Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Pupil gauge to measure exit pupil

  • Please log in to reply
57 replies to this topic

#1 Nobodo

Nobodo

    Mariner 2

  • *****
  • topic starter
  • Posts: 241
  • Joined: 31 Jul 2016
  • Loc: Rural Douglas County, Colorado, USA

Posted 16 December 2016 - 01:45 PM

Has anybody used a gauge like this to measure dark adapted exit pupil?

 

It seems simple enough in concept - a single eye is looking through two holes with a measured blockage between them, and when the size of the blockage increases to the point that it is blocking the pupil, that's the exit pupil size.

 

Does it work?

If it does, it seems like it would be simple enough to print your own, cut them out, etc.

 

Thanks,
Mark.

 



#2 J A VOLK

J A VOLK

    Apollo

  • *****
  • Posts: 1042
  • Joined: 24 May 2006
  • Loc: So. Cal.

Posted 16 December 2016 - 02:18 PM

yes - bought a similar one from S&T many years ago - the trick is you can't read it in the dark when your eyes are dark adapted, I guess you can get close in dim light or with a red light. You can get a pretty good idea by comparing the brightness of a diffuse object (nebula/galaxy) with eyepieces giving different exit pupils. For me, it is clearly obvious that images are dimmer @ 7mm than 5mm.

Edited by J A VOLK, 16 December 2016 - 02:21 PM.

  • Nobodo likes this

#3 Kendahl

Kendahl

    Apollo

  • *****
  • Posts: 1361
  • Joined: 02 Feb 2013
  • Loc: Omaha, Nebraska

Posted 16 December 2016 - 02:33 PM

You can measure pupil diameter using a set of drill bits. With one eye closed, hold a bit at arm's length in front of a star. Pupil diameter lies between the largest bit that doesn't quite cover the star and the smallest bit that covers it completely.


  • musicengin and Nobodo like this

#4 Nobodo

Nobodo

    Mariner 2

  • *****
  • topic starter
  • Posts: 241
  • Joined: 31 Jul 2016
  • Loc: Rural Douglas County, Colorado, USA

Posted 16 December 2016 - 02:42 PM

I'm 58 years old and averages say my dark adjusted exit pupil should be around 5.8mm or so.

In my f4.6 telescope a 25mm gives 5.3mm exit, 32mm gives 6.7mm, and 40mm gives a nasty 8.5mm exit.

With the 32 and especially the 40 I can see dimming, so use those more often than not with a barlow.

 

With binoculars I see a brighter image in 7x50 than in 10x50.  Go figure!

 

Thanks,

Mark.



#5 Nobodo

Nobodo

    Mariner 2

  • *****
  • topic starter
  • Posts: 241
  • Joined: 31 Jul 2016
  • Loc: Rural Douglas County, Colorado, USA

Posted 16 December 2016 - 02:44 PM

You can measure pupil diameter using a set of drill bits. With one eye closed, hold a bit at arm's length in front of a star. Pupil diameter lies between the largest bit that doesn't quite cover the star and the smallest bit that covers it completely.

That makes perfect sense.  I'll have to give that a try the next night I can actually see stars (cloud cover the past few days, grrr).

Thanks!

Mark.



#6 havasman

havasman

    Cosmos

  • ****-
  • Posts: 9423
  • Joined: 04 Aug 2013
  • Loc: Dallas, Texas

Posted 16 December 2016 - 04:06 PM

I measured using calipers in front of a mirror in a darkened room after acclimating with my eye indirectly lit with the dimmest level red led on a Rigel Starlite mini. I had my ophthalmologist measure but "dark room" to them isn't dark and he measured 3mm. I thanked him and chuckled later.

I'll be 66 before the end of the year and got a bit over 6mm. I decided I can live with that just fine.


  • Joe1950 and Nobodo like this

#7 Nobodo

Nobodo

    Mariner 2

  • *****
  • topic starter
  • Posts: 241
  • Joined: 31 Jul 2016
  • Loc: Rural Douglas County, Colorado, USA

Posted 16 December 2016 - 04:19 PM

I measured using calipers in front of a mirror in a darkened room after acclimating with my eye indirectly lit with the dimmest level red led on a Rigel Starlite mini. I had my ophthalmologist measure but "dark room" to them isn't dark and he measured 3mm. I thanked him and chuckled later.

