This chart auto-calibrates to the response of each magnetic sensor over time. As it does, accuracy should improve.
Thanks to Vic for the pictures
This telescope was my first "serious" one. I'd previously built several smaller scopes including a "copy scope" and bought a 60mm f/7 refractor to look at Halley's Comet (IT's actually a neat wee scope to use!). I even tried grinding my own 4" reflector It took FOREVER to grind, and the mirror had a big hole in the middle. I still have the grinding tool though. But it wasn't really until I'd been to the local astronomical society a few times, that the thought of a larger scope crept into mind.
The pivotal moment was one night when they had scopes out looking at Saturn. Everyone was queuing up to look through the giant 15" Dobsonion reflector. It took forever to get to the eyepiece, and you had to keep the queuing moving. My one glimpse revealed a dazzling, featureless ball. Yeah, it had rings, but as I stumbled away trying to regain my night vision, I felt cheated by the experience. I moved away, and as my sight returned I realised that I was standing next to an old brass refractor, that had been abandoned in favour of other, larger telescopes.
Sighting along it, I aimed at Saturn, thinking that I might get a more leisurely look. Satisfied with the aim, I peered into the eyepiece.
There floating slowly across the field of view, was Saturn. Instead of a bloated glaring disc there was a ivory yellow world. You could see a pale, tan band of cloud on it's surface. The rings were the same pale ivory yellow, and you could see clearly that there were not a single ring around the planet, but two, one inside the other. Away to the right a small bright star - it's moon Titan.
The chatter and the hub-bub at the other telescopes seemed to fade away. I was captivated. Somehow looking at this tiny speck of light in the sky that was a world, the whole universe of stars seemed to open up around me, until all I could see was Saturn drifting in front of me.
From then, I wanted a refractor. In spite of chromatic abberation, and cost, and gawky mountings, I was going to have my own.
I started this telescope way back in 1984. The objective lens is a 4" f/15 air spaced doublet from Edmund Scientific. Back then they produced heaps for the amateur and their catalogues were always fascinating to look through. It seemed to take ages to get here and it wasn't until the local customs duty at what would have been 80% of the purchase price was paid, that I finally had it in my hot little hands!
Getting the lens was the easy party. The hard bit was making the telescope! Sad to say it took something like 8 years for that to happen. At one point i paid a fortune for castings to be made for the lens cell but the machinist who was going to do it for me never fronted up with the finished goods. So things sat in limbo for a while.
The theme of this whole project became "low tech". I had no access to a machine shop so all those wonderful books and magazine articles on refractor construction were interesting and adsorbing to look at, but no use to me. I had also not long left my job, so I didn't have the funds to splash around. Whenever I needed anything the first port of call would be the scrap metal yards and second hand shops.
The first order of business was making the lens cell. Just how I was going to satisfy the basic mechanical requirements had me stumped for ages. Eventually I came across a couple of books that advocated lens cells made from wood, but I wasn't convinced that they would be a mechanically stable enough solution. Eventually I came up with an idea for a lense cell that required no castings to be made. The only drawback was that it would still require a visit to a machine shop, or access to someone with a lathe. However the work required would be minimal and at a fraction of the cost of having castings poured and machined.
What we have is a flat circular ring made from heavy gauge non-ferrous material like brass or aluminium (or even stainless if you're feeling extravagant). The internal diameter is marginally smaller than the OD of your object glass (so the object glass has a ledge to sit on). If you want you could mill a small lip in the top surface for the object glass to sit. At 3 equidistant radii (60 deg), holes are punched and drilled for 3 pieces of angle. These will form the "sides" of the cell. A smaller flat piece of brass (or whatever) forms a retaining clip to hold the the lens in. You may want to line surfaces that touch glass with cork or similar. EVERYTHING is tightened securely. If using an airspaced objective, it may be necessary to tape around the gap between the lens elements so dust and moisure does not get in. Don't forget to punch, drill and thread holes for the push/pull alignment screws. The object glass is quite exposed in this design, so you will need to take care of it's placement and when you access the push/pull screws that contact with the actual glass is eliminated or minimised.
In the end I didn't have to use this design as I eventually found a meter length of brass bushing that was the perfect dimensions for the Cell and so had a piece cut off. I opted to go to a machine shop in the end to have it made. The cost for that was a couple dozen cans of export beer!
So with that part done the next bit to worry about making a focusser. Again, it had to be a low tech approach as there was no way I was going to be able to make threaded sleeves or a rack and pinion.
One of the options I toyed with was a crawford focusser. However all the designs I saw still seemed to require access to a lathe. So more thinking and doodling was in order. Eventually hit on this idea:
(rev 1/5/05) The two main parts of the focus mechanism are the drawtube and the outer sleeve. Nylon bearings are slipped between the two and held in place by bolts threaded through the outer sleeve that contact sockets in the nylon bearing and press it gently against the drawtube. Lug "B" is threaded to accept the threaded rod. Lug "A" has a clearance hole drilled into it and locknuts on the end of the threaded rod hold it in place. Turning the threaded rod causes lug "A" to be pushed or pulled from lug "B" and thus moves the drawtube. The only parts of the assembly visible from the tailpiece of the refractor, are the eye end of the drawtube and a handle attached to the end of the rod. A slot cut into the drawtube about 30mm wide accepts filters that are cut from photographic filters that come as a plastic sheet about 70mm square. They just drop into the slot and sit on the inside surface of the drawtube nicely.
This was the mechanically simplest solution I could find for someone with little or no access to a machine shop. I made use of nylon washers (I made these from sheet stock) to smooth the motion of the threaded rod. However there is some backlash (about a quarter turn) so a future improvement might be the use of a spring attached to the inner sleeve to provide some tension and eliminate this. The design is a "long" one and is best suited for a refractor. It may be too gawky to adapt for a reflector.
These two steps were mechanically the most complicated. Once they were done the rest of the telescope construction proceeded very smoothly.
I had already decided that for ease of use, I was going to make an alt-az mounting. However because of the length of the refractor I also knew that it was going to have to be hoisted up fairly high in order to get to the eyepiece without hunching on the ground. To this end, I came up with a the simple truss design for the tube of the telescope. The lense cell and focusser are mounted in boxes and two aluminium beams (50mm x 25mm) make up the "sides" of the telescope "tube". This made for an extremely light and surprisingly rigid structure that could be hoisted up onto it's mounting easily. I had thought that maybe the combination of the heavy lens cell mounted on the end of metal beams might induce a tendancy for vibration and flexure, but vibrations were dampened in 3 seconds or so when the side was given a hefty whack! I would think that in larger designs it would not be rigid enough so a more substantial truss might be in order. This design also reached thermal equilibrium very quickly.
There were only a few disadvantages to this design. The lens was exposed on both sides, so it required two close fitting caps and extra attention to dust protection. The open design of the tube meant that whilst it was ok in a dark location (I used the carpark of the block of flats we were staying at) if there was any light or if I wanted to observe anything in the daytime then I needed to make a box that fitted in the truss with baffles to eliminate glare and skyglow.
In the end, I mounted the objective and focusser into a traditional tube (5mm heavy wall PVC pipe reinforced internally with wooden rings (10mm thick) that serve as light baffles) as at times I had trouble convincing people it actually was a telescope! I managed to scare a new neighbor once because she couldn't work out what the crazy man was doing in the garden in the middle of the night with the shiny metal contraption. I'm sure it looked very suspicious! *chuckle* One of my friends used to call it "Vaughn's Deathray..." Hmmm! :-)
Some pictures of the refractor before it's refit. This picture shows the brass lens cell mounted in it's box. To the left is the removable front cover for if the cell needs to come out. You can see the aluminium side beams and the dobsonion-style altitude bearings with red endcaps. The steel ruler is for scale (300mm). As the box physically encases the lens cell, it was going to be the solution to physically protecting the lens if I was going to go for the open low-tech cell design. The eyepiece end of the scope. You can see the inner sleeve and it's attached piece of angle. If you look carefully you can just make out the white nylon bearings that hold it in the brass inner sleeve. The focusser seen from the eyepiece end of things. The finder is made from a cannablised binocular (30mm) and simply mounted on a length of aluminium bar. Not fancy, but a very simple and easy to use solution. You can also see the pier that the scope sits on. It's a 6 foot length of salvaged chimney flue (Probably stainless, couldn't tell thru all the soot and tar!) 2 x 4's (You can just make out one at the bottom of the image) jutt out to form tripod legs, and are bolted to more 2 x 4's that join onto the base. Without the scope on, the whole contraption looked like a rocket sitting on 3 wooden tailfins! The interior of the pier had more 2 x 4's wedged in to rigidify it. It was not portable at all, but it was extremely solid and steady, only took an afternoon to make and it served me well for a year before someone vandalised it knocked it over. I abandoned the pier where it was and designed and built a tripod. In reflective moments I like to think that maybe after all these years the pier is still sitting up at the carpark, forgotten and overgrown by the weeds and grass. :-) The object glass cell seen from the eye end. Visible are the push/pull screws. I used four for the simple reason that I found it a 100 times easier to visualise the alignment of the OG for collimation in terms of an X-axis and Y-axis! ( I guess it's because people think in terms of up-down, left-right >:-) )
When the pier was destroyed I decided that it might be worth the effort to make a tripod. From experience I realised that a permanent location is ok, but portablility to avoid trees and the like is a plus. Also, the tripod could be locked away securely.
The design phase of the tripod took very little time. It was probably a matter of weeks. I'd decided to carry on the theme of the 50mm aluminium beams from the scopes tube. As you can see from the picture the legs are made from 2 metre lengths of 50mm aluminium beams (I think the wall thickness is 2.5 or 3 mm?). These were expensive (all up approx $300 NZ Dollars) but worth it. The legs are bolted together with non-ferrous rod and aluminium tube to create legs that are light and extremely rigid. The tripod head is built up from 12mm plywood (No particular reason for that thicness, it was just available) which was screwed and glued together so the head was rigid and did not flex (about 4 layers). As you can see, the legs of the tripod are designed to opened wide, increasing the stability of the whole structure. Nylon rope attached to the legs stops them from splaying out. Wooden legs would probably work just as well of course.
The second picture gives you a closeup detail of how the tripod legs attach to the tripod head. The thick nylon rod is cut at an angle which allows the legs to be joined to the brass angle and turn smoothly. The altazimuth head is made from plywood screwed and glued to thickness and aluminum arms that suport the altitude bearings of the refractor. You can see the refractor refitted into it's new solid tube. It still uses the original lense cell and focussing mechanism. The finder is larger (50mm) which makes life much easier.
How does this more traditional design compare to the original? Well, it's certainly heavier, although I can still lift it up onto it's mount. (I hate to think what the beastie would weigh if it were a traditonal brass tube!) Because it has more mass, it does take longer to cool down. I generally give it 40mins to an hour to be sure, when the previous truss design would be cool in 20 - 30 mins. More mass means that the alt-az head has to be more rigid. Unfortunately this isn't the case and the supporting cradle can physically warp. I guess screwing and gluing another layer or two of ply will solve that.
However it does "look" the part and I now no longer have to worry about freaking out any more neighbors with the "ray gun". The objective is quite securely protected within the tube and doesn't suffer from dust like it used to.
And there have been occasions when the sky as been right and the evening quiet, that Saturn has looked as it did that very first time.