Introduction
Following my 16 inch telescope project for visual observation, of which you can see the results here, I was finally able to enjoy deep-sky visual observation… but like all astro buddies, the temptation to go further was strong—to see more objects, to see more colors, to truly “see” and no longer just “guess” at the most classic objects… in short, to take the experience a step further.
This project began in the summer of 2013. The 400mm was complete, and it had received its new mirror, made by Franck Grière. Pure bliss, and usability that perfectly suited this “portable” telescope.
Then came the desire to go bigger… Some deep-sky objects are still quite disappointing, like the Crab Nebula, for example, or certain galaxies whose structure I’d like to see. Others, like the Orion Nebula, are pure joy—you can spend hours observing them… but apparently, with a larger scope, you can see colors and even more details.
And now that I’ve successfully built a telescope, I know I can master the different aspects of Dobsonian construction, so I can tackle something bigger without worrying about whether I’ll see it through to the end.
Next was the choice of diameter: I quickly resolved this dilemma—there needed to be a real quantitative leap to make it worthwhile. So, the choice was made: a 600mm at F/D 3.3, so I wouldn’t have to climb the Eiffel Tower every time I wanted to look through it…
The specifications are different from the 400mm. Of course, the telescope must be transportable—in the sense that it must fit in the car—but this time, I’m prioritizing sturdiness so the mount can match the mirror I want to use. It’ll be heavy (I’m estimating around 80 kg), but mounted on wheels for transport (loading into the station wagon with a ramp). All that’s left is to proceed…
Project Definition
The design followed the same approach as the T400: pure optics using well-known tools (Newt, Plop, etc.), then 3D modeling on the computer, with a healthy dose of inspiration from what experienced artisans and amateurs do (ah, Dobson Factory…). But I’ll still adapt it to my own style, with a few points that seemed important to me:
- The structure will, of course, be tubular and symmetrical with 8 tubes. It’s not theoretically perfect, but in my opinion, it simplifies fabrication. In particular, it limits the number of different parts, which is important (machining 4 or 8 identical parts is always easier than making unique pieces).
- No wood for the tube structure, except for the secondary ring. In particular, the entire primary cage is made of metal (aluminum mirror cell and steel frame). The tubes will be carbon fiber for good thermal stability. On the other hand, the rest of the structure (rocker and base) will be standard plywood to achieve a solid, not-too-heavy, and easy-to-build result.
- The mirror cell will be entirely machined from aluminum, with 18 support points distributed in 6 triangles on 3 rocker arms. Each triangle is mounted on a ball joint, and each rocker arm on bearings to minimize friction and best support the back of the primary mirror.
- The mirror is laterally held by a steel cable (the chosen mirror cell structure didn’t allow for secure lateral bearings).
- Finally, for user comfort, I’m considering motorization at several levels (collimation and tracking, of course). Handling this kind of machine is not the same as pushing a 200mm scope with a strap brake…
The construction allowed me to innovate on certain points since the T400: some parts will be arc-welded steel (it’s less difficult than it seems). This allows for a robust enough structure to support the ~40 kg primary mirror. All tube attachments are machined aluminum to get parts tailored to my needs without any concerns about durability.
The mirror will still be well “protected,” as it will receive a baffle to shield it from stray light, but above all, from the crowds of people who gather around during public observing nights. This might even make the light shroud optional (we’ll see).
Specifications
Here’s a summary of the specs for my T24:
- Primary: 24 inch (600mm) aperture and 6.5 feet (2000mm) focal length (F/D 3.3).
- Optical design: Newtonian with a parabolic primary and a 150mm flat elliptical secondary (25% linear obstruction).
- Primary mirror by Mirro-Sphere, in a triangular cell with 18 support points, entirely on bearings and ball joints.
- Collimation via the triangular frame of the mirror cell, and thus from the front. Motorization under study.
- Truss (tubular) structure, 36mm carbon fiber, with quick-release attachments for disassembly. Assembly and collimation entirely tool-free.
- Alt-azimuth Dobsonian mount, with a lowered rocker and wheeled base for transport.
- Overall dimensions: 3.2ft wide (to fit in the trunk), 7.2fr tall. Estimated weight: 180 lbs.
- Construction time: From design to first light, about 18 months in my spare time, with an “intense” crafting phase over the last three months (evenings & weekends).
Construction…
Or how to properly execute a project, from design to field… it’s all >> here <<