I'll be 66 before the end of the year and got a bit over 6mm. I decided I can live with that just fine.

I should try that with calipers; sounds like it would be challenging but worth a try!

 

I asked my ophthalmologist about dark adapted exit pupil and had to explain to her it's role in astronomy.  She was intrigued (seemingly) but had no suggestions for how to measure it.  All she really had to offer was "your eyes seem average so probably have the average exit pupil for your age".  I thought about telling her how useless that comment was but decided not to.  :)



#8 Joe1950

Joe1950

    Cosmos

  • *****
  • Posts: 8699
  • Joined: 22 Aug 2015
  • Loc: NJ, near Philadelphia, PA

Posted 16 December 2016 - 04:21 PM

I asked my ophthalmologist to do the same thing but while fumbling around in the dark he tripped and fell into his lens grinding machine, making a spectacle of himself. 


  • rockethead26, Mike G., Dartguy and 5 others like this

#9 havasman

havasman

    Cosmos

  • ****-
  • Posts: 9423
  • Joined: 04 Aug 2013
  • Loc: Dallas, Texas

Posted 16 December 2016 - 04:26 PM

I asked my ophthalmologist to do the same thing but while fumbling around in the dark he tripped and fell into his lens grinding machine, making a spectacle of himself. 

eye exam and free floor show!


  • Joe1950 and lakland5 like this

#10 Nobodo

Nobodo

    Mariner 2

  • *****
  • topic starter
  • Posts: 241
  • Joined: 31 Jul 2016
  • Loc: Rural Douglas County, Colorado, USA

Posted 16 December 2016 - 04:36 PM

I asked my ophthalmologist to do the same thing but while fumbling around in the dark he tripped and fell into his lens grinding machine, making a spectacle of himself. 

Hilarious, but maybe that was the doctor's way of getting your eyes wide in amazement, ready for measure!  ;)


  • Joe1950 likes this

#11 Alex McConahay

Alex McConahay

    Cosmos

  • *****
  • Posts: 7884
  • Joined: 11 Aug 2008
  • Loc: Moreno Valley, CA

Posted 16 December 2016 - 05:12 PM

Get yourself a filefolder (or some relatively stiff opaque paper file folders are made of--cardstock--but make it thin).

 

Cut a long, thin, trapezoid out of it. By long, I mean maybe eleven or twelve inches--or whatever the width of the file folder. By thin, I mean eight or nine mm wide at one end, down to one mm or so at the other end.

 

Label the thing according to width. Mark it where it is 2 mm wide, 3 mm wide, all the way to about 8 mm. 

 

Go out at night, close one eye, pick a bright star, and center it in the open eye.

 

Hold the trapezoid in front of your pupil, with the narrowest point in front of the pupil. You will be able to see the star because, although the center of the pupil is blocked, the edges of your pupil can still see the star. Wiggle the trapezoid back and forth a bit. If you can see the star constantly, your entrance pupil is bigger than the trapezoid at that point.

 

Move the trapezoid down a little bit, so that the wider part of the trapezoid is in front of your pupil. You can still see the star. Give it the wiggle test.

 

Move it again, and--poof, the star goes out. The width of the trapezoid at the point where the star disappears is the point at which the trapezoid is just wide enough to cover your pupil. Measure that point, or interpolate from your width markings. Note that the wiggle test at this point has the star blinking. At some point, when directly centered, the trapezoid is big enough to block your whole entrance pupil.

 

Move it further down (past the point where the star first disappears), and the wiggle test hides it even when you wiggle. You want the point where it is JUST wide enough to hide the star, but no wider.

 

Now, beware, if you put the trapezoid too far from your pupil, you will get a false reading.

 

And, of course, if you are not steady, you might not be putting the trapezoid right in front of the pupil, and you can see the star from light getting in on one side or the other of the trapezoid. The wiggle test checks for this.

 

But, you get the idea.

 

Try it with dark adapted, and light adapted eyeballs.

 

Alex


Edited by Alex McConahay, 16 December 2016 - 08:36 PM.

  • Joe1950 and musicengin like this

#12 drollere

drollere

    Vanguard

  • *****
  • Posts: 2328
  • Joined: 02 Feb 2010
  • Loc: sebastopol, california

Posted 17 December 2016 - 02:25 PM

the drill bit method kendahl recommended can also be done with allen wrenches. if drill bits, be sure to use the round shank, not working or clamping ends of the bit. with the allen wrench, just hold the wrench so that the short end is pointing directly away from your eye, and measure with the long end.

