[ATM] How good is good enough?

["Scott Milligan" <starzkey@charter.net>]

John Sherman wrote (a while ago):

 

Hi Scott,
 
>rated in the laboratory as diffraction limited (as evidenced by a >Strehl
ratio evaluation > 0.8), and optics that test measurably 
>better than that
 
Ah, but is 80% Sr REALLY diffraction-limited? And if so, why can we see the
difference when optics are made better than that?
 
John, if we accept that the practical implication of "diffraction limited"
is to establish a threshold beyond which further improvements to the
wavefront result in asymptotically decreasing improvements in contrast
transfer & resolution, then I think that both theory and experience have
shown that a Strehl ratio of 0.80 correlates well with such a threshold.
That is not the same thing, I think, as saying that no further improvement
is detectable under all observing conditions.
 
 
>Now I realize that anything like a controlled experiment designed to 
>answer the question at what point (figure accuracy vs. aperture) 
>should a visual observer stop worrying about optical quality issues 
>is probably not achievable by a group of individuals scattered 
>across time and space. 
 
>1. Some attempt should be made to quantify and communicate the 
>atmospheric seeing limit FOR THE APERTURE BEING USED to report the 
>observation. 
 
Indeed, trying to decide on standards verbally is likely to be futile. So
that's why I believe the best way is to set up a bunch of scopes together,
and do side-by-side testing. It reduces the variability of the subjective
rating of the atmosphere. What one person thinks is excellent seeing is bad
enough to make the other person go indoors and watch tv. 
 
I agree with your observation here wholeheartedly, and was hoping to hear
from individuals who might have performed such a comparison, particularly
from owners of scopes larger than 12" aperture.  My own observing time
lately has been limited to mostly solitary excursions, and so side by side
comparisons haven't been easy to come by.
 
Your other four points are also very important to the cause (I think that is
part of the reason for the mirror-testing round-robin which has been ongoing
for a while). And still the best way I know of are side-by-side comparisons,
so each issue can be equalized as much as possible. 
 
I will try to join in your experiment, but it just doesn't seem doable by
emails.
 

Are you in or near California?

 

Unfortunately no, I am in Massachusetts.  

 

I will be rather busy this week with work and getting ready for Stellafane,
however I did think of a couple more observing experiences for which I have
a reasonable data trail from the bench test side of things.  I will try to
organize these and post them after Stellafane, if not before.

 

 

Scott Milligan

 

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