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Building a new mount looking at drive systems

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#1 scadvice

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Posted 23 February 2018 - 12:48 AM

Hi CloudyNighter’s

I’ve been out of observational and astrophotography for 38 years  and want to get back into it. I’m rebuilding my old telescope which unfortunately years back went through a bad divorce at the demise of  the single arm fork mount I built. It was in Ski and Telescope around 1980 or so. All I have is the old tube and mirror assembly with a 12.5” full  thickness F5 mirror I ground. The mirror is in the process of being cleaned and it looks like I may need to be re-polished. If I determined it does, I most likely will regrind it to a F4. or 4.5.

 

I’ve decided I’m going to build a new GE mount and I picked up with some nice (series one size) Byer’s rough castings and new drive gears. I done a design on an old CAD SW I have and it’s just about finished. After I do a functional review of that design I will start the machining process most likely the first week of March in my home machine shop.

 

My problem is the electronics as I’m near 40 years out of date. I’m currently thinking of a Sidereal  Technology  system and talked to Mel and I believe Dan was in the background as I asked questions. I was wondering if anyone is using their Dual Servo Telescope Controller II system? I like to get some feedback from you and others. Also, what other comparable systems are out there? Likely I'll stick with the SiTech but want to update myself on the available systems.

 

Thks


Edited by scadvice, 23 February 2018 - 12:49 AM.


#2 555aaa

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Posted 23 February 2018 - 02:12 PM

The SiTech system with servomotors is pretty well known. I have a system that uses commercial servo drives and you can pm me for more info. There is a home brew project called onstep that uses stepper motors also. Bisque makes a system with hardware that I think is called TCS.
I think one of the first questions though is how big is the mount and OTA, how big of a worm wheel, what is the slowest slew speed you are ok with. FYI on my system I run 24V servos but it can run anything including brushless servomotors.

#3 scadvice

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Posted 24 February 2018 - 01:13 AM

Hi Viking,

 

Have you ever seen a series 58 Byers mount? This one will be the big brother to that one I think a series 1 mount, about 1 ½ to 2 inches bigger on the castings. I believe Byers made the castings for the amateur market, also the the gear and worm.  But as I understand, later decided to go with the larger series II so he never finish these.

 

What I’m building will carry and guide solidly around 100lbs. The Polar shaft will be cone shaped and have five inch bearing forward and a one and nine sixteenths on the aft end. The dec axis which is actually a series 1.5 casting (so I was told) it's shaft will be slightly cone shaped with a two and three quarters inch bearing forward and a one and three eights bearing down on the aft side.  I will be guiding no more than 60 lbs with the main 12.5" reflector, a piggybacked Celestron C5, and the rest of the gear.

 

I have not done the math but I'm comfortable with a slew rate of 6 degrees/sec, however I'm more interested the resolution obtainable.

 

BTW this will be the third mount I’ve made so I’m not completely new at it.  Just out of date on where computer controlling them is at and trying to fast track myself up to speed…

 

I’ll look into the onstep and the Bisque. Thanks


Edited by scadvice, 24 February 2018 - 06:14 PM.


#4 scadvice

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Posted 26 February 2018 - 03:24 PM

Correction: I'm comfortable with a slew rate of 4 to 5 degrees/sec....



#5 555aaa

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Posted 26 February 2018 - 09:34 PM

This thread might be useful:

 

https://www.cloudyni...-speed-control/

 

You might ask to re-post this over in the ATM forum; there are more DIY types over there then on the mount forum.


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#6 JoshH

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Posted 26 February 2018 - 10:14 PM

I use SiTech on my MI-250, a friend uses it on his home made mount, both serve one controllers, we love them and the software. 



#7 Chucke

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Posted 28 February 2018 - 12:05 PM

I use a Servo 2 on my big mount with a 15" Byers drive (see avatar).  Works great.  I've had it for several years.  It is one of the first Servo 2 units sold since I pre-ordered it when it was first announced.  The only issue I've had is there may be something wrong with the PEC sensor circuit.  I'm not really sure and haven't yet been motivated to dig into it to determine whether or not it is user error.  The software is great and Dan along with others on the user group offer first rate service and advice.


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#8 scadvice

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Posted 01 March 2018 - 07:45 PM

Thanks Ranger and all of you for your continued comments. Viking, I did put it on the ATM site.

I did some calculations on the tick's and slew rates available with my worm/gears, Sidereal Tech's servo's, ect and with direct drive to the worm, that came to 5,313,057 ticks which equals just a hair over 14 deg/sec slew rate possible on direct drive! Too fast… not likely going to do that!  So, at Sidereal Tech's suggestion I’m going to do a two to one further reduction to put me at 10,626,114 ticks. When I  bench test the mount I'll start the slew at around 3 deg/sec and adjust from there. (FWIW -- 4 deg/sec = 180 deg in about 50 sec with accel and decel factor and 35 sec at 6 deg/sec.)


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#9 555aaa

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Posted 01 March 2018 - 09:07 PM

I think that will be okay. In my experience, the issue is more with providing good tracking at low speeds. If you are at about 10.6 million motor tics in 360 degrees of RA rotation, that means during tracking you are at about 123 tics per second, and assuming this is a 500cpr encoder (2,000 quadrature tics per rev) that is 3.7 rpm at the motor. So it is pretty demanding of the servo controller to provide precise control down to that speed but also run up to max speed for slewing. In the Bisque TCS design they use a 10:1 motor reduction and 100 oz-in servomotors (at the gearbox output) on the small drive. So with a 360 tooth worm and a further 2:1 on belts that is 7200:1 or a motor speed of 5 rpm during guiding. They use 48VDC.  The SciTech DC brushed motors are roughly 10:1 but aren't super high torque, about 20 oz-in at the shaft I think. They appear to also support 24VDC and you might check if that is an option.


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#10 scadvice

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Posted 01 March 2018 - 11:09 PM

I am considering running at 24v and your close on the torque  I believe it it's around 25 oz-in at the shaft at 19v. I think the whole system may run on 24v but I have to check the manual to be sure.



#11 DuncanM

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Posted 02 March 2018 - 01:21 AM

Personally, I'd prefer a stepper system for it's lower cost. IIRC, there's a fellow selling an inexpensive microstepper controller that it Eqmod compatible.



#12 555aaa

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Posted 05 March 2018 - 02:04 PM

One of the nice things about servos is that you can measure current to determine and monitor balance. My mount doesn't have clutches so that's how I balance.

#13 scadvice

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Posted 05 March 2018 - 08:56 PM

One of the nice things about servos is that you can measure current to determine and monitor balance. My mount doesn't have clutches so that's how I balance.

That's interesting, I never would have thought of it but it makes sense. Do you have any references I could read on the subject?  One of the issues in designing a mount is how to easily release the clutches that is reliable for balancing without taking a panel or cover off to loosen the springs.




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