Posted By Ed Zarenski
Acuity is the apparent separation results for a range of varying doubles measured with one or more pieces of equipment. The results will fall into a fairly narrow range. This would be the observer's acuity range.
“The resolution of the eye is 3 to 5 minutes of arc; therefore a minimum magnification must be utilized to enlarge the image sufficiently to exceed the eye’s resolution.” C. R. Kitchin, “Telescopes and Techniques” Springer-Verlag, 1995. Other sources will quote lower limits. My personal records of many doubles puts me right in the middle of this range. Possibly some observers can achieve better results.
In my best binoculars I can rarely achieve apparent separations of less than 200 arc-seconds. I’ve recorded many observations between 210 and 240 arc-sec. Some telescope results are slightly better, possibly attributed to the better resolution of telescopes. The best results I’ve achieved with very good optics was about 160 arc-sec, equivalent to a 2” double at 80x. The closer the doubles or the wider the mag delta, the harder it becomes to achieve these results.
So my usual visual acuity is 3 to 4 arc minutes. I am not capable of seeing separation in a 4 arc-min pair naked eye; I cannot see e1-e2 Lyra (207") as separated. Optics improve the point source resolution and contrast making it easier to see separation.
I've read one binocular observer was able to split doubles of 4 seconds magnified 20x = to apparent separation 80 arcsec. My results do not indicate this cannot be accomplished, but they indicate it’s clear if we wish to compare our ability to perceive resolution with our instruments, we must first establish the visual acuity of the individuals. Anyone ever split Castor 4” or Algeiba 4.4” with 20x or 25x binoculars?
We could test acuity with a list of doubles. Some that might get better success with binoculars are 17CrB at 6” or 7CrB at 6.3”, E(sigma)953 in Mon at 7.1”, Mesartim y Ari at 7.8” or y Del at 9.6”. A wide range of binocs might be able to see these close matched doubles. Smaller binocs try 100Herc at 14.2” or any one of many around 20”. With telescopes, for each double record the lowest magnification that splits it.
Some may achieve acuity of 2.5 arc minutes or 150”, or less, requiring a magnification of only 150”/9.6” or 16x to see separation in yDel.
I’m very interested in collecting results from all observers that might lead to some realistic conclusions concerning acuity. Having determined acuity, we should be able to compare results of resolution tests from various individuals, as long as all other measurement criteria are held constant.
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