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TCF-S3 Temperature Compensating Focuser

The TCF-S3 Focuser

Above: Optec TCF-S3 Focuser with the provided Hand Control and Temperature Probe (182,502 bytes).
Click on image to see enlarged view (72,577 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."

the TCF-S and TCF-S3 Focusers Overview Optec developed such a system with the original TCF-S Focuser which earned for the company much acclaim since its introduction in the year 2000. And now the TCF-S has a new big brother - the TCF-S3. This 5 lb. focuser has all the same features of the TCF-S but is simply scaled up to provide a larger clear aperture and increased payload capacity. With the ever increasing size of CCD cameras and their associated instruments the need for a precision digital focuser that can manage the weight and is compatible with most of the camera control software available is now met. Even though it is designed for massive loads on big telescopes, a 3-inch to 2-inch reducer (stock number #17804) is available to accommodate Optec and third party standard 2-inch accessories.

Left: Optec TCF-S and TCF-S3 Focusers (36,014 bytes).
Click on image to see enlarged view (353,631 bytes).

Mechanically, the TCF-S3 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. No corners were cut with this model; the massive drawtube has a 3.00-inch inner diameter and is made from 7075, the strongest of the commercial aluminum alloys. To prevent any slippage common to Crayford style focusers the drive rod is pressed onto the drawtube with a force of about 500 pounds. The friction drive handily manages 10 lb. loads in a vertical orientation, and 15 or even 20 lb. depending on the orientation. The TCF-S3 can take this extreme pressure because the drive rod is made from a ground bar of high speed steel with a diameter of 0.375 inch. Hardened rails of stainless steel are embeded in the drawtube to engage the drive rod and take the strain.

the TCF-S and TCF-S3 Focusers

A geared stepper motor rotates the drive shaft so that the total linear travel of the drawtube is 1.00-inch, with a resolution of 0.0001 inch. All gears are made from 303 stainless steel. Like the TCF-S, a control box with IN and OUT pushbuttons, serial computer interface and digital display control all functions of the TCF-S3 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. All other features of the TCF-S3 are similar to those of the TCF-S. It uses the same cables, temperature probe and the new remote keypad. It is also compatible with the current Optec control software TCF-S.exe.

Right: TCF-S3 with gear cover removed. All gears are made from 303 stainless (45,997 bytes).
Click on image to see enlarged view (369,836 bytes).

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-S3 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-S3 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 and TCF-S3 systems 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 or TCF-S3 focuser.

Initially a simple learning procedure is used to find the temperature coefficients specific to the user's telescope system. The TCF-S3 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-S3, either coefficient can be selected with a simple slide switch.

The TCF-S3 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-S3 via a serial port to provide truly automatic focus control. And MaximDL/CCD Version 3 and later** by Cyanogen provides remote and automatic focus control with the TCF-S3 focuser too.

    * For the users of CCDSoft, it is necessary that the maximum number of steps allowed by the program be increased from 7,000 to 10,000. This is done as follows:

    • 1. Open "SBFocuser.ini" in \Windows folder

    • 2. Under the TCFS section, add the following MaxPos line: [TCFS] MaxPos=10000

    ** For users of MaxIM, it may be necessary to copy over the TCFS ASCOM driver with the one linked below. You can find the driver using the following path:

    • C:\Program Files\Common Files\ASCOM\Focuser

    • The new dll can be downloaded by clicking here

    • Unzip the dll and copy to the folder described above.

At the end of an observing session, the TCF-S3 focuser remembers the last temperature and focus position. When the TCF-S3 unit is turned back on for a new session then the TCF-S3 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. A MAXfield 3" Filter Selector is coming in 2007 for the TDF-S3.
(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-S3 Focuser:

    Solid, heavy-duty Crayford-style 3" Focuser.

    Automatically compensates for focal shift due to temperature changes.

    Capable of supporting instrument packages more than 15 pounds.

    Easy to use hand controller with push-button focus control.

    Weight of the TCF-S3 is 4 lbs 8 oz. (2.6 kg).

    Backfocus taken up is from a minimum of 4.75 up to 5.75 inches with total travel of 1.0 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-S3.

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

The TCF-S3 is a modular system, to put a complete system together one will need optional hardware and cabling:

  • Hardware fittings to attach the TCF-S3 onto a telescope (Astro-Physics, Celestron 11/14, or Meade 10/12/14/16, 3 inch custom, or 2 inch slip fit, etc.)

  • Hardware fittings to attach a CCD Camera or other accessory onto the TCF-S3

  • Serial Port Coverter

  • Control Cable from Hand Control to TCF-S3. This may be short if you expect to work near the telescope, or long for remote operation.

  • Cable to connect Hand Control to the Serial Port Converter (at the PC). This too may be a short or long cable.

A typical TCF-S3 3" Focuser system arrangement will include:

Stock No. Description Price (2008)
17802 Meade 12" Telescope Mount, 3" Diameter Ring. $89.00
17670 Model TCF-S Temperature Compensating Focuser. $1,789.00
17804 3" to 2" Adapter. $79.00
17662 2" to 1¼" Adapter. $55.00
17471 12-ft. Control Cable, Hand Controller to TCF-S3. $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: $2,081.00

Only a small number of telescope mounts are available to fit the 3 and 4 inch Meade thread and the Celestron C-11/C-14 3 inch thread. This focuser is only intended for large scope with massive mounts. Refer to out Optec price list for a list of available production mounts. Other telesope mounts can be made custom to order.

Future 3-inch products from Optec will include a filter wheel, optical telecompressors and field rotators.

Please refer to our TCF-S3 and Accessory price list section to see a comprehensive list of the available fittings, cables and other options for the TCF-S3. 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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