29 replies to this topic

[B l a k S t a r]

Seemingly silly question, how does one go about measuring the diameter of a fully dark adapted pupil?

 

Can't see, too dark for ruler - quick turn on the light for a second - nope.

 

Glow in the dark (red) rule ?  maybe.

 

Photograph the eye and compare to a measurement of a nearby feature distance in the light? could work.

 

How do people usually do this ?

[DHEB]

In their book "Astronomy hacks", Robert and Barbara Thompson recommend using a set of Allen wrenches (hack #7). The method goes by looking at a star (naked eye) and setting an Allen wrench in front of an eye. One begins with a wrench of small size, say 4 mm. By doing this the star image should split in two. Then one goes up to the next bigger wrench, say 5 mm. Continue trying until you find a wrench size that completely blocks the star light.

 

I have never tested this method but sounds very reasonable.

Edited by DHEB, 09 April 2020 - 10:44 AM.

[happylimpet]

I heard someone suggest using drillbits - if you have them - which come in a range of thicknesses, 5mm,6mm,7mm etc, maybe 0.5mm sizes also, and holding them in front of your eye and finding which one JUST makes a star invisible when perfectly centred. Or something like that!

[DHEB]

I heard someone suggest using drillbits - if you have them - which come in a range of thicknesses, 5mm,6mm,7mm etc, maybe 0.5mm sizes also, and holding them in front of your eye and finding which one JUST makes a star invisible when perfectly centred. Or something like that!

Yes. Exactly the same concept as with Allen wrenches!

[Second Time Around]

I started by getting dark-adapted enough for my pupil to open to its maximum.  This takes just a couple of minutes, full dark adaption being a chemical process and takes much longer.  Then I got my wife to take a flash photo whilst I held a ruler up to my eye.  The flash is so fast that there's not time for one's pupil to react.

 

Do make sure though that you don't have the flash setting to red eye, and turn off pre-flash (often used to autofocus in poor light).

[*skyguy*]

Hex keys (Allen wrenches) are probably the best "simple" method to determine dilated pupil size. However, pencils usually have a diameter around 6mm and can be used for a quick test.

[B l a k S t a r]

good suggestions with the wrenches and drill bits.  somewhat Frankensteinian fun.

[ButterFly]

Same as the drill bit test really, but a little easier to get done.  You can print on a card some boxes of given diameters, arranged vertically, then cut out an area around them.  Then just slide up and down to find the right diameter.  You can turn on the red light afterward, keeping your finger on the right slot, to check the measurement.  A big long triangle rather than separate boxes is harder to manage becuase of tilting issues.

 

One half or one quarter mm increments is usually good enough.  Even at a dark site, the sky glow (and direction) can make a difference.  What is relevant is how big the pupil is at the eyepiece.  You can shift your eye around to get a sense of whether your pupil is bigger or smaller than the eyepiece pupil.  Try the measurement using a toilet paper roll around your eye to shield the sky glow to get a sense of what it is at the eyepiece.

[LDW47]

Seemingly silly question, how does one go about measuring the diameter of a fully dark adapted pupil?

 

Can't see, too dark for ruler - quick turn on the light for a second - nope.

 

Glow in the dark (red) rule ?  maybe.

 

Photograph the eye and compare to a measurement of a nearby feature distance in the light? could work.

 

How do people usually do this ?

Next time you go to your optometrist get them to measure it, it only takes a minute or two at the most !  Clear wide open skizes and eyes !

[Tom Polakis]

I started by getting dark-adapted enough for my pupil to open to its maximum.  This takes just a couple of minutes, full dark adaption being a chemical process and takes much longer.  Then I got my wife to take a flash photo whilst I held a ruler up to my eye.  The flash is so fast that there's not time for one's pupil to react.

 

Do make sure though that you don't have the flash setting to red eye, and turn off pre-flash (often used to autofocus in poor light).

This is exactly what my wife and I did.  We have "documentation" that our pupils opened to between 6mm and 7mm.  Don't expect to see a single thing in that darkened room after the flash goes off.

 

Tom

[Jon Isaacs]

This is exactly what my wife and I did.  We have "documentation" that our pupils opened to between 6mm and 7mm.  Don't expect to see a single thing in that darkened room after the flash goes off.

 

Tom

 

This is essentially what I did except that I did it by myself. 

 

I used my trusty old Coolpix 4500, The 4500 has a swivel in the middle so I can rotate the screen 180 degrees and see the screen, the original selfie camera. 

