Big Dobsonian Telescope

Up    Home

On a lark, I was trolling through Cloudy Nights Classifieds and decided I wanted to get a mirror to make an 8 or 10 inch Dob.  I ran across an OTA from a Meade Starfinder.  It was an olde enough model to have been made in Irvine, so the optics should be pretty good AND it was located in Phoenix, not too far from my house.  I drove up and purchased it as it was clean and in very good shape.

The OTA came off of an Equatorial mount, so the fist thing to do is to make some trunnions for the altitude bearings:


3D Printing TGrunnion  Completed Trunnion

3D printer to the rescue!  I printed the trunnions with PETG filament



Trunnion with inserts   Trunnion being attached

I installed Helicoil inserts for attachment strength.  Trunnion installed on the OTA.  The tube is Sonotube



Finished trunnions  Machining the bearing plate

OTA with both trunnions installed.  The next step was to design and build altitude bearings.  Some hard rubber rollers were obtained from Amazon and a 3/16" thick aluminum bracket was machined



Bearing Roller Bracket   Completed Bearing assembly

 The rollers are 1.2" wide and just under that in diameter.  The rollers have twin bearings and are mounted to the plate on 10-32 button head Allen bolts.



Bearing assembly   Bearing rollers showing the flange 

My able bodied assistant provides a test fit of the bearing assembly to the trunnion.  A thin roller flange was machined from 0.050" aluminum, this will prevent the OTA from walking from side to side.  Clearance is adjusted by using some AN washers between the flange and the bracket.



Routing the side panels  Completed side panels

The next step was to make the side panels of the base.  These were CNC routed from 0.750" MDF.  The paper thin flashing has not been removed from the side panel in the right hand picture.  The roller bearing assembly is being test fitted on the panel. A short video of the side panel being routed can be seen here: 



Roller bracket installed   Mocked up side panels

Here we see the roller assembly attached to the side panel and the flange can be seen on the right hand roller.  The side panels were clamped to angle irons and then clamped down to my fabrication table to determine the required spacing.  I estimated the spacing and then my assistant and I hoisted the OTA into place.  I was off by less than 1/4"....


The next step was to make the base board and ground board.  To cut a nice even circle I made a router compass.


Router Compass  Ground Board

A piece of Plexiglas was drilled and bolted to the bottom of my plunge router base and the radius was calculated.  A 1/8" metal dowel pin is hammered into a hole on the material and the appropriate hole is selected in the base.  From there it was a simple but very dusty process to cut the disks.  On the right is the ground board.



3D Printed Foot   Completed Ground Board

I designed a foot and printed a set of three with my 3D printer.  I left a slight flat spot on the tip of the foot for stabilization when sitting on a flat hard surface.  The balance of the shape helps when setting up on soft ground.  The feet were printed using Hatchbox PETG filament.



Base and Ground Boards  Azimuth Pivot   Pivot Bolt Fastening Detail

Shown left is the Base board and Ground Board with foot.  The Azimuth pivot is a grade 5 3/8" NF bolt 4 inches long.  The bolt was tack welded to a steel dick which was screwed to the underside of the Ground board.  It was windy, please ignore the croppy weld...



Azimuth bearing design   Installed Azimuth Bearing

 The Azimuth bearing included an integrated fixture flange and was 3D printed using Hatchbox PETG filament.  This as a snug fit into the base board and after installation a 3/8" chucking reamer was run through the bore.  The od of the stem is 0.500" and it projects about 0.080" above the top surface of the base board to help locate the spring.




Test hardware for Azimuth Pivot   Azimuth Encoder Retention Nut

I tested my spring selection using a plain 3/8" NF nut.  Satisfied that the spring I chose will work, I proceeded to make the nut that would be used to secure teh two parts as well as serve as a mounting point for the planned encoder.  A length of 0.625 Aluminum hex stock was used.  One end was drilled and tapped 3/8" NF.



Encoder Mounting Nut   Installed Encoder Mounting Hardware

The encoder end of the nut.  Hardware shown on right prior to installation.  You can see the nose of the pivot bushing sticking up about 0.150" above the base board.  This prevents the bottom of the spring waling around.



Installed Encoder Hardware   Installed Encoder

Installed Encoder/Ground Board nut, spring and washer.  Encoder has now been mounted.  Turns out these are too tall and the  Azimuth encoder will hit the bottom of the OTA.  I ordered some US Digital S2's.



