I have now finished bench-testing my third dew controller (had to wait for another humidity sensor to arrive as the one I had appears to have been faulty). So, I think it’s time to start looking at Robert’s focuser controller.
The Sourceforge site with all the details is a bit overwhelming at first because there are a number of different designs: myFP2E, the wifi version which has actually moved to a separate project on Sourceforge; myFP2N with a touch-screen display; myFP2M which is a ready-made controller with various options and the standard DIY version which also has a number of hardware options. There being no big friendly “Start here to build your own focuser” button, the documentation here is probably the place to start.
I have a number of OTAs that I’d like to be able to fit a controller to, varying from my Photoline 72 which is about f/4.75 when fitted with a focal reducer, to my C9.25 and PST at f/10 or even my 127 Mak at almost f/12. Obviously the size of the critical focus zone at f/4.75 is only marginally larger than a gnat’s whisker. In order that I might hang on to what remains of my sanity I think it makes sense to keep the hardware the same, which probably makes something like the Nema17 with a 27:1 gearbox the obvious choice. Whilst I might get away with the 5:1 gearbox on some of the OTAs, there’s no real cost advantage to using the lower reduction ratio gearbox and I think I’d probably still end up needing micro-stepping for the faster OTAs.
I did consider not using a gearbox and using timing gears and a belt drive instead, but I’m not sure that really gains me anything other than potentially being able to hang the motor under the focuser on ‘fracs and newts rather than have it poking out of the side.
So, for the moment at least, my plan is to use the Nema17 with 27:1 gearbox from OMC across the board, which means I’ll be using the DRV8825_HW203 driver and need the appropriate circuit design.
On the electronics side, unlike the dew controllers I intend to build these on strip board. I do actually have one circuit board for a revision of the controller that is a couple of revisions behind, but I’m thinking I may save that for a combined travel dew/focuser controller box. Circuit diagrams and layouts are available in the STRIPBOARDS directory, but it’s not immediately obvious which are the relevant reading. I started with “BOARD OPTION DRV8825 HW203.pdf”, which I think is the main document covering most of the DRV8825 builds. I’m not actually too worried about displays, temperature compensation and suchlike as the focuser will be remotely operated and software will take care of refocusing properly, but having a set of buttons to adjust the focus manually would be good. The documentation for the buttons (in the OPTIONS directory) says that pressing both buttons together resets the focuser position to zero and beeps to confirm that it is done. I was wondering what the buzzer was for…
The main build document doesn’t cover exactly which options I want, and in fact suggests that the strip board layouts may be out of date when compared with the “everything” design, so one of the first jobs would seem to be to compare the various circuits, work out what does what and remove the bits I don’t need or add the bits I do and see how that affects the strip board design. I think “Stripboard DRV8825-HW203 FULL” is not that far off, though I don’t need the firmware programming switch or the temperature sensor. Some of the other designs appear to have a 1N5408 diode and a 100uF electrolytic capacitor across the supply and ground that I’m not fully sure I understand the reasons for yet. Hopefully I can work that out in time.