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Fullerscope MKIV mount Goto conversion?

 

Hello everyone, 

So this is my first post here. I am David. I've been practicing armature astronomy for about 10 years now, but have looked up at the stars my whole life. I use slow refractors. My pride and joy is a Skylight 102mm F15 telescope. 

I've recently purchases a mount that will do my Skylight 102mm F15 proud. A Fullerscope MKIV mount. It comes with Beacon Hill 6 1/2" worm gears, and matched worm drives. However I wish to add a goto system to this mount. I was recommended to look at the On Step system. I read the page. It impressed me. The software seems very good. I'd go as far to say even the best in class! 

Importantly this will be my first goto system, I have no previous experience of GoTo. 

For those that don't know, this mount is very heavy cast aluminium mount. The mount was design to be matched with a 6" F15 refractor or 16" Newtonian or Cassegrain. I'm told with a 6" F15 these mounts are rock steady. It will need high torque stepper motors.

This is where I need help. How do I decide what stepper motors I need to go with the mount? And how do I make sure on choosing the stepper motors that it will work with the On Step hardware and software?

Howard Dutton
 

On Fri, May 8, 2020 at 04:01 AM, david boyt wrote:
This is where I need help. How do I decide what stepper motors I need to go with the mount? And how do I make sure on choosing the stepper motors that it will work with the On Step hardware and software?
Hopefully there are good quality bearings inside and it moves smoothly with little force, the worm/wheels are smallish looking for such a massive mount.

How many teeth do those wheel gears have?

 

Hi Howard,

Good to hear from you. This mount was made over 40 years ago. I don't think it uses modern bearings. I can try and find the information out for you, as to what bearings it uses.

The worm wheels are 6 1/2 " in size. They are 360:1 I believe. This coming week the mount will actually arrive. I will then count the teeth on the worm gears.

Fullerscopes back in the day used to use synchronous motors that were rated at 8 Lb/in or 0.9 Nm. 

David

Dave Schwartz
 

The most recommended stepper motor https://www.omc-stepperonline.com/nema-17-stepper-motor/nema-17-bipolar-0-9deg-36ncm-51oz-in-0-9a-5-4v-42x42x40mm-4-wires.html?mfp=149-step-angle%5B0.9%5D with a 3:1 belt reduction (exactly what I use on my EQ-5) will match that spec.

On 2020-05-08 11:06 a.m., david boyt via groups.io wrote:
Hi Howard,

Good to hear from you. This mount was made over 40 years ago. I don't think it uses modern bearings. I can try and find the information out for you, as to what bearings it uses.

The worm wheels are 6 1/2 " in size. They are 360:1 I believe. This coming week the mount will actually arrive. I will then count the teeth on the worm gears.

Fullerscopes back in the day used to use synchronous motors that were rated at 8 Lb/in or 0.9 Nm.

David

Drew 🔭📷🚴‍♂️
 

At initial glance this mount looks like a great candidate for OnStep conversion. Firstly, there is no such thing as "modern bearings". Bearings have been of the same specifications for a 100 years at least. The real question is: is it bearing or bushings? Both are quite capable of producing a good mount. When you receive the mount you will need to disassemble both axis. Bearings may need to be replaced due to corrosion, etc. Bushings and bearings both will need a good cleaning and regreasing. Here modern lubes such as "Superlube" come into play.

If the torque specs are right finding the right stepper motor and pulley combination should be easy. I myself would look at the nema17 400step/rev 2amp motors (StepperOnline #17HM19-2004S1) using the higher current rated drivers such as the tmc2209. Using 60/15 tooth pulleys will give you a 4:1 initial reduction. The result will be a drive capable of astrophotography. The torque generated should be able to handle more than the mount mechanicals can.

The real obstacle will be interfacing the motor to the mount. From the looks of it you could use a stock stepper motor bracket (StepperOnline #ST-M1) with minimal machining required.

This old mount looks like it has great "bones" for a OnStep conversion. I am quite jealous.

