| HN Mirror

People don't talk about it now but when he first started the "established" telescope making folks spoke derisively of him, his techniques, and his telescopes.

Why?

To start with "we" - he and his students - made the mirrors out of old portlights (the glass in portholes), so it was "assumed" they would flex (since they were thinner than store-bought mirror blanks) and would be subject to thermal issues.

Then of course was the fact the telescope tube was made out of a heavy cardboard concrete form called a "Sonotube", which you'd waterproof and paint - paint color and pattern choice being one of the most creative parts of the project. The "diagonal" - the mirror which directed the light path 90-degrees out to the eyepiece - was mounted on a 1"-2" dowel with 3 slots cut into it and held in place by wood shingles.

The mirror mount itself was a 3/4" piece of plywood with 3 bolts in it, which you'd use to collimate the mirror once it was mounted in the tube.

And then the mount. Not only was it "alt-azimuth", it was made of plywood. You built a box around the tube, and two circles on the box fit into 1/2 circles in the mount.

There are more details on the Stellafane page - https://stellafane.org/tm/dob/index.html - but those are even fancier than the ones we made!

But Dobson's ultimate heresy was his approach to figuring the mirror:

Instead of using a "Foucault Tester" to measure and figure the mirror, he'd mount the polished mirror in the telescope and point it at a point source of light - usually the sun's reflection off a ceramic power line insulator.

By moving the image in and out of focus and looking for bright rings in the image, you could tell the shape of the mirror and whether is had hills or valleys in the figure. The end result was a parabola accurate to 1/2 or 1/4 wave (he said he could get it to 1/10th wave, and I have no reason to doubt it).

To the folks used to using much fancier foucault or even more advanced testing methods on much more expensive mirror blanks this was impossible and widely derided and, frankly, made fun of. People weren't very nice.

But when they took the mirrors and tested them with their foucault and diffraction testers they got a big surprise - the curves _were_ accurate and of high quality. And, _big_ - people regularly made 16" telescopes this way, and the San Francisco Sidewalk Astronomers had a portable 24" for goodness sake.

(I think people kind of forgot he used to be a physicist, and probably knew a thing or two about light).

The other big beef was the alt-azimuth mount. Not only did it not have setting circles to find things in the sky by RA and Dec, it wouldn't automatically track, so it could never be used to take pictures (you can get Dobsonians which will do that today natch now that we have computer controlled stepping motors).

But the point was _none of that mattered_: He wanted to make telescopes for people to look through, not take pictures with. So if he could build a telescope he could wheel out into Golden Gate park, set up in 15 minutes, and have 100 people see stars, planets and nebulae, that was The Win.

And teaching regular people - including kids - of both genders - how to make their own telescope, well that was almost as good. A big part of that was it was _affordable_, which meant many, many more people could make telescopes than otherwise. In Dot-Com vernacular, he grew the TAM (Total Addressable - or would that be Astronomical - Market), well, astronomically.

(Bada-Bing, I'm here all week folks).

But seriously, I can tell you from experience, no astrophotograph you take will ever, ever, compare to seeing Saturn, or M31, or any one of many other things with your own eye, and in a telescope you built.

Sorry for the long screed - got started and stirred up some memories there.