It ain't Tom Swift and His Giant Telescope, but . . .

Feb 23. The 10-inch is no longer a project (insofar as any telescope is ever not a project) but a regular part of the stargazing toolbox. I shortened the mirror-end truss, snapped a third 10-24 tap (find them all! win a prize!), installed washers to help the springs in the mirror mount, polyurethaned exposed wood (which is likely temporary), and made every effort to lock down the receiver for the Feather Touch adapter.

Just as I balanced the 10-inch on the G11, a 737 passed almost overhead. Its contrail provided a target to show that things still focus where they did before I shortened the mirror truss. I tossed off a temporary baffle to go behind the diagonal. It's a scrap of plywood left over from the dulcimer project. I painted it black and taped it in place.

So... let's go shoot something that the Canon can handle in lots of moonlight without benefit of a narrow-band filter. M35 & NGC 2158 will do.

Guiding is a bit off: lots of back and forth in RA with an occasional kicker in Dec. The OTA works well -- feels solid, shows only the expected degree of coma in the corners of the 20D's frame (on second thought, the optics are not collimated well enough to say this is coma rather than misalignment). Erratic guiding is the problem that must be solved.

Flex in the Linhof? Imbalance somewhere? Guiding at too short a FL? Too slow a correction rate? Should I put the big G11 tripod back to work? Rain is forecast.

Feb 24. After much pushing, pulling, rocking, and feeling, I've decided that there's too much weight (or too much moment arm, too much inertia of some kind) for the Linhof to handle. I can feel (and see) the tripod twist under very modest changing loads. Remove the guidescope, it improves. Remove one 11 pound weight, it improves even more. We're just that close to its limit. A load applied at the guidescope's top mounting point produces a lot of what feels like bending; the deformation is transferred and redirected in complicated ways all the way to the ground. I couldn't begin to guess how far or in what direction pointing might change as the guiding motors switch on and off. Just get rid of as much of this motion as possible and not worry about it.

I c-clamped the guidescope here and there to test balance and clearance and eventually decided to remount it on the bottom of the corrector frame. The lowered CG allowed me to dispense with one 11-pound weight. I'll deal with the imbalances this produced. I seperated the finder scope from the guidescope and mounted it on the center frame diagonally opposite the guidescope and as far aft as possible -- the better to offset at least some of the guidescope's weight. I moved the AstroZap dew shield from the front of the corrector (where, despite its slight weight, it contributed quite a lot to the instrument's instability) and wrapped it instead around the primary mirror, inside the truss, where it is easily removed, secure, and makes a dark baffle for the primary. It lends its weight just where most needed. Good deal.

I asked Amy to twist collimation wingnuts while I watched from the eyepiece tube. We spent no more than two minutes at this but made a substantial improvement. I took aim at the Moon and panned around with the hand control. The mount seems to move crisply, with less rebound, better. The field is noticeably flatter: focus on craters midway along the terminator and the Moon's limbs are sharp at the same time. We'll see how it all works when the clouds go away again.

Feb 25. Replaced the plywood with a less weighty and much darker backstop: a piece of 0.12" acrylic, textured on one side, smooth on the other and holding on its smooth side a swath of self-adhesive Protostar flocking cloth. You can cut this acrylic neatly on the table saw, and the Protostar flocking adheres instantly and very firmly.

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Cameras behind Telescopes:
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Text & Photos by David Cortner
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M35 & NGC 2158
Canon 20D, 3x300, RAW, ISO 400
(Doesn't look any better big.)

Smaller, lighter, better?

 

 

 

 

Observing kit when in the
throes of modifications.