Optec 2" TCF-S Temperature Compensating Focuser, Serial Version with RS-232 Capability

"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 & holds the focus steady throughout the night."

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 movementof the drawtube. A pair of pushbuttons control the direction of focus & the DRO (digital read-out) displays the current position. The TCF-S focuser can easily handle cameras and instrument packages weighing up to 10 pounds.

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 & monitors temperature with a resolution of 0.1°C.

For a typical Schmidt-Cassegrain of 8 to11 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 & requires frequent re-focusing during long exposures. A typical RGB exposure sequence can last one hour making it imperative that the focus be checked & corrected aftereach filter change.

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 two different f-ratio configurations) to be calculated and stored in the EPROM memory. Once learned, either coefficient can be selected with a simple slide switch. You can set the focus manually at any time.

The digital nature of the TCF-S allows opportunities for truly intelligent focusing. With the serial version TCF-S focuser, programmers can communication directly with the focuser's controller (located in the hand control) via any serial port.

The simple ASCII protocol is easily programmed & 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.