Having got the structure of the dome and room completed my attention turned to the pier that would serve as a base for the telescope mount replacing the tripod. Obviously this structure is crucial in the design of any observatory that is going to serve as a platform for astrophotography.
It has to be solid and rigid and preferably allow for a certain degree of alignment of its payload to correct for deviations from the true horizontal.
Furthermore its seating surface has to match perfectly the base plate of the selected mount, in my case a Celestron AVX.
An exact cardboard template was made of the perch face on the tripod top and accurate caliper measurements were made of the various holes and features of the disc.
…back to the scrap yard
Obtaining the various bits and pieces required for making the pier necessitated several visits to a couple of local scrap yards .
So armed with my trusty caliper gauge I ventured forth and returned home with two brake discs. One place charged me 5Eu for a rusty downtrodden specimen I spotted half buried in mud but the second place gave me one still housed in half an axle mount ..for free. Going by the caliper readings both were close to the 61mm that was the base diameter of the conical bottom end of the AVX.
Some kind soul on a forum I had read had mentioned that the brake disc of an old Skoda Favorit matches closely the size of the AVX base plate but the ones I found came from unknown sources. Lady luck was on my side this time and on dismantling the axle I discovered that the diameter of the hole on that brake disc was just made for the AVX mount nose cone – hairbreadths off a perfect match! The other disc brake had a slightly narrower hole and was kept in reserve.
Other scouting visits to the junk yard empire eventually rewarded me with a hoard consisting of four disc brake plates and a section of steel tubing that fitted precisely inside one of the larger discs.
I sliced off the base of the largest of the discs, the one that fitted the diameter of the steel tube so as to use it as a circular flange around the tube providing it with a rim for anchorage to the concrete plinth that would eventually form the floor of the observatory .
Now came a tricky problem! Welding the mild steel tube section to the cast iron rim from the brake disc is a challenge for an amateur welder like myself. Cast iron is very finicky and does not take kindly to being offered a sparkling stick. My faithful 6013 rough and ready welding rods usually cause it to cough and splutter, smoke, boil and misbehave!! I tried to chicken out and convince an expert welder friend of mine to do the task but he said I needed special rods for that and he could never guarantee the result. Moaning, pleading and various other ploys did not help and so back home I went with my prized collection of scrap to formulate a Plan B.
On the way I asked at a hardware store if they stocked these ‘special rods’ and I was told that these would need to be ordered and could only be supplied in bulk at a very hefty price!
It was Youtube and various welding forums that saved the day and provided the answer.
I cleaned the two surfaces to a shine and after clamping them I checked for plumb and tacked the two together using the 6013 rods. I then heated the two surfaces with my propane/oxygen torch until red heat at which point having changed to low hydrogen 7012 rods I welded around the tube with my stick welder while continually torching around the joint to prevent rapid cooling. This trick prevents the two dissimilar metals at the weld from undergoing large temperature gradients resulting from the different rates of cooling. It is this that usually causes the joints to fail and cracks to develop in the welds. A little prayer mumbled under my breath certainly helped as well 🙂
And hey presto a short pedestal complete with four mounting holes!
Long stainless steel threaded sections were fixed to the mounting holes and a plywood plate was interposed underneath the pedestal. This would allow the anchoring studs to be sunk into the concrete cylindrical plinth and the surface of the concrete to be leveled. ( to be continued)