Re: PMC-Eight for other mounts?

Jerry Hubbell - Explore Scientific VP Engineering

On Sat, Feb 8, 2020 at 01:36 AM, Charles Botkin wrote:
I believe that the drive pulses and gear ratios can be adjusted to match almost any mount ? Or did I read the information wrong?
Doing atm work is something I do enjoy, but why would I buy the G-11 kit and not the EZOS2-GT and supply my own motors and brackets ?

Hi Charles,  thanks for your question. Yes, the PMC-Eight can be configured and used on any mount as a DIY project.
The PMC-Eight Programmer's Reference provides a lot of good information you would need:

The retrofit for the G11 is only for those that have a G11 that they want to upgrade, it includes shipping the mount back to us for the mechanical upgrades and for servicing. We also test the mount and bring it up to the level of the new G11 PMC-Eight mounts that we sell. We install and test everything and you get a like new updated G11 PMC-Eight mount back. That is why the cost is more than you might spend if we just offered it as a DIY kit, which we do not do.

There are a couple of things you can do since we do not sell the controller boards separately and that will save you quite a bit of money compared to the G11 PMC-Eight upgrade. I think it is important to state that if you go the DIY route we would not provide any warranty for the PMC-Eight system installed on your DIY mount.

Depending on the size of the mount you want to add the PMC-Eight system to, you can either buy an iEXOS 100 PMC-Eight mount with the small controller board (Model 1A-01C) and NEMA 11 motors or the EXOS 2 PMC-Eight with the large controller board and the NEMA 11 motors. If you opt for the EXOS 2 PMC-Eight, then you can buy your own larger NEMA 17 motors and use it with the large board controller (Model 2A-06B). Either way you would need to purchase your own belt/pulley system or you could use the ones provided if it would work on your DIY mount.

The basic equation for calculating the total motor counts which is the main value needed to scale the gear train correctly is:

Total Motor Counts = Ring Gear Teeth x Pulley Ratio x Motor steps/rev x 32 microsteps/step

So for the G11 this is:  360 x 1 x 400 x 32 = 4,608,000 total counts. The arcseconds/microstep = 1,296,000 / 4,608,000 = 0.28125 arcseconds/microstep
For the EXOS 2 this is: 144 x 4.5 x 200 x 32 = 4,147,200 total counts. The arcseconds/microstep = 1,296,000 / 4,147,200 = 0.3125 arcseconds/microstep

The only other thing to set is the motor current. This is not user configurable, but I do provide versions of the firmware that set different motor currents. You can use those to select the motor current and then set the total motor counts in the ASCOM driver (it is hardcoded for our mounts, I would need to update the driver to support custom motor counts, or you could change the driver yourself) or the ExploreStars application which allows you to set the motor counts.

I have built a prototype Celestron CGE PMC-Eight mount for a friend of mine that we are using at our MSRO Observatory. It works very well. 

Jerry Hubbell
Vice President of Engineering

Explore Scientific, LLC.
jrh at
1010 S. 48th Street
Springdale, AR 72762

Author: Scientific Astrophotography: How Amateurs Can Generate and Use Professional Imaging Data
             Remote Observatories for Amateur Astronomers: Using High-Powered Telescopes From Home

Mark Slade Remote Observatory (MSRO) IAU MPC W54 Equipment
Wilderness, VA
: ES PMC-Eight G11 + Telescope Drive Master (TDM)
Scopes: ES 165 FPL-53 ED APO CF, ES 102 FCD100 ED APO CF
Cameras:  QHY174M-GPS + FW, QHY163C
Misc: 3-inch 0.7x Focal Reducer Field Flattener, Filters: Luminance,
Red, V-band Photometric, Diffuser, 200 lpmm Spectral Grating

Software: MaxIm DL 6, Cartes du Ciel, Astrometrica, AstroImageJ, AutoStakkert!

Join to automatically receive all group messages.