The birth of Longdrop - by Johan Smit - May 2011
In 2010 a good friend, mentor and fellow telescope maker, Louis Barendse passed away. Together with some colleagues from our telescope making class, I helped his widow sort out Louis’ telescope making stuff. As any serious ATM’er knows, that was a major exercise because we tend to collect stuff - just because we can and one day it could possibly be used in a telescope.
Well, we did the task and when we were finished went through the workshop one last time. At that point, we found a mirror in a cell on the top of some shelves. I eventually purchased the mirror; when I tested it, I found that it was a 10” F9. On the Foucault test it seems quite reasonable: smooth with just a hint of correction, as can be expected of such a slow mirror.
Now I had a mirror and an idea started brewing in my head. This mirror which had been hidden for an unknown number of years just had to see starlight. So, in memory of Louis Barendse, I set about making a telescope.
I soon realised that it would be a very tall telescope and a truss assembly was required. Fortunately the mirror cell was heavy - solid ½ inch thick steel plate - so weight at the bottom end was a given. I had some 18mm pine shelving plank. Another friend of mine donated some 10mm plywood from old packing crates. So, wood I had, in abundance.
The mirror box was made out of the shelving plank. It turned out nearly square with a height of about 500mm. Because I had to fit the original cell at the back, the box was made much larger than normal. This turned out to be favourable in the end: I had enough space for mounting the trusses and the scope gained some stability.
The top end was made out of 10mm plywood. It was also generously proportioned and can stack on top of the mirror box. Due to the generous proportions of the two main components stray light is well controlled and a skirt is not needed.
The two boxes were strapped to my ladder and the whole assembly was manoeuvred to achieve focus on some distant buildings. That enabled me to determine the exact truss lengths that would be needed.
The shape of the mirror box resembles a pit toilet seat and the resemblance was promptly noted by my colleagues. Such an article is called a long-drop in South African slang. And so the telescope got a name.
I am a poor metal worker, stingy and lazy. Buying aluminium and making pole mounting and joining mechanisms were not considered as a viable option. And I had an abundance of light plywood. So I decided to make triangles to act as trusses. Some time with my trusty jigsaw and I had workable wooded trusses. The bottom wide section is fastened with wing-nuts on to bolts glued into the inside of the mirror box. The top end sits on little shelves that also have glued-in bolts, and wing-nuts again served to fasten the lot.
As luck would have it, the balance point ended just about at the top of the mirror box. So a fairly standard rocker box and ground board was made. The cut-outs for the mirror box top and bottom did service as side bearings. 200mm PVC sewerage pipe sections were cut open and stretched around the bearing to provide a smooth surface. Longdrop stood tall and proud for the first time.
That same evening saw first light on the stars. Despite much stray light (no light shields fitted yet) it performed better than my wildest dreams. Rigel and its companion were cleanly split, and some other favourites (the jewel box and M42) showed that I had a good working scope.
Then came the part that I dislike the most ... painting. My son recommended that we paint it red. So we settled on a signal red and matt black colour scheme. It was stripped and two layers of undercoat and several layers of topcoat were applied. The inside of the top cage was lined with black cardboard and we now had a scope that looks as impressive as it performs.
It has since seen service at our national Karoo star party in Britstown and was exhibited at Scope-X, where it was awarded a prize for the unconventional truss assembly. It was also used at every other viewing opportunity that I could attend and everyone was equally impressed at the views that it provided.
Pros and Cons: The whole assembly works better than I imagined. It is stable. It does not vibrate. Any vibrations introduced dampen out very quickly. And what amazed me most is that it stays in collimation. After a few thousand kilometres of travel and many assembly/disassembly cycles I hardly ever had to tweak collimation. It just comes together perfectly every time.
It does take much time to fasten 12 wing-nuts (from start to looking takes 10 minutes), so it is quite feasible and the simplicity in making the scope and the stability makes up for the effort in assembly.
Movements were not as smooth as I wished for, so a few tweaks was done and now it is an absolute pleasure to control.
The altitude bearing surface was changed to Teflon (cheap furniture sliders).
To improve the Azimuth movement a flat steel ring (38mm x 3mm flat bar) was added to the bottom of the mirror box. I forced the steel around the ring and welded the ends together. This runs in V-groove pulleys that sit directly on the ground spaced at 120 degrees. That made the movement too smooth, but the addition of a folded up face towel between the ground board and the rocker box provides just the right amount of friction.
The finder scope system is still clunky. I am in the process of improving it and soon Longdrop will be perfect.
Conclusion: Using the scope I found another reason for its name. If you lose your balance on the ladder, it is a long drop to the ground. So, if you find the original origin too vulgar you can use this one.
If you consider making a truss tube telescope, I can recommend going this route. It is simple to make with the minimum of tools and is as stable as anything else I have seen.