Intro: 6' F8 Newtonian With Sled Focuser on EQ Mount
Here is a project of mine, an all-metal build of a 6" f8 telescope and mount. This scope was constructed from salvage, scrap and miscellaneous parts over last winter. This project is pretty far out there in terms of found parts, so I don't expect anyone to immediately set out to find identical parts to construct an identical version. I view the creative process as more about finding sources of inspiration for my projects and this build is intended for similarly minded individuals.
If you ask me, the coolest parts to put together were the sled focuser and the mount. It took a while to get all of the components together, but they went together with minimal work!
As I have entered this build into an Instructables contest sponsored by a laser company my first thought is that with this scope and a clever mounting bracket, I will etch things from insanely far distances. OR all the folks in my astronomy club (Kankakee Area Stargazers!) will get cool name tags for when we do astronomy outreach at schools. We could take our club business cards to the next level. I can also foresee some interest in folks wanting to see my likeness etched on any thing they own (provided its smaller than 12x12x24 of course...)
This project started when a good friend of mine went digging under his deck and pulled out a homebuilt telescope given to him decades ago by an ATM (Amateur Telescope Maker). The scope was incomplete and the build was crude. The telescope body had been cut freehand with a hacksaw, the mount was chipped, cracked and missing whole sections. The mirror was good and the focuser was an unusual brass assembly I later learned was made here in Chicago in the late 50's early 60's.
The first step was to straighten out and square up the tube ends. I have a workshop in my basement but I am not a skilled tradesman. I have some bench tools, but mostly hand tools and perseverance. These days you would find a machine shop or a friend with a chopsaw and square up the tube in a couple of seconds. I like this method however... Take a long strip of paper(I use a roll of butcher paper) and wrap the tube a couple times around. Line up the paper edges and you will have a perfectly square line to cut from. I then use a hose clamp to guide the hacksaw around. Start by cutting a groove once around then continue to cut a channel around until you break through. This technique gives your cube a square end that is better than any chopsaw with little finishing required!
Step 2: Sled Focuser Construction
A sled focuser is a style of telescope focuser in which the eyepiece holder and secondary mirror moves laterally along the telescope tube. The advantage of a sled focuser is reduced secondary mirror size because you can minimize the distance from the eyepiece to the secondary mirror. Reducing the secondary mirror diameter increases effective primary mirror diameter and increases image contrast/brightness. You don't see many sled focusers because mounting the secondary to the focuser via a post can produce image spikes and can make for a complicated build.
To begin, I figured some rough dimensions and used a holesaw to drill a couple of holes. I used my jigsaw with a metal blade to make a slot...
Step 3: Sled Focuser... Continued
Originally I thought the brass focuser that came with the scope was missing a barrel and thought that making it into a sled would be a nice way to resurrect an old part. I found out later that the missing barrel was tucked inside the focuser but by that time my mind was set!
Cruising through Ebay I came across Chinese linear bearing assemblies for DIY 2d routers... Coupled with a heavy focusing rack from an old stereo microscope and I had the makings of a neat mechanical setup. Keep in mind the brass focuser already had the secondary mirror post attached. I only had to make sure that the diameter of the focusing slot could accommodate the light cone of my optics!
I drilled and tapped mounting posts into the rack and used a bit of threaded rod to attach the rack to the focuser. The linear bearings had to be redrilled and retapped to accept US threads. Pics are of the pieces and the focuser getting mocled up.
Step 4: Sled Focuser Continued
I sanded a radius into the bearing post mounts and screwed everything together. The secondary most and mount is all brass. The assembly works really well and the brass has been shined up a bit too!
Step 5: OTA Assembly
The primary mirror is a 6"dia f/8 Edmund mirror from the early 60's and sits in a cast aluminum mirror mount. I don't have pics of that part, but luckily those things haven't changed much in 50 years.
To mount the tube for the mount I wanted to do something a little different then standard tube rings and a dovetail. I had come across a custom made thick aluminum dovetail that overbuilt even for a much larger scope.
One of the complications of an eqmounted Newtonian telescope is that the eyepiece can end up in some really awkward placements so a rotating tube is a necessity. Unfortunately rotating rings are really expensive and not easy to make from scratch so I came up with this convoluted method of constructing 6 channels that can double as carrying handles. To fasten the tube to the dovetail, I built a secondary clamping assembly to the dovetail that the channel fits around. So I have a gothic looking many spined OTA inspired by the Romulan mining ship from that first Star Trek movie...
