I have a 2 year old GM-8 and was out Saturday night as the temps got close to freezing. I think the grease in the mount got close to freezing too. The mount started getting real quiet -- not silent, but quiet -- and the 15-second exposure stars started looking a little like comets. It's a 492 system. I have the encoders with the Nexus system and it kept dropping out and saying the telescope was not responding. I thought it was a cold Nexus or iPad, but now I think it was its way of saying the telescope wasn't moving or at least keeping up. Is that expected from the factory grease? The axes were for sure stiffer when I was taking everything down than it was when I put it together earlier in the evening. Not hard, but more firm.


Thanks!


Bob

Bob,

If your mount is only two years old the gearbox lube is probably fine because the gears and lube are all sealed up in a nice closed clean environment that operates at a very low load and speed. The gearboxes can add very little drag to the stepper motors and there is no good way to re-lube them without drilling out the plastic or probably in your case, steel rivets to disassemble them and re-lube. You can check the gearboxes once disassembled by turning the gearbox output shaft by hand, there is always going to be some drag and probably a spot where it wants to slow down a little bit but that additional resistance is unimportant as you are working the gearbox backwards and the stepper does not see this resistance. Remember that the gear box is a 25:1 ratio so as long as there is no gritty feeling while turning you are good to go.


One other thing, I'm assuming your GM8 came equipped with the Losmandy HP worms installed but that you do not have a one piece worm block installed on the RA axis. The reason I say this is that either of the OPW designs improves or resolves at least four of the five drivetrain design issues which significantly affect the tracking error numbers of Losmandy G11 and GM8 mount drivetrains.


#1 Worm bearing alignment
#2 Worm bearing preload
#3 Worm mesh adjustability
#4 Alignment of gearbox output shaft axis to worm gear axis
#5 Gear mesh adjustment


Hope this helps.


Chip

Dooh! I knew that!?

But this just makes my comments even more relevant as the integrated stepper/gearbox is even better sealed than the servomotor gearboxes. Additionally with the higher gear reduction at 150:1 this reduces the changes of the gearbox being an issue even more given the slower speed. These are not stressed and should last almost forever barring severe environmental or physical damage.?

There is one thing Bob did not mention (or I missed) was the power source Bob used when the problem was displayed. If Bob was not using a power supply plugged into utility power and Bob used a conventional lead-acid battery that was not at full voltage air temperature may have had an impact on the #492 drive system.?

As much as I enjoyed using my Losmandy G11 and GM8 in push-to mode with their #492 drives even if well balanced I did not feel there was much headroom left ! in the digital drive system. Used visually the #492 never missed a beat but obviously the my leaky brain is less picky than a camera collecting data.?

Thoughts?

Chip

?

??

Dooh! I knew that!


But this just makes my comments even more relevant as the integrated stepper/gearbox is even better sealed than the servomotor gearboxes. Additionally with the higher gear reduction at 150:1 this reduces the changes of the gearbox being an issue even more given the slower speed. These are not stressed and should last almost forever barring severe environmental or physical damage.


There is one thing Bob did not mention (or I missed) was the power source Bob used when the problem was displayed. If Bob was not using a power supply plugged into utility power and Bob used a conventional lead-acid battery that was not at full voltage air temperature may have had an impact on the #492 drive system.


As much as I enjoyed using my Losmandy G11 and GM8 in push-to mode with their #492 drives even if well balanced I did not feel there was much headroom left in the digital drive system. Used visually the #492 never missed a beat but obviously the my leaky brain is less picky than a camera collecting data.


Thoughts?


Chip

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The motors (SAIA or Hurst) operate on 12V – the regulator is just there for the digital stuff. The Darlington transistor array (the 18 pin DIP at the top of the PCB) is connected directly to the 12V line, before the regulator.

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The electronics of the 492 are pretty simple (although if they re-engineered it with modern parts it would only have two chips). So unless the unit is really old (mine has the 2006 sticker on it) I wouldn’t worry about that. In a few more decades maybe you’ll have to worry about capacitors.

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The grease in the motors should be good to much colder temperatures than you’ll ever see in CA – I’ve used my 492 system in Northern Illinois for years and we get down to -20F from time to time. That is -29C. Then the operator is the problem, not the mount.

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And at half-stepping I have never seen a problem even with lunar imaging at 2800mm+ focal length and 2 micron pixels.

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The Hurst (now NiDec) motor specs say -10 to +40C (approximate) and don’t forget that the motors generate their own heat, although only ? watt per the label. Not a lot of power but it is in a confined space. Starting from a cold soak is another matter but, again, I’ve never seen a problem in practice. Hurst, after all, is in Northern Indiana, not California or Shanghai. We’re not sunbathers around here.

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So I wouldn’t go messing with the motors until after everything else (mesh of gears, bearing and axel grease, connectors, batteries) were checked out. Opening up the cases is a bad idea to my mind. If I did that I’d be ready to buy new ones in case I screwed it up.

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As to batteries, well, the Lithium composite cited is not recommended for storage below freezing. Operating (where it generates its own heat) they are rated down to -4C (not as good as the motors for what that is worth – you never know how much engineering margin is being applied) and will lose 20 to 40 percent of the capacity as a function of on the discharge history at -20C depending on whose research you read.

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So I’m sticking with lead acid for now, personally.

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But I’d look at the mesh and mount grease plus connectors (ugh, who decided modular phone jacks were a good idea for field equipment?)? and ?cables, then the batteries. The motors? The last thing I’d look at. Could be, of course.

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Happy Holidays,

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Mark Christensen