[APML] Robo-Focus (was: The Van Slyke "Slider" in detail)

[Matt BenDaniel <matt@starmatt.com>]

At 10:40 AM 1/19/2002 -0800, Alan Voetsch wrote:
>...Robofocus...

The problem with focusing by hand is that your fingers cannot sense very fine movements (0.001") of a focuser. Obviously we can still achieve critical focus, but it can take a while. Then as the temperature changes, the focus will shift.

One approach is to attach an dial micrometer to the focuser. If you make good records of how much the focus shifts as a function of temperature, you can easily make temperature corrections throughout the night. If the micrometer is left permanently attached, or if it can be reattached in a calibrated way, you may be able to reliably focus just by using the micrometer.

I went another direction. I just received a Robo-Focus today. For a refractor w/R&P, you remove one of the focuser knobs and attach the Robo-Focus motor. Robo-Focus works well with newts and SCTs, too.

For manual focus (AP or visual), you use a focusing tool (KEF, SureSharp, loupe, ...). You operate the focuser with a hand control. You can still manually adjust the course focus with the other knob. The Robo-Focus allows you to make very fine movements of the focuser. This speeds up the focusing. When powered on, the Robo-Focus locks the focuser to prevent shift.

But this kind of focuser allows you do a lot more if you connect it to a PC. You can operate the focuser from software and/or read its absolute position. You can also configure the Robo-Focus to automatically compensate for temperature shifts. This should mean that you only need to focus once during an entire session. You can even write your own Visual Basic programs to control the focuser.

If you're using a CCD, there is software from multiple vendors that can process short exposures from the CCD and control the focuser. They can automatically achieve critical focus in a few minutes.

For more info see:
http://www.homedome.com/
--
Matt BenDaniel
http://starmatt.com


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