 

I took the photos in the closet and waited a minute or two before taking a photo.

 

To take the photo, I put the camera in the manual focus mode so there was no preflash, I held a standard about 0.375" wide next to my eye with one hand while I flashed the photo with the other. 

 

As Tom said, don't expect to see anything for a while.  

 

It took me a while to get a good sharp photo so I blasted myself several times. 

 

Once I got a sharp photo, I transferred it to my computer and used an image processing program to inspect it. Rather than trying to count pixels, I used a pair of digital calipers to measure the size of the image of my pupil and the image of the standard on the screen.  I then calculated the size of the my pupil based on the actual width of the standard.  

 

I did that about 3 years ago. I got interested in measuring it because I was trying to figure out why under some circumstances, faint nebula appeared brighter with an 8.2mm exit pupil than they did with a 7mm exit pupil.  It just didn't make sense for someone nearly 70.  

 

What I found was that it seems my dark adapted pupil was closer to 8mm than to 7mm.  Unusual but this is not without precedent.  In one study, there were over 200 participants and the results were grouped by their decade.  There were 30 people in their 60's, the average pupil was 5.6 mm but the range was 3.5mm to 7.5mm. 

 

Given the lock down, it might be worthwhile repeating. I am 3 years old, smarter to be sure, but my eyes..

 

Jon

[havasman]

My ophthalmologist measured my "dark adapted" pupil at 3.5mm. But his darkness was not dark, simply what is useful to his examination needs. He's very accomplished and trustworthy but astronomy is unfamiliar territory for him.

 

I use a set of calipers held below my eye and open to a value near the expected range of my pupil as the reference. I measured 6.5mm a couple of years ago. My calipers came from Harbor Freight and cost little.

Edited by havasman, 09 April 2020 - 07:38 PM.

[ascii]

I used the photographic technique also.  I made sure the pre-flash was off, held a metric ruler near my face in a darkened room, and used the timer function on a tripod mounted camera.  The ruler didn't have to be held right at my eye, just the same distance from the camera as my eye.  Once I got the photo onto the computer, I used a paint program to cut and paste a section of the ruler.  While the pasted image was still able to be moved around by the mouse, I slid it over to the image of my pupil to make the measurement.  I got 6.5 mm as my measurement.

[B l a k S t a r]

Interesting responses involving photos and manual measurements. Makes me wonder how much extra pupil diameter can be measured after the initial "mechanical" dialation occurs, after the chemical adaptation of 20+ minutes. 

 

My allen wrench collection is rather scattered but my vernier calipers should do nicely.

[Cotts]

Take a piece of thin cardboard - the cereal box kind is perfect.

 

Draw a very long, skinny isosceles triangle with a base of 10mm and an altitude of 20cm...  EAch 2cm from the base the width of the slot will be 1mm less.  make a scale in pen or sharpie....

 

Cut out the triangle with a sharp blade.

 

Dark adapt at your favourite dark sky site.  Place the slot with the wide, 10mm end in front of your eye.  Look up at the sky.   Slowly slide the device so that a narrower and narrower slice goes in front of your eye.

 

You will hit a point where you will be able to see the device impinging on the edges of your field.  Grab the device at that point with thumb and forefinger and read the scale....

 

Sky and Telescope used to sell such a thing and I have one around somewhere.

 

Measured mine at 5.5mm

 

Dave

[Myk Rian]

I asked my ophthalmologist. 8mm.

Myk

[tau neutrino]

During a routine eye exam, my optometrist dilated my eyes. I asked her to measure the dilation when the exam was done. It came to 8mm at age 50.

[Jon Isaacs]

I think one has to be skeptical of chemical dilation.  This is not the natural process of dilation. 

 

As far as techniques to measure your dilated pupil.  I find using a gauge difficult.  The closer you are to the edge, the smaller the effect, close to the edge, the effect is essentially zero.  

 

That is why I decided to use a photograph.  There is no need to try to judge what I am seeing and whether the image is being blocked or is allowed to pass, I measure the image on a large computer screen at my leisure.

 

Jon

[Tom Polakis]

I dug up our pupil measurements from seven years ago that used the flash-photography technique.  Scale bars are 7mm.  Mine, on the bottom, fell just shy of 7mm, while my wife's were very nearly 7mm.

 

Tom 

 

 

[B l a k S t a r]

I dug up our pupil measurements from seven years ago that used the flash-photography technique.  Scale bars are 7mm.  Mine, on the bottom, fell just shy of 7mm, while my wife's were very nearly 7mm.

