Re: [ATM] Telescope Focusing at Public

[Mark Holm <mdholm@telerama.com>]

Actually, it is a bit more complicated than whether a person is near 
sighted or far sighted.  Both of those can be remedied with a small turn 
of the focusing knob, and the observer can dispense with glasses.  With 
no glasses, eye relief problems are minimized.  The viewer can see all 
or most of the field of view.

The next most common vision problem is astigmatism.  Turning the focus 
knob will not help astigmatism.  Still, it would be nice to know if 
there are situations where an astigmatic observer can take his glasses 
off, in order to minimize eye relief trouble.  Sky & Telescope has a 
discussion on page 132 of the September 2004 issue.

The discussion quotes Richard Buchroeder as saying that, whenever the 
telescope's exit pupil measured in millimeters is less than 1 / Sqrt(D), 
then one can remove eyeglasses without undue image degradation from 
astigmatism.  D is the Cylinder correction part of the eyeglass 
prescription measured in diopters (the way they are commonly measured, 
at least in the U.S.)  Note that a person may have very different 
astigmatism in each eye.  The angle of the astigmatism, also part of the 
prescription, does not matter for this purpose.

You could make up a table or chart for any particular telescope of the 
astigmatism limit versus magnification, or astigmatism limit versus the 
eyepieces you have available to use.  Then, if your viewer happens to 
know his astigmatism number, you can give him sound advice.

Example:  200 mm diameter f/6 telescope

Eyepiece Focal Length   Magnification   Exit Pupil   Maximum D
       25 mm                  48        4.17 mm        0.058
       18 mm                  67        3.00 mm        0.111
       12 mm                 100        2.00 mm        0.250
       10 mm                 120        1.67 mm        0.359
        6 mm                 200        1.00 mm        1.000

Maybe Nils Olof Carlin could tell us the common range of astigmatism 
values in the population.

It is quite common to have both near/far correction and astigmatism 
correction in eyeglasses.  If the viewer doesn't know their astigmatism 
number, then the only practical way to find out if they are better off 
with or without their glasses is to have them try it both ways.  They 
will need to refocus for the with and without situations.  Whichever 
allows them to see most clearly, is the one to go with.  Just tell them 
that it does depend on telescope size and magnification, so they won't 
think they have one answer for all situations.

-- 
Mark Holm
mdholm@telerama.com


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