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The image shows the back of the objective cell. The longer pull screws rotate freely in the objective cell flange. While the push screws fit in an an M5 thread for adjustment of collimation. The pull screws being slacked off slightly to allow movement of the entire objective in its cell and the re-tightened to take up any slackness.
I also added Nyloc nuts to the longer, pull screws which hold the lens and dewshield to the plywood rings. Which are split, glued and wrapped around the main tube. The small flange on the end of the main tube prevents these rings from pulling forwards. The result is a much stiffer arrangement where the collimation does not alter with pointing altitude. Had I known I would later remove the lower flange at the focuser end to shorten the main tube I need not have split these plywood rings.
This image shows [from left to right] the outer dewshield, inner dewshield base/counter-cell and the sandwiched plywood base rings wrapped around the main tube. The cut-down, T-nut is just visible on edge where it bites into the plywood ring and accepts the the M6 x 60mm Pull screws. The dewshield base is sandwiched between the objective cell flange and the plywood rings. With the push screws pressing against the inside of the dewshield base.The near 3lbs total weight of the [now doubled] dewshield pushed the balance point towards the objective. Though not enough to make it look too "lopsided."Thankfully the longer dewshield helped to improve the visual balance.
In the image below I have fitted the complete OTA to the old Fullerscopes MkIV mounting to re-check its balance. I discovered I now need a slightly heavier polar counterweight, on the Declination shaft, to compensate for the increased weight of the new and longer dewshield.
The blue cord loop is just a security device to ensure the tube will not slide down through the tube rings when they are loosened off. The cord is more psychological than practical because the tube refused to slide down through the rings when I was trying to adjust the balance point.
The scale still looks rather modest in this image despite the sheer size of the instrument when standing close to it.The weight of the OTA now makes it quite a struggle to lift into the mounting rings. Though I no longer try to lift the OTA with it lying horizontally in my arms. The climb up the stepladder is now too high and too precarious to take the risk. So now I set the mounting pointing at the N.Pole and lower the main tube vertically into the open rings. The thick plywood, packing rings in the metal tube rings are really quite snug and refuse to easily relinquish the tube. I may have to sand them to allow the tube to be more easily moved up and down.
It was suggested on the iStar scope forum that I fit pan head screws above one of the tube rings to stop the tube sliding down and to give a location point. While I was considering fitting another pair of tube rings with a long handle between them. I do not have a useful handle anywhere near the middle of the tube at the moment. Which means I have to carry the OTA in the crook of my elbow. There is no shortage of handles down near the focuser end but they are too far from the balance point to be very useful. So a central handle would certainly help but would require the focuser and baffles are slid right out of the maim tube to be able to fit handle securing screws or nuts and load spreading washers inside the main tube.
Click on any image for an enlargement.
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