer in any case.

#4 TheBigEye

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Posted 01 October 2012 - 08:53 PM

Hi Lynn: Thanks for your comment. I'd like to see a pic of your modified Schaefer! I was hoping for an "electronic" solution to my problem, rather than having to modify the gear train. The Losmandy 492 for a G11 runs at 50% of the rate needed to drive my Schaefer mount at the correct RA rate. If I hit the 2X button, it tracks perfectly. Of course, the G11 has a 360-tooth RA gear vs. the 180-tooth version on the GM8, which should actually require a slower rate. Hmmm. You may be right! I would like to see your gear reduction modification.

Thanks!

- Chuck

#5 srosenfraz

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Posted 01 October 2012 - 10:22 PM

Hi Chuck -

I just happened today to install a set of Hurst stepper motors to my Schaefer mount (good timing!). Scott Losmandy had one of these motors that ran at 1/2 RPM when used with the 492 controller, so he sold me the set along with the 492 Digital drive. I'm not sure where he found the motor, but you might want to see if you can hunt one down that runs similarly. I'm guessing the motor has different gearing internally. If you need info on the model number or anything else, let me know.

Now, I ended up with a bit of a problem. Everything works fine except...... the RA motor is running in the wrong direction!

My mount has always had a single motor. When I'm standing in front of the mount (north side) looking at the mount, the motor is on the right side. OOC, which side of the mount is your motor on?

#6 Lynnblac

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Posted 02 October 2012 - 12:01 AM

Chuck, For pictures Click on on my username and look at my forum posts 'schaefer'. Lynn

#7 srosenfraz

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Posted 02 October 2012 - 12:38 PM

Follow up to my posting - I spoke with Scott Losmandy this morning. There's a switch inside the controller to reverse the direction of the motor. So, it looks like I should be able to have my "conversion" running correctly tonight!

#8 srosenfraz

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Posted 06 October 2012 - 06:01 PM

Another followup - I was able to use the new stepper motors with the Losmandy 492 controller. Everything worked like a charm. It sure is easier to center objects with up to 16x slew rates, and my guiding seemed to be much more consistent than I had with the asynch motors.

#9 TheBigEye

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Posted 07 October 2012 - 12:54 PM

Hi Scott: I sent you an email this morning. Just got to your forum posts today - sorry for the delay. There are some really great upsides to doing a stepper motor conversion using the Losmandy 492. For one thing, it's virtually a "bolt on" motor change. And, apparently Losmandy is still supporting the 492 controller, although they are now using a different stepper motor (SAIA). I don't know anything about the new motors, but the Hurst synchronous and stepper motors have a long track record of reliable performance. Obviously, the multiple tracking rates offered by the Losmandy 492 drive system really provides a step up from the old AC synchronous motors' lilmited range of drive speeds. And, if you should ever have a motor failure, you don't need to be an electronics bench expert to change one out.

The only potential downside I see to the stepper motor conversion relates to "motor start delay." This would onlly be an issue for astrophotography, and then only if you're making a correction for declination drift. The Hurst AC synchronous motor start up is almost instantaneous. But, the stepper motor can take up to a full second to start, after you push the button. Two solutions: PERFECT polar alignment (no dec correction needed) or having a very high power guiding magnification that picks up a dec tracking error before it shows up on the CCD. I would be interested in your observations on that.

#10 srosenfraz

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Posted 11 October 2012 - 06:48 PM

Hi Chuck - I've only been able to use it 2 nights so far, but both nights I had very little trouble. I'm using an autoguider and Phd, and it seems to have compensated for the delay fairly well. The only minor issue I'm seeing right now is that the RA motor is a 1/2 RPM, but the dec motor is one of the original GM11 1/4 RPM motors. So, when I go to calibrate the autoguider, the dec motor takes a long time to calibrate (probably an extra minute or so). I'll probably end up replacing that motor with another 1/2 RPM motor, so that should eliminate that problem.

It also means that the slew rates in RA are much faster than they are in Dec. So, that taking a little bit of getting used to. Nonetheless, it sure beats centering objects at guide rates!






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