Finger Lakes Instrumentation

 

The FLI Professional Focuser

Design Goals
The FLI Professional Focuser was specifically designed for modern CCD astronomy. It was assumed that the CCD camera could be mounted very close to correct focus, making a thin, short-travel design practical. A serious CCD camera may weigh several pounds, so the focuser has to safely carry that weight, plus focus it accurately. Computer control was considered a necessity, as all other imaging operations are carried out from the keyboard, not at the telescope. Autofocus logically follows, as FFT or other algorithms can establish exact focus at the CCD faster and more accurately than the cut and try process commonly used. A stepper motor was chosen to drive the unit, rather than the simple DC motors used in most other designs. This allows known and repeatable positioning, in turn allowing more sophisticated focusing algorithms to be used. Extreme mechanical resolution was considered a must, as low f/ numbers and high resolution CCDs make even a small focus error detectable. 
The design of the FLI Professional Focuser is simple and elegant. Three leadscrews set the distance between two parallel plates. The plates are, of course, bored and threaded to receive the camera and telescope adapter. The three leadscrews are driven simultaneously, thus moving the plates closer or further apart. Resolution is dependent on the pitch of the leadscrews and the precision with which they can be driven.
The details of a practical design are a bit more complicated. The leadscrews are double-ended, having both left and right hand threads. The plates are tapped to match. Light shields are necessary, along with a motor mount, cover, and electrical connections. Closely fitted pins and journals prevent rotation or lateral shift between the plates. Finally, three stages of reduction are employed for high mechanical advantage and extreme resolution.
Some other design issues that might be of interest include rotational symmetry to enhance stability, and strict attention to proper kinematic design principals. Accurate machining on the most modern numerical controlled equipment proved essential for smooth motion, and instrumentation grade timing components were used to insure that the leadscrews turn exactly in phase. Materials were chosen for low wear and minimal maintenance under extreme environmental conditions. Finally,a unique stepper control system was designed to allow both manual adjustment and easy computer interface.

Summary
Though the technique of parallel leadscrews has been used in industry for decades, we believe this is the first time it’s been applied to an astronomical focusing device. FLI has made very  specific tradeoffs in terms of travel and installed height to produce a CCD-specific focuser of unique performance. You will find this focuser to be one of the thinnest available, with resolution and position accuracy second to none. Though designed for CCD astronomy, the unit may also be used to advantage in any scientific application requiring precise motion between two objects: laser focusing, collimation, spatial filters, etc.

Design References
Though a patent could probably be secured on this design, there is significant prior art on parallel leadscrew arrangements; such a patent would not hold up to any serious challenge. The following references illustrate similar and related mechanical configurations that may be of interest. Mechanisms, Linkages, and Mechanical Controls by Nicholas P. Chironis (McGraw-Hill, 1965) shows several variations of parallel screw actuators with both driven screws and driven nuts. Various LH/RH screw drives are also shown. See pgs. 57 (missile lift mechanism), 185-5, 187-14&15. Fine Woodworking magazine, August, 1998 (#131) describes an adjustable height workbench surface using four leadscrews and a chain driven configuration. See the rear cover page. Surface planers (wood) from various manufacturers have also used a parallel screw and chain drive arrangement for many decades to set the vertical height.
Burleigh Instruments Inc. describes the advantages of rotational symmetry in their interferometer designs (1975-77) in various advertising materials.
Note that there are several variations of leadscrew that can be used. The FLI design uses equal pitch LH and RH screws, however a differential screw is also possible. By using thread pitches that vary only slightly from each other, extremely high resolution can be obtained- far in excess of most astronomical needs. Though very fine threads will also produce high-resolution motion, friction remains constant. The FLI design uses low loss reduction stages between the stepper and moderately pitched leadscrews to provide a much smoother operating design.