Focus Control at the Laptop
Submitted: Monday, 10th December 2007 by Dennis Greeve
Focus is paramount for good astronomical photography and trying to achieve it while at the telescope making adjustments and viewing the results on the laptop computer a metre away is a frustrating hit and miss affair.
There is an answer, a very simple remote focus control and the picture below is my approach to the problem and it works!
Having built the Crayford type focuser from Jim Sapp’s web page, a magnificent design that provides a smooth non shake focus movement, it was a simple job to use one of the focus knobs as a pulley and a bracket to hold a small geared motor with smaller diameter pulley and an O Ring belt providing a 2:1 reduction drive.
Its success is the One-Shot controller that advances or reverses the focal tube a small amount with each push of the switch.
This simple circuit can be found on Dave Johnson’s web page, Circuit Designs; with the addition of a power transistor BD681 to drive the small YG2734 geared motor.
The circuit is built on a 60 mm x 50 mm Vero Board and powered by a 12v DC rechargeable battery, which also powers the equatorial platform, illuminated finder scope, laser finder and cooling fan. Colour coded RCA plugs and cables are used for all connections; they are neat, and positive.
A Power Panel clamped to the telescope side bearing, provides for these connections. It also contains the voltage reduction circuit for the red illumination leds and the laser finder. Each pulse - push of the square red button switch, advances the focal tube 0.1 mm – 0.004 inch - and the slide switch reverses the action. The O Ring belt slips on the small diameter motor pulley allowing the focuser to be turned freely for the initial focus.
Why not use a stepper motor? The smallest available stepper motor will not drive this type of focuser that requires a reasonable spring loading on the focal tube and theYG2734 motor is less than half the price of a suitable stepper motor and the One-Shot circuit is far simpler and cheaper to build.