 

whichever you use, pass the object left to right a few inches in front of your pupil. if the star seems to twinkle but not extinguish, the object is too small. if the light entirely blinks out, the object is too large. your pupil size is between the smallest diameter that blinks out and the largest diameter that flickers.


  • *skyguy* and Nobodo like this

#13 Nobodo

Nobodo

    Mariner 2

  • *****
  • topic starter
  • Posts: 241
  • Joined: 31 Jul 2016
  • Loc: Rural Douglas County, Colorado, USA

Posted 17 December 2016 - 06:23 PM

Get yourself a filefolder (or some relatively stiff opaque paper file folders are made of--cardstock--but make it thin).

 

Cut a long, thin, trapezoid out of it. By long, I mean maybe eleven or twelve inches--or whatever the width of the file folder. By thin, I mean eight or nine mm wide at one end, down to one mm or so at the other end.

 

Label the thing according to width. Mark it where it is 2 mm wide, 3 mm wide, all the way to about 8 mm. 

 

Go out at night, close one eye, pick a bright star, and center it in the open eye.

 

Hold the trapezoid in front of your pupil, with the narrowest point in front of the pupil. You will be able to see the star because, although the center of the pupil is blocked, the edges of your pupil can still see the star. Wiggle the trapezoid back and forth a bit. If you can see the star constantly, your entrance pupil is bigger than the trapezoid at that point.

 

Move the trapezoid down a little bit, so that the wider part of the trapezoid is in front of your pupil. You can still see the star. Give it the wiggle test.

 

Move it again, and--poof, the star goes out. The width of the trapezoid at the point where the star disappears is the point at which the trapezoid is just wide enough to cover your pupil. Measure that point, or interpolate from your width markings. Note that the wiggle test at this point has the star blinking. At some point, when directly centered, the trapezoid is big enough to block your whole entrance pupil.

 

Move it further down (past the point where the star first disappears), and the wiggle test hides it even when you wiggle. You want the point where it is JUST wide enough to hide the star, but no wider.

 

Now, beware, if you put the trapezoid too far from your pupil, you will get a false reading.

 

And, of course, if you are not steady, you might not be putting the trapezoid right in front of the pupil, and you can see the star from light getting in on one side or the other of the trapezoid. The wiggle test checks for this.

 

But, you get the idea.

 

Try it with dark adapted, and light adapted eyeballs.

 

Alex

That sounds great, but having not tried it yet my only concern is that one side of the trapezoid is higher/lower than the other side, throwing off the measurement.  Do you actually see the markings on the paper while you are doing the test?

THanks,

Mark.



#14 Nobodo

Nobodo

    Mariner 2

  • *****
  • topic starter
  • Posts: 241
  • Joined: 31 Jul 2016
  • Loc: Rural Douglas County, Colorado, USA

Posted 17 December 2016 - 06:32 PM

the drill bit method kendahl recommended can also be done with allen wrenches. if drill bits, be sure to use the round shank, not working or clamping ends of the bit. with the allen wrench, just hold the wrench so that the short end is pointing directly away from your eye, and measure with the long end.

 

whichever you use, pass the object left to right a few inches in front of your pupil. if the star seems to twinkle but not extinguish, the object is too small. if the light entirely blinks out, the object is too large. your pupil size is between the smallest diameter that blinks out and the largest diameter that flickers.

Wonderful idea.  I definitely have more sizes of allen wrenches in that range than drill bits, plus the wrenches have a built-in handle to get your fingers out of the way.

 

The forecast for last night was clouds, but I noticed as it got dark it was perfectly clear.  So... I took out a telescope, binoculars on a parallelogram, and set out some drill bits, then went back inside and ate supper with my wife.  An hour later when I went back outside it was completely clouded over and starting to snow.  Figures.  It's cloudy again tonight so I'll try this as soon as possible.

 

All these suggestions gave me an idea too.  In Photoshop create a page that has black circles in sizes from about 5mm up to 7mm, with sizes underneath.  Then print that out on transparency paper.  I'll give that a try too, and maybe compare it to the drill bit / allen wrench / trapezoid methods described in this thread.


Edited by Nobodo, 17 December 2016 - 06:33 PM.


