Hi Eric:
If you have access to a 3D printer, here is a link to the STL files for the geared focuser I designed for my WhiteCat (identical in construction to your RedCat, just a different color - and $30 cheaper when I bought it..
). See previous page in the same thread here.
www.dropbox.com/sh/urepva7jg2eq15a/AADkq...DnhqS9KyQW1ZTva?dl=0
The only file not in there is the motor mounting block, which I had designed for an analog motor driven by FCUSB, so you would have to rig one yourself, but that is probably the simplest part to design and fit onto the included rail. Once I have a litte more time again, I will design one and put the file into the same folder. If you beat me to it, it would be great if you would share the stl file as well.
I would suggest going with the clamped design for several reasons: Very easy to put on and take off, much higher tolerance, doesn't have to fit snug off the bat, since you use a screw and nut to tighten it, and it won't slip at all on the rubber ring of the telescope, so no contribution to backlash from that end. Also, the focuser doesn't have to travel very far. As you have also found out, the RedCat has a very narrow focus range, which is difficult to adjust by hand. For autofocusing the ring has to barely move 5 degrees between the extreme focus positions of the autofocus process, to the unwieldy looking extension on the focus ring I designed is not going to conflict with the base plate or anything else. You definitely do not need a contiguous geared ring around the telescope! You manually adjust the telescope to be roughly in focus, then slip on the ring, tighten it with the screw at the 0 degree position on top, move the motor on the rail to engage the gears tightly and you are set.
Inspired by your discussion here and by Kevin's helpful hints, I thought I might give it a try and adapt the same design to a stepper motor.
I used the Moonlight driver compatible Focuser design that Robert Brown generously put in the public domain:
www.dropbox.com/s/ht01zhe3jor9swp/SolderlessFocuser.pdf?dl=0
I soldered it together yesterday from a few components. In the image below, I am using a buck converter to step down the 12V input voltage from my standard 12 V power supply to 9 V, mainly because that is what was printed as input voltage on the DRV8825 shield (although the datasheet says any input up to 45V is fine) and because I was not sure whether my cheap Arduino nano clone (($ 3), would take a Vin of 12V or go up in flames trying. Probably, though, the buck converter is not necessary and one can feed the rig directly from the 12V input.
It worked like a charm once I set the current limit on the DRV8825 shield by adjusting the Vref as described here:
www.makerguides.com/drv8825-stepper-moto...er-arduino-tutorial/
For the moment I have the Vref set at 0.33 V, so a max current limit of ~660 mA, but I will see if I can go even lower without affecting motor functionality. If I am losing steps, I can go up to 0.5V, i.e. 1A current limit. I tried that and that also works. Trade-off is that the motor produces more heat, which I obviously want to keep as low as possible, since it is connected to thermoplastic 3D printed gears. You DEFINITELY have to adjust the max current limit! I forgot that and the motor would hum and shake and get extremely hot. I was lucky that it did not fry the stepper driver on the shield. Fortunately, that had overcurrent protection. When I checked, the max current limit was set at 3.2 A, with the motor rated at 1.4A. I almost burned my fingers just touching it.
But once I had adjusted the max current limit, it immediately worked great with the Moonlight Driver in Indi right out of the box, no further modifications required. You can add an OLED, Temperature probe and In/Out LEDs by just uncommenting the respective lines in the header of the code. I used the Focuserv176_DRV8825_HW203_OLED.ino file contained in the zipped firmware folder:
sourceforge.net/projects/arduinofocuscon.../ARDUINO%20FIRMWARE/
The OLED is inexpensive and it gives you more detailed information about the state of your focuser than simple LEDs:
Here a link to a movie of the set-up which obviously still is a prototype, i.e. I need to build a box around it. But it works!
www.dropbox.com/s/7pwzxom3mpi0z1m/Arduin...serInAction.m4v?dl=0
I used it with a very basic NEMA17 motor (~$6 from Amazon) and microstepping all the way to 6400 step/revolution, fully adjustable from 200 step/revolution in 2x increments all the way to 1/32, i.e. 6400 steps/revolution. That equates to 0.05 degrees/step, which is way more than necessary, given the mechanical tolerances of the gears.
In the movie I only demonstrate 1/32, 1/16 and 1/1 ratios. I think the optimal ratio for use with the geared focus design and Hy's linear focuser is probably going to be 1/16 or 1/32, mainly because I don't want the focuser to move at too high a speed on those gears.
I fully expect that to work on my geared focuser design (see picture above earlier in the thread), since all I really have to do is replace the analog motor with the stepper motor.
I would be interested to hear whether you are using the geared design and how it works for you. Please send me a PM when you do or if you have questions.
Best of luck!
Jo
PS: One more thing: The motor wire connections did not match the pin order on the DRV8825 shield, so I had to cut the wires and connect them in the right order. The colors were correct, though, with Black=1A, Green=1B, Red=2A and Blue=2B.
Attached a screenshot of the Control Panel settings and the Focus Module with Linear selected. I highly recommend the linear setup with helical focusers as in the RedCat. Basically eliminates all backlash problems.