My new best friend J. B. Weld and I take up refractor focusing.

December 15-16. More housekeeping: I lubed the G11 worm and put the cover back on (mostly so I wouldn't lose it), and I finished cutting, drilling, filing and gluing the parts for the micrometer bracket for the refractor focuser, made a "Wodaski mask" to try out for final focussing, and sanded the rust off the bare-metal counterweights and coated them with Minwax fast-drying polyurethane; let's see if it keeps them satiny rather than rusty for a while.

The micrometer... from some aluminum barstock, I built a split-block holder for a Starrett 1/2-inch micrometer head (eBay). The holder bolts to a bracket epoxied to a clamp that tightens down as needed around the rim of the refractor's focus knob. The micrometer's probe contacts the refractor's back plate which provides a precision stop. Rough focus however you like; then clamp the micrometer down and spin its drum to find final focus. Stop trying to make sense of that description and just look at the picture.

As with the 200mm's focusing rig, four turns of the micrometer's drum moves the probe 1/10 of an inch. The micrometer is mounted 3.25 inches from the center of rotation. So one revolution is good for 1/40 of an inch out of 3.25*2*pi total required for one revolution of the rack and pinion, so we get a slightly greater reduction than the 200mm enjoys: about 800:1. To a decent approximation, one turn of the refractor's focus knob moves the CCD 0.8 inch. Which means that one turn of the micrometer drum moves the CCD one thousandth of an inch. Which means, happily enough, that each of the 25 divisions on the micrometer corresponds to a micron's movement of the CCD. This is overkill, but why go halfway?

According to Ron Wodaski's book (The New CCD Astronomy), the range of good focus is determined by the f-ratio of the optical system. The size of the Critical Focus Zone in microns equals F^2 * 2.2. (The results are close enough to those I derived from first principles back in 2001 and are so much easier to compute.) At F6, the CFZ is 79 microns. At F4.5, 45 microns.(but since the compressor moves...?). Nevermind: get the focus into the zone where a robust spin of the drum doesn't make any difference, and you've arrived. This is quite a change from the way it's been so far: get close and I'm afraid to more than breathe on the knobs lest I do more harm than good. It also means that for tri-color or mapped color imaging, I can probably work out rough and ready guidelines (shoot luminance, then three turns out; shoot red, two turns in, shoot green, etc) rather than trying to read fine-print on the micrometer by a faint red light.

BTW, Ron's formula implies that with the 200mm wide open, the focal plane has to be brought to the CCD within 9 microns. At F2.8, the CFZ is twice as deep. Calculate how much tilt is tolerable in the Nikon F-mount of the Mandel wideagle adapter on the CCD. Maybe I am worried over nothing by the play I can feel there. More likely I could do with brace of some sort while I figure out a robust repair. Don't worry, more J-B Weld is on order.

The mask... I took some advice from Ron's book and (finally) cut a focusing mask consisting of two triangles rotated with respect to each other -- when the spikes produced by each triangular opening all point to a common center, you're focussed. Aluminum foil, masking tape, one of the rings cut from the A-P's tube when I shortened it years ago, and more polyurethane. Details to follow, if it works.

Guiding is under control; focus should be solved; what's left? Polar alignment. A red LED illuminator for the reticle would be a welcome improvement. Actually, one came with the G11 when I bought it, and I probably last used it 14 years ago. And here it is!

 

Eta Geminorum, gross focus with
Ron Wodaski's recommended mask,
middle 5% of ST2000XM CCD frame.

CCD is about 1/2 inch outside focus in the
top frame, damn close in the bottom one.

 

 

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Text & Photos by David Cortner
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