Robotic Paramount MyT Polar Alignment Update

This is an all-in-one combined post about the Robotic Polar Alignment project I initially published on June 6, 2022.   All the pieces are working perfectly, and excellent PA is achieved in just a few steps.

Disclaimer: This is a DIY project that carries no guarantees of any kind, and is published as a personal article.  Anyone working with electricity, heat, hand and power tools, must exercise proper precautions and safety measures.  The author is not responsible for any damages or injuries.  Use common sense, and do not undertake actions, for which you do not have proper skills, or are not sure how to perform - seek help, practice and learn first.  Do not alter anything you cannot afford to lose.

Sources of Parts and Tools: Author gets no compensation from any online store, outlet, vendor or merchant, all product references are purely informational. Do your homework, check reviews, and please share if you find something better, cheaper, higher quality.  I provide links to sources wherever possible, but things get out of stock often, so it may be necessary to source them elsewhere.  The good news is most of the parts are widely available, and are interchangeable with minimal modifications.

Now to the actual project:

Project Description

The Paramount MyT from Software Bisque is an excellent mount, with which I enjoy astroimaging deep space objects.  However, its polar alignment routine involves manually adjusting several hard to reach knobs, while observing the result of these adjustments on a laptop screen - hardly a fun task in the field. With encouragement from friends and family, I have undertaken a project to make this process remotely controlled and automated.  Having considered a few options, I have come up with the initial set of criteria for this design:

• No modifications to the Paramount MyT itself
• Use standard, easily available components and tools at reasonable cost
• Open source software with standard settings and options
• Portability/Mobility for fast set up and dismantle while traveling to dark sites (around here anything without four street lights above your head qualifies as a dark site)
• Sturdy design that survives humidity, cold, transportation vibrations
• 12V power, USB connection, and easily available drivers for all popular OSs (I use Windows)

New Worm Gear Azimuth Drive

This is the latest azimuth robotic polar alignment drive that I have redesigned, this approach should work on a fairly wide variety of systems.  Since I use the Paramount MyT, I added the capability to rotate the entire mount about the pier, as follows:

• Added a PTFE (Teflon) washer cut with scissors out of a 1 millimeter thick PTFE Sheet I bought here - this makes the pier plate rotate fairly freely, while maintaining sufficient stability.  I tried a thrust bearing before that, and it was too thick, and would probably not work without modifying the plate, which I did not want to do.
• Designed the worm model in Fusion 360, illustrated below, and modeled the motion of the gears.  I used this technique to create the worm gear for this inexpensive worm.  I have since bought a McMaster-Carr worm that was listed in Fusion 360, but this one is working really well already.  
• 3D printed worm gear and brackets, all files are on Thingiverse.
• Drilled the 5mm hole through the worm, and installed onto a 5mm x 200mm steel rod.  
• Inserted the rod on one side into a 5mm x 16mm x 5mm bearing then into this pillow block.  The pillow block is attached to the "bearing bracket" 3d print, which goes on top of the existing hole and uses the existing MyT bolt.
• The other end of the rod is inserted into a 5mm to 8mm coupling, and then coupled to the stepper motor.  I wound up using a 14:1 geared stepper from Newegg, as it takes quite a bit of torque to turn my fully loaded imaging train setup.
• Note on the motor bracket: while the 3D printed version worked, it flexed quite a bit, so I wound up machining it by hand from a piece of 6061 aluminum cut from this angle.  This bracket takes most of the force that moves the mount, so it has to be sturdy and rigid.  The bearing bracket is fine 3D printed, although I wound up printing it in PA-CF just in case.

Video of the Az drive animation in Fusion 360:

Altitude Drive with Universal Joint

• Universal joint 8 mm to 8 mm
• Two clothesline hooks, I got mine at Lowe's
• Four wing nuts to fit on the hooks
• Small steel plate to mount the hooks and motor bracket
• 3/16" screwdriver bit to drive the MyT Altitude screw

I added a steel geared motor bracket, to help rest the motor on the MyT, and attached the small steel plate to the bracket, in order to attach the hooks.

Mounting on the MyT 

Mounting the Alt drive: 

Undo the lower wing nuts and hook onto the pin on each end.  Insert the 3/16" bit into the Alt adjustment handle, it will slip in with maybe just a slight turn of the handle to align.  Then tighten both wing nuts - you want the motor to be clear of the red MyT body.

Mounting the Az drive: 

The first part is to attach the worm gear.  It has six countersunk holes to align with the 8-24 screws in the MyT tripod pier, but I needed to get longer screws (1 1/4" were fine).  The gear just slips over the tripod pier's circumference.

I used a PTFE (Teflon washer, described above) to reduce friction between pier plate and pier.  The worm itself holds position nicely during imaging.

Once the worm wheel and pier plate are attached, slip the Az drive brackets over the rear holes in the MyT, and secure with the regular MyT winged bolts.  It is important to ensure that the worm gear meshes fully into the worm wheel before everything is tightened, this is accomplished by a slight rotation of the whole assembly about the pier.

When tightening, it is OK to put a little tension on the worm rod to ensure it stays in place during operation.

Control Electronics - Arduino Shield with GRBL

The cheap Clone CNC V4 shield I got with Arduino Nano has a few issues, which are fairly easily overcome.  These fixes are described here and here.  There is no need for microstepping jumpers, so just remove them.

Install Arduino IDE, download GRBL (help on GRBL installation and use Google for more), but make sure to modify cpu_map.h for the clone board's pins.

Solder your 12V connector to the Mot GND (-) and Mot VCC(+) connectors on the board.  Make sure you get polarity right.  I measured with a meter for the barrel jacks connected to my power supply to be sure.  Be safe, use alligator clamps etc., protect yourself from touching any live wires.

I recommend tuning the A4988 reference voltage to the motor you will be using.  The link btw, is from the Ardufocus project, which may lead you down the path of designing your own open source precision focuser and other explorations.  Here, we are just concerned with setting up the optimal stepper motor drive current.

The X motor cable is connected to the Az motor, and the Z to the Alt motor.  There is no need for the Y cable in this design.

Software

I wrote two Python programs to work with this PA system - one for manual control and one for fully automated centering of the APA star, following a TPoint run.  These are available at https://github.com/storcium/polarbot

You will also need Ken Sturrock's library for interacting with Paramount SkyX available at https://github.com/kenneth-sturrock/PySkyX_ks