Robotic Paramount MyT Polar Alignment

This is an obsolete post, the new version is here

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.  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 nothing from any online stores, outlets, vendors or merchants, product references are purely informational. Do your homework, check reviews, and please share if you find something better, cheaper, higher quality.  Rather than provide links, I often include descriptions, so you can search for your preferred sources for these parts.

Now to the actual project:

The main idea is to control the polar axis motion using common components and software used in CNC machines or 3D printers.  This can be adapted to use on almost any mount.

The Paramount MyT from Software Bisque is an excellent mount, with which I enjoy learning precision astrophotography, but that enjoyment does not go as far as contortion gymnastics involved in reaching MyT's polar alignment knobs while taking pictures of APA star, or adjusting the Polemaster Polaris position.  With encouragement from friends and family, I have undertaken a project to make this process remotely controlled.  Having considered a few designs, I have come up with the initial set of criteria for this design:

• No modifications to the Paramount itself, outside of normal Software Bisque options
• 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 Windows (but possible to use on Linux and other platforms)

Under my imaging train load (one standard Bisque counterweight to balance a Versa plate with scope, focuser, camera, guider), the standard NEMA 17 stepper motors used in CNC and 3D printers, are too weak.  I did not want to use NEMA 23 motors, so wound up ordering three 14:1 geared stepper motor sets from Newegg.  This worked very well for my loads (up to 1.5 Newton-meter), but higher ratios may be required for heavier gear.

The MyT has two screws for azimuth adjustment, which must be turned together to move the mount, one screw goes clockwise and the other counterclockwise.  For example, to move the mount to the left, you must turn the right screw counterclockwise, and the left screw clockwise.  Refer to the mount's user guide for details.  Each screw has a socket, reachable by a "deep" screwdriver bit.  I wound up just cutting an IKEA disposable hex wrench into two 2-inch straight pieces, which work perfectly fine in a 5mm coupling hole (more on couplings below).

Altitude adjustment is done by one screw, which has a 3/16" hex socket, easily reachable by a standard screwdriver bit.  To navigate around the mount's relief, I used a flexible drill socket extension.  Interestingly, the cheapest extension performed the best, probably due to its flexibility.  

Parts List:

• Three 14:1 geared stepper motor sets from Newegg
• CNC Shield V4+ with Nano 3.0 A4988 Stepper Motor Driver Board for Arduino.  The one I got is no longer listed as available, but search for a similar one, or use any CNC/3D printing solution that can drive stepper motors.  See below for important information about the cloned Arduino CNC shields and program options.
• Three A4988 driver boards, if not included with the shield above.  Make sure to use the heatsinks.
• Arduino Nano, or a clone, note that it must be Atmega 328 compatible (so the Nano Every will not work), in order to be compatible with GRBL
• Cables to connect the motors to the Arduino shield.  I had some laying around 5 pin (motor side) to 4 pin (PCB side), standard 4 pin to 5 pin ones used in 3D printers.
• Project box to house the shield, I used this one: LeMotech ABS Plastic Junction Box Dustproof Waterproof IP65 Electrical Enclosure Box Universal Project Enclosure Grey with PC Transparent Clear Cover 3.3 x 3.2 x 2.2 inch (83 x 81 x 56 mm)
• Brackets for the stepper motor.  Note that these are the geared motors, where the mounting holes are closer to the center than the non-geared ones.  I could not find the brackets with the geared holes for fast enough delivery, so I got three Alloy Steel 42mm Stepper Motor Mounting Bracket Fixed Base Fixing Seat for Nema 17 and drilled holes in them to fit the gearbox screw holes.
• 2PCS Linear Rails 400mm Linear Guide Rails with 4PCS SBR12UU Slide Blocks, CNC Rail Kit, Linear Rails and Bearings Kit for Automated Machines and Equipment.  These are used in DIY CNC and 3D printers, and are easily found online, they look like this:

• Couplers for stepper motors to connect to the hex bits.  Use the ones appropriate for your bits.  The stepper gearboxes are 8mm, also use 8mm for1/4" hex bits (I used the 1/4" drive 3/16" bit for the altitude axis), and the bits I cut form the IKEA wrench for the azimuth screws fit into 5mm holes.  So I used (one) 8mm to 8mm Bore Rigid Coupling Set Screw L25XD14 Aluminum Alloy,Shaft Coupler and (two) 8mm to 5mm Bore Rigid Coupling Set Screw L25XD14 Aluminum Alloy,Shaft Couplers.  There is a whole lot of choices of these on Amazon and eBay, and other sites.  I started with flexible couplers, but ended up using rigid ones for more precise movement.
• A spring to pull together the slide blocks on the linear rail.  I went to Lowe's and just got the one that looked right, for less than $1.  The goal is to keep the slide blocks engaged in the hex sockets of the two azimuth screws, like so:

• Brackets to mount the linear rail to MyT.  I got two black steel brackets at Lowe's and drilled holes in them to match the holes on MyT, and I'll be making new ones from a piece of aluminum for looks and sturdiness.  The Lowes' brackets work fine for now.  It is important to assemble motors on sliding blocks, and fit to the MyT screws to make sure they line up, then mark and drill the holes.  
• 5mm screws to mount the motor brackets to sliding blocks.  I had to slightly enlarge the holes on my brackets with a drill, for the screws through them.
• A box of screws and washers is very helpful.  I got some M3, M4, M5 metric screws and nuts, and some fractional ones as well.  These are available in hardware stores, Amazon etc.
• 12V feed for the Arduino shield to feed motors.  I soldered a 2.1mm barrel jack to the connectors on the shield, as shown below, but any 12V source will work, make sure that it can supply enough power to drive the motors you are using
• USB for PC or your hub for Arduino to computer connection

Altitude Adjustment Assembly

I have redone the Altitude assembly to be much simpler and sturdier:

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

Motor Assemblies

For the RA axis motors, I first marked and drilled the holes for the motor gearbox mounting screws in the motor steel brackets.  Next, I enlarged the holes on the other side of the bracket to be able to attach them to slide blocks.  

First, attach to slide block, then attach the motor (otherwise you won't be able to reach the slide block screws easily).

Attach the motor side of shaft coupling with two included hex set screws.  Make sure there is enough space on the other end of the coupling to insert and secure the hex bit, which will drive the MyT polar alignment screws.  I first inserted both, then tightened set screws.

The 8mm to 8mm coupling is connected to both the motor shaft and the flexible drill bit extension.

Mounting the Slide Rail

The slide rail needs to be mounted to two of the four screws, which attach the MyT to its pier plate or base (rear of the mount).  For the first iteration of this project, I bought some right angle steel brackets from Lowes and drilled a few holes in them to properly position them.  Your main goal is to make sure the hex bits connected to motors align perfectly with the holes in Az axis screws.  I had to measure and drill a few times to obtain a perfect fit.  You will want to smooth edges of the holes with a larger bit, or a countersink bit, to avoid scratches on the MyT parts.

Mount the slide rail to the MyT and test the motors on slide blocks to fit the hex bits into MyT's screws.  Adjust as needed.  Once the perfect fit is attained, secure the brackets, and engage the motors' hex bits fully.  It may be necessary to rotate MyT's knobs in and out for the hex bits to seat themselves.  Once seated, tighten the knobs with reasonable force to the axis lever (like you would when you are done with polar alignment).  

Arduino Shield

The cheap Keys Studio 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 (the full step is running at 14:1 ratio by the gearbox), 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 preferred 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.

Download and install a Gcode program, such as UGS.  

Connect the Nano USB cable to the computer and note its serial port via e.g. Device Manager on Windows.  Connect to this port via UGS:

Try to initiate motor motion on each of your 3 motors by sending some steps via X, Y, and Z axes.  For this test, do not have the motors engaged in MyT knobs yet - we are just making sure they can turn in each direction.

Once you are happy with the motor controls, disconnect the Arduino, turn off the power to the motors, and engage the motors' hex bits into the MyT knobs.  Reconnect all cables and UGS.

For my setup, I connected the X motor to the left Az side, Y motor to the right Az side, and the Z motor to the Alt knob screw.  With such setup, Altitude is controlled via the Z axis, and Az is controlled either by the bottom right diagonal arrow (X+ and Y-) or the top left diagonal arrow (Y+ and X-).  This will maintain tightness of the screws against the adjustment lever.  Do not use the other diagonal arrows.  You can experiment with other types of movement (X only or Y only), but I found the two diagonals to be the easiest option.

Calibration

You can calibrate UGS to treat MyT's "ticks" as mm for scale, this is accomplished in UGS setup wizard, or firmware settings (in the Machine menu).  This makes it easier to follow the SkyX polar alignment recommendations, which are given in ticks.

Testing

The first full test should be inside, with the mount's payload exactly as you will have it in the field, all equipment and counterweights attached.  Try moving the axes in each direction, and observe the motion.

After this is completed, I recommend packaging your controller shield and connectors into a project box, such the one I listed, or any other sturdy enclosure.  Find a way to attach this enclosure to the mount.  I am currently using stick-on magnets, you may want tot try this or Velcro, glue, nuts and bolts etc.  Do not let your controller dangle on wires in the field, that is not fun.  Test in driveway, back yard, other safe and well lit area to make sure all comes together and works well.

Dark Site Use

Once the MyT base is assembled (pier/tripod, plate, mount, equipment etc.), attached the motors as before, and connect to computer.  Run all your usual setup, which for me is a rough polar alignment, focus, filter, exposure, and a TPoint model.  I do use a Polemaster with these motors initially, which is unnecessary, but it makes APA a rather small adjustment later on.

Finally, operate your motors to achieve the precise polar alignment.  You can always turn the knobs by hand, even with the motors attached.  The motors may remain engaged after alignment is completed, or you can disengage them and remove, or just disconnect power, or keep everything connected - whatever suits your needs best.

Assembled Components