My 6" f7 home/club made mounting.
Posted: Wed Oct 20, 2021 10:42 am
Some of you may have heard me talk about the home made telescope I made when I was a teenager. I often use the OTA on my EM200 GOTO mount but today, I brushed off the cobwebs of it's original push to visual Porter-Springfield inspired mounting. The original Porter-Springfield mount designed in the early 20th century used an offset tube and an extended counterweight to divert the focus down the optical axis to a diagonal on the polar axis. This resulted in a fixed eyepiece position facilitating sketching at the telescope. The following illustrations show a newtonian and cassegrain implementation.
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The Late Great Cliff Duncan, president of theATM club of Queensland and the designer of the whole series of mountings, used the broad bearing surface principle from the Porter-Springfield for stability but did not want to implement the polar axis eyepiece position. Cliffs design mantra was that the diameter of the bearing disc surface should be the same as the mirror. So the bearing discs on my mount are 6 inches to match the six inch mirror.
Most of the parts were cast in aluminium. Each member would buy their own Aluminium ingots at the local metal supplier and then everybody would pitch in and help with the various tasks, conditioning the sand, making the sand moulds, firing the furnace. Cliff was ingenious. His furnace used a blower fan to aspirate a mix of sump oil obtained free from his local garage and kerosene. The fuel tank was hoisted into a tree by a pulley then the fuel gravity fed down and into the industrial blower and into the furnace. The club would have "foundry" weekends when the club furnace would be fired up, sand moulds formed and laid out then poured and cooled. My nickname in the club was "Hacksaw Joe." This sounds derogatory but was given to me to acknowledge that I could cut the feeders off the castings with more precision than most of the members - a process known as fettling. The more feeder metal that stays on, the more time it takes to machine it off. But it the hacksaw ever ran into and damaged the casting, it would be discarded and thrown into the remelt pile.
Cliff owned all the equipment and generously made it accessible to club members. The foundry was in Cliffs back yard. Under his house were two lathes, drill press, linisher and other hand and machine tools. There was no milling machine but Cliff used a variety of techniques on the lathes to do what milling had to be done. There was also a small optical polishing room and an optical testing tunnel for Foucault and Ronchi testing. Final figuring was done by testing with a Coudé mask over a Foucault test.
The parts that were cast and machined were mirror cell, finder brackets, focuser base, cradle and yokes, rotation rings, bearings, angle bracket between the bearings, aluminium legs, zinc counterweight, and the ATMQ badge. The spider was fabricated (machined on lathe) from brass bar and spring steel arms were brazed into the body. Brass tubing was also use to make the upper part of the focuser.
The mount has just five pieces(below).
The cast aluminium pier legs have milled slots that hold the leg bolts in place so that you only need one spanner to attach the legs to the pier. I know I know, there are two spanners on the grass. 
Despite the four 6" diam 1" thick bearing discs. The equatorial head is quite light. Below I am lifting it with only a little difficulty using the fingers on my right weak hand. I have an arthritic wrist and fingers. I have not weighed it but I'd guess 7kg.
The head attaches to the pier with a hand tightened 5/8 bolt and ball washer then the final latitude angle is adjusted with 4 3/8 allen head bolts that surround the main bolt to adjust. There is no azimuth adjustment but the mount is easy enough to nudge around to the pole and it is only a push to so precise polar alignment isn't important.
The counterweight is made of cast zinc and attaches by hand without tools.
The counterweight shaft is machined from 1.5" steam pipe. If you look carefully you can see a slot milled up the side. Inside the pipe is a 5/8" W threaded rod which is turned by the steampunk cast aluminium wheel at the end of the counterweight shaft. As the thread turns, it drives a travelling nut with a protruding fork that pushes and pulls the counterweight up and down the shaft. A really beautiful if arrangement allowing very precise adjustment of RA balance to make pushing very smooth. The large disc bearings can be pushed with one hand on the counterweight shaft to smoothly track the scope even at 300x magnification or more. The counterweight shaft is not mechanically part of the dec so using the counterweight shaft won't ever mover the scope in dec .
As soon as I set this up it looked like it was about to rain so I covered the mount even though the mount is basically waterproof.
I will show you theOTA and rotation ring system tomorrow.
Joe
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The Late Great Cliff Duncan, president of the
Most of the parts were cast in aluminium. Each member would buy their own Aluminium ingots at the local metal supplier and then everybody would pitch in and help with the various tasks, conditioning the sand, making the sand moulds, firing the furnace. Cliff was ingenious. His furnace used a blower fan to aspirate a mix of sump oil obtained free from his local garage and kerosene. The fuel tank was hoisted into a tree by a pulley then the fuel gravity fed down and into the industrial blower and into the furnace. The club would have "foundry" weekends when the club furnace would be fired up, sand moulds formed and laid out then poured and cooled. My nickname in the club was "Hacksaw Joe." This sounds derogatory but was given to me to acknowledge that I could cut the feeders off the castings with more precision than most of the members - a process known as fettling. The more feeder metal that stays on, the more time it takes to machine it off. But it the hacksaw ever ran into and damaged the casting, it would be discarded and thrown into the remelt pile.
Cliff owned all the equipment and generously made it accessible to club members. The foundry was in Cliffs back yard. Under his house were two lathes, drill press, linisher and other hand and machine tools. There was no milling machine but Cliff used a variety of techniques on the lathes to do what milling had to be done. There was also a small optical polishing room and an optical testing tunnel for Foucault and Ronchi testing. Final figuring was done by testing with a Coudé mask over a Foucault test.
The parts that were cast and machined were mirror cell, finder brackets, focuser base, cradle and yokes, rotation rings, bearings, angle bracket between the bearings, aluminium legs, zinc counterweight, and the ATMQ badge. The spider was fabricated (machined on lathe) from brass bar and spring steel arms were brazed into the body. Brass tubing was also use to make the upper part of the focuser.
The counterweight is made of cast zinc and attaches by hand without tools.
I will show you the
Joe
