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NGC 1502

Measuring dark adapted pupil

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Hello. We are all well aware that part of dark adaption is that the eye’s pupil dilates. Very often, choice of telescope / eyepiece / magnification  and the resulting exit pupil is taken into account when we know the size of our dilated pupils.  Many articles on the subject say that as we age, then our pupils don’t dilate as large.  But how to accurately measure the size of our own dilated pupils is important, not the average for our age.  So trawling the ‘net, I found that one of many suggestions is to ask our eye examiner when attending an eye test. But there can be a snag in doing it that way. I found the following -

“During my routine eye examination, I asked the ophthalmologist if he would kindly measure my dark adapted pupil.  He readily agreed, and dimmed the lights as far as possible  in his very well appointed examination room. However, fumbling around in the dark, he unfortunately tripped and fell into his lens grinding machine and made a spectacle of himself “.     Very sad indeed.

Ed 👀

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I asked this very question recently attending an annual eye test appointment and in response the examiner asked why I wished to learn this. Upon explaining I was informed that this examination was common among amateur astronomers, but would require specific instruments not available at the opticians, perhaps through a private eye hospital / clinic.

Edited by scarp15

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It's really simple....

Get a set of spiral drill bits, graded in 1/2 mm steps. Go outside (your pupils will adapt within a few minutes, not so the retina), and look, one eye covered, at a bright star. Hold, beginning with about three millimeters, a drill bit (blunt side up), 15 to 30 cm in front of your eye (you may as well go farther away with it -stars send parallel light rays out of infinite distance). You will see the starlight, passing the bit's edges and entering your eye. Now, work your way up to larger bit diameters; you will still see the starlight. At a certain point, the used bit will block all the starlight, and the star's image will have disappeared. Repeat two or three times; the bit's diameter just below the occluding bit is the diameter of your maximal dilated pupil. Now test the other eye. I've assessed for my eyes 6.5 mm (left eye) and 6 mm (right eye) that way - very satisfying at age 66. It's astonishingly precise, when repeated.  Learned this from an article by German amateur Uwe Pilz years ago.



Edited by Nyctimene
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On 13/12/2018 at 16:50, scarp15 said:

 . . . would require specific instruments not available at the opticians . . .

Yes, I assume few opticians keep sets of drill bits among their equipment   :-)

Stephan, many thanks for passing on the drill bit tip, which seems the obvious and logical way how to measure dark adapted pupil diameters.

Which made me wonder, and go into my bathroom with the light off, equipped with one of my bicycle rear LED lights for testing . . . .

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It's more difficult to do if we have myopia, as the star will be in focus ( with specs on ), but the bit blurry.

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