Main tube: Pt.4 Countercell completed.

*

Tuesday 28th: Overcast with rain, on and off, all day.

Continued smoothing the cell now it has three laminated rings. It wants to be tight enough to grip the tube while not being so tight that it causes damage. Nor jamming while the counter cell  is being slid along the entire, 120cm length of the main tube. 

The recess in the front ring worked well. Leaving the rear of the lens cell flange just slightly proud of the counter cell. That means there is enough of a rudimentary seal with the main tube in normal use. Without compromising the small movement required for collimation. Which might have broken the desired seal.

The collimation screws, shown here, are too long but all I had available at the time. I will also need to order some M5 T-nuts for the back of the counter cell. To ensure there is no rotation of the nuts during collimation adjustment. The spikes on the T-nuts will dig into the plywood on the back of the cell to lock the nuts safely into position. I have loads of larger T-nuts but no more M5.

The plywood counter cell will sit immovably, surrounding the front end of the main tube and stopped by the small tube flange. With the front ring slightly projecting while carrying the objective lens in its cell. This objective lens, support system was used on the 7" refractor and it worked well. While safely avoiding the need for a turned aluminium counter cell with unwanted, extra weight and quite possibly, considerable expense. An aluminium dewshield will fit over the plywood adapter ring.

Next, I must make a tailpiece for a 2" screwed [WO] fitting. I shall "borrow" the existing screwed fitting from the present, 6" f/8 as it will no longer be needed. Perhaps I had better start looking for a suitable saucepan to fit over the tail end of the new telescope for a cosmetic lift? I also need to think hard about a truss support for the far end of the PST etalon/filter stack.

I'm thinking about using a finder alignment ring for the tail end of the stack. Only an open truss will provide the necessary hand clearance for adjusting the filter stack. Though I shall, of course, use the existing motor drive to the PST etalon and arrange motor driven focusing for the helical focuser. The latter needs an independently mounted motor to avoid the belt running off the moving focuser drum. I can probably fix the focuser motor to the truss or a suitably placed, support ring.

I have ordered some shorter, M5, stainless steel, hex socket head bolts and ss T-nuts for collimation. Shortening bolts and screws is time consuming and fiddly. It also denies access to the longer item if needed. So I'll add the longer bolts to my stock containers and have them handy in future. If I calculated my lifetime's collection of fixings at bubble pack prices I'd be a millionaire!

Talk about a license to print money! Them! Not me! Even with added postage it is far cheaper to buy such stuff online from specialist dealers. I try to buy stainless steel whenever I can. Mild steel and flash plated fixings begin to rust before they are out of the pack. A total waste of time and money on anything exposed to damp air or the night sky. The price is no higher for stainless steel online than the rip-off, decorative zinc, bubble pack rubbish in the DIY outlets.

 *