[ATM] Flexed mirrors

["Bill Kelley" <flex@commspeed.net>]

ATM friends,

About 16 years ago an article of mine was published in Sky & Tel about a 
simple improvement in a 6" F8 sphere by mounting it on a soft ring at it's 
periphery and applying tension at the center to further correct the mirror. 
Later a mirror I made with a star shaped puller gave much better correction. 
Alan Adler published a simple doss program in Sky & Tel that predicted the 
optimum puller shape, required tension, and much other information. 
Occasional notes have been published that further verified the idea. No ATM 
posts or publication elsewhere has found verifiable fault with mirror 
flexing. Any time any thing new is posted about flexing it seem to stir many 
angry responses and a few calming replies from others who know better.

So it is time to poke the hornets nest once again.

Why flexing for the ATM ? :
  1. A spherical surface is the easiest to make.
  2. A spherical surface is the easiest to test.
  3. Extreme smoothness can be achieved because the tool and mirror fit in 
all positions of the polishing stroke. The larger
      and faster the mirror the worse the fit, and consequent small scale 
surface defects. The resultant light scatter reduces
      resolution and explains why many refractors provide better resolution 
than reflectors.
  4. Among the many other advantages your flex mirror can be detuned to an 
appropriate ellipsoid  for near range imaging.

At 90 years of age and 16 years since the introduction of "microflexing" 
what started me on this rant?

I was out the other night with a dozen local club members. Commercial scopes 
up to a C14, one 8" home made reflector,
and one 6.5" refractor. The refractor guy and I with my home made 13.2" F4.8 
flexed reflector were pleased at how many stars we could see in the core of 
M13. None could be seen in the other scopes. The guy with the 8" had done a 
decent job of parabolizing the way it is always taught and still couldn't 
resolve anything in the core of M13.

The newbie ATMer does not want to become a professional, he just wants to 
make a telescope that works very well with a minimum of grief in the making. 
The ATM list is always full of newbies trying to parabolize. Flexing has 
stood the test of time. Let us give them a break and let them be succsessful 
with a flexed telescope. At least for their first one, and keep them 
energized and staying in this wonderful, fading hobby we so love.

Bill Kelley




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