 

Tom 

 

 

That looks precise.  Did it hurt? 

[Starman1]

I use a Sky & Telescope Pupil Gauge.  They sold them many years ago.

It is similar to this one:

http://www.usneurolo...how&ref=NO_JDPG

You hold it up to your eye and slide it up and down until you see the edges of the holes *just* touch.

My dark-adapted pupil: 4.5mm.

Now, that is looking at the sky at night because you need to have some light to see the edges of the holes.

I can feel my dark adapted pupils contract when I look at the sky because the sky is so much brighter than the ground.

I try to only look at the ground and through the eyepiece so my night vision stays good.

(and that is at a dark site)

So I suspect it is more like 5mm when I am looking through the scope.

 

My daylight pupil is about 1.5mm (I am VERY sensitive to light due to having light blue eye color), so my night vision pupil diameter is, if 5mm, is about 11x as large as my daylight pupil.

The average increase in sensitivity of the eye is many thousands of times at the retina (I've read up to 90,000 times as sensitive as day vision when you have achieved the

scotopic limit).  As you can see, pupil diameter dilation is simply a tiny figure in comparison.  I'd say it's largely irrelevant.

Reaching maximal retinal sensitivity is the name of the game, and that means:

--no PC, tablet, or phone at the eyepiece

--no red light or any light if possible, or only an extremely dim red light if required, and never directly into the eye.

--no looking at the sky before a really dim object search, for at least 5 minutes, even though you are already dark-adapted.

--full dark adaptation--no dim object viewed until at least an hour and a half after sunset at mid-northern latitudes, with you by the scope, dark-adapting as it gets darker.

--no Moon in the sky and preferably no Venus or Jupiter, either.

--a black cloth over the head at the eyepiece, or hands cupped around the eyepiece to block ALL peripheral light.

 

You can experiment if you observe near some woods.  Walk into the woods in the middle of the night and continue to stare at the ground for at least 5-10 minutes.  Then turn around and look at the clearing you just walked out of.  Bright, isn't it?  Now, look down.  You can no longer see what you were just looking at and the only light that damaged this night vision was the light in the clearing from the sky!

The sky itself is much brighter than that, and can really knock down your night vision capabilities.  Imagine how much damage to your night vision the use of a bright red flashlight or tablet does.

And, through all of that, your pupil probably stayed the same.

 

So what is the value of determining your pupil diameter?  To pick a lowest power eyepiece, I guess; to match your pupil size with the exit pupil of your lowest power eyepiece.

other than that, I can't see any purpose to knowing your pupil diameter because it has so little to do with your scotopic vision.

Edited by Starman1, 12 April 2020 - 05:10 PM.

[Tom Polakis]

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.

.

 

You can experiment if you observe near some woods.  Walk into the woods in the middle of the night and continue to stare at the ground for at least 5-10 minutes.  Then turn around and look at the clearing you just walked out of.  Bright, isn't it?  Now, look down.  You can no longer see what you were just looking at and the only light that damaged this night vision was the light in the clearing from the sky!

The sky itself is much brighter than that, and can really knock down your night vision capabilities.  Imagine how much damage to your night vision the use of a bright red flashlight or tablet does.

And, through all of that, your pupil probably stayed the same.

 

So what is the value of determining your pupil diameter?  To pick a lowest power eyepiece, I guess; to match your pupil size with the exit pupil of your lowest power eyepiece.

other than that, I can't see any purpose to knowing your pupil diameter because it has so little to do with your scotopic vision.

.

.

.

 

Yeah, the sky at the darkest sites is gray.  I know that dew limits the ability to observe under a dark hood for many people, but it sure helps you see faint detail through the eyepiece.

 

Now I'd like to do the flash-photography pupil measurement while looking up at the sky rather than in a pitch-black room.  Probably get a similar result.

 

I like knowing my pupil diameter as a curiosity more than anything.  As you say, it doesn't have much to do with how well you're dark adapted.

 

Tom

[Second Time Around]

I use a Sky & Telescope Pupil Gauge.  They sold them many years ago.

It is similar to this one:

http://www.usneurolo...how&ref=NO_JDPG

You hold it up to your eye and slide it up and down until you see the edges of the holes *just* touch.

My dark-adapted pupil: 4.5mm.

Now, that is looking at the sky at night because you need to have some light to see the edges of the holes.