 

Hi Dave,

That's good to know about the 3:1 reduction, as I know that between 3:1 and 2:1 is usually used on this mount. If I were to use 3:1 what would the final reduction be? 974?

David

 

Hi Drew,

Well when I said modern bearings what I really meant is I don't think this mount uses ball bearings or roller bearings anywhere. It uses bushes I think. This is a picture of a Fullerscopes MKIV mount, that has been taken apart for restoration. It is not my mount.

Drew 🔭📷🚴‍♂️
 

Hard to tell from that picture. Doesn't appear to be a complete disassembly. I would lean towards it being bushings. There is nothing wrong with bushings. Look at a turbocharger. 100k+ rpm's on bushings. What will matter is their precision and condition.

Drew 🔭📷🚴‍♂️
 

Also take a good look the drive I suggested. You have the room for it. For very little if any extra money you will gain more torque and tighter tracking. The commonly used motor and pulleys are for the Vixen GP type mounts (EQ6.CG5, LXD75, etc). They work for these mounts because they provide enough torque and fit into the space available. You have a more open design mount, use it.

Dave Schwartz
 

If you used 3:1 belt reduction (I use 16/48 tooth pulleys but you probably have the space for 20/60 tooth) you would have 360*3=1080 mechanical reduction if the gears are 360 tooth. With a 400-step motor tracking at 32 microsteps it gives 38,400 steps per degree and a tracking resolution of 0.19 arcsec which is well below the minimum for imaging. You can go for the 1.68A 400-step motor... no problem having more power than you need... but then the recommended driver would be the TMC5160 which will run nice and cool even at the recommended ~40% of the 1.68A (=0.672A). Plus the TMC5160 256 microstep interpolation will give you super smooth tracking which is bound to be useful on a long FL scope (although 1500mm is not especially long).

On 2020-05-08 11:52 a.m., david boyt via groups.io wrote:
Hi Dave,

That's good to know about the 3:1 reduction, as I know that between 3:1 and 2:1 is usually used on this mount. If I were to use 3:1 what would the final reduction be? 974?

David

Howard Dutton
 

On Fri, May 8, 2020 at 10:19 AM, Dave Schwartz wrote:
You can go for the 1.68A 400-step motor... no problem having more power than you need
Overall reduction is on the higher side vs. typical so these 1.7A higher current/lower inductance are a good fit.

I would try NEMA17 (size, ~60 oz-in holding torque usually) first... if they are up to the job w/ an 3:1 or 4:1 secondary reduction great, if not switch to NEMA23.  These motors are not terribly expensive.

I'd also get a set of S109 stepper drivers to run things with at first.  They are cheap and easy to use and can run either the 1.7A NEMA17 as well as some NEMA23 models.  Once you settle on a design you can refine from there.

A little trial and error is nothing to be afraid of.

 

Hi,

Thanks for the replies guys. So I was hoping to use the Onstep universal mount controller version 3, how does that alter things with the above motors? I was intending to use the android app, for my phone or blue tooth to connect it to either my laptop or phone. So that I could use Sky Safari 6 pro. 

I'm worried a 4:1 reduction would be to slow for slewing? I'm going to look at all motor suggestions and power levels. I know a NEMA23 comes in 1.5 Nm and 1.8 degrees? So I think 200 steps?

David

Howard Dutton
 
Edited

On Fri, May 8, 2020 at 12:13 PM, david boyt wrote:
I'm worried a 4:1 reduction would be to slow for slewing?
That depends on your expectations and how much force is needed to move the mount around, there are unknowns here.

To give you an idea though... A 1.5A NEMA17 400 step motor I looked up makes 20 oz-in pull-out at 600 RPM (2.5 deg/s speed for the proposed 4:1 design.)

