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Old January 2nd 19, 09:41 AM posted to sci.astro.amateur
Paul Schlyter[_3_]
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Default Let's Photograph Comet 46P Wirtanen

On Tue, 01 Jan 2019 07:51:15 -0700, Chris L Peterson
wrote:
If all telescopes always had the same focal length it would be
practical to do that. Then the aperture could easily be calculated
from the focal ratio and the implicit focal length. Now, this is

not
the case of course, but in the case of visual acuity tests the
Snellen chart is always at the same standard distance.


That's what you're missing. The test is NOT always performed from

the
same distance, and as a result, the distance is a meaningful part of
the test reporting.


OK I can buy that. But the test is usually done with the standard
distance. If done at a different distance, a differently sized
Snellen chart will be needed.

And when the standard distance is used, it can still be implied. If a
non-standard distance is used, that can be added as a note. "1.0 at
3m" contains just as much information as "3/3".

The no - standard distance might contain fractional meters btw. Would
you then want ti write it as, say, 3.56/3.56?


Diopters. Always. (By definition, a diopter has units of reciprocal
meters, so could always be considered "metric".)


Among amateur astronomers there's a curious mixture of units.
Telescope aperture is often given in inches, while focal lengths is
often given in millimeters.