Re: [ATM] Re:How much of cell induced deformation is too much?

["john sherman" <atm@mail.johnspics.com>]

Hi Vlad,

>you may be tired of reading books, but it's all 
>different story when it comes to writing :) 

I'm sorry. I hadn't done much writing in quite a while (due to too much reading), and I didn't realize till later how long that post was.

>Let me address the core of our disagreement: 
>
>My point was that after the error has been reduced to 
>a certain (very low ) level, there is no practical 
>gains from investing into further error reduction. 

So we only disagree on how low that level is, apparently.

>, and we arrive at the final Strehl of ~0.50. 

Maybe I'm wrong here; Is it correct to say that 50% Strehl means ~43% of the light is in the Airy disc, and ~57% is elsewhere?? Shouldn't that be horribly obvious when you look at an in-focus star at high enough power to see the Airy pattern?

>It is almost as bad as 1/2 wave of spherical aberration.

The interferogram from my old mirror showed almost 1/2 wave of spherical and a tiny bit of astigmatism. The printout called it 85% Sr, not 50%. But that's on a mirror's wavefront, not a telescope's final wavefront. Maybe it's different somehow?

>Primary needs to be closer to 0.80 (actual) Strehl, 
>to begin to make a difference to the worse. 

I still don't agree. However, I can understand how someone who lives in poor seeing might say so. The guys in Florida sometimes talk about regularly getting quarter-arcsecond seeing. Perhaps a trip to the Winter Star Party would be fun?


Good luck,

John










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