#15 Redbetter

Redbetter

    Cosmos

  • *****
  • Posts: 7551
  • Joined: 16 Feb 2016
  • Loc: Central Valley, CA

Posted 18 December 2016 - 03:51 AM

One of the issues with the drill bit/allen wrench method is the geometry kills the precision even with the steadiest hand/eye.  For 7mm exit pupil a 6mm stripe will result in 3 magnitudes of dimming if my geometry is correct, 6.5mm will result in 4 magnitudes loss.  (Remember, calculate the circular segments on both sides after subtracting the blocked segment, then convert the area blockage to magnitude loss.)  Even the lesser value will render most bright stars averted vision in NELM 5 skies without accounting for the loss of monocular vision vs. binocular.  And then there is the issue of the most receptive parts of the retina being the most vignetted, which will tend to intensify the effect and probably make averted vision more problematic.  I would expect the overall estimate to run perhaps ~1mm short of actual.  


  • Jon Isaacs and Nobodo like this

#16 Alex McConahay

Alex McConahay

    Cosmos

  • *****
  • Posts: 7884
  • Joined: 11 Aug 2008
  • Loc: Moreno Valley, CA

Posted 18 December 2016 - 08:30 AM

>>>>>>one side of the trapezoid is higher/lower than the other side,

 

Actually, I use it with the trapezoid vertical, but the concern remains the same, only expressed as left-right. This is what the wiggle test gets around. If you are off center, you wiggle, and the star either stays there (i.e., the strip is not wide enough there to block the whole pupil) or it blinks out (i.e., the strip is wide enough there to block the whole pupil). Very rarely would anybody be able to hold his or her head and hand steady enough at just the right place. So, the wiggle test is an important part of the operation.  

 

>>>>>>>Do you actually see the markings on the paper while you are doing the test?

 

No, you move the strip along, and then when you have the right place, grab it with your other hand right where it is in front of your pupil. Measure where that point is.

 

Alex


  • Nobodo likes this

#17 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 77148
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted 18 December 2016 - 09:04 AM

I used a scale and a camera.  I went into a closet, let my eye dark adapt for a minute or so, held the scale next to my eye and snapped a photo.   It took a few shots to get a reasonable photo.  One must wait between exposures to allow the eye to dark adapt.  Anyway, I then transferred the photo to my computer and counted the pixels, using the scale for calibration.  I calculated my pupil was right at 7 mm.  

 

5640910-Eye pupil 1.jpg

 

I agree with Red, techniques that block the light are susceptible to significant, systematic errors.  When I thought about it, it seemed to me that I needed to actually somehow measure it directly.  The camera technique seemed to work reasonably well, it depends on the camera, the focusing method it uses. Ideally, it would be manually focused with no preflash.  In my case, there was some illumination prior to the exposure.. 

 

Under dark skies, when using aggressive filters, in particular the H-Beta, sometimes I find I see more with an 8mm exit pupil than with a 7mm.  I am not sure what that is about, whether it's image scale for the particular object that compresses the boundaries but there's something going on there. It's difficult to imagine that my pupil still dilates to something more than 7mm.  

 

Jon


  • Redbetter and Nobodo like this

#18 drollere

drollere

    Vanguard

  • *****
  • Posts: 2328
  • Joined: 02 Feb 2010
  • Loc: sebastopol, california

Posted 19 December 2016 - 11:17 AM

One of the issues with the drill bit/allen wrench method is the geometry kills the precision even with the steadiest hand/eye.  For 7mm exit pupil a 6mm stripe will result in 3 magnitudes of dimming if my geometry is correct, 6.5mm will result in 4 magnitudes loss.  (Remember, calculate the circular segments on both sides after subtracting the blocked segment, then convert the area blockage to magnitude loss.)  Even the lesser value will render most bright stars averted vision in NELM 5 skies without accounting for the loss of monocular vision vs. binocular.  And then there is the issue of the most receptive parts of the retina being the most vignetted, which will tend to intensify the effect and probably make averted vision more problematic.  I would expect the overall estimate to run perhaps ~1mm short of actual.  

i'm unclear what the point is here. the stipulated phenomenological criterion was that the star image either "flickers" or is entirely extinguished. that's not a gradation percept, so "dimming" is irrelevant.

