7" f/12 iStar refractor 18: Almost finished!

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The Dan Lim glue proved to be up to the job and the plywood, tube packing rings were held firmly. Though I still added the security cords for safety after putting the OTA back on the MkIV mounting. My earlier attempts at balancing the OTA had been thwarted by slippage through the sponge lined rings. I discovered that the tube would still slip even if the rings were clamped tight and the foam strips glued in place. There is just not enough friction regardless of the pressure applied.

Now I was able to grip the tube tightly thanks to the firmness of the 30mm wide plywood packing rings. I could also use the sliding weight to really fine tune the tube balance. Giving the focuser a push allowed the OTA to swing freely on the declination axis. One soon gets a feel for how far the tube swings in any particular direction. The symmetry of the total arcs of the swing is strongly dependent on tube balance. I was very pleased with the freedom provided by the MkIV as I moved the telescope around.

The balance has improved to 20" above the top ring to the dewshield base. While the lower tube is 27" from the lower tube ring to the focuser back plate. The rings are mounted on 23" centers on the substantial and heavily ribbed, Fullerscopes saddle casting.  I do have a 50mm DIY brass finder which would certainly help the cause of  improved balance. Though at the cost of something else to lift! A longer dewshield would redress the appearance of being slightly "bottom heavy."

Here is a closer look at the MkIV mounting, the counterweights and relative scale of the 8" x 77" main tube. The 50 year-old Fullerscopes mounting doesn't look too bad despite its years out of doors under a series of small tarpaulins. Though the 6" wormwheels and motor cases are no longer as bright as they once were just after I finished the MkIV's restoration.

The center of the declination tee is now 80" above the ground when set horizontal. Ground clearance with the tube vertical and the star diagonal in place is 32". I can only just reach the lower tube ring from the ground. Reaching the upper tube ring needs the stepladder. Which is slightly unnerving when fitting and removing the OTA. While it rests in the open tube rings there is a point where it might slide out.

A third tube ring clamped permanently to the tube just above the top ring would act as a stop to prevent the OTA sliding towards the ground. Though the extra tube ring itself would carry a weight penalty for lifting and balancing the tube. An alternative might be a cord loop fixed lower down where it could be easily reached to anchor the tube. Or, a third tube ring could be clamped just above the lower tube ring. A fourth ring fitted just below the top, opening ring could allow a strong handle to be mounted between the two to to aid carrying. These rings would not be attached to the saddle but remain with the OTA. This would increase the weight of the OTA but not affect its balance point. The extra rings would ensure the OTA was fitted in exactly the same place each time [for balance] but they would still be movable [or completely removable] if major changes required it. The problem is often that fitting the OTA to bare tube rings leaves no position indicator for the exact balance point.

The pier pipe is 7" in diameter x 5' tall from its base to the massive welded flange on top. The abbreviated dewshield reaches well over 10' high when pointing at the Pole Star and the pier wheels at their lowest [braked] setting. I welded the heavy pier together myself. In retrospect I would have had four legs/feet instead of three for even greater stability. Relative to the pier center a triangle has a much smaller radius between support points than a square. In real life a four legged pier will never rock between the two highest feet on the ground. 

I was initially quite worried about the appearance of the pressed seam of the main tube but have grown to enjoy it for its honesty. It adds an industrial quality in keeping with classical refractors of the 19th century. The tubes of which were often bolted or riveted together. Their obvious means of construction were not ground away nor hidden simply for the sake of appearance.

The massive, but now freely mobile pier is shown here with the jacks lowered to bring the ends of sturdy, welded legs into contact with the tyres. Thus affording excellent braking against unwanted wheel movement and reducing tire flex simultaneously. It has occurred to me that I could fit some more clamps and jacks with plates, on the opposite side to the wheel clamps, to act as stabilizers. They would have to be raised to their full extent to allow the wheels to caster during movement but would certainly kill any tire flexure. Until I can test the entire instrument at high powers I have no idea how much of an issue the tyres really are in practice. The tires have a relatively low pressure limit of 35psi.

In real life the sheer size of the instrument is quite impressive. The scale is rather lost in this image taken from a distance. The telescope lacks another foot of serious dewshield. Which would help to add some extra 'stature.'

I am really rather pleased how well the whole instrument has turned out. Particularly after the early searching for secondhand cooking pots and pans from the charity shops. Finding a local source for the galvanized steel, main tube was a huge breakthrough. This certainly moved the project on far more rapidly than I imagined possible. Taking the pier movement seriously, for the first time, has resulted in a practical level of mobility. This will pay dividends when I want to observe an object hidden by all the local obstructions like the house and all the surrounding trees and hedges. Not being able to move the pier had been an enormous limitation on my observing anything at all.  

Click on any image for an enlargement.

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