I can feel my dark adapted pupils contract when I look at the sky because the sky is so much brighter than the ground.

I try to only look at the ground and through the eyepiece so my night vision stays good.

(and that is at a dark site)

So I suspect it is more like 5mm when I am looking through the scope.

 

My daylight pupil is about 1.5mm (I am VERY sensitive to light due to having light blue eye color), so my night vision pupil diameter is, if 5mm, is about 11x as large as my daylight pupil.

The average increase in sensitivity of the eye is many thousands of times at the retina (I've read up to 90,000 times as sensitive as day vision when you have achieved the

scotopic limit).  As you can see, pupil diameter dilation is simply a tiny figure in comparison.  I'd say it's largely irrelevant.

Reaching maximal retinal sensitivity is the name of the game, and that means:

--no PC, tablet, or phone at the eyepiece

--no red light or any light if possible, or only an extremely dim red light if required, and never directly into the eye.

--no looking at the sky before a really dim object search, for at least 5 minutes, even though you are already dark-adapted.

--full dark adaptation--no dim object viewed until at least an hour and a half after sunset at mid-northern latitudes, with you by the scope, dark-adapting as it gets darker.

--no Moon in the sky and preferably no Venus or Jupiter, either.

--a black cloth over the head at the eyepiece, or hands cupped around the eyepiece to block ALL peripheral light.

 

You can experiment if you observe near some woods.  Walk into the woods in the middle of the night and continue to stare at the ground for at least 5-10 minutes.  Then turn around and look at the clearing you just walked out of.  Bright, isn't it?  Now, look down.  You can no longer see what you were just looking at and the only light that damaged this night vision was the light in the clearing from the sky!

The sky itself is much brighter than that, and can really knock down your night vision capabilities.  Imagine how much damage to your night vision the use of a bright red flashlight or tablet does.

And, through all of that, your pupil probably stayed the same.

 

So what is the value of determining your pupil diameter?  To pick a lowest power eyepiece, I guess; to match your pupil size with the exit pupil of your lowest power eyepiece.

 

Exactly that. Much better than guessing or assuming 7mm.

 

Of course having an even lower power may effectively mean you're looking through a smaller aperture scope, but you'll probably have a wider field of view.

 

O

[starblue]

This is S&T's Pupil Gauge product, copyrighted 1992.

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[Jon Isaacs]

The average increase in sensitivity of the eye is many thousands of times at the retina (I've read up to 90,000 times as sensitive as day vision when you have achieved the scotopic limit).  As you can see, pupil diameter dilation is simply a tiny figure in comparison.  I'd say it's largely irrelevant.

 

 

I'd say yes and no.. 

 

-  The difference between a 4.5 mm pupil and a 7.7 mm pupil is still 1.2 magnitudes.  That's relevant. 

 

- The vast majority of critical observing is done at exit pupils smaller than a 4mm exit pupil.  That means a large dark adapted pupil is rarely an advantage.     

 

I like knowing my pupil diameter as a curiosity more than anything.  As you say, it doesn't have much to do with how well you're dark adapted.

 

I agree.  The pupil dilates very quickly, less than a minute, probably less than 30 seconds.  Dark adaptation has little to do with the diameter of one's pupil.  As Don says, the biochemical processes result in orders of magnitude (10x) times more sensitivity, I have seen numbers even higher than 100,000.  

 

I decided to measure my dark adapted pupil to resolve a puzzle.  When I had my 25 inch F/5 Dob, I bought a used 41mm Panoptic with the idea of maximizing the TFoV to make star hopping a little easier.  I realized I would probably be giving up some aperture but I figured I could give up some just to gain a wider field.

 

As finder, that didn't really pan out, it was only 10% wider than the the 31mm Nagler.   But what puzzled me was that under some circumstances, the 8.2mm exit pupil was clearly brighter than the 6.2mm exit pupil of the 31mm Nagler and also brighter than the 7mm exit pupil of the 35mm Panoptic.  That just didn't make sense for someone not far from 70.  I figured it was probably some sort illusion or something, it just didn't make sense.

 

I decided to talk it over with Glenn LeDrew and we came to the conclusion it was time to measure my dark adapted pupil.  

 

That's when I devised my photographic method.  And the result was my dark adapted pupil was something like 7.7-7.8 mm.  

 

That solved the puzzle.  

 

That said, I don't think it's much of an advantage, I have some level of cataracts and my gut feeling is that my eyes are not particularly sensitive. At times it's an advantage for me but not often.

 

Jon