 

the geometry of the light intercept is *not* determined as projected from the pupil past the allen wrench onto the celestial sphere, but from the star past the allen wrench through the pupil. you could easily hold the allen wrench at arm's length with no significant change in the geometry (no significant change in the distance from the occluding object to atmospheric layers of refraction).

 

one of the main themes on CN is "how accurately can we measure, if measurement accuracy matters more than measurement value?" and apparently something like that motivation leads redbetter astray. because in this case, we are measuring an aperture (the pupil); averted vision is a completely unrelated sensor effect, and an entirely different consideration.

 

another hoary CN theme, to paraphrase a great film, is: "we don't need no stinkin' utilities." that is, it's more important to derive a measure, than to have a useful application of the measure. generally, pupil aperture is relevant only to issues of limit magnitude or (another favorite CN theme) "maximum useful exit pupil," and it's difficult to see encumberments in that direction when NELM can be assessed directly (by star detection at the site) and in commercial equipment it is quite rare to be able to achieve exit pupils greater than 5 mm.



#19 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 77148
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted 19 December 2016 - 12:03 PM

i'm unclear what the point is here. the stipulated phenomenological criterion was that the star image either "flickers" or is entirely extinguished. that's not a gradation percept, so "dimming" is irrelevant.

 

Bruce:

 

The point is that as the object blocking the star approaches the diameter of the pupil,  the light drops off very significantly before you reach the actual diameter.  At 6.5 mm of a 7 mm dilated pupil,  an accurate measurement requires that there be two strips that are 0.25 mm wide.  To make an accurate measurement this must be maintained until full extinction,  otherwise,  misalignment can result in an inaccurate measurement,  both sides need to be aligned with the pupil's  edge. 

 

Maybe your hand is a steady one,  maintaining alignment to a small fraction of a millimeter is not so easy for me.. 

 

Measurement accuracy,  if you are going to make a poor job of it,  why do it at all.   I know you are not much on large exit pupils and such but ones dilated pupil diameter is worth knowing for those of us who are.  I don't know what you do for a living,  I retired after spending most of my life in research.   Making a good measure is better than making a flawed measurement.. 

 

Jon


  • Redbetter likes this

#20 Alex McConahay

Alex McConahay

    Cosmos

  • *****
  • Posts: 7884
  • Joined: 11 Aug 2008
  • Loc: Moreno Valley, CA

Posted 19 December 2016 - 12:37 PM

Hey, Kids, before we go too far down this path.....

 

How sensitive does one need this measurement of pupil diameter? Aside from bragging rights, ("My seven year old has a wider diameter dilated pupil than your grandmother!!!") practically speaking, what does it matter?

 

Does my strip of file folder cut into a trapezoid, or various drill bits, allen wrenches, and even photographs, measure more accurately than the noise one is likely to introduce by non-standard conditions of the measurement (How long were your eyes dark adapted? How dark was the light you were in during dark adaptation? Shaky hands not being able to center the file folder/drill bit? Seeing the night of observation?..........)

 

I mean, really.   Does anybody worry about these things when out under the starry skies?

 

I guess that's why it is called "Cloudy Nights."

 

Alex



#21 Nobodo

Nobodo

    Mariner 2

  • *****
  • topic starter
  • Posts: 241
  • Joined: 31 Jul 2016
  • Loc: Rural Douglas County, Colorado, USA

Posted 19 December 2016 - 04:34 PM

I really only care as far as not wasting money on eyepieces goes.

I definitely have eyepieces that waste light for me, and the view through them isn't nearly as enjoyable as it should be as a result.  There's space between ones that I don't think are losing light and ones that I can tell for sure are, so it would just be nice to not guess so much around that.  If I'm looking to buy a new eyepiece in the 'gray zone' more info is better.

 

Thanks,
Mark.



#22 Alex McConahay

Alex McConahay

    Cosmos

  • *****
  • Posts: 7884
  • Joined: 11 Aug 2008
  • Loc: Moreno Valley, CA

Posted 19 December 2016 - 04:41 PM

That is a good point.....but I do not think the difference between a 4.9 and a 5.1 mm exit pupil is all that big a deal, and the various methods we have mentioned here is probably accurate enough to get you in the ballpark for viewing purposes. Furthermore, there are so many variables regarding exit pupil (how much adaptated one is) that one need not be much more accurate than maybe half a mm. Face it, we are talking about a range of maybe four to eight mm from one human to another. And those are way outlying numbers.

