The 1.5-3mm Exit Pupil 'Sweet Spot'

Greetings,

I have read in this forum that an eyepiece 1.5-3mm exit pupil hits the 'sweet spot' for the viewer. I have looked for (but may have missed) an explanation of why this is so. Why not a 4mm or 5?

Thanks

-David

In my opinion, the "sweet spot" is greatly dependent on the object to be viewed and the 1.5-3mm may turn into a "sour spot" ;-). For instance, 1.5mm exit pupil is way to bright and too low of a magnification in both my 8" Dob and my 4" TV-102 APO ;-) when viewing Mars. Even with DSO, some objects can benefit from less exit pupil (i.e. more magnification) such as GC and PN ;-).

Perhaps for this reason, many of us have a eyepiece case full of eyepieces (eg. Floyd needs a suitcase ;-).

Ron B[ee]

I believe that the sweet spot refers to the fact that the human eye has a maximum resolution at an entrance pupil of about 2mm.

Dave Mitsky

There is a chapter in 2005 Observer's Handbook on exit pupils (p45)...

From my limited observing experience, larger exit pupils 5mm - 7mm provide brighter images but also brightens the sky also, so unless you have dark skies (i have suburban pollution) its not so good...

Smaller exit pupils darken the sky, and 3mm is balance of darkening the sky (for contrast) and darkening extended DSOs...

Under 1mm exit pupil, you're extended objects are too dark, and the high magnification/small exit pupil combo is bringing out imperfections in the optics, your eyes, increasing diffraction, reducing contrast, magnifying atmospheric turbulence etc.??? Good for seeing dimmer stars (points of light) though and also separating doubles...

In the Backyard Astronomers Guide, Dickenson and Dyer refer to a 2mm exit pupil as being optimum in terms of the eye's ability to resolve detail. So the optimum EP for an f/10 SCT would be 20mm, 10mm for a f/5 reflector , etc.
"This rule isn't hard and fast, but it serves to point out which eyepiece you should invest in most heavily. If you can afford just one top-end eyepiece, make it the one with a focal length about twice your scopes focal ratio."
I bought a 20mm Nagler for my 8" f/10 SCT last year during the sale, with this in mind. Fortunately, it seems to work fairly well in the f/4.3 Starmaster, as well. I guess my sweet spot EP for it would be my 8.8mm UWA. Though I'd really like to try a 7mm T6. Anyone?
Linton 8)

Hi David et al,
Sorry to come in late on this thread. There have been a couple discussions recently on this topic. One of the better ones was on [email protected] in late May/early June.
Chris is correct that the 1.5 to 2mm value comes from the Maksutov publishings in the 1920's through 1940's.
Maksutov published his findings based on his own visual observation and testing. Hopefully his eyes were good! Average? Who Knows.
Note this value is based on the Central Vision vs Averted Vision Rods & Cones of the eye.
From the posts on the ZOC group:
"For Maksutov's eye...best resolution comes at around 1.5mm pupil size...a bit over 2 arcminutes. At 5mm pupil size it's down to almost 4 arcminutes. (From personal experience of various eye exams where my pupils have been dilated or constricted...big pupils, 7 - 9mm, give horrible images, but 1.5 - 2mm is awful sharp.)"
"Maksutov goes on to show how the data from this table can help one actually determine what magnification/exit pupil/scope aperture is really needed to resolve a given angle. (Note: Maksutov did these test's on an optimum brightness artificial double star. If he made the double star brighter or fainter...his resolution ability dropped.)"
Continued in part 2

Continuation - Part 2
"Maksutov also made studies of angular resolution of faint objects, i.e. transitioning from central/foveal vision to averted/rod cell vision. Keeping pupil size constant at 5mm, he lowered brightness of his test target. The best resolution, as stated above, was a bit less than 4 arcmin. At the lowest detectable light levels (threshold of detection)...resolution was down to 2 degrees, repeat, 2 degrees."
"I have briefly summarized a small bit of what Maksutov took pages to carefully, thoroughly cover. This is a complex subject. (And I have not addressed contrast sensitivity at various light levels or size/spacing of test targets.) Maksutov ends his section with this appeal:
"It’s especially important to know how to apply the theory discussed above when designing new optical instruments: consumer, military, geodetic, etc., so that they are equipped with the best eyepieces for their given observing tasks. But we are powerless to give any strict guidelines until there appears statistically reliable material on the resolving power, aberrations, and inhomogeneities of the average observer’s eye. Therefore I must repeat my appeal from 19 years ago to astronomers, who are most able to undertake such studies: “test your eyes’ performance and report to the author your objective and thorough findings!”"
There's a WHOLE bunch more in the thread, excellent reading. My Thanks to the Carl Z. Group!