Introduction: Homebuilt 6" F15 Refractor and Mount
Hello fellow Instruct-ables... What better way to celebrate space than to look at it through a really big telescope? I have been an ATM for quite a while and this is my latest project, a big refracting telescope on a big mount.
Step 1: Gathering Parts
The mount is made from mostly easy to find scrap parts I found on line. I spent a lot of time looking for scrap pieces that were close to sizes I needed until I found that the metal merchants would cut pieces to size for pretty cheap. I used common power tools such as a small drill press, router, circular saw and chop saw. Working with aluminum is a lot like working with very hard woods, the cutting feels the same but slower.
The chassis is made of 1/2" sheet aluminum. The bearings are pillow block bearings. The Main(RA) shaft is 2-5/8" diameter aluminum and the smaller(dec) shaft is 1-1/6" steel. The large worm gear was purchased cheaply from an astronomy website.
Step 2: Routing Aluminum
This is a step that I thought about for a good long while. An equatorial mount has to have some adjustment for the angle of altitude in order to track properly. The angle for your location is your latitude, Where I am the mount needs to be set at 42deg. I decided to make the angle adjustable about 5 deg either way. To make these small arcs I used a circle-cutter attachment on my router and made quick 1/8" passes until I cut through each side of the chassis.
Step 3: Basic Chassis Assembly
I trimmed the mounting tabs from the large pillow block bearings and ground down the sides with an angle grinder. After some drilling and some tapping, I bolted the base chassis up...
I added a brace and tapped a hole for an adjustment bolt. Turning this bolt allows the angle to be adjusted then clamped down when it's right.
Step 4: The RA Shaft
My large pillow block bearings were sold to me as 3"ID however when they arrived they were closer to 2-5/8"ID. I was able to find 2-5/8" aluminum cylinder and had it pressed into a 6" dia 1" thick disk. Pressing the shaft in is a very strong and stiff junction but is a job for a metal shop with a press, My local machine shop charged me $20.
Step 5: The Dec Shaft
I had the 1-1/16" dia x 24" long dec shaft pressed into a 2-1/2" x 2" x10" Aluminum bar. For the correct alignment of the tube, this unit has to be trued on a lathe. Another $20 to the machine shop.
I felt like Thor when I carried this into the house.
Step 6: The Dec Shaft Assembly
The dec shaft rides in two 1-1/16"ID pillow block bearings mounted to a 1/2" aluminum plate which is bolted to the RA shaft. I added a 4"wx14"l aluminum bar to mount the tube rings.
Step 7: Mounting of the Large Drive Gear
In a big scope the accuracy of the tracking is directly tied to the size of the worm gear. Big worm gear means smaller errors. A big worm gear also needs a smaller drive motor. This gear is clamped between two circles of HDPE which is actually cutting board material. I routed two 12" circles to act as clutch plates as well as a smaller 6" donut for the gear to ride on. This assembly is clamped tight via two threaded shafts. This clutch arrangement allows the tube to be moved anywhere while the motor turns for effortless pointing and tracking.
Step 8: Worm Drive Assembly
The worm drive is built from pieces of aluminum bar cut from left over material. The worm gear came from another smaller drive and happened to fit. The shaft rides in oilite bushings. Gear slack is taken up by a screw that presses the shaft from the side and prevents lateral movement of the gear. Each piece of this unit took a couple of tries because to avoid binding, some precision was involved. By precision I mean drilling holes over and over until finally the shaft stopped binding!
The motor is a synchronous timing motor which runs at a set speed from household voltage. These are used in older or simple telescope drives because it uses the frequency of your house voltage to keep accurate speed. Adjustments of the speed can be accomplished by the use of a square-wave frequency generator, a simple circuit that allows you to speed up or slow down the motor for tracking.
Step 9: Tripod Hub
The tripod hub is made from two disks of aluminum bolted to three leg mount bars. The top of the hub is machined flat. The bolt hods the chassis to the hub.
Step 10: Oak Tripod Legs
The tripod legs are built up from oak flooring boards discarded as scrap by a neighbor. They are as long as the shortest piece so abou 48"l.
Step 11: Mount Assembled
All the different assemblies together as a mount head.
Step 12: Finished Mount
After some fiddling, refitting, stiffening sanding and painting, the finished mount....
Step 13: The Heart and Soul....
Ah, now it's time for the cool stuff... The Objective! This lens was made by Istar Optical, at the time a relatively new manufacturer of telescope optics. The lens has shown itself to be an excellent performer, especially for the price. Currently the objective in cell costs about $515.
Step 14: The Tube...
The tube is a section of aluminum tube made by Hastings Irrigation. I had to special order the size as shipping an 8' l tube costs as much as the tube itself...
The focuser is a Synta refractor focuser painted blue for looks.
The interior is baffled via 7 knife-edge baffles spaced 12" apart. The baffle assembly was built from plans I drew of the optical path. The baffles are kept out of the light path by about 1/4" or so.
Step 15: Objective Cell Mounting
The objective needs to be mounted in a collimatable ring. This ring was routed from three pieces of HDPE drilled and tapped for three push-pull screw arrangements.
Step 16: Sliding Dew Shield
Traveling with a 9'l tube is tough, traveling with an 8' tube is much easier, fits right in the mini-van. To make things easier the dewshield slides over the objective to shield the lens from stray light.
Step 17: How Does It Perform?
This refractor is an achromat which normally would not be suitable for photography. Due to it's large focal ratio (f15) and long focal length(2276mm) this scope is a superb planetary performer. This is a photo of Saturn taken with a Celestron Neximager, a webcam-based imaging camera. This photo is a stack of 1000 frames in Registax. There is a white blob which is a storm imaged on the surface of Saturn from 3 blocks outside of Chicago!