 

Alex



#23 Nobodo

Nobodo

    Mariner 2

  • *****
  • topic starter
  • Posts: 241
  • Joined: 31 Jul 2016
  • Loc: Rural Douglas County, Colorado, USA

Posted 19 December 2016 - 05:22 PM

That is a good point.....but I do not think the difference between a 4.9 and a 5.1 mm exit pupil is all that big a deal, and the various methods we have mentioned here is probably accurate enough to get you in the ballpark for viewing purposes. Furthermore, there are so many variables regarding exit pupil (how much adaptated one is) that one need not be much more accurate than maybe half a mm. Face it, we are talking about a range of maybe four to eight mm from one human to another. And those are way outlying numbers.

 

Alex

There are 60+ year old people on here claiming they have measured their exit pupils to be 7mm or more, and others saying 5mm.

That's quite a range, considering the exit pupil of various eyepieces in different telescopes.

I tried the trapezoid method and the allen wrench method last night under dark skies with plenty of adjustment time... I doubt under those conditions there's really a lot of difference in one set of eyes one night to the next.  In my experiments I don't really see how either is accurate to more than +- 1 mm or so; not very close at all in my experiment last night.  Perhaps others have better technique.

Considering the expense of eyepieces, more info is better in my opinion.

 

Thanks,

Mark.


  • Jon Isaacs likes this

#24 Redbetter

Redbetter

    Cosmos

  • *****
  • Posts: 7551
  • Joined: 16 Feb 2016
  • Loc: Central Valley, CA

Posted 20 December 2016 - 05:40 AM

 

i'm unclear what the point is here. the stipulated phenomenological criterion was that the star image either "flickers" or is entirely extinguished. that's not a gradation percept, so "dimming" is irrelevant.

 

 

the geometry of the light intercept is *not* determined as projected from the pupil past the allen wrench onto the celestial sphere, but from the star past the allen wrench through the pupil. you could easily hold the allen wrench at arm's length with no significant change in the geometry (no significant change in the distance from the occluding object to atmospheric layers of refraction).

 

one of the main themes on CN is "how accurately can we measure, if measurement accuracy matters more than measurement value?" and apparently something like that motivation leads redbetter astray. because in this case, we are measuring an aperture (the pupil); averted vision is a completely unrelated sensor effect, and an entirely different consideration.

 

another hoary CN theme, to paraphrase a great film, is: "we don't need no stinkin' utilities." that is, it's more important to derive a measure, than to have a useful application of the measure. generally, pupil aperture is relevant only to issues of limit magnitude or (another favorite CN theme) "maximum useful exit pupil," and it's difficult to see encumberments in that direction when NELM can be assessed directly (by star detection at the site) and in commercial equipment it is quite rare to be able to achieve exit pupils greater than 5 mm.

 

 

Once again you let personal animosity toward me get in the way of reasoning.  I am concerned about the accuracy because I would like to have a decent estimate of the value.  Basing decisions on low ball exit pupil estimates will make it more challenging to observe some objects, because I won't be employing the best eyepiece for my eye on a given target, and in fact wouldn't even likely consider it.  (More importantly it could factor into future equipment design/selection.)  The same issue applies to recommendations for others.  You must not have tried the test yourself or you would be aware of some of the errors in your claims.  I've done enough experiments in my life that I try to analyze what doesn't work well and improve my method.  I am sorry that is offensive to you, but it is your problem, not mine.

 

So let's look at some of the errors in your assumptions and why I commented in the first place.  I had tried this drill bit test before the topic was started and had noticed the reduction in light entering my eye on both sides simultaneously.  Considering the problem I concluded that if full diameter blocks 100% of the light, then 100% - X will block some different percentage based on the geometry of the blockage.  In this case it is a rectangle of X width completely blocking a section of a circle leaving sections on either side.  From that one can determine how much light is lost and convert it to magnitudes.  It doesn't require one make assumptions about NELM, only the geometry and the comparison of the flux from the unblocked star to the blocked one, easily converted to delta magnitude.   (If held close to the eye as you suggest then there will be a shadow effect to contend with, and I have seen some recommendations online that suggested doing it this way...again, I had experimented with this aspect and found it unsatisfactory.) 

