Re: [ATM] undercorrecting?

["John Sherman" <atm@johnspics.com>]

Hi There, 

>OK, so I missed the real point there.  Sorry about that.

Mike, that's cuz the real point was muddied afterwards. The later post said:

>>a slightly undercorrected mirror, warmed 2 degrees 
>>Celcius at the back side, goes from 0.66 to 0.99.

Notice the "warmed".. 
And notice that 0.66 Sr is called "slightly" undercorrected!

>even a mirror cooled with fans, will never reach equilibrium 
>because of radiation of heat from the alumiumized surface

I find this impossible to believe. Someone will have to prove 
it to me. Surely you've heard of "wind chill factor".

>radiation gives a 2 degrees C difference in temperature between 
>front and back side of the mirror, 

How can a shiny, reflective surface radiate that much heat? How was this number arrived at? Theory? Guess? Was the mirror in a solid-tube scope, or a scope with a shroud, and thus insulated from the sky? Or was it in a totally open minimalist tube? Was the ambient temp perfectly stable the entire night of the experiment? Perhaps this temperature difference is really caused by a mirror warmer than ambient, but exposed to the sky, while insulated at the back?

I have heard that many makers make undercorrected mirrors for two reasons: 1) They are afraid of overcorrected mirrors,  2) It takes less time - as soon as the mirror gets anywhere near a parabola they ship it and start on the next mirror. Then they use "radiation" and so forth as an excuse.

John






Is there some reason why these people have to include one or even 
two complete posts in their reply? Too lazy to cut it down?
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