10" f/8 Mirror and cell

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I have applied some sticky felt pads to the cell backplate to support the primary mirror on a 100mm (4") circle. These felt pads sit on a waxy paper and are usually sold for protecting furniture against scratching by decorative objects, like vases, etc. The pads are firm, extremely sticky and easily stacked to the required height. Though I needed 3 layers the adhesive makes them completely stable. Being quite slippery the felt will allow the mirror to slide freely so that it does not distort on its supports. I shall add some pads to the 3 side supports, as well, to limit lateral movement without pinching. Hopefully eliminating the chance of the mirror tilting forwards without having to add diffraction-raising clips over the surface of the mirror.

This is one of the disadvantages of the new, smaller support circles suggested by "Plop". The mirror can easily tip on such a small support circle. Fortunately(?) I have a much larger circle of felt pads where the collimation (coach) screw heads sit. This circle was deliberately placed on a much larger radius to achieve slow motion collimation adjustments. The soft pads on the screw heads were just extra insurance against the mirror dropping while being installed or removed. Though they did raise the necessity for another layer of support pads further in.

The mirror support "gold standard" for many decades was a 0.7x Radius circle. Or 3 points 120 degrees apart on a circle 7" diameter under a 10" mirror. The new "computer calculated" standard is only 0.4R. Make of it what you will. The pads can always be removed and replaced further out if the they ever change their minds.

Here the cell backplate has been removed to confirm the mirror fits perfectly between the felt tabs on the three lateral restraint brackets. The pictures I took with the mirror in place were fuzzy. Probably the camera focusing on what was in its reflection. It is raining today so I couldn't go outside where there was more light for a greater depth of field.

The doubled up collimation springs and the wing nuts are also shown. The springs fit between the back plate and the (pot) cell, resisting the downward pressure of the mirror. Adjustment of the wing nuts aligns the primary mirror accurately with the secondary mirror in the telescope tube.(i.e. Collimates the Newtonian optics)

One day, if I'm in the mood, I might sink the lateral, mirror support angle brackets below the mirror cell backplate. This would lower the mirror a little if it was ever necessary for improving the balance of the OTA. Which I seriously doubt will ever be necessary.

Now I just have to work out how to get the mirror safely into its cell! There isn't remotely enough room for my fingers between the 10" (254mm)  mirror and the very deep, 27cm (10.6") inner pot diameter! The best method might be to lay the pot on its side and slide the mirror in on a couple of thin battens. If I get it right the mirror will ride in as if on a fork truck. Once the mirror is safely between the edge restraints I can bring the pot upright. All without having touched the mirror surface. The cell pot is held to the main beams by two screws with split spring, locking washers under the nuts for security. The notches for the screw heads are seen on the left of the backplate in this picture.

I have obtained some M4mm threaded rod and some 40mm expanded polystyrene sheet. The first is for the collimation tilting screw on the secondary mirror cell. The foam is for a plug which will be fitted to the original lid of the mirror cell pot. This will seal and protect the mirror from falling objects, dust and damp when not in use.

I shall have to do some homework on the latest thinking on first surface mirror protection. Some telescope designs had a soft pad resting directly on the mirror. I think the 18" Fullerscopes at Charterhouse had a direct contact pad.  Usually blotting paper and chamois were involved to remove any moisture. Some people sealed the top and sometimes the bottom  of the closed telescope tube. It is decades since I last had a reflector so ideas must have changed. As have protective coatings, I would imagine. My lid and almost airtight plug might seal in the dew from the last observing session.

Should I cover the plug with absorbent material? Taking the protective lid indoors to get warm while I am observing should help to remove dampness when it is replaced after observing. I could cover the foam plug with real chamois to increase absorption. Can dry chamois absorb moisture from the air? I imagine so. The fan opening in the rear of the cell will allow some air movement to remove remaining moisture. Do I need a cloth dust filter on the fan? Am I being paranoid? Probably. I knew it was a mistake to turn back to the dark side of Newtonians again. Where will it all end? I could even start making mirrors again! Eek!

Well it seams little has changed since I used a plastic, snap-on lid, food tub. I built it right into the 9-point mirror cell to house my F3.8, 8.75" primary mirror back in the 70s. I was quite impressed with my idea at the time. It fitted the mirror perfectly and protected it from almost anything likely to happen to it in the truss skeleton tube. Then there was my use of expanding rubber Rawlbolts to hold the aluminium tubing to the supporting rings Perhaps I should have patented my ideas and become hugely rich? I could have become a household name and moved with the stars! ;-))

Cutting the polystyrene sheet for the lid plug went well. I used a home made beam compass to mark the circle and then cut round with a fretsaw. Nipped outside to blow the dust away, et voilà, a perfectly fitting circle. Now I just need to buy some genuine chamois. I wonder if there's any point in padding it with cotton wool for greater absorbency? Probably not unless I make it a mirror contact pad. Now I have to think about a suitable glue. I have some water based contact adhesive...

Or should I use a clamping system so the chamois can be removed and rinsed if it gets dirty? It might get accidentally dropped or placed face down on a dirty surface, or even the ground. Large washers would stop screws from pulling through. Countersink the screws and washers to avoid contact with the chamois if they should ever rust over time. Clamping is best I think. Trim the excess chamois and trap it between polystyrene and the aluminium lid. The chamois will increase the friction of the plug in the pot. Reducing the risk of it falling out while the telescope is being moved about. I may need to reduce the polystyrene plug slightly to get a nice fit in the pot.

Self tapping screws would be neater on top of the lid but wouldn't hold in the polystyrene. I could use a circle of thin plywood between the lid and the polystyrene. The chamois would still be trapped as before but between the plywood and the lid. The screws would bite in the plywood. The polystyrene would be glued to the plywood.

I know I'm rambling here but thinking it through like this saves making mistakes with time lost and materials wasted. Think twice- cut once.  Until I finish the cover I'm not keen to put the primary mirror in its cell. It just feels too vulnerable with the lightweight lid only located by a loose, pressed taper. You know what they say: Location-location-location.

Update: The only real chamois I could find cost £18(equiv) in a builders merchants. I'll keep looking!

Chamois for £10 equiv big enough for the job has been well rinsed and hung in the shade after washing in pure soap flakes.

Plugging on: T-nuts and screws to hold the polystyrene to the lid. Over long screws to pull the nuts into the foam. The extra freedom will allow me to tuck the chamois under the lid. Then I shall swap to shorter screws to clamp the chamois in place between lid and foam. Well, that's the theory. If the clamping pressure is not enough I'll find some thin  ply for the T-nuts to bite into.

The cover seems to work well. Snug, not too much friction but stays firmly in place. I used a large spatula to push the chamois into the gap between the lid and the polystyrene. Then re-tightened the screws. Provided I don't snatch the lid off, perhaps creating some suction the lid comes off easily by hand. I have made no special effort to reduce folds and creases. I'll do it properly when I feel the need.

Click on any image for an enlargement.

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