That's 20*1080 = 28800 oz-in = 150 lb-ft [note: this was for a 3:1 not 4:1, but the end result isn't too different.]  Which is a lot... but worm-wheel losses can be pretty brutal and that thing I bet doesn't move effortlessly on sleeve bearings when loaded up with a 16" 'scope. And you might do well with slower slew speeds (say 1 or 1.5 deg/s) since the worm/wheels look small and delicate in relation to the mount and mass it will carry.

I'm going to look at all motor suggestions and power levels. I know a NEMA23 comes in 1.5 Nm and 1.8 degrees? So I think 200 steps?
The NEMA23 size comes in 200, 400, and even 800 step.  In any case don't pick just any stepper motor, run the design by us first.

Howard Dutton
 

On Fri, May 8, 2020 at 12:13 PM, david boyt wrote:
So I was hoping to use the Onstep universal mount controller version 3, how does that alter things with the above motors?
I have no idea what that is.

 

Hi Howard,

The Onstep universal mount controller version 3 can be seen here OnStep Goto Kit


I was hoping to use a motor that suits that mount controller and my mount.

David


On Friday, 8 May 2020, 20:59:50 BST, Howard Dutton <hjd1964@...> wrote:


On Fri, May 8, 2020 at 12:13 PM, david boyt wrote:
So I was hoping to use the Onstep universal mount controller version 3, how does that alter things with the above motors?
I have no idea what that is.

Howard Dutton
 

Oh, an Instein controller gotcha.

Doesn't he use TMC2130 drivers?  They are not suitable for NEMA23 frame motors.

 

Hi Howard,

The information in the page said " Main MCU used is a Robotdyn Mega 2560 Pro Embed, this board, unlike standard Arduino Mega2560 boards, incorporates a dedicated quartz crystal oscillator for MCU, this feature ensures precise tracking at temperature changes. Additionally a external mode switch has been added to enable/disable MCU bootloader. The built-in wifi module is a ESP-07S, an ESP8266 based wifi module with better functionality; ipex antenna connector and electromagnetic shield.TMC2130/5160 Stepper drivers for smooth and silent movement up to 256 real microstepping, to save MCU workload, during tracking, the drivers are configured to high microstepping and during slew are configured to low microstepping, process is automatic. The drivers are in thermal contact with the upper case for optimal heat dissipation. "

David



On Friday, 8 May 2020, 22:40:23 BST, Howard Dutton <hjd1964@...> wrote:


Oh, an Instein controller gotcha.

Doesn't he use TMC2130 drivers?  They are not suitable for NEMA23 frame motors.

Khalid Baheyeldin
 

On Fri, May 8, 2020 at 11:43 AM, Drew 🔭📷🚴‍♂️ wrote:
The result will be a drive capable of astrophotography.
The mount should be capable of astrophotography after converting to OnStep.
However, that F/15 refractor is definitely not suitable for deep sky astrophotography (galaxies, nebulae).
Planets and star clusters should not be an issue though.

Howard Dutton
 

On Fri, May 8, 2020 at 03:15 PM, david boyt wrote:
The information in the page said
The TMC5160 is capable of running most NEMA23 motors.

I don't really follow Instein's controller features sounds like it will be ok though.

 

Hi Howard,

So to go back to an earlier post, if the worm gear has 360 teeth and is indeed 360:1. If I were to use 3:1 then it would give me a slew speed of 1.6 deg second. With 4:1 reduction is 1.25 degree second?

The reason I was thinking to use NEMA23 motors is due to the higher torque values, but if I can get away with using a NEMA17 then good!

If this helps for a base line. There is a company that states for a direct drive from the motor to the worm, with a worm gear of 359:1 ( the original Fullerscopes gear, which mine are not, mine are hopefully 360:1 from what I've been told ), that a motor that produces 1.5 Nm is needed for a Fullerscopes MKIV mount ( my mount ). 

I understand if I use 2:1 reduction then I only need a motor that produces 0.75 Nm, and for 3:1 a motor that produces 0.5 Nm. 

I'm not set on the Instein controller, its just a controller I am aware of. If you know of another product which is better? Or you build them. Then I am all ears.

David