The TCF-S by Optec is a temperature-compensating focusing system. As the night cools down, the optical tube of most telescopes contract, shortening the optical path, and necessitating re-focusing. If one measures the shift in focus with temperature throughout a night (as can be done automatically using the Optec), a curve of focal position (or shift) vs. temperature can be generated. This can then be used by the "smart" focuser to automatically shift focus position as the temperature drops. The TCF-S can store two different curves (for focus position vs. temperature), and independently, automatically, change the focus position as the temperature decreases throughout the night - without computer control. Currently, I am not using the temperature compensation features, as I focus regularly throughout the night using FocusMax via CCDAutoPilot. However, I do intend to investigate the temperature vs. focus slope for both of the scopes I am using (12" LX200R and FSQ-106EDX).

The 2" TCF-S was HUGE! It also weighs a lot, so can only be used on scopes and mounts that can support a significant amount of weight in the imaging train. (this, plus a big camera, filter wheel, adaptive optics, and rotator make for a tremendous amount of weight for the scope to carry; it therefore is critical that the scope is balanced on the mount). However, if I thought the 2" version was huge, take a look at the comparison between the 2" and 3" models (as shown on the cover of the TCF-S manual) in the photo below! Now that I have a large-format camera (STL-11000 with 24x36 mm CCD), I may need to consider switching to the 3" TCF-S focuser, to minimize vignetting, due to the extended 2" tube of the TCF-S (and it's relatively forward position relative to the CCD).

... of course, with all the extra weight, I may just have to get a bigger telescope to hold it!

The TCF-S system includes the focuser, control box, cables, software. and both telescope-side and camera-side adapters. As of Fall 2009, Optec introduced their integrated version that does not require the separate control box, the TCF-Si. The complete TCF-S focuser system is shown in the photo below:

In my initial testing of the TCF-S (on a night with really good seeing), I was able to move from star to star, refocus, and obtain a FWHM of almost exactly 2 arcseconds EVERY TIME! I was never able to do this using FocusMax and the RoboFocus on the LX200 scope; in fact, I typically couldn't focus to better than 2.5-3.5 arcseconds (perhaps due to seeing, but I never saw better focus until I bought the TCF-S). The TCF-S is a very precise system, with step size of 0.00008" - about 2 microns! The "critical focus zone" (CFZ) of a scope is just dependent upon its f#, and for the f/10 LX200 is about 220 microns, while for the f/5 FSQ it is about 55 microns. Therefore, these correspond to 110 steps and 28 steps, respectively, of the TCF-S to get through the focus zone; i.e., this focuser provides a lot of 'resolution' in focusing, and allows very precise and repeatable positioning of the optical system within the CFZ.