A remote focuser model DF-2 by Finger Lakes Instrumentation, was tested for overall operation and stability at the request of M.W., who kindly provided the loan of the equipment.  The unit tested was serial number 0203, manufactured 12-02.  It was equipped with a USB interface, an AD-2 for attachment to an SBIG CFW-8, and the current release of software.  It was also accompanied by a set of Vlier screws.  These screws are designed to provide a pre-defined force to keep the two plates of DF-2 rigidly separated and locked.  While these screws would prevent any focus movement, they do provide a mechanism to lock the focuser, if there were a concern about movement.

Handling the focuser, there is a perception of a small play or wobble between the two plates.  While this is not significant, it was thought that it may impact image accuracy.  To this end, a series of tests were run using an RCOS instrument rotator.  This device was used with my 10" RC, an SBIG ST-8E and CFW-8 combination.

The camera was assembled to the DF-2 via the FLI AD-2 adapter.  The AD-2 is held in place via two setscrews, whose location is accessed via the edge of the front plate.  While FLI did add a cut-out on the plate to clear the CFW-8 motor, it was located in such an orientation to still require removal of the CFW-8 to access one of the setscrews.  A better approach would have been to properly locate the cut-out so as to allow the CFW-8 motor cover to not block the set screw access.

The camera and DF-2 as attached to the Instrument Rotator (IR) via all threaded connections for rigidity.

The plan was to expose a star field while the camera was being rotated to assess any axial instability.  For all of the following images, the image was exposed for 110 seconds after commanding the IR to rotate through 360°, which takes approximately 100 seconds.  Four sets of images were taken at each elevation of 45° and 72°.  Images were taken with CW and CCW instrument rotation and with the Vlier screws engaged and not engaged.  The images below are the results of those exposures. 

Even close examination of these images show no significant departure from round.  This indicates to me that the focuser does indeed preserve axial alignment.  It may be that the play or wobble felt when holding the DF-2 in hand may be reduced or removed by the preload provided by the camera weight.

An additional test of FocusMax v3.0.12 (beta) and the latest ASCOM driver gave good results to achieve focus on this F/9 system.  Note that the DF-2 is a very fine positioning focuser and should give good results at F/5 systems, such as the BRC-250, FSQ-106, etc. as well as longer focal length instruments.