10" f/8 Steady progress on the Planetary Newtonian.

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I have redesigned the primary cell support to make the alignment far more reliable. Still some small details to attend to. A strip of aluminium inside the cell will help to reduce distortion of the pot wall which forms the primary cell when the wing nut is tightened.

The mirror cell springs have also been upgraded with much stiffer examples. The mirror collimation screws now have no slack movement. The OTA now rests on three rubber door stops instead of its collimation screws. A small detail  but it keeps the screws clean and the OTA is more stable in vertical  storage.

I dragged it all outside to the mounting and took some pictures in the bright, winter sunshine. It's a shame the hedge in the background is so transparent and scruffy at this time of year. Unfortunately I had no other suitable background which didn't involve a lot more work.

The measuring pole leaning against the OTA is 230cm high to the red tape. About 7'6" in old money. This is as high as it gets but only when pointing at the Pole Star. Southerly objects will always be much more accessible from the ground.

The OTA can now be quickly dismantled via the wing nuts holding each cell to the twin spars or beams. The same goes for the OTA fixings to the 2' long [60cm] cradle on the Fullerscopes MkIV mounting. I just loosen the wing nuts, rotate the thick Tufnol clamping plates and lift the OTA away. The OTA can be very easily slid up and down in these clamps to balance the OTA longitudinally. Handy if a heavy eyepiece is inserted with a Barlow.

The new handle fitted to the primary cell is very useful. Previously the OTA tried to turn itself "upside down" whenever it was lifted. The handle makes carrying the OTA almost effortless thanks to the low balance point with the mirror in place.

The cooling fan is missing in this picture. It normally sits on the back of the cell [pot] to speed up the mirror blank temperature equalization with the night air. Though the OTA is stored in unheated accommodation to avoid large temperature differentials. That said, the trend is usually towards falling temperatures at night.

The large fan cooling holes in the cell are ideal for lifting the mirror in and out while it is resting on a heavy brass tube. The tube stands vertically as the entire cell is lowered over it. The mirror is then placed face up on the top of the tube. The complete cell can then be lifted carefully until the mirror enters the cell and rests on its three support pads. Finally, the cell can be lifted free of the brass tube and set aside with the mirror safely in place. There is not sufficient room for fingers on each side of the mirror blank. So the mirror cannot be lowered into the cell by hand.

I am sticking to a 3-point mirror support for the moment having looked at Plop and other websites for advice. I used adhesive furniture protector pads for the support points and the lightly sprung edge supports. These pads have a felt-like finish offering low friction. So the mirror can settle slightly and slide if need be. Note there are no clips over the front of the mirror to avoid unwanted diffraction effects.        
               

The upper cell shows the curved vane spider to the secondary mirror support. The secondary cell clamping wing nut is seen between the spars. Thanks to the angle profile "rails" between which it rests, there is no chance of movement relative to the spars.

The OTA innards still need to be sprayed matt black to kill all reflective surfaces near the optical path. Presently there is only sheet of thin, black foam opposite the focuser.

The overall, aluminium construction ensures lightness and [hopefully] a very long life without obvious corrosion. So many telescopes deteriorate badly over time that I was determined to avoid such problems if I could.

The vintage, Fullerscopes MkIV mounting rests on a temporary, slotted angle iron stand. This low pier is the minimum height possible without the OTA striking the ground when pointing vertically overhead east or west of the mounting. Here, the telescope is pointing south west. Where Venus and Mars are presently situated in the evening sky. They would be visible if it were not overcast and raining. 

I have yet to fit a strong plate to the top of the pier to support the MkIV base more firmly. More procrastination over a more suitable pier material has put a damper on progress. I could cast a solid concrete base with cast-in, anchor screws to hold the mounting. Because of the desperate need for mobility around the garden I would need to be able to push a sack truck underneath the cast block.

I have been rather repeating myself in discussing my long focus, 10" ultra-lightweight, planetary Newtonian. Most visitors here will not read every single post from start to finish. So they might miss the thinking behind some of the important details. 

I am now seriously considering a Hargreaves strut. This will join the tip of the declination shaft and the upper end of the OTA. This will hopefully help to kill any flexibility between the declination shaft and the very long OTA. Since the two points remain fixed, relative to each other, there is no need for the universal joints seen in some designs. The only real requirement is that there is no slop or flexibility and that the strut can be readily removed for storage of the instrument after each use.

Click on any image for an enlargement.

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