From its design many years ago, this mount was always intended to carry a large refractor so I designed it with the heaviest bearings and the most rigid structure I could imagine. Yet once it was complete, I realised I could have imagined a lot harder! Never underestimate how solid a telescope mount needs to be, especially with big refractors. This 50kg welded steel structure does a pretty good job but I can see why the great makers of old like Thomas Grubb used over a ton of cast iron to park a 7" refractor on.
I've included a lot of detail about the construction process for the interest and amusement of my fellow astronomers but please be aware that what I did wasn't necessarily the best approach. It was just what I could get to work with the resources and experience I had at the time.
Large setting circles were an important aspect of the design, for both aesthetic and practical reasons. In my youth, I had a 60mm equatorial refractor with piddly little setting circles that were no use to man nor beast. so here I wanted to achieve a setting resolution of better than one degree. It then becomes possible to set the telescope such that any object will lie within the field of view of the main scope. in effect, it's a Victorian style "go-to".
I began with a sketch of what I'd ideally like the end result to look like.
Then some engineering drawings to explore how it might actually work
The hand sketched parts were converted into dxf files using Illustrator then sent to the waterjet cutters where a jet of water and fine abrasive under ridiculous pressure can cut through 5mm steel plate like a hot knife through butter. I'd never believe it would work unless I'd seen it!
The support block of the mount made from welded 3mm steel sheet. The mount itself attaches via an M20 bolt through the centre of the latitude adjustment. In the background, Jeanne Hébuterne watches the process with mild amusement.
For the mount to work properly the RA and declination shafts must be exactly bang-on perpendicular to each other. With a welded construction like mine, that's virtually impossible to achieve unless there's a bit of post-weld machining to square everything up.
The counterweight shell. Notice the black nylon core that forms the bore down which the declination shaft will slide and the nylon tip embedded in the locking bolt so that it doesn't chew up the polished shaft when tightened. The counterweight was filled with lead to get it up to 20kg. Please be aware that hot casting 20kg of lead is very, very dangerous! Don't attempt it unless you're an expert caster. Spilling a 20kg ladle of hot lead down the front of your pants is going to be an unpleasant experience and the slightest drop of water can cause a steam explosion that will throw molten metal in your face!
The finished counterweight being polished in the lathe
The decorative ball at the end of the declination shaft doubles as a safety stop preventing the counterweight falling off if the screw comes loose. Again a safety measure. The gravitational attraction between 20kg of lead and the Earth is a physics lesson your foot won't appreciate :)
Just some of the hundreds of parts that make up the telescope mount
Early stages in the assembly - testing how the various parts couple together and move. Notice the numbers on the circles are written with black texter. At this point I hadn't found anyone who could do the engraving.
Most houses have a special room with wide bench-tops for assembling large telescope mounts and if you're a tidy worker those rooms can even double up as kitchens :)
The secret to a good mount? Every part bigger than every other part - it can never be too solid. The end plate is held on the declination axis with a high tensile m24 bolt 18" long.
This was the most difficult part of the whole job, getting the circles engraved. Lot's of people will CNC engrave a flat plate but it's hard to find someone who one can handle a circle. I was very lucky in that the university I worked for had lots of old 1940's machine tools and some very skilled old craftsmen who knew how to use them. With much gratitude to Herb and Peter, the engraving did finally get done.
The RA circle has numbers engraved on screw-on plates rather than directly into the circle so that the order can be reversed for northern or southern hemispheres.
The declination circle and its indicator
Close up of the RA drive chain
The electronic drive for the mount consists of a simple relay switched 12V DC motor for declination adjustments and a stepper motor drive for tracking and slewing in right ascension. It's not cutting edge technology by todays standards, but it works perfectly well for visual observations and basic astrophotography.
Most of the drive control electronics is contained within the "Victorian" hand controller so that it can easily be unplugged and taken to the workshop bench for any maintenance or improvements that become necessary.
Of course just because your technology is relatively primitive doesn't mean you don't have to debug it a bit to get it working!
This is a 2 minute exposure of the globular cluster m13 taken at prime focus of the 7" refractor with the mount driven but unguided. It isn't nearly a good as the best commercial astrophotography mounts, but as you can see, it definitely does the job!
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