Slashdot Mirror
Cold War Spoils: Amateur Builds Telescope With 70-Inch Lens
First time accepted submitter 192_kbps writes "Mike Clements, a long-haul trucker from West Jordan, Utah, built the largest amateur telescope ever with a whopping 70 inch primary mirror he purchased at auction. The entire telescope is 35 feet tall, 900 pounds, and he hopes to tour it in parks. As a hand-turned Dobsonian the telescope lacks the photographic capacity and tracking required for professional astronomy but the views must be breathtaking." (Are there other compelling candidates out there for "largest amateur telescope ever"? The 71" scope listed by nitesky.org appears to be dormant.)
He put a reflective metal coating on a purchased piece of glass with the proper final curvature.
Just a heads up for you non-Brits. There will be truth in this article... somewhere.
Technically Lord Rosse (https://en.wikipedia.org/wiki/William_Parsons,_3rd_Earl_of_Rosse) was an amateur, and his telescope was 72 inches.
I read it the same way.
They probably cut the mirror and polished the glass, and then the edge chipped.
A chip in the glass could be a fatal injury for a spy satellite as the article suggests was the intended use. Such telescopes use active optics to improve image quality; they apply pressure over the glass to bend it slightly. A chip could have micro-cracks and other damage that would easily spread across the surface. Without the actuators deforming the glass the image won't be as clear, but it would be good enough for a hobby telescope.
Once the glass chipped they likely just stopped the process, so the new owner would need to add the mirror surface on his own.
//TODO: Think of witty sig statement
Here is the scoop on the 70" telescope. Mike Clements purchased a polished but uncoated mirror that is 70" across that was intended for a spy satellite project that was cancelled. A huge uncoated mirror is not a telescope anymore than (car analogy - wait for it...) a V8 engine is a racecar. Building a good performing telescope (collimation tolerances are measured in thousandths of an inch) is a significant task, a huge telescope like this is a major engineering feat. What's more this is a transportable telescope. It is possibly the biggest transportable telescope in the world. This telescope is more powerful than any telescope that existed before 1917 (when the 100" Hooker telescope saw first light).
Successfully silvering the mirror using updated 19th Cedntury mirror coating technology was nifty too.
Second class citizen of the New Gilded Age
Cloudy nights thread and a another news article.
It was silvered with a spray-on solution using a weed sprayer; much too large for the regular vacuum deposition chambers.
-R C
"'Tis great confidence in a friend to tell him your faults, greater to tell him his." --Poor Richard's Almanac
Reflecting telescopes use front-side mirrors. The glass is just there to provide the shape; it is not part of the optical path.
Why? It's in microgravity and temperature controlled.
When you're in orbit, "temperature controlled" is a slippery concept. You've got direct sunlight on one side, dark space on the other side, temperatures to the fourth power fighting it out, and no air to redistribute heat -- and an hour later, the sides will have switched.
Remember how when it first went up, the hubble had problems focusing clearly? The designers forgot that its mirrors would be deformed/reshaped by the lack of gravity. Essentially, the hubble's primary mirror was optically designed to work as a telescope mirror on earth, not in space.
Uh, no. That would've been an amateur mistake to make and didn't happen.
Instead, the amateur mistake made was not to properly verify that the grinding
machine was actually grinding correctly. They even ignored measurments by
another instrument showing a faulty shape, assuming the instrument to be faulty instead.
And skipped the final post-assembly check to save time and money.
The mirror simply was ground extremely precisely into a wrong shape, and nobody noticed.
But as always in cases like this, the whole story is more complex and consists of a lot of
things not going as planned. It's a good and instructive read.