7" f/12 iStar folded refractor 7: The flat mirror retaining shells:

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Despite it being 38F in my workshop and suffering from Sciatic pain I decided to press on with the flat mirror cells. Or rather the shells which will contain the optically flat folding mirrors. These 4" & 5" baking  tins had already pledged their donorship to my cause. First I used a pair of screw adjusting bow compasses to mark the center of each base by means of arcs. I then used these as a center to draw a circle around the edge of the base. Ensuring that I left a generous width of rim to safely start my chain drilling. Chain drilling by eye is an inexact science so it pays to leave plenty of meat behind.

I used three small drills in turn in my dirt cheap, Chinese, bench, drill press. It must be nearly 30 years old by now but still behaves at least as roughly as it did as new. By using a couple of large nuts bolted down as guides on the drill table I was able to ensure concentricity of my first circle of closely spaced holes. Stops makes it so much easier than relying on eye alone. The first and smallest drill is inevitably springy due to its very small size. So it pays to be patient and allow it to start drilling where it will. Even if that results in slightly uneven hole spacing it hardly matters.

Once the first ring had been drilled I changed to the next drill size up and went round again opening up all the previously drilled holes. It is important to let the drill find its own center. Trying to force it to go where you'd prefer will only bend the drill. The larger drills to follow will ensure the disk is safely cut out. Thicker or stronger materials should have very much more care put into hole spacing on the first ring. Double-pointed center punches are handy for this job in thick, tough material. By steadily increasing the drill size on each run the gap between the holes is reduced by the difference in its own radius. In my thin alloy a third drill managed to break through almost all the way around. So that a light tap with a hammer knocked the ragged edged disk right through. I then used a large, coarse, half round file to smooth the ragged hole in the base of each baking tin.

In thicker and/or tougher materials a hacksaw blade should always be run along the previously drilled holes. This will ensure the disk is cleanly separated and can fall out of its own accord. Forcing a partially separated disk from the parent material will usual be costly if separation is not carried out properly. Patience and care will always be rewarded in a minimum of waste/wasted materials.

Once the edges of the large holes were filed reasonably smooth I moved on to the lathe. Had I left the edges ragged they might have caught in the lathe tool. Holding such flimsy items in the lathe chuck requires some care. Concentricity is essential or the resulting rim will be lop-sided. An eccentric hole means loss of folding mirror aperture. Which could be critical for mirror position in the folded OTA.

A more fastidious builder might have used wooden/plwood/MDF mandrels to hold the tins concentrically and more securely. I decided to bypass this step to avoid making wood dust in my cold and damp workshop. Fine wood dust sticks like glue to cold, bare metal and causes rapid surfaces rusting by retaining moisture. In warm weather this is not remotely such a problem and the wood dust can be simply brushed off.

I used the 4" 3-jaw chuck for holding the 4" tin and my larger 6" 4-jaw chuck for the 5" tin. Tightening the jaws carefully avoided denting and permanent marking of the thin alloy walls. I cut very slowly and carefully throughout to avoid the tins being snatched out of the chuck jaws. The soft aluminium wanted to build up on the tip of the ceramic tool. So I just scraped it off at intervals with my finger nail after withdrawing the tool. Progress was slow but eventually I had reached a 98mm diameter hole on the base of the 4" can, leaving a 1mm wide rim. I finished off with rough abrasive paper wrapped around a cylindrical object. This safely removed the burrs which had formed both inside and outside the rim.

Soon I had a clean but much narrower rim without any burrs. Leaving only the thickness of the base material.

I stopped my cutting on the 5" can when I had reached 123mm. A 1mm wide retaining rim is fine for a 125mm mirror blank. Again I used the abrasive paper with the cylindrical former to avoid cutting my fingers as the tin was rotated quickly in the lathe. Burrs are always razor sharp so safety must be seriously considered. Sharp metal can cut through abrasive paper without any effort at all. Better it does this to an inanimate scrap object rather than a 6 hour wait at the A&E in a distant city!

I was also careful to keep the abrasive paper square to the tin to avoid damaging the anodizing. Even if it is only cosmetic it helps to maintain a longer life. So many telescopes look awful after a few short years of constant exposure to the cold and damp.

Unlike the 4", the mirror blank is a tight fit in the 5" tin. So I will have to split the shell to make room and allow for shell expansion and contraction. The Zerodur blank will hardly expand or contract at all relative to the aluminium. So one doesn't want to sacrifice optical surface quality by squeezing the mirror blank unduly! Screws into the cell base through the split shell will ensure there is no loss of strength. An alternative would be to seek out a slightly larger 5" baking tin.

Now I have my mirror shells finished I can start seriously thinking about the cell bases. I have yet to decide if I am going to retain the rolled rims on both baking tins for added strength. Or cut the tins down to greatly reduce their depths. If I do keep their present depths then some packing will be required behind the mirrors. This packing might well inhibit air circulation around the thick glass. Which in turn might reduce the glass cooling in typically falling temperatures. Resulting in thermal convection currents forming from the faces of the folding mirrors and rising into the light path.

I could drill a series of holes around the shell to allow air to reach the mirror blanks. Only a high quality hole saw will make a decent job in such thin, soft material. Trying to use twist drills will only tear the alloy shells to shreds and leave ugly burrs! It might still require a wooden mandrel to allow the holes to be cut neatly. Better perhaps to allow air to circulate freely behind the mirrors by means of large ventilation holes in the collimation cell bases.The backs and fronts of the mirror blanks have a much larger surface area than the edges alone.

Click on any image for an enlargement.

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