TMB-130 Signature Series
Focuser Assembly w/MoonLite High Resolution Stepper Drive

  I found the stock focuser to work pretty good once I got it adjusted correctly. But, I needed an automated way of focusing during the phases of photography. I wanted not only a functional setup, but one that was clean and uncluttered looking. So I sent the stock focuser to MoonLite Focuser to be retrofitted to their high resolution stepper drive. This replaces the stock micor-focuser assembly. By coincidence their motor coupling was the same size as the TMB focuser housing diameter and perfectly in alignment but would not fit into the housing. The folks at MoonLite made a custom coupler that slides into the TMB focuser housing and locked in place with the stock setscrew.
  I used these stepper drives on my other OTA's and have been completely satisfied with their very accurate and repeatable performance. It uses an ASCOM USB controller that interfaces with FocusMax or other programs that support ASCOM focusers. The motor housing can also be rotated to any position by loosing a setscrew and turning the housing. One nice benefit is that the motor locks the gear weight so there is absolutely no tube creeping.
  I also had the rear 4" - 2" adapter drilled for 2 additional set screws. This aids tremendously in an even clamping force around the band to hold the camera in place. The longer screws make securing the equipment much easier than the single short screw design.

TMB130SS focuser with MoonLite stepper drive The drive position can be changed by loosening the set screw
and rotating the housing.
Focuser with drive housing vertical. Focuser with drive housing horizontal
Focuser mounted on OTA Closeup of stepper drive mounted on OTA
Top OTA is an Orion ED80 with the stepper drive
Though not part of the stepper drive upgrade I had the rear 4"- 2" adapter plate drilled for 3 screws instead of one. The screws are longer than the stock one to make tightening easier and more secure when a camera is used.

FocusMax V-Curve Measurements and Caliper Measurements

  Positioning accuracy must be repeatable for reliable automated focus operations. Three passes of V-Curve measurements using FocusMax and with full equipment weight were performed to test the accuracy of the MoonLite stepper drive and also measure the profile characteristics of the stock TMB focuser draw tube. The images below show a very repeatable and consistent profile.

  Physical step measurements were conducted using a Mitutoyo caliper with .001" gradients. The stepper drive ASCOM interface was set to use full steps (vs. half steps). A total of 10 passes were made to verify consistency. A group of 20 steps measured .005" or 127 microns. This breaks down to 1 step equaling .00025" or 6.35 microns. To determine the critical focus zone (CFZ) the numbers applied to the formula CFZ= focal ratio (squared) X 2.2 the TMB 130's F/7 CFZ = 107.8 microns. This equals 16.9 steps of the Moonlite focuser. The Focus Max V-Curve measurements display the Position Intercept difference of 17.X counts so the math and physical measurements fall within the CFZ area.

FocusMax V-Curve run #1 FocusMax V-Curve run #2 FocusMax V-Curve run #3