Re: ATM undercorrection

[Ken Lowther <s0109366@cc.ysu.edu>]

On Fri, 30 May 1997, Mel Bartels wrote:

> It is true that a mirror gains correction when cooling.  But then what? 
> I don't buy this business of undercorrecting a mirror so that it's
> figure is marginally better for an hour or so while cooling.
> 
> What do you guys think?  Do any of you amateurs actually on purpose
> leave your mirrors undercorrected?  If so, what do you do from midnight
> on?
> 

Neale E. Howard, Revised Edition

"At this point there is considerable variation in the practice of mirror
making.  There are those who say mirrors must be corrected less than these
theoretical values since temperature effects encountered when the mirror
is in use tend to deepen the curve.  Telescopes are used mostly in the
early evening when the temperature is falling and the mirror is
contracting.  During this time the colder external areas of the mirror
exert a squeezing action which deepens the curve.  After the whole mirror
has coolded to a uniform temperature it resumes its normal shape, so that
a mirror properly corrected for 75 degrees above zero will have the same
figure as at 10 degrees below zero.  But we are using it during the
contracting period and therefore must expect a deeper curve than we
actually put on in in the workshop.  For this reason, some workers correct
a mirror to only 95 percent of the theoretical value, expecting  that
changing temperatures will do the rest.  This is not necessary with Pyrex
mirrors, since their coefficient of expansion is so low that the effect of
temperature changes is almost negligible.  So correct your mirror all the
way and let temperature effects do what they will.  The theory here is
that the wise astronomer does not use his telescope during a period when
the thermometer is jumping around, but uses it at as near a constant
temperature as possible, secure in the knowledge that a properly
corrected mirror will give him the best results."

sorry for any typo's

Ken Lowther  
s0109366@cc.ysu.edu
Youngstown, Ohio  USA