24 f3.7 Encased GoTo Dobson

 

GoTo - Testdrive

 

Andr Heijkoop
Strijen, NL
http://www.astrosurf.com/aheijkoop/

 

Contents

Preface. 3

The azimuth drive. 5

The drawing board. 7

Bill of Material 7

How it is done. 8

Preassembly of the servomotor. 8

Grinding the groundboard. 8

Assembly of the roller bearings on the groundboard. 11

Gluing the toothed belt around the groundboard. 12

Assembly of the pulley and servomotor. 14

The power supply. 17

Testing the azimuth drive. 18

The altitude drive. 19

The drawing board. 19

In height adjustable roller bearings. 19

View inside the mirrorbox (topview). 19

Drive wheel inside the mirrorbox (side view). 20

View inside the mirrorbox (front view). 21

Bill of Material 23

How it is done. 23

Altitude roller bearing. 23

Measuring the place for the drive wheel inside the altitude bearing sector 25

Drilling the chamber for the drive wheel 28

Measuring the place for the drive wheel on the mirrorbox. 30

Installing the gear train. 31

Installing the worm gear reductor and servomotor. 34

Finished. 34

The drive wheel as seen from the outside. 34

Ready to Roll 35

Credits. 37

 

 

Preface

I always fancied the big Dobsons and the marvelous images they show me through the eyepiece.
I was sold when I had my first look in 2008 through a 80cm Dobson at a French starparty
the RAP. I had to climb a wobbly ladder, but the views are still printed in my memories.

IMG_6516

At that time I had a nice 14 f5.7 truss Dobson on a homemade equatorial platform:
14" f5.7 on a equatorial platform.htm

IMG_6568

In 2010 I started building my own 24 f3.7 truss Dobson. Two years later this was the result:

24 inch f3.7

If you like to know how it was build, please have a visit at this website:
24" f3.7 truss Dobson

I soon realized a GoTo for this big Dobson was necessary. Not because I couldnt find the objects by hand but to maximize the little time I have under dark and pristine nights. I saw numerous Dobsons equipped with a fully operational GoTo system build in. They all showed me the same, protruding servomotors, electric wires, encoders, large 12V batteries.
And yes they all work as advertised. But I didnt want my footprint of the 24 Dobson any larger, and the GoTo system had to be almost invisible as you looked at the Dobson.

I had to do better

The azimuth drive

I started with the less complicated azimuth drive. I had the idea to use toothed belts and pulleys as they are easy to install.
In designs of others the timing belt is loose around the groundboard and friction between the belt and groundboard is needed to pull the rockerbox around.

http://siderealtechnology.com/16InchNightSky/16InchUnderside.jpg

The problems with this design are:

         As you can see in the picture above the belt has to be supported by the white blocks , otherwise the belt drops to the floor.

         The servomotor is attached on the outside of the rockerbox and makes the overall footprint of the Dobson larger.

         You have a chance of slippage. Slippage is not a problem if you install encoders on the axis, but I dont want extra encoders, just the encoders on the servomotors must be enough.

In my design the timing belt is glued around the circumference of the groundboard and together with a pulley acts as a large gearbox.
The problems with this design are:

         The circumference of the groundboard has to be as perfect as possible.

         It is best the pivot point in the groundboard has no slack.

 

The drawing board

Dimensions in mm

 

Bill of Material

1.       Timing belt 2535-T5-10, length 2535mm, pitch 5mm, 507 teeth, material polyurethane

2.       Pulley 30-T5-10F, pitch 5mm, 30 teeth, material aluminum

3.       Worm Gear Speed Reducer, type A 2B 7-H480, 1 to 48 reduction

4.       Spindle, diameter 16mm, length 114mm, material RVS 304

5.       Servomotor, brand Pittman, type GM8224D309-R1

6.       Shaft coupling, material aluminum

7.       Needle bearings, type NK 5/12 TN, inside diameter 5mm, outside diameter 10mm, width 12mm

8.       Dowel pins 5 m6, length 24mm

9.       Heavy duty hose clamp

 

How it is done

Preassembly of the servomotor

04 - Speed reducer - Motor koppeling

Pictured above the preassembled servomotor and 30 teeth pulley. The worm speed reducer in between has a 1 to 48 reduction.
To install the preassembled unit in the rockerbox I used two 10mm squared messing rods.

Grinding the groundboard

In my design the groundboard has to be as round as possible also the pivot bolt has to be without slack. Im glad I installed the Astrosystems pivot bolt when I made the 24 Dobson. This pivot bolt is free of slack. To get a round groundboard I grinded the circumference with a Bosch PVS 300 AE sander. The sander is attached to the rockerbox with a screw clamp, and by turning the groundboard around you can get a perfect round groundboard easily.

05 - Rond schuren Gondplaat

It took me three sanding belts before all uneven spots were gone and the diameter was big enough for an endless timing belt with a pitch of 5mm.

08 - Finished, drie schuurbandjes verder en 5 mm van de omtrek

Lessons learned:

         When I measured the groundboard it was already a bit too small in diameter. I realized when I stopped grinding at that moment the belt should be a bit loose around the groundboard. I didnt see it as a problem. Later I did regret my decision and wished I took the time to grind the groundboard to a better suited diameter (one tooth less).

         Grind first and order a suited belt later. I did the other way round and had to order again because the first had the wrong circumference.

Assembly of the roller bearings on the groundboard

To turn a Dobson by hand it is common to use Teflon on Formica for the azimuth bearing. To turn the Dobson with a small servomotor the torque needed would be too much. Instead of Teflon, roller bearings are preferred.
I used six small needle bearings with an inside diameter of 5mm. I attached these bearings on 5mm dowel pins. In the groundboard I made 16mm chambers for the bearings with a hand drill. With a router I made the little grooves for the ends of the dowel pins.

09 - Monteren van de rollagers

 

Gluing the toothed belt around the groundboard

The diameter of the groundboard was a little too small, and as a result the toothed belt was a little loose around the groundboard. To compensate I used polyurethane glue to attach the timing belt around the groundboard. Polyurethane glue fills the gaps and I hoped that the glue evenly expanded around the circumference of the groundboard.

13 - Aanbrengen van de tandriem

The end result is not perfect, but still good enough.

15 - Aanbrengen van de tandriem

Assembly of the pulley and servomotor

Before drilling the holes I had to make sure I had the good position for the pulley.

18 - Plaatsbepaling van pulley

20 - M6 inserts

Above you can see the M6 inserts in the bottom of the rockerbox for connecting the servomotor and pulley. In the big hole in between you can see the toothed belt around the groundboard.

23 - Motorsteun

In the picture above you can see the end result of the azimuth drive.
The gearbox is bolted to the rockerbox with little RVS M6 bolts. The servomotor is attached to the rockerbox with a heavy duty hose clamp.

The power supply

In my goal to have as little cables around the telescope as possible I had to find a solution for a suitable power supply. The SiTech servo system runs on 12 to 24 volt DC. The servomotors are rated 19 volt DC. Im not exactly an electrician by profession and asked others about their opinions.
After searching the web for a power supply that could deliver 18 volt, Henk came up with the idea to use a 18V Li-ion battery as the power source for the servo system. I already had a cordless Li-ion Makita Flashlight laying around which could be adapted as a holder for the battery. With a handsaw I removed the battery holder from the flashlight and installed the battery holder on the rockerbox.
But a question arose, could the servo system run a complete night on one 18V 4Ah Li-ion battery?
I didnt took the risk and placed two battery holders in parallel. When the servos are running on one battery, I have a second battery on the battery charger. When the first battery runs low on juice, I place the full battery in the second holder and remove the (nearly) empty battery without losing power to the servo system.

35 - Dubbele Makita Li-ion batterij houder
And yes Im a sucker for symmetry!

Testing the azimuth drive

With a working power supply I could test the azimuth drive.
The first test was not too bad, the tracking speed was OK, but the movement was a bit jerky. After a lot of searching and time I found the problems. Some of the needle bearings pushed against the sides of the chambers in the groundboard. I resolved this problem by cutting 2x6 pieces sleeves from old Festo 8mm PE tubing and placed them on the ends of the dowel pins.
Also a gain setting in the SiTech software was wrong.
After these small changes the jerky movement was gone. The azimuth drive draws 1.5 Amps, which is not too bad.

The altitude drive

After the positive test with the azimuth drive I was confident enough to start with the much more complicated altitude drive.
I didnt want a servomotor hanging on the outside of the rockerbox. I wanted to try to mount the servomotor inside the mirrorbox with the drive wheels running over the radius of the rockerbox, something that was done never before.

 

The drawing board

In height adjustable roller bearings

 

 

View inside the mirrorbox (topview)

Dimensions in mm

 

 

Drive wheel inside the mirrorbox (side view)

Dimensions in mm

 

 

View inside the mirrorbox (front view)

Dimensions in mm

 

Bill of Material

1.       4x Roller bearings, Abec 9, outside diameter 20mm, inside diameter 8mm, width 7mm

2.       4x Aluminum U-profile, 20x20x3mm, length 70mm

3.       4x Aluminum bar, 25x20mm, length 30mm

4.       2x Timing belt 610-T5-10, length 610mm, pitch 5mm, 122 teeth, material polyurethane

5.       2x Timing belt 360-T5 10, length 360mm, pitch 5mm, 72 teeth, material polyurethane

6.       2x Pulley 30-T5-10F, pitch 5mm, 30 teeth, material aluminum

7.       2x Pulley 60-T5-10, pitch 5mm, 30 teeth, material aluminum

8.       2x Pulley 20-T5-10F, pitch 5mm, 30 teeth, material aluminum

9.       Worm Gear Speed Reducer, type A 2B 7-H400, 1 to 40 reduction

10.   2x Spindle, diameter 16mm, length 81mm, material RVS 304

11.   Spindle, diameter 12mm, length 700mm, material RVS 304

12.   4x Igubal flange bearing, EFSM-16

13.   2x Igubal flange bearing, EFSM-12

14.   Servomotor, brand Pittman, type GM8224D309-R1

15.   Shaft coupling, material aluminum

16.   Heavy duty hose clamp

 

How it is done

Altitude roller bearing

To turn a Dobson by hand it is common to use Teflon on Formica for the altitude bearing. To turn the Dobson with a small servomotor the torque needed would be too much. Instead of Teflon, roller bearings are preferred.
I had a hard time figuring out where to put these bearings, and how to attach them on the rockerbox.
Luckily I made the wooden altitude bearing sectors a bit bigger than the usual 180 degrees, which gave me the opportunity to mount the roller bearings at the exact end of the sector radius.
I used four Abec 9 roller bearings with an outside diameter of 20mm and a width of 7mm. I used aluminum U-profile and aluminum bar to make them adjustable in height.
The picture below explains it better:

#10 - Altitude roller bearing

I used messing M6 inserts and RVS M6 bolts for fixing the bearing support to the rockerbox.

Measuring the place for the drive wheel inside the altitude bearing sector

In my design the place for both drive wheels had to be exact. And to make it even worse, I wanted the drive wheels inside the wooden altitude sector, so they would not be visible if you look from outside.

#22 - Aftekenen plaats drivewheel

Just barely visible in the picture above, the small line and point where a little centerhole will be drilled for the drive wheel.

#23 - Centergaatje voor de Forstnerboor ø 50 mm

Drilling a small centerhole for the drive wheel. The little hole will be barely visible and is the marker for the drilled chamber for the hidden drive wheel.

 

Drilling the chamber for the drive wheel

To drill the chamber for the drive wheel in the altitude bearing sectors I had to buy a special forstner drill bit with a diameter of 50mm.

#26 - Blindgat met de Forstnerboor ø 50 mm

 

#28 - Altitude bearing met ingebouwd drivewheel

In the picture above you can see how the drive wheel is placed in the altitude bearing sector.

 

Measuring the place for the drive wheel on the mirrorbox

#29 - Altitude bearing met ingebouwd drivewheel

In the picture above you can see the contours of the altitude bearing sector on the mirrorbox. I had to adjust the balance point of the Dobson for the added weight of the gear train and servomotor inside the mirrorbox. In the meantime I glued the timing belt inside the radius of the rockerbox.
The drive wheel is placed on the timing belt to drill a hole in the mirrorbox for the spindle.

 

Installing the gear train

#30 - Altitude bearing met ingebouwd drivewheel

The hole is drilled for the spindle and the flange bearing is attached to the inside of the mirrorbox with four messing M6 inserts and RVS M6 bolts.

 

#32 - Tandriem overbrenging

Assembly of the rest of the gear train.
I had to make wooden supports for the flange bearings.

 

#34 - Een blik in de spiegelbox

Installing the worm gear reductor and servomotor

#35 - De servomotor ingebouwd

Above a view at the gear train from behind. The worm gear reductor and the servomotor are installed in the mirrorbox and ready for a test in this mirrorless and trussless setup. In order to get a reasonable balance the secondary case is directly bolted to the mirrorbox.

 

Finished

The drive wheel as seen from the outside

 

#33 - Drivewheel inside the altitude bearing

If you look closely you can see the drive wheel between the altitude bearing sector and the radius of the rockerbox.

 

Ready to Roll

 

GoTo - Testdrive

I had my first test on march 7, 2015 with the 24 f3.7 Dobson with the Encased GoTo. The electronics are barely visible and for the rest the Dobson looks like it is still hand driven. No motors and encoders on the outside of the Dobson, just one wire for a connection with the ArgoNavis unit or for a connection to the PC.
The test went better than expected. No corrections had to be made to the altitude drive.
The tracking at 180x was smooth. I had Jupiter in the eyepiece for an extended amount of time.
I used a 2x Powermate with a Denk II binoviewer, a 2x Powermate with a turret and four orthos, a ParaCorr with a Pentax XW14. All combos could easily be exchanged without any balance problems.
The servomotor for the altitude motion runs at about 0.7 Amps without any noticeable extra heat, which was a concern when I started this project.

Credits

A project of this size is not possible with the help of others.
I especially would like to thank the following persons:

         Cyrille Thieullet, Astromist

         Henk Prein

         Gary Myers, StellarCAT

And I got a lot of help from the many persons at these forums:

         The SiTechservo group at Yahoo! Groups

         The Dutch Astroforum