Archimedes 10" f/5.4
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- Sharp bright images
|Inspired by the Sky & Telescope article
in June 1990 about Tom and Jeannie Clark's 14.5-inch "Pocket
Scope", I decided to mount my first self made mirror, a 10"
f/5.4 as a Dobson. In Germany at that point in time Dobsonians were seen
as exotic light collecting potato cases and not as precision
instruments. For me, it was the only way to go. Many others changed
their mind too after looking trough it and asked themselves, why
they spend so much money for their 4" Zeiss refractors. Today
60% of all scopes at European telescope meetings are Dobsonians!
I tested the mirror with a rudimentary Foucault Tester of a piece courtain rod +bicycle bulb on mimileter paper (see original test chart from 1990 and later FugureXP analysis with the original data). An interferometer test in 2021 reveald a good match with the old method.
Many years later I remounted this mirror into the much more compact version "Travel Archimedes" and made for this initial one a new 10" f/6.25 planetary mirror. It became a really good one at approx. Lambda/10 wave (see Foucault test analysis.pdf). Though, it is amazing, how sensitive the star test is: I still can see defects in extrafocal images.
Following the Clark design, the focuser is removable with 2 hand knobs in order to make the secondary cage fit into the mirror box which fits itself with the trunnions removed into the rocker. The collapsed box is 40 x 40 x 50 cm (16" x 16" x 20") and can be transported in an airplane, as I did on a trip to Tenerife.
Since I used 15 mm (0.6") Birch plywood for the mirror box and rocker and a heavy 80 mm finder, the total weight went up 28 kg (62 pounds). Later I replaced the annoying star diagonal finder by a Telrad, as you see in this pictures at left. What a difference to the latter travel design at 10 kg (22 lb), but at star parties other people feel more comfortable, to hold some more material in their hands.
The diagonal is 54 mm (2.14"), so at 21% linear obstruction not very small. I am not enthusiastic about exceedingly small secondaries, the mirror Quality is what really makes the difference. It's kept in place by a Novak secondary holder and spider.
The trusses are fixed at the top and low end with Ø 17 mm (0.67") aluminum studs with a bore to accept the M6 (0.24") bolts. The studs are epoxied into the trusses. This method is very stable and keeps collimation, but requires to screw and unscrew the 16 clamp knobs. See the later developments at my other designs. Despite that, with some practice, set up still takes no more than 8 minutes.
The altitude trunnions are covered with smooth polyurethane laminate which shows a bit more friction, but works fine in combination with the relatively small diameter bearings. In the following minimalist designs I used larger bearings. For the azimuth I changed later to pebbled "gold coated" aluminum sheet, which proved superior.
If you consider, to build your own first Dobsonian, there is not much to make wrong, if you follow a similar design.
Be aware, that a
minimalist Dobsonian is by comparison for several reasons by far more
difficult to make:
In memory: My very first mirror in Astrotreff Forum (German).