(In the daytime, of course)
I took a chance on a "Yardage Pro" rangefinder at the consignment shop
($14) and quickly replaced the battery that had a 2016 sell-by, so it wouldn't
"juice out" out the battery compartment. With new battery...shazam! Works great.
I'ts a laser time-of-flight widget, so angles are no problem.
So it's easy now to place things like shampoo or Dorito ingredient lists at a
solid spot and get the distance, then figure the "total acuity", scope+ep+me....
As an example:
From an 80mm x 400m scope, focal-extended to 80x,
a 1.2mm font can just be read at 39 meters (39000 millimeters).
Optometrists say you need 5 line seperations per-height
to read a sans-serif font, so the resolution is at least 1.2/5 = 0.24 mm.
The angle for this is arctan(0.24 / 39000)
or 3.526e-4 degrees, or (times 3600) 1.27 arc-seconds.
(I assume I could do slightly better, but it was starting to blur)
According to several books, the Airy disk is:
"For an 80-mm refractor, 1.4 arc seconds."
So while the eye can likely see better than the Airy disk
(likely due to the brain knowing letters, and cortex processing),
the results are pretty close.
One thing I observed:
----there was very little chromatic aberration at 80x (1mm exit pupil)
for an 80mm F5
---the thing is, an 80mm scope gets fairly dim at night past 80x anyway,
and the atmosphere here can having seeing wobble of 2-6 arc-seconds
I would need a bigger aperture AND a better sky to do more power...
I do a fair amount of day observing as well, so the rangefinder will be
valuable in looking at the ranges where the sometimes-brutal
"horizontal seeing" kick in. Over a mile, the slushing.can get to 30--70 arc-seconds!
Edited by MartinPond, 15 March 2019 - 01:10 PM.