Step 6: Constructing the EQ Mount Head
One of my side hobbies/obsessions is finding other ATM's projects on Craigslist or Ebay and finding a way to complete them or use them. As an ATM, I know how easy and fun it is to start projects and how easily one gets distracted. Combine that with the fact that ATM's tend to be older and often stuff comes up for sale for mere shade of the work and materials cost that were put into them.
Some time ago I purchased an old telescope mount that was cobbled together from all sorts of stuff. At it's core this mount used an old slo-syn stepper motor body as the bearing assembly for the RA axis. I loved that part, but the rest of the mount was crude by comparison. I disassembled the mount and stuck that motor body on my parts shelf. Some years later I was cruising the 'Bay and once again stumbled on an old mount build and what looked like the exact same kind of build, namely a large motor can used as an RA axis. The nice part about this particular mount was that there was a drive attached! A very nice Pacific Coast Instruments clutch and worm drive! I picked the thing up for essentially the cost of shipping. When it arrived I was surprised to find that what thought was a motor body was actually a custom machined RA axis with a solid steel shaft in the style of a Porter mount. A Porter mount is a style of Telescope mount in which one end of the mount axis is a very large bearing and the tail end is a small diameter. The design gives you maximum strength and rigidity in a very compact package.
Step 7: Telescope Drive
Here are some pics of the telescope drive... It turns out the mount was never working. The drive parts were binding and the bearing was seized. I spent a couple of days filing and grinding to get everything working with the appropriate clearances. I was rewarded with a robust and simply built RA axis.
Step 8: Mount Attitude Frame
If you know telescopes, then you know what the company Astro-Physics is known for the highest quality finely engineered Telescope mounts. Sometimes someone buys a $10,000 telescope mount and decides they don't want it to be portable. When that happens, they have this part left over and I bought it. :) I had to do some adjusting and redrilling to make the sides accommodate the diameter of my mount. That's when I found out what $10,000 mounts are made off... The answer is very tough heat-treated aluminum which is an absolute bear to drill and tap...
Step 9: Mount DEC Axis
The RA axis uses the first motor can that I mentioned in previous steps... This motor had the brass armature and coil intake, but had the central shaft removed. The motor can fit into another cast aluminum body which had it's sides cut away with a bandsaw. The motor spun in between 2" bearings.
At this point I had to add a counterweight shaft so lacking a lathe I took everything over to a machinest friend on mine and he enlarged a couple of holes to accept my 1" steel shaft. I had a couple other metal disks and donuts that were drilled and tapped to bolt together and finally to the dovetail mount you saw previously...
Step 10: Mount Coming Together...
The motor frame gets bolted to the RA axis front plate, the counterweight shaft gets bolted to the dovetail and everything else bolts to everything else and viola! The mount begins to take shape...
Step 11: Counterweights
One of the side benefits of knowing people with machine shops is they give you scraps and junk.... I was able to use a huge 15lb brass donut as well as the innards from another huge motor to use as counterweights. I press fitted everything together and shoved them on my drill press. Some filing and sanding and I have awesome brass counterweights.
P.S. this is the most dangerous step in this whole build. DO NOT put 22lbs of brass in your hobby sized rill press and hold your belt sander up to it. That is A LOT of mass spinning very quickly. Ever seen the Mythbusters with the Cannonball? yeah...
Step 12: Mount Pedestal
Mount pedestals have been supplanted by tripods in modern telescopes. Tripods can be stiffer and lighter and can take up much less space than a pedestal mount.
However, I think pedestal mounts are cool, so that's what I used.
The pedestal legs I used were from the original 6" build that I was given for this project. They had been buried for decades someplace that cats liked. The first order of business was to scrub and degrease the legs so they smelled like metal and nothing else.
The legs bolt simply to a tube from another scope. The tube is 7" dia x 2' long but is thin walled and will be replaced by something stiffer at a later date. The mount head sits on a plate that came off another scope mount (a C8 fork mount) that happened to fit the ID of my tube exactly. I added some spacers and ran a 1/2" threaded rod from the feet all the way to the mount head to tie the whole thing together.
Step 13: The End.... or Maybe the Beginning??
Lastly i added a brass tube counterweight system... completes a scope I lovingly refer to as "Metalstorm!" or as my wife calls it, "The Shiny Heavy One"
Thank you for reading..