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TCF-S Temperature Compensating 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."
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.
Above: The TCF-S Focuser mounted on the MAXfield 2" Filter
(Click on any image to enlarge)
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.
Automatically compensates for focal shift due to temperature changes.
Capable of supporting instrument packages up to 10 pounds.
Easy to use hand controller with push-button focus control.
Weight of the TCF-S is 2.5 lbs.
Backfocus taken up is from 3.5" to 4.1" for a total travel of 0.6 inch.
Built-In 4-digit DRO (Digital Read Out) display.
Simple Learn procedure for determining temperature coefficients.
Shut-off procedure stores and remembers last position.
CCDSoft Version 5.0 and later compatibility.
MaxIm DL/CCD Version 3.0 and later compatibility with TCF-S.
The TCF-S is protected under U.S. Patent No. 6,327,081.
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.