Unique to the TCF-S 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 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 or more making it imperative that the focus be checked and corrected after each filter change.
A simple setup procedure is used to find the temperature coefficients specific to the user’s telescope system. The TCF-S 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-S allows opportunities for truly intelligent focusing. With the TCF-S focuser, programmers can communicate directly with the focuser’s controller (located in the hand control) via any serial port. The simple ASCII protocol is easily programmed and has been implemented in many popular imaging and camera control software packages. For example, CCDSoft Version 5 offers the @Focus routine which communicates with the TCF-S via a serial port to provide truly automatic focus control. MaxIm DL/CCD version 3 and above provides remote and automatic focus control with the TCF-S focuser.
At the end of an observing session, the TCF-S focuser remembers the last temperature and position. When the unit is turned back on for a new session, the TCF-S 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. It is possible to observe many days without the need to focus again.
The TCF-S comes with its own PC control program for Windows. The source code for the program is available in the TCF-S folder after the install program is run. Users are encouraged to write their operating program using the TCF-S.bas routines as examples. You can download the setup program for TCF-S_Control by going to our Software Download page. The navigation button is on the left.
TCF-S/S3 Control Box
This control box powers the stepper motor, reads the temperature sensor, provides bi-directional serial communication with a PC and allow the user to remotely control and read the state of both TCF-S and TCF-S3 focusers. In order to attain proper connection, three cables must also be obtained: power, focuser control, and PC serial connection.
For normal operation, the MODE and LEARN slide switches are placed in the RUN and MANUAL position as shown in the diagram. To teach the focuser a new temperature coefficient, the LEARN switch is placed in the LEARN postion and the MODE switch is placed in AUTO A or AUTO B.
The position of the focuser in units of “steps” is shown on the DRO display. For the TCF-S, position ranges from 0 to 7000 steps which corresponds to a maximum travel of 0.60 inches. For the TCF-S3, the range of position is from 0 to 9999 steps which corresponds to a maximum travel of 1.00 inches.
Important Note! This control box is intended to be used to allow control of a focuser through a computer or from a desktop, not as a hand controller! For handheld operation of a focuser, purchasing the remote keypad is recommended.
