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Fully dark adapted pupil size?

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#1 Ken Watts

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Posted 21 October 2019 - 10:58 AM

I have read a lot about matching eyepiece exit pupil size to the dark adapted pupil of the eye size when looking for faint objects.  My question is: How do I determine my fully dark adapted pupil size?  The reason I ask this is if I have a maximum pupil size of 5mm, why would I purchase an eyepeice with a 6mm exit pupil?

 

I recently purchased an XT8 withe a 2-in focuser and all my current eyepieces are 1.25 in.  

 

Any education you can provide would be appreciated!

 

Clear and steady skies,

 

Ken W



#2 rowdy388

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Posted 21 October 