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Re: Wonderings On Spin Casting Parabolic Mirrors, Multiple Mirror Telescopes

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Posted by David Allen/">David Allen on February 1, 2000 00:25:01 UTC

: : Formula mis-sent, read L=g/2w squared

: : : : SPIN CASTING: : : : I've been checking out info on liquid mirror telescopes and found mentioned that a rotating slab of fluid naturally takes on a parabaloid. Varying RPM will change focal length, the relationship being

: : : 2 : : : L = g / 2w

: : : where; L = focal length : : : g = gravity : : : w = angular speed ( in radians/sec )

: : : (12 RPM is about 10' focal length, faster RPM = shorter focal length, recommended 6 RPM range for large mirrors. See Liquid Mirror Telescope @ : : : http://math.iupui.edu.m261vis/LMirror/LMirror.html).

: : : I've no intention of spinning a couple of meters of mercury, but they seem to have used this principle to : : : cast the rotating paraboloidal pan or bowl to hold the mercury, which allows use of mere millimeters of murcury (a skin of mercury on top of a rough paraboloid), saving much weight (Mercury being HEAVY). : : : Also Opticast corp has patented some kind of spin casting process using a gypsum cement substrate with hard thermosetting plastic (or some such) layers on top for the aluminized surface. They offer a 1/4 wave 6" f4 for $58, aluminized and SiO coated.

: : : I WONDER if there's a low temp material that will take a polish of optical quality (at least, for a first surface mirror) that I can spin on a grinding machine-like rig, allowing hardening while rotating. Will it need finishing or polishing? RPM tolerances are only as critical as are precise focal length requirements. I can live with my mirror being f5.8 or 6.2 instead of f6.0 if I dont have to spend thou's on 4 decimal precision motor speed control, but how much will small : : : speed changes during hardening affect the curvature without it? I would guess that the less precise the motor speed control, the more finishing required. : : : I guess the major points to address would be:

: : : Finding a substance suitable for spin casting, polishable to optical standards, aluminizable(?), not requiring molten glass temps, and thermally stable. : : : (Could you avoid the necessity of polishing with Precision motor control?)

: : : Where does accuracy of motor speed reach the budgetary point of diminishing returns? How close is close enough?

: : : It just seems to me that Nature has given ATMer's a natural phenomena to make it easier to enjoy Her wonders, and it's up to us to find a way to use it. : : : It's easier to watch it spin than to grind it.

: : : : : : MULTIPLE MIRROR TELESCOPES:

: : : Has anyone heard of an amateur made multiple mirror 'scope? I'm talking multiple primaries, not folded scopes. I KNOW that mirror focal lengths must be matched, and there are BIG collimation headaches, and that mirrors off axis have coma probs (Long focal length may not be so bad), but I can get 4 or 5 finished 6" mirrors for less than a 12" mirror grinding kit. Finances aside, are there reasons it's not done? Are there any out there? I understand that : : : it may not be feasible at the amateur observer scale, but as an intellectual/educational excercise, perhaps? : : : Maybe because no one else is doing it is reason enough.

: : : : : : Any feedback? : : : Thanks in advance, Cly Roth

: I know what you mean by this. I am looking for spin casting mirrors also. I have gotten this far in my quest for an inexpensive mirror. : First of all, you can use a potter's wheel for spinning your glass. I don't know how accurate it would be, but the good part is, accurate polishing is very much easier to accomplish on a spinning potters wheel. Theoretically, junk pyrex glass can be thrown into a high temperature mold made from a type of concrete. This mold should at worse, crack. The concrete is very inexpensive also. : How do you heat it up? This is another hypothesis. Make a dome out of the concrete to cover the glass In order to keep it warm. Underneath the glass You will make a concrete foundation; most of it solid, but the upper part with mostly space because you may heat it from the bottom also. Melt the glass by using a high temperature device(map gas, propane gas etc)and begin melting it by putting the flame directly on it. Mapp gas can melt it, but it is very slow to do so. That is where the dome comes in. Melt the glass and place the dome over it and insert the blowtorch into a hole you will have made for that. You will also have a hole(possibly at the top of the dome) for venting the gas. When the glass is melted put it immediately on the potter's wheel and spin it until it solidifies. You will figure out yourself how fast to spin it.

: The bottom line is: if it works, make more of them and sell the glass to other amateur telescope makers.

: This message will only cost you one concave 12" or better piece of your glass product. : good luck : david

Hi! I couldn't resist telling this. I have thought of making a reflector out of sugar. They make panes of glass out of sugar for special effects. I would love someone to do it with a telescope reflector. David Allen

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