The following notes describe the building of a powerful, fast, lightweight telescope. By powerful, I mean it is of sufficient aperture to grab a LOT of light. By fast, I mean that its focal length is aggressively small (below f/3), resulting in a compact, lightweight, telescope that can be easily moved between locations.
My last telescope was a modest, 8” f/7 Newtonian with a Dobsonian mount. It was luggable owing to the fact that I cut the optical tube assembly into two, that can be easily re-assembled in the field. Still, that resulted in two tubes each about 36” in length: luggable, not portable. And the base was heavy and solid.
The impact to me was that I tended not to move the scope around from its primary viewing location, the cottage, to back home in the city during the winter when the cottage was closed. This meant I missed a lot of great viewing.
Besides that, I was always pining for more aperture. 8” was nice, but I was always envious of images taken through 12”, or better, 16” scopes. And as I developed more disposable income, I secretly wanted to treat myself to a Meade 16” LX200. Luckily (I suppose) I never quite had that much disposable income.
Fast-forward to the summer of 2015, when I randomly came across Mel Bartels’ website , and this awesome telescope:
I was blown away by its elegant design, and even more blown away by the compactness of the optics. This was my new scope. And what you are reading now is my odyssey to having it.
The Meniscus is the Thing
There are, of course, many components to a telescope that contribute to its weight. But as aperture increases, the weight of the glass in the mirror becomes more and more dominant. So a key motivator to having a large but lightweight telescope is to somehow reduce the weight of the mirror.
There are various approaches to reducing the primary mirror weight, such as making mirrors from composite materials, or casting mirrors with open honeycomb cells. But a more accessible approach taken by Mel Bartels is to make the primary mirror a meniscus: like a big contact lens. The (empirically observed) benefit of the meniscus is that it is inherently stiffer than a flat piece of glass and, consequently, the meniscus can be thinner for the same overall stiffness. Indeed, Mel’s mirrors are generally 1/2” to 3/4” for mirrors 13” to 24” in diameter (!), where a traditional mirror in those same dimensions would be 1 to 2 inches (at least). This results in dramatic weight reduction.
But a meniscus has another important benefit. As the mirror gets faster, the curve in the glass is deeper. For a traditional glass blank, that means that the difference in glass thickness between the edge and centre is substantial. This creates differential cooling in the mirror, affecting its initial performance. It also means the glass needs to be inherently thicker to allow sufficient glass to be behind the centre of the mirror. Both these issues become non-issues in a meniscus because the glass is uniformly thick (thin!) from centre to edge. You have a stiff, lightweight glass that cools quickly for use.
You can read more about the benefits of meniscus mirrors here .
How to Make This Telescope
The assumption is that general telescope-making techniques are familiar to the reader, so I won’t actually describe how to make a telescope. But these pages will detail how to make this telescope, with the nuances related to fast, meniscus mirrors.
The details described will include:
- creating a computer-controlled kiln;
- slumping glass into a meniscus;
- grinding, and polishing, the mirror;
- figuring (parabolizing) a fast mirror;
- building the telescope tube and mount.
Note that this is currently a work-in-progress, so not all details will available. For an up-to-the-minute status of this project, you can follow the build on the Cloudy Nights forum.