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Old 08-10-2021, 10:08 PM
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Dieselten (Ned)
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Join Date: Jul 2009
Location: Wollongong
Posts: 10
First Light Modified 12" F5 Dobsonian

About ten years ago I purchased a home-made 12" F5 Dobsonian truss-tube 'scope with several eyepieces and a Telrad. The vendor assured me it was a good high-contrast 'scope, but added it had a few design and construction issues.

Time was to prove it was indeed a good high-contrast 'scope, with a decent GSO primary, a secondary of unknown origin but quite acceptable and a JMI single-speed focuser. The eyepieces were all Plossls, but good ones for someone taking their first steps in observational astronomy. The clarity of a good Plossl still pleasantly surprises me.

I used it in the heavily light-polluted suburb I live in, but it was heavy and awkward to set up. The UTA was way too heavy and over-built, the entire 'scope was top-heavy, resulting in the CoG being too high, and the altitude-bearings were too small (8" diameter) and mounted very high on the rocker-box; in other words, over-built and just way too heavy. Here's a couple of photographs of what it used to look like.
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Lockdown afforded me the time and motivation to take this 'scope in hand and radically alter it, with the aims of reducing the weight as far as possible, making it easier to set up and dismantle, and improving usability.
The old UTA was removed, and a totally new one built, using two plywood hoops and plywood spacers, with a mounting-plate for the focuser and Telrad. The "Swiss-cheesing" was done using Forstner-bits, drilling from both sides to get very clean holes. That worked really well.

Here's the new UTA during construction:-
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and here it is finished and varnished:-
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The ground-board was also way too heavy, and could benefit from slimming down by removing two triangular pieces, thus:-
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and after modification (which removed1.6Kg of weight in a few seconds!):-
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The old altitude-bearings were too thick, too small and too heavy. I made new ones (15" diameter) and lightened them by routing away un-necessary wood:-
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So, once it had all been put manufactured, varnished, painted with blackboard-paint where it was needed and assembled, it looked like this, waiting for "first light" in its new form:-
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First light took place after dusk on Friday 8th October 2021, using Jupiter in the field of my Explore Scientific 11mm eyepiece to collimate the Telrad.
It took a bit of fiddling to get the planet centred, but once it was and the Telrad was collimated, I spent a few minutes observing Jupiter and saw the Great Red Spot for the first time, noting the four Galilean moons with little Io hugging the planet.

My go-to eyepiece these days is the Explore Scientific 11mm 82-deg FOV, and the 'scope was balanced principally for this eyepiece. That said, the new altitude-bearings work with a 42mm Panoptic (heavy!) right down to a 9mm Orion (very light). (The Formica for the bearings came from Astrogoods in the US, my thanks to Mark for his help.)

Saturn was partly obscured by the clouds, but I spent a few more minutes observing it as the planet faded in and out due to high cloud, then I packed up and put it all away, extremely pleased with what had started out as a rather unpromising evening to attempt "first light".

In the process of modifiying this 'scope, I removed over 7kg of weight, moved the CofG down to a point 15mm below the top of the rocker-box, lowered the height of the assembled telescope in the rocker-box and generally improved both the functionality as well as the aesthetics. I'm more than happy with the result because it makes the telescope so much easier to use.

Lockdown has been a test of endurance for most of us, but without it, (and the patient gathering of some woodworking tools over the past few years), I doubt I would have attempted this makeover. (It occurs to me that this thread might belong better in the "Equipment Discussions" forum, and if the moderators wish to move it I have no objections at all)

Last edited by Dieselten; 08-10-2021 at 10:14 PM. Reason: Spelling & Grammar
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