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

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Posted by Cly Roth/">Cly Roth on November 1, 1999 05:52:28 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 @ :

: 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.


: 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

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