 

But it does get interesting when one determines his/her NELM in town and considers the target star and the dimming calculated above.  Do you really think you can get any useful measurement on a 4th or 5th magnitude star in the city?  Can you use an averted vision star at all for this test and get a useful result?  So my conclusion was that I needed to try this sort of test with nothing dimmer than 1st magnitude, the brighter, the better and if possible keep the star from dipping into the averted vision range during the test.

 

Tonight Sirius was well enough positioned to give it a go.  After reading your misguided rant I went outside and tried the test with what I had handy:  3/16", 1/4" bits and 6 and 8mm allens.  I held each at arms length.  Unfortunately I did not have a 7mm or 7.5mm tool located for the test.  Some dimming was apparent even with the 3/16", and more so at 6mm and 1/4".  At 8mm it was completely blocked.  So what is my exit pupil less than 10 minutes after stepping outside on a moonlit night in the city with 5 NELM skies at best?  At least 6.35mm (1/4"), but less than 8mm from what I saw shifting the latter back and forth.  Not very precise, but good enough to recognize that 7mm exit pupil is not an unreasonable target for my eye.  

 

I am not sure how you came to the conclusion that "in commercial equipment it is quite rare to be able to achieve exit pupils greater than 5 mm."  Every scope I have can exceed 5mm exit pupil with commercial eyepieces, including the f/10 SCT (5.5mm).  In others I frequently use 6 to 7mm and have one now that has given me good results at ~7.9mm exit pupil for Barnard's loop and other large nebula complexes.  Perhaps what you meant to say is that with your equipment it is quite rare to be able to achieve exit pupils greater than 5mm.  Narrow field/long ratio instruments have more difficulty with it than other types.

 

You keep railing on CN folks for trying to measure things, yet your expertise is based on doing just that with double stars.  Measuring position angles, separation, detection limits, aperture resolution, geometry, calculating orbits, are all part of that.  Yet you complain that folks are trying to calculate, estimate, measure, and predict too much.  Talk about the pot calling the kettle black.  :ohmy:  


Edited by Redbetter, 20 December 2016 - 05:41 AM.


#25 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 77148
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted 20 December 2016 - 09:27 AM

That is a good point.....but I do not think the difference between a 4.9 and a 5.1 mm exit pupil is all that big a deal, and the various methods we have mentioned here is probably accurate enough to get you in the ballpark for viewing purposes. Furthermore, there are so many variables regarding exit pupil (how much adaptated one is) that one need not be much more accurate than maybe half a mm. Face it, we are talking about a range of maybe four to eight mm from one human to another. And those are way outlying numbers.

 

Alex

 

Alex:

 

I doubt that using dimming techniques one would be +/- 0.1 mm.  0.1mm is 0.004", about the thickness of a human hair. I think we're talking +/- 0.5mm or more.  I measure a flat wooden tooth pick at 0.050 inches thick, 1.25 mm.  

 

As far as dark adaption goes, the eye dilates quite quickly to it's maximum diameter in low light conditions. I do not think that it varies much due to lighting.  What does vary is the longer term photo chemical processes and that varies a great deal.  

 

Measurements of populations seem to indicate a range of from about 4mm to 8mm, in terms of light gathering, that's a factor of 4.  I think it's worth knowing to some sort of reasonable accuracy for a couple of reasons.  If one is buying eyepieces, it's worthwhile understanding what you are buying and how it fits your telescopes and you as an individual.  And when choosing and using eyepieces, it's helpful making the choices and understanding why one is seeing what one is seeing.

 

For me, as a senior citizen with a number of fast scopes and longer focal length eyepieces, I did want to know.  The general thinking is that one's dilated pupil diameter shrinks with age and at 65, people talk about 5mm dilated pupil diameters.  My 12.5 inch F/4.06 with the Paracorr and the 31mm Nagler provides a 6.6 mm exit pupil.  Just what was going on there...  If my eye dilated to 5mm, the scope was working about 9 inches, it might make sense to go with something with a shorter focal length.. 

 

So I measured it.  It's worth knowing.  It turned out it was about 7.0mm.  Somewhat unusual for someone my age but it meant that combinations like the 41mm Panoptic in my 80mm F/6 do make good sense in viewing large faint objects.  

 

Knowing is better than not knowing, particularly when one is spending real money, when one is out under dark skies and trying to figure out what's going on.

 

Jon


  • Nobodo likes this


CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics