Hypatia - a Crayford style focuser for the Hadley Telescope

I am proud to present to you “Hypatia”, my take on a focuser for the Hadley Telescope ecosystem. It combines what I consider to be the best features in other designs, and some ideas of my own:

• the Crayford style of focuser - which uses rollers and is driven by a pinion - provides super smooth action and allows for quick and precise focusing
• a printed drawtube allows for fitting a collet, which holds your eyepieces very securely and lets you easily swap them; 
• the drawtube has built-in endstops so it cannot fall out
• all files were designed with 3D printing in mind - supports should not be necessary

Instructions

Print settings

The models were designed to be printed in this orientation:

The threads on the drawtube and the nut are designed to be easily printable, a layer height of 0.2 - 0.3 mm works well. The main focuser needs to handle some force, so I printed it with 4 perimeters. I used a 0.6mm nozzle for all models. 

Print the drawtube and the collet in black, for best optical performance.

Heat set inserts

For the heat set inserts to be securely installed, they need to have enough plastic around them. I have included modifier files for the main focuser body and the knobs, so you can easily set the infill to 100% where the heat set inserts will be inserted. They should automatically be in the right place (it works for me in Prusa Slicer).

Assembly

After printing

Install the heat set inserts in the knobs and the focuser body. Good alignment for the inserts for the bearings is important; it can help to insert a bolt through the hole from the bottom end, screw the heat set insert partially on, and then install it using your soldering iron. This way the insert will be aligned correctly.

If necessary, clean up the print. Make sure the bushings fit in the holes and can move smoothly, make sure the screws for the bearings fit through the hole.

Drawtube and collet

To get a perfect motion and grip with the collet, sand the inside of the drawtube and the outside of the collet lightly with some sandpaper. Clean the dust, then coat the outside of the collet with some candle wax, I lit a candle and used a q-tip to apply it. You can apply some to the threads on the drawtube too. The fit was nice before, but after I did this it was just amazing. 

Cut the aluminum strips to size, make sure they are nice and flat so that you have smooth action. Glue them onto the drawtube using super glue, and check that they sit flush.

Installation

• ⚠ Before working on your Hadley, take some safety precautions. Put on your mirror caps, and work with the telescope laying horizontally so that if you drop anything, it won't fall onto the mirror ⚠
• Install the bearings on the main focuser body using the M4x20 screws. Tighten them so they have little play, but still turn smoothly. Insert the nylon screws into the top, but only partially.
• Now screw the body onto the LTA of your Hadley; you will not be able to reach the screws after the drawtube is installed, so make sure it is attached securely.
• Attach 1 of the knobs to the pinion and fix it in place using the grub screw, and add one of the bushings. Have the other bushing nearby.

• Insert the drawtube into the main focuser body, then insert the pinion. This order of operations is important, since the pinion locks the drawtube in place. Add the other bushing and the second knob, fix it in place too.
• Tension the pinion using the nylon screws. Make sure the pinion sits horizontally, and that all bearings sit flush with the aluminium strips. 
• Click the top flange of the collet into the collet nut, then screw it onto the drawtube.

Bill of Materials

Acknowledgements

Many thanks to the lovely community on Maff's Hadley discord server who inspired me to make this focuser, and gave me some smart insights to make this project a success.