Unique to the TCF focuser, an electronic controller system monitors the telescope's tube temperature and compensates the focus accordingly. A small temperature probe is attached to the side of the telescope tube and monitors temperature with a resolution of 0.1°C. For a typical Schmidt-Cassegrain of 8 to 11 inches aperture and f/10 focal ratio, the back focus will move approximately 0.010 inches for every 1°C change in telescope temperature. It is not unusual during an observing session for the ambient temperature to change by as much as 10°C within the time span of a few hours. This change in focus due to temperature is a serious problem for most telescope designs and requires frequent re-focusing during long exposures. A typical RGB exposure sequence can last one hour making it imperative that the focus be checked and corrected after each filter change.
A simple learning procedure is used to find the temperature coefficients specific to the user's telescope system. The TCF system allows for two different coefficients (corresponding to two different f-ratio configurations) to be calculated and stored in the EEPROM memory. Once learned, either coefficient can be selected with a simple slide switch. A manual mode allows the user to set the focus manually at any time.
The digital nature of the TCF allows opportunities for truly intelligent focusing. Using TTL level electronics, a programmer can control the focuser from any PC. Exact focus can be found by optimizing a stellar centriod’s diameter.
At the end of an observing session, the TCF focuser remembers the last temperature and position. When the unit is turned back on for a new session, the TCF computes a new position using the current tube temperature and moves to that position. Assuming no changes to the optical configuration, the object will snap into sharp focus.