|
|
RoboFocus Mounting and Clutch Coupling
With respect to functionality and cosmetics, I could not bring myself to use the stock mounting plate and double-back foam-core tape provided with the RoboFocus so I designed a custom mounting plate that is fully adjustable. While I was at it, I added a precision clutch/shaft coupler with adjustable torque.
The RoboFocus motor is bolted to a black anodized aluminum plate. The plate is attached to a Losmandy Dovetail Adapter which slides on a male dovetail plate secured to the side of the scopes which allows for front to back adjustments. Vertical motor position adjustment is achieved with vertical slots machined into the plate through which the motor mounting screws pass. The motor shaft is easily aligned to the rack and pinion focusing shaft with these adjustments.
The RF motor shaft is coupled to the scopes
focuser shaft with a zero-backlash, adjustable-torque, shaft coupler
which has a stainless steel bellows portion which allows some degree of
freedom for offset shaft coupling or eccentricity without binding. There are also
SS shaft adapters on either side of the coupler needed to fit the 10mm
bore of the clutch to the 8mm Takahashi focuser shaft and the 0.25"
motor shaft.
The clutch has a firm click-detent which releases if
the torque setting of the clutch is exceeded. The torque setting
is adjustable over a range that is appropriate for the motor and focuser
and can't be set so high as to damage anything. The RoboFocus
doesn't loose its position sense even if end of focuser travel is
reached as the click-detent disengages, and when you reverse motor
travel the clutch reengages exactly where the focuser bottomed out.
This also allows manual focusing with the focuser knob. The
resting torque of the motor resists the manual rotation force applied,
the clutch releases, and you can then focus manually as needed.
When done, just turn the focuser knob back (or run the motor) and the
clutch reengages where you left off with no change of the RF position
count with respect to the position of the scopes focuser. A slip
type clutch would not do this, resulting in a relative change between
motor and focuser position when the clutch operates.
Using a RoboFocus step size of 3, I get .0005" resolution per step on the FSQ. This was measured with a highly accurate electronic dial indicator. This is more than enough resolution for accurate and repeatable focusing of the FSQ at its native f-ratio. Too small a move per step takes too long to move the focuser, doesn't improve the focusing accuracy and slows down the autofocusing routines completion time. Lowering the RF step size to 2 or 1 would give even finer resolution if you thought that might be beneficial.
The blue color of the RF motor is close to a standard anodizing hue and makes for a nice match on blue anodized aluminum knobs if you're so inclined. It also nearly matches the Tak blue parts and labels. I liked the result so much I added a mirror image RF setup to the FS-78 guide scope. Both motors are remotely controlled using focusing software on separate serial ports on the Ethernet Serial Server.
Precision zero-backlash, adjustable-torque, flexible clutch coupling, McMaster-Carr part #6613K13.
Custom shaft adapters fit the coupler bore to the motor and focuser shafts and provide additional length.
RoboFocus motor mounted on adjustable position aluminum plate.
.
Dual RoboFocus.
Revised 11/8/02
Arrakis
Observatory
©2002