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The arrival of my 4" and 5" 1/20th wave flat mirrors from Nova Optical in the US prompts me to post yet another monologue. See the link in the next post for Nova Optical. For those who have not read the entire saga I need to reduce the load on my aging bod. Carrying a long, straight tube out to the mounting involves several operations including dragging, lifting and handling considerable weight. Not least is having to lift the entire OTA above my head into the waiting tube rings. So I am going to fold the optical path into three shorter sections with two flat mirrors. The folded OTA will not need to be raised nearly so high so that I can "get under it" to look overhead. In fact I can probably sit comfortably on a chair when the folded telescope is mounted at a suitable height for observing with a star diagonal.
I need a means to support the folding, optical flats so that they do not sag or become strained from their containment or support. Mike Lockwood has much to say on the subject and his wise words have guided my own choices. Though he was talking about large Newtonian secondaries the same conditions apply. The 2nd folding mirror will be suspended face down just like a Newtonian secondary flat. Except that the 2nd folding flat will be round rather than an ellipse. Or any other similar "oblong" form typically used for large Newtonian secondaries. Grinding a traditional elliptical secondary out of a huge, round blank means lots of very careful work and expense. So some secondaries have only the corners relieved from a basic, rectangular blank. Strain may be built up or relieved accidentally in the mirror blank if any heat is caused by grinding. Some might consider silicone adhesive to retain the flats but it has limitations and great care is required to avoid an unsuitable supporting material. The adhesive must be flexible enough to avoid causing optical strain in the mirror blank itself. This usually means applying thick blobs of adhesive with easily removable spacers to ensure the blobs are not flattened by the weight of the mirror blank. Thick blobs will act rather like coil springs once the adhesive has cured and the temporary spacers removed. Which will allow some lateral movement when the backing plate [or board] inevitably warps, contracts or expands.
The backing material must also be suitable for the silicone without loss of strength or adhesion. Which might mean solvent treatment or special priming to avoid the adhesive peeling off under load. A large secondary could easily ruin a very costly primary mirror [or objective] if it fell the entire length of the OTA when the telescope was pointing vertically!
Mike Lockwood goes on to suggest a surrounding metal shell instead of using silicone adhesive. But one with a full surrounding rim which is carefully flattened and smoothed. The intention being to offer an evenly supporting rim around the entire edge of the secondary mirror. Sagging or warping of the suspended mirror blank must be minimized. Equal edge support all around the edge is probably the best and easiest way to avoid it.
If the baking tins prove to be undersized I can easily split the shells with a fine saw like most other commercial, elliptical secondary mirror shells. The shells can then be snugged up with screws to contain the mirror at the base of the anodized aluminium cans without distorting the glass. If the cans prove oversize then that is no problem at all and suitable packing can surround the glass blanks. I just hope they aren't too undersized!
When they arrived I found the 4" can provided a perfect fit with "shake" clearance. I had placed the mirror face up on a suitable plastic object to lift it safely off the table. The baking tin was then lowered very slowly, gently and squarely over the raised mirror. [Just as is done with object glasses to lift them out of their cells or to replace the glass elements afterwards.]
The 5" tin is slightly oval and refused to slide onto the mirror blank. I measured the tin and confirmed at least a 1mm of ovality. So there is still hope of it fitting if I can just make the tin perfectly round. If not I shall just have to slit the baking tin for clearance. Both mirror blanks are of exactly the correct diameter.
I shall have to make up some rough plywood mandrels for the lathe to hold the tins. Then I can cut an opening in the base of each can to leave a neat rim to contain its respective mirror. Both mirrors will be similarly supported so that the folded refractor OTA can be placed down in any orientation. Without any risk of a folding mirror taking a "nose dive" out of its collimation cell. I feel the encircling rim is kinder, more theoretically sound and more secure than traditional edge straps. Which usually have bent over tabs to stop the mirror blanks escaping. It is inevitable that such tabs will damage the mirror coating over time and may even add unwanted diffraction effects if they fall withing the light path.
The flats are shown resting on top of their respective baking tins. Which will hopefully be turned into retaining shells for their collimation cells. The remarkably strong, rolled rims will provide a perfect stop for a recessed [plywood?] ring to prevent the tin flopping about as the telescope is moved around. With a flat base behind the ring the mirrors will be held securely but gently. All it needs now is to turn the tin's bases into open rims to safely retain the mirrors. Which will of course be snuggled inside the tins with their faces almost flush with the open bases. The quality of these baking tins is truly remarkable. I am completely unable to make any impression on the ovality by pressing with my hands. It seems almost a shame to have to spray the anodized aluminium tins matt black before they are used in anger. I am a great believer in using found objects instead of starting from scratch with cruder raw materials. Or even machining a component from the solid. If I were to remove the rolled edge, the cans would still have more than enough strength to support the substantial mirror blanks. Note the almost complete lack of taper in these 3" deep drawn objects. The cell bases are likely to be Birch plywood turned to fit the can at the normally open end. The cell base board may be supported on traditional springs much like a reflector's main mirror cell. Or held by a hinge on one edge of the base and tilted for collimation via a sprung screw or push/pull screws. Rotation of the tilted cell will allow for lateral tilt during collimation.
Mirror cell hinging [hinge-ing] has several potential advantages: It is likely to need less depth than a conventional three spring cell. It avoids having to put a preset tilt on the mirror cell to match the required angle of reflection. It may also provide more secure collimation for an OTA. Particularly one which needs to be brought out from storage and returned there after the telescope has been used. It should safely avoid sagging of the folding mirrors as the springs settle and adjust to the suspended mass of the mirror blank and its cell. All pulling and pushing at different orientations of the OTA. Including being placed "nose down." [Objective facing downwards.]
Click on any image for an enlargement.
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