400 mm telescope building

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After months of reflection, reading, and sleepless nights browsing the web, I finally jumped in the bandwagon and started building my own telescope ...

Without rewriting the Dobson "revolution" story, which is btw very nicely depicted here, I will limit myself in summarizing reasons why I chose to build my own telescope, and choices that were made for that :

Theoretical principes being nowadays completely mastered, anybody skilled enough can succeed in telescope building.
Even if we are in a "mass" market for astronomy today (which is very good), only a custom made telescope can fully answer to its user needs (transport, visual or photographic use, kind of object observed, operating, etc...).
A self made telescope remains cheaper that a bought one, giving access to larger diameters...

Getting started...

Once the decision is taken, there is still need to think a minimum about the way this is going to be built. Thanks to the Internet, I found all the necessary information, as well as numeros examples from fellow ATMs, that allowed me to get started with confidence. Now it's my turn to share my experience on this adventure...

On the design side, I spent some time imagining how I would like my instrument to be, and how would I use it. This step must not be overlooked, and necessary time has to be allocated to it, else the risk of getting to a hazardous construction gets high, with the telescope builder giving up the construction. This step of reflection and design, with the help of CAD, led me to workaround many issued (but not all), and made me gain much time on the making phase.

Some details about the design are explained here

On the building side, there is room for choice : wood or metal based structure, or a mix of both, all being glued, soldered, screwed together... As for me, I'm comfortable with a saw and I am equipped for metal machining (lath, mill). But I never soldered anything else that electric wires. Then it will be a wooden structure, with aluminum assembly for all mechanical parts (mirror cell, truss structure, etc..). This combines lightweightness, stiffness and reduced price.

Details about the building are described here

Characteristics

Here is summarized the specification of my T400 :

- Primary : 400 mm aperture, and 1800 mm focal length (F/D 4,5).
- Newton optical design, with a parabolic primary and elliptical plane secondary mirror of 88 mm size (22% linear obstruction).
- GOS primary mirror (the same which equip Meade Lightbridge units), on a 18 points mirror cell, and "Strock" style collimation system.
- "Truss" structure (using tubes), which can be mounted in a matter of minutes. Mounting and collimation can completely be done without a screwdriver.
- Alt-az mount (Dobsonian type), with an oversized rocker and pseudo-annular base (with no central pivot).
- Overall size : 54 cm width, 1m70 length (no need for a ladder ;-)). Estimated weight : 40 kg.
- Building time : 3 months, working during week-ends. Design time : about 3 months from the idea to the CAD drawing completion.

Inspiration sources

Just as other fellow ATMs, I widely got inspired by existing telescope buildings. I'd like to greet the following ATMs, from whom I "stole" ideas :

Laurent, aka Basque en Balade on Webastro (see the building of his "Monstro")
Avet on Webastro (see his site Astro Dob)

But I can also mention the following sites to know everything about amateur telescope making :

Alt-Az - the reference website in Dobson making in France
website of Serge Bertorello which is also a reference
the website of Magnitude 78 club, for the building of the travel telescope (the know so famous "Strock")
Webastro website & forum, which has many amateurs & experts :).
Astrosurf website & forum, which is the other big amateur community in France.
The Dobson Factory website & blog, spécialist of big aperture Dobson making.

Progress status

Project is complete, and the telescope is now fully functional, and has been operated on the field for more than 2 years, with its new mirror set. Just enjoying it now.

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