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TCF-S Temperature Compensating Focuser

The TCF Focuser
Above: Optec TCF-S Focuser with 2"-1-1/4" Reducer. Not shown are the provided Hand Control and Temperature Probe (182,502 bytes)

"One of the single greatest detriments to taking truly outstanding astronomical images is the focus shift due to changing thermal conditions. A small change in ambient temperature can cause otherwise sharp stellar images to bloat in size ruining the final image and wasting precious observing time. Someone needs to develop a focusing system which compensates for these temperature changes and holds the focus steady throughout the night."

Overview Optec has developed such a system. Mechanically, the TCF-S is a robust Crayford style motorized focuser with high repeatability. The Crayford design allows for a solid friction roller focusing system with no play and very little backlash. Optec’s implementation is ideal for applications which require exact focus such as CCD imaging or film astrophotography. A geared stepper motor rotates the drive shaft with one step rotation of the motor equal to a 0.00008 inch movement of the drawtube. A pair of pushbuttons control the direction of focus and the electronic DRO (digital read-out) displays the current position. This readout can be read on the provided hand control, or remotely with suitable optional control software. The TCF-S focuser can easily accommodate cameras and instrument packages weighing up to about 10 pounds (4.7 kg). Its big brother, the TCF-S3 Temperature Compensating Focuser is designed for use on larger telescopes, and can accommodate loads twice as heavy!

Temperature Compensation Most telescopes are made with components that may expand or contract as the ambient temperature changes. This can produce a change of focus over the course of a long exposure, or series of shorter exposures. Unique to the TCF-S focuser is an electronic controller circuit and thermal sensor which monitors the telescope's tube temperature and notes any changes which then cause the TCF-S motor to make minute adjustments to compensates the focus accordingly.

For an example of the effects of thermal changes, consider the focus changes of a typical Schmidt-Cassegrain of 8 or 11 inches aperture and f/10 focal ratio; the back focus will move by approximately 0.010 inches for every one degree C (1°C) temperature change in telescope temperature. It is not unusual for the ambient temperature during an observing session to change by as much as 10°C within the time span of a few hours. The change in focus due to temperature is a serious problem for most production telescopes, a typical RGB exposure sequence can last one hour making it imperative that the focus be checked and corrected after each filter change.

The TCF-S system includes a compact temperature probe which is attached onto the side of the telescope tube. This probe is sensitive to changes of temperature with a resolution of 0.1°C. It transmits temperature information through a 20 inch (51cm) long flexible signal cable into the electronic controller of the TCF-S focuser.

Initially a simple learning 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 the operators choice of two different f-ratio configurations) to be calculated and then stored in the EPROM memory. Once learned and programmed into the TCF-S, either coefficient can be selected with a simple slide switch.

The TCF-S also provides a manual mode which allows the user to set the focus manually by push buttons on the Hand Control or remotely with appropriate software at any time.

The digital nature of the TCF-S3 allows opportunities for truly intelligent focusing. With the current OPTEC TCF-S.EXE, version 1.21 software, users can communication 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 and later by Software Bisque offers the @Focus  routine which communicates with the TCF-S via a serial port to provide truly automatic focus control. And MaximDC/CCD Version 3 and later by Cyanogen provides remote and automatic focus control with the TCF-S focuser too.

At the end of an observing session, the TCF-S focuser remembers the last temperature and focus position. When the TCF-S unit is turned back on for a new session then the TCF-S computes a new position (using the current telescope tube temperature) and moves focus to that position. Assuming there are no changes to the optical configuration, the object will snap into sharp focus.

left view

rear view

right view

Above: The TCF-S Focuser mounted on the MAXfield 2" Filter Selector.
(Click on any image to enlarge)


Image of TCF Focuser
The TCF-S Temperature Compensating Focuser shown with the MAXfilter attached onto the axial port
of a Meade 10" LX-200 telescope, with a SBIG PixCel ST-237 CCD camera.

Advantages and Capabilities of the TCF-S Focuser:

The TCF-S is protected under U.S. Patent No. 6,327,081.

The TCF-S is a modular system, to put a complete system together one will need optional hardware and cabling:

A typical TCF-S 2" Focuser System arrangement will include:

Stock No. Description Price (2008)
17455 Meade 12" Telescope Mount, 3" Diameter Ring. $59.00
17670 Model TCF-S Temperature Compensating Focuser. $789.00
17662 2" to 1¼" Adapter. $55.00
17471 12-ft. Control Cable, Hand Controller to TCF-S. $39.00
17695 PC Serial Port converter, DB-9 to RJ-12. $15.00
17671 6-ft. Reverse Cable, RJ-12 to RJ-12. $15.00
  Total Package Cost: $972.00

Please refer to our TCF-S and Accessory price list section to see a comprehensive list of the available fittings, cables and other options for the TCF-S. Or contact Company Seven to obtain a complete suggested package configuration and cost estimate.

For further reading and technical information:

  1. TCF-S & TCF-S3 Temperature Compensating Focuser: A technical manual for theory of operation and operating procedures. The Optec TCF-S 43 page illustrated instruction manual. Copyright Optec Instruments, from Company Seven's Library. Download size is 2,093,399 bytes (in Acrobat Reader ".pdf" format).

  2. Focus Changes Of 8 inch Schmidt-Cassegrain Telscopes. Charting of temperature and associated temperature changes measured over fourty (40) degrees by Optec. Copyright Optec Instruments, from Company Seven's Library. Download size is 24,559 bytes (in Acrobat Reader ".pdf" format).

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