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What are Celestar 8 Declination motor's two speeds?

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#1 InductorMan

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Posted 08 February 2021 - 04:33 PM

Hi Folks, I've been trying to retrofit a homemade declination motor unit onto my old C8 Basic (the base model, no PEC, no AstroMaster; Dec motor is supposed to run from a 3.5mm phone jack).

 

I finally found a motor that runs smoothly both at the slow and the fast slew rate voltages (which one accesses by either just pushing the button, or pushing the button and then also pushing the opposite direction button, respectively).

 

Does anyone know what Dec knob RPMs these two rates are supposed to produce? I did some searching already and found the following relevant threads: 

 

https://www.cloudyni...mi-motodec-rpm/

https://www.cloudyni...lination-motor/

 

However these threads seem to be about either the aftermarket motors, or the PEC model of the drive (don't know if that's supposed to be the same rotation rate as the basic Celestar's drive). 

 

Folks seem to be recommending about 10rpm on the knob. But is this supposed to be the slow/default rate, or the fast rate? The fast rate seems to be at least 4x faster than the slow rate. I guess in practice I feel like 10 rpm slow and 40 rpm fast would be OK, given how I use the hand knob, if one used pretty short button pushes: but I have a fantasy that maybe one day I'll try manually guided astrophotography, and so I don't want to end up with settings that are way too fast.

 

Don't want to make it be 10rpm and 40rpm if it's supposed to be 2.5rpm and 10rpm. Anyone with real practical experience know what the two speeds are supposed to be?



#2 InductorMan

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Posted 09 February 2021 - 11:04 AM

Oh, follow-on question. I'm confused by the Celestar 8 Basic's Dec motor behavior, maybe someone can clear up for me what's going on. Each time I reverse the motor direction, the controller automatically runs the motor at high speed for about a half of a second in the new direction before switching to low speed. It'll do about a cumulative half second no matter what. So if I was previously running in the "up" direction, and then push the "down" button several times briefly, each of these brief pushes is at the fast speed until the controller is "satisfied" that it's run at the fast speed enough.

 

Why does it do this?

 

My guess is that this is to take out lash of some sort in the intended mechanism. But that's still confusing. Even with a very high reduction ratio (making high speed 10 rpm and low speed 2.5rpm), this amount of reversal is still about a tenth of a turn of the knob. I can't imagine a geared transmission being sloppy enough to require this much lash compensation?

 

I've also seen some pictures of older C8's with a belt driven motor, and I guess maybe this amount of lash compensation is meant to work with a belt drive? But I even tried that, using a Lego drive belt, and at a reasonable rate there wasn't enough stretch/lash to take up this much lash compensation.

 

And besides that seems pointless to try and compensate belt stretch: the belt stretch is going to vary massively based on friction, and balance.

 

So what gives? Why does the Basic's little Tangent controller board run the motor like this? Am I just grossly underestimating the reduction ratio of the original drive? Or did the original drive just... suck?

 

I guess the other thing I haven't tried is to actually mount the motor for an observing session, so I also don't know if maybe the end-shake of the tangent arm screw is enough to take up this much rotation; it probably is, actually. But in practice it feels like scope imbalance usually preloads the tangent arm in one direction or another, I haven't ever really noticed lash in visual observing use.

 

Any insight anyone can give into the Celestar 8 Basic's original Dec motor (or whatever it was they sold aftermarket that would be screwed into those two holes, since the scope didn't sell with a Dec motor) would be appreciated!


Edited by InductorMan, 09 February 2021 - 11:07 AM.


#3 Jaimo!

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Posted 13 February 2021 - 12:05 AM

Moved from mounts.



#4 Foundationer

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Posted 14 February 2021 - 08:14 AM

I don't think there is a correct answer to your question.

I have a Celestar 8 that I equipped with the Celestron motodec.

It behaves exactly as you described and is used for guiding not really slewing.

Some models of the motodec were sold with a pot on the end so you could

adjust the speed a little bit. Not really sure what the benefit would be since

all you really do is release the dec lock, aim, relock and guide. So if you are

going to diy your own, make it the speed you like! If it's really fast you are 

going to run out of tangent arm quickly. Good luck, great scope, love mine!

On a side note, if you can find an AAM and a set of encoders, it turns this

scope into a super accurate, super fast push towaytogo.gif

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#5 InductorMan

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Posted 17 February 2021 - 09:39 PM

Thanks for the reply Foundationer! Did you find that the motodec has any of the sort of "way overcompensating for gear lash" thing that I was worried about, when you reverse direction?

 

Basically I was seeing that with the provisional (Lego!) gearbox that I was using, you can't reverse the direction of slow/guiding-speed travel without it jerking.

 

I set up my ratios so that the speeds pretty much match the RA axis, and when I do that, the tangent screw sorta has a "minimum quarter turn" it'll go at the fast speed, whenever I reverse directions. You can definitely see it in the eyepiece distinctly at 170x, so I feel like this wouldn't be good for guiding astrophotography. Does the original do that?

 

Also, is it using a belt drive internally, or direct gearing? or what?

 

I have retrofitted encoders onto mine, and I love it! It's amazing. I am still a total amateur (scope used to belong to my dad, but I didn't use it frequently until recently). But I've seen five times as many planetary nebula in one night with those encoders than I'd seen before in my life! well... I've seen five in a night, and before I'd seen one! lol.gif

 

But yeah, 100% agree. Encoders+SkySafari are amazing.


Edited by InductorMan, 17 February 2021 - 09:39 PM.


#6 Foundationer

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Posted 18 February 2021 - 12:42 PM

Thanks for the reply Foundationer! Did you find that the motodec has any of the sort of "way overcompensating for gear lash" thing that I was worried about, when you reverse direction?

 

Yes, mine is not that dramatic, just under .5 seconds I would guess.

 

I set up my ratios so that the speeds pretty much match the RA axis, and when I do that, the tangent screw sorta has a "minimum quarter turn" it'll go at the fast speed, whenever I reverse directions. You can definitely see it in the eyepiece distinctly at 170x, so I feel like this wouldn't be good for guiding astrophotography. Does the original do that?

 

Yes, but again, it's not that dramatic. My dec knob only turns maybe 1/16th of a rotation before it settles in.

At normal speed it takes 1 minute 20 seconds to do a single shaft rotation.

 

Also, is it using a belt drive internally, or direct gearing? or what?

 

It is a self contained gearmotor. The motor speed can be adjusted slightly by a potentiometer.

The motor inside is a  2 RPM Hankscraft Motor. You can get Hankscraft motors all over the 

web, but the actual mounting and mating hardware might be a different story.

 

A mounting collar screws to the fork arm. A small metal cylinder attaches to the dec shaft.

A female version with an o-ring inside fits over the small cylinder and the body fits around

the mounting collar, just push it on and hand tighten the mounting screw.

 

I'm guessing the speed changes when reversing are a function of the electronics in the scope

to do exactly what you are assuming, address backlash. So maybe play with your ratios a bit?

Good luck! Thanks for making me learn more about my own scope!

 

 

 

 

 

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#7 InductorMan

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Posted 19 February 2021 - 01:26 AM

All right, excellent info! Thanks again!




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