paulbrowne21 days ago
14 requests, 500kb total size
I also recommend anyone wanting to grind their first mirror to read about modern ways of testing in addition to all the classic books (Texereau, Sam Brown, Lecleire) about mirror making.
Bath interferometers changed the game and allow to reach λ/10 wavefront with certainty and repeatability compared to Foucault testing. They are affordable and there's a healthy community around DFTFringe, the de-facto standard interferogram analysis software at interferometry.groups.io
You can also find a Foucault + Ronchi + Bath combo tester's plans on Printables.com and a companion three-axis-table, allowing great testing ergonomics for a low cost if you have access to 3D Printing.
The best resources on how to setup a Bath Inteferometer can be found on the GAP47's website (french, but machine translatable) and GR5's YouTube channel.
Have fun :)
Currently we have some tiny photon sieves, around 1.5mm aperture ~f/14. The next step is going up to 60mm @ f/6.5. The end goal, and I don't know how achievable this is, will be a very large aperture panelized scope. We've discussed making something unsteerably large, sticking it in a field and using the Earth's rotation to sweep the sky.
There's a little bit of trickery to reduce harmonics, though I'm not sure how it'll perform in practice. Please get in touch if you have experience doing diffraction simulation. After first light I plan to write everything up.
Do not hesitate to post results on cloudynights or your club's website, I'll refresh it from time to time ! I do not think I have enough diffraction experience to help though.
Best of luck for your endaevor.
https://louisville.edu/micronano
After the next prototype run, I'll probably use them or MIT's Nano Fab lab.
https://en.wikipedia.org/wiki/John_Dobson_(amateur_astronomer)
I later went on to make a 16" and then "fell off the wagon" and bought refractors, equatorial mounts and cameras. But I never could have gotten started without him.
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.
Bringing telescopes out of the rarefied world of astronomers where they were "precious" to professional and amateur alike is what I see as his greatest legacy.
I build "public friendly" scopes as a result. If anyone is thinking of a new mount for a Newtonian may I suggest looking up "Sudiball" mount as they allow you to accommodate a wider range of eyepiece heights for a given target. (so parents are less likely to put their kids in a half-nelson screeching "DON'T TOUCH DON'T TOUCH" as they poke them in the ear with your scope)
Btw, for those very interested it looks like they have a yearly convention in VT, with registration opening May 1 — https://stellafane.org/convention/2025/index.html
The keynote speaker last year was talking about the James Webb Telescope build, absolutely fascinating. There are tons of things to do. They also have a competition which judges on various aspects of telescopes. I have really enjoyed growing up around this convention.
Involves chipped paint and household washers.
https://hackaday.com/2020/04/29/test-equipment-shim-washers-and-a-30-year-old-space-telescope/
Simon Winchester also covers it in great detail in his book Exactly: How Precision Engineers Created the Modern World
If you're interested in precision making and how it all came to be it's a very joyful read.
It's not life-and-death, but it's pretty satisfying nerd work.
Real amateurs modify their whole house to fit in the telescope, apparently.
Younger I made a 6" scope from a bought mirror set, and the first time I used it I caught one of Jupiters moons occulting in realtime.
Aperture rules until the work to set up means it sits unused.
The objects you mention are bright (and small) and can be seen in anything, including nothing, which fills department stores with small scopes known to amateur astronomers as "Hobby Killers".
So immerse yourself a bit, pick up some language and basics, then find a local club to try before you buy, clubs or generous members may have a loaners scopes so you can figure out where you want to be on the what you can see v.s how much effort you can put in scale.
cheers
With visual telescopes, bigger aperture is better. Not only do they collect more light but they are able to resolve more detail. 8" or 10" Dobsonians will offer much better viewing, but at the expense of size and cost.
Also, a solid-tube 6" Dobsonian may outperform the Heritage 150p in areas with a lot of streetlights, as the collapsible design can let in a lot of stray light. I made a shroud for mine, which helps, but it isn't perfect. You can't store a 6" solid-tube Dobsonian on a shelf though.
Finally, don't underestimate a pair of 10x50 binoculars. You won't get a great view of the planets, but the skies are much more impressive through them.
And the website seems to answer that question :)
https://stellafane.org/tm/dob/ota/tube.html
(that said, there's a LOT more to it...)
Impressive stuff though, coming from a former professional astronomer who never built a telescope from scratch.