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- To: owner-atm@shore.net
- Subject: BOUNCE atm@shore.net: Non-member submission from ["Mark VandeWettering" <markv@tick>]
- From: owner-atm@shore.net
- Date: Tue, 22 Oct 1996 14:55:34 -0400 (EDT)
>From atm-owner Tue Oct 22 14:55:29 1996 Received: from tick.pixar.com (tick.pixar.com [138.72.30.65]) by relay1.shore.net (8.7.5/8.7.3) with SMTP id OAA13081 for <atm-digest@shore.net>; Tue, 22 Oct 1996 14:55:28 -0400 (EDT) Received: by tick.pixar.com (Smail3.1.28.1 #15) id m0vFlwm-0003PCC; Tue, 22 Oct 96 11:53 PDT From: "Mark VandeWettering" <markv@tick> Message-Id: <9610221153.ZM1593@tick> Date: Tue, 22 Oct 1996 11:53:03 -0700 In-Reply-To: Steve Scampini <scampini@hp-mpg.an.hp.com> "ATM Correcting Mirror using Resistors" (Oct 22, 11:04am) References: <9610221504.AA09957@hp-mpg.an.hp.com> Reply-To: markv@pixar.com X-Mailer: Z-Mail (3.2.0 26oct94 MediaMail) To: Steve Scampini <scampini@hp-mpg.an.hp.com>, atm-digest@shore.net Subject: Re: ATM Correcting Mirror using Resistors Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii On Oct 22, 11:04am, Steve Scampini wrote: > Subject: ATM Correcting Mirror using Resistors > Hello Everyone, > The article was written about a technique used by Alan MacRobert, an > Associate Editor of Sky and Tel. He has written many very well thought > of articles and books on amateur astronomy including an article on star > testing which seeming reasonable to me. In essence, this amounts to an "appeal to authority" to prove a point. While Mr. MacRobert may be an associate editor of Sky and Tel, in this instance I think his idea is loony. :) > I have visited Alan and used his scope including trying out the heating > technique on his scope. While I claim to be no expert on star testing, > it sure looked to me like it was able to significantly reduce the slight > touch of spherical aberration the mirror (a Galaxy, if I remember) had. An interesting data point, to be sure. Since I wasn't there, so I won't dispute that. > As far as the heating issues, keep in mind that the resistors are sandwiched > against the mirror under a piece of foam insulation thus minimizing the > heating of the air on the back of the mirror. Also, I think (and could be > wrong) that you do not have to keep dumping increasing wattage into the > glass to maintain the effect - just keep the back hotter than the front. Glass, while an excellent insultor, does conduct heat. The heat that is applied (even perfectly) to the front will diffuse to the front, eventually reaching an equilibrium temperature when the heat input = heat radiated. Any significant correction in figure can only occur when the temperature is changing, as that is what introduces stress on the glass. There are those I know in the optics business that claim that optics tend to have a "preferred" temperature, that for instance, a flat manufactured at 70 degrees should be used at 70 degrees, because it tends to acquire some residual curvature at other temperatures, even when at equilibrium. I have never heard of this before, but would like to hear other opinions as to this. If this were true, then Mr. MacRobert may be utilizing a similar effect to develop correction in his mirror. That being said, I remain intensely skeptical. Mark -- Mark T. VandeWettering Telescope Information (and more) Email: <markv@pixar.com> http://webspace.com/markv/ <markv@webspace.com> Clear Skies!
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