Azimuth Rollers   Dob Driver Azimuth Motor

Support for the Base on the Ground Board consists of two metal drawer rollers obtained from McMaster Carr (p/n 1714A3  $2.59) and the Azimuth friction drive from the Dob Driver kit.  The McMaster rollers were a duplicate of what came in the DobDriver kit.  A notch needed to be cut from the Base Board to accommodate  the drive wheel.  1/4" button head Allen bolts were used on the side of the unit and they threaded into metal inserts screwed into the Base Board disk.  10-32 button head Allen bolts were secured using T-Nuts for the vertical fasteners. 



Azimuth Roller I      Azimuth Roller III   Dual Bearing

The drawer rollers turned out to be a poor choice as the base had to be spectacularly level to operate without stalling.  Pulling the base apart I engineered a roller using sealed ball bearings.  I CNC'd a bracket from 1 1/2" x 3/16 Aluminum angle and a bushing to mount the bearing.  The bearings are 20mm x 6mm with a bore of 8mm (608RS skate board bearings).  The design was the same height as the roller so it was just a matter of swapping the parts.  With the drawer rollers the MAXSPEED value was 5140, with these new rollers it was in excess of 10K.  When the drive is disengaged the scope is so much easier to move by hand.  After testing I decided to stack two bearings for a larger footprint.  I did this mostly because I did not want to have a single bearing start to wear a channel in the ground board.  In either case the loading is low enough that this is most likely overkill.  Plus black bearings look cool!




Azimuth Motor Over View   Test Setup for the Azimuth Drive

I need to finish cutting the one side panel.  The rest of the Dob Driver II was dug out and wired up to test the Azimuth drive.  I had previously wired the hardware and bench tested the system before settling on the use of the Dob Driver II.  Here's an Action Video of the Azimuth drive...



Azimuth Lifter   Lifter, Underside View

I decided to use a roller for the Az motor lift instead of the threaded knob with the Teflon pad.  This would allow the drive to be disengaged quickly AND have it pivot smoothly.  A smaller steel drawer roller from McMaster Carr (p/n 1714A2) was used.  I have a box with about 30 pounds of electronics hex standoffs and decided to use those as they were ready to use other than needing to have the fastener size bumped up from 6-32 to 8-32.  The two standoffs pictured are a little over 2" long.  Two holes were drilled in the Base Board and then the base was reinstalled on the Ground Board.



Lifter top side   Completed Lifter Bracket

The top of the lifter bracket poking through holes drilled in the Base Board.  A small length of aluminum angle was bolted to the top of the standoffs.  This balances the load and keeps the roller level underneath.  A piece of aluminum channel acts as an alignment guide for the push rod.



K=Lower portion of the lifter completed   Belcrank design

The lever clamp will act as the locked/free control for the azimuth drive.  When the lever is flipped over the drive wheel is lifted ever so slightly above the ground board.  A bell crank will be used to do a "right turn" in the lifting control mechanism.



Lift Cam   Lift Mechanism I

I made a test part on the 3D printer to determine the ratio of the vertical movement.  Not strong enough to lift, but an assist with a screwdriver between the base and ground board allowed me to determine I was close.  The right image shows the completed test set up.



Completed Lift Mechanism   Lift Mechanism Completed II

Left side shows the lift in the raised position.  The drive wheel is just under an 1/8th inch away from the ground board.  A magnet was added to help keep the lever in the unlocked position.  Right side shows the drive wheel lowered and ready to move the scope in azimuth.



OTA Test Fit   OTA Test Fit II

Amazingly my balance calculations were spot on.  I had to use a bag of dried beans and trust the advertised weight of the Telrad which I did not have at the time I installed the trunnions.  Yes, the Meade logo is upside down due to the OTA coming off of an equatorial mounting.  I have a plan...



Drive Wheel Spacer I   Drive Wheel Spacer II

The three loose spacer bushings were definitely going to be an issue out in the field, easily lost in the gravel and the dark.  To fit the OTA into the duffle case I got from Orion, the wheel needs to be removable.  I 3D printed a spacer to permanently attach to the drive wheel. 




Altitude Pulley Hardware I   Altitude Pulley Hardware II

The drive wheel is attached  with tri-wing knobs.  I used green Loctite to secure some short pieces of 1/4-20 threaded rod into the brass inserts molded into the phenolic knobs.  Almost like welding.  No bushings to lose in the dark and no tools required.




Azimuth Drive Installed   Azimuth Drive Installed II

The last major mechanical aspect was to install the azimuth drive.  I had previously machined the 11" belt drive pulley on my CNC Mill.  My god what a mess, I try to avoid doing wood on the machine.  I spent more time cleaning up than I did prepping and cutting the part.  I will say, it is as accurate as it could be...



A Little Humor    Sky Safari on Tablet 

A little humor.  The OTA came off an Equatorial mounting and the MEADE logo was upside down.  I made a vinyl decal with my Cricut crafting machine. 

In testing the DobDriver now that everything is fully assembled, I was disappointed to discover the Azimuth stepper would stall more often than not when moved in the Easterly direction and to a certain extent when moved in the Westerly direction.  Damn!!  This is where RTFM comes into play. 

Turns out you can train the drives and set the maximum slewing speed based on the mass of the mounting. The DobDriver is put into MAXSPEED mode and you slew the loaded axis by constantly holding the button down.  It starts off slowly and gradually increases the speed until the stepper motor stalls.  The device remembers the point at which the motor stalled and will not allow the speed to get to that point.  When you move an axis during normal observing by holding down a button the speed gets gradually faster, no speed changes are needed to configure.  

In altitude you train starting with the tube in the horizontal orientation.  The DobDriver raises the OTA while increasing the speed until stall occurs.  My balance of the OTA coupled with the roller bearings caused the stall point to occur close to vertical.  With the Azimuth, you train in both Eastern and Western directions of movement with the OTA pointing straight up. It actually sets a maximum speed just under the stall point of the stepper motors and in the case of Azimuth they can be slightly different depending on the direction.

The guy who designed this thing is one real clever SOB. Works fine now....



PICALL Programmer   PICKit2 Programmer  


I have used Sky Safari for a long time and recently bought one of their SkyFI III Wireless interface modules to use with my ETX125.  Dave Ek now has a Bluetooth version of his ubiquitous Encoder Interface and I picked one up.  The rub was it was not fully assembled or programmed.  The radio module needed to be soldered onto the board and the PIC programmed and installed. 

I dug out my trusty (and dusty as you can see) PICALL programmer only to discover it would not run under Windows 10.  I had purchased a clone of the MicroChip PICKit2 to have at home.  We use these at work to program our product's interface modules.  Getting the PICKit2 to run on Windows 10 apparently takes a magic wand and some secret decoder ring magic I do not posses.  I have a Windows XP machine here whose sole purpose in life is to run my Rimage Prism Plus CD/DVD label printer.  This machine runs the PICKit2 just fine...

I managed to get it working, but the build quality of the board from FAR Circuits left a lot to be desired.  A lot of dead bugged components and the 5v regulator got very hot with the encoders connected.

I decided to roll my own.  I use Sprint Layout 6 for circuit board design so I started there.  The radio I selected was the FeasyCom FSC-BT986, easily obtained from Amazon and not very expensive.   Working with Dave's posted encoder circuit and the documentation from FeasyCom for the radio I laid out a nice two sided board that had a 12volt input and a 5v regulator for the Encoders and 3.3v for the radio module and the low voltage version of the ubiquitous PIC 16F628A microprocessor.  I used OshPark for the board fabrication as my skills were exceeded when it came to the header for the radio module.



Beevo BT Board I   Beevo BT Board II

Here is the very nice, very purple circuit board that appeared after about 12 days.  I elected to go surface mount and used through hole only for the PIC and the 5v regulator.  I added in circuit programming (header at the top left) and soldered the PIC to the board.  I was not confident of my SMD soldering skills  to use a SOIC PIC and figured I was pushing my luck as it was with the BT radio module.

The populated board ran the first time and no changes in design were needed.  Yellow LED is master power and the blue is the radio status.  I went with the same connector (and pinout) as the Far Circuits board just to prevent rewiring the encoder cables.



EK Box Tester   Board and 'Droid

Here it is up and running with the EKBox Tester program.  You enter the encoder resolution and click Set Resolution.   Encoders are not attached in this shot, but do work when I have them plugged in.  In Sky Safari you need to select Basic Encoder System as the telescope, click the Bluetooth radio button and you are good to go. 



Test Print of Housing   Board Installed in Housing

I 3D printed a test housing using what was loaded in the printer, hence the wild pink part.  The final part was printed in clear (it's not...) PETG filament.  The two round areas are open almost all the way to the surface to act as lenses for the LEDs.  The raised portion is a recess for a PC mount tactile switch for the reset pins on the radio module.   Three supports were put in the design to hold the board at the correct level.  Two tabs were added to screw the unit down.  A few dabs of hot melt glue retains the board.  Small Anderson Powerpoles will be used for all 12vdc connections.  The finished product is shown on the right and below.  Pretty spiffy if I do say so myself.

   Module up and running   Encoder Cables

Here I have the encoder interface powered up.  Red LED is power indicator, the Blue is the status LED for the Bluetooth radio module.  Push button is the reset for the Bluetooth radio which I recessed into the top of the housing on my final version. Next step was to get encoder connections set up.  I used a RJ45 feed through connector.  Stripping back the cables quite long allowed me to poke the wires through one at a time and have room to work with needle nose pliers and no large cable jacket in the way.  After the crimp (very solid) the ends are cut away.

 Completed Encoder Cable   Altitude Encoder Socket

Here's the completed encoder cable.  Had to have fun with the Rhino label maker and the tubular heat shrink labels!  The Altitude encoder needs to be removed for transport and storage and the small pin connectors on the encoder are not something I want to cycle, so a short cord terminated with a DE9 will be used.  I laser cut and engraved some 1/8" Plexiglas sheet for the purpose



 Encoder Socket Altitude   Encoder Brackets 

Shown is the installed encoder socket.  I chose the venerable and durable DE9. Next step was to make the stabilization brackets for the encoders.  Laser cut from 1/8" Plexiglas.  Small holes are for tie wraps to secure the wiring.   A short whip with a mating DE9 will be attached to the altitude encoder.



    Altitude Encoder Attachment   Altitude Encoder Installed   Altitude Encoder Installed II

A threaded insert was installed to secure the encoder bracket.  A spacer and a thumb nut will secure the encoder and make removal easy.  Finished Altitude encoder installation on the right.




Azimuth Encoder Installation   Bubble Level Installation

The installed S2 encoder for the azimuth.  A threaded wood insert and spacer was used to secure the bracket in the same manner as was used on the Azimuth Encoder.  Bluetooth encoder module has been installed and the wiring routed and secured.  Power wiring is yet to be installed and secured.  Anderson Power Poles are used throughout for power connections.  Right side is the terminal block for the Dob Driver which I elected to retain.  I need to shorten, route and secure the motor cables (grey).  I also had some 1.25" round bubble levels and using a Forstner bit cut a shallow pocket in which to install it.  Leveling the base is not excruciating critical but close will be nice and the level adds a nice touch.      {;o)




Main Panel I   main Panel II

Getting closer....  I laser cut the main panel, this will be located to the left of the Altitude drive motor.  The DA15 connector is for the hand box of the Dob Driver II.  Labels laser etched in the panel do not show up in the picture, the left switch is power for the DobDriver II and the right is for the Bluetooth wireless encoder module.  The Powerpole connector is a convenience outlet for 12vdc.  The Powerpoles are a press fit in the laser cut opening and a couple drops of solvent weld secures them.



Main Control Panel   Power Input Panel

Main panel now installed, the next step will be to installing the Power Input panel on the back part of the rocker box.   I mounted a 5V power port above the control panel to provide charging options for the cell phone or android tablet that will be used for the Sky Safari planetarium program.  Two USB A outlets with a combined maximum output of 3 amps.  A 3 amp fuse is being used,  dual Power Poles for other accessory connections at the battery location.




Mani Panel Back Side   RE-Routed Altitude Motor Wiring 

Here I have started to clean up the wiring.  I will likely leave extra cable lengths as to disassemble the scope will require removing ends of terminals from the cables.



Finnished Wiring  Finished Wiring II

Control panel wiring completed.  Right side shows the overview of the completed wiring.



Cover Design  Cover Installed

I decided a cover to prevent shorting of the exposed terminals of the Dob Driver II system was needed.  I designed (LH image) and printed one from PTEG filament on my 3D printer.  It also neatens up the area, fewer loose looking wires.




Encoder Module Wiring   Battery Box

Final encoder wiring now that the Altitude encoder cable was shortened.  Battery Installation, upper PowerPole socket is a spare that is also wired to the input.



Dob Driver Controller Pocket I   Dob Driver Controller Pocket II

I 3D printed a pocket using PETG to hold the hand box for the DobDriver II



Wheelie Bar   Wheelie Bar Construction

When using the scope at home, I decided that a wheeled base (Wheelie Bar) made the most sense.  I found some thin wall tubing that fit nicely on the 3D printed feet.  Cross pieces were cut from 3/4 mild steel square tubing and welded into place.  Some pieces of 3/16 mild steel plate were used for the caster mounting.  The 3" urethane casters I selected lock in both wheel and caster rotation.  They seem to hold the scope without any noticeable play.



Telescope   Telescope and Proud Owner


Now that the telescope mount is ready for a pre-painting test of the drive, optics and Software it has been raining for the last 24 hours.  Don't tell any of the astronomy clubs in the Phoenix area, I don't want to be blamed...   I added a Celestron Right Angle Illuminated finder.  Scope with the proud owner....



Zinsser Primer   Painters Touch  

The next step after testing is done will be to pull it all apart and paint the MDF used for the construction.  I will fill the porous edges of the wood with body putty and then finish all surfaces with primer sealer.  I selected the Zinsser Primer Sealer.  Top coat will be Rustoleum Painter's Touch 2X Ultra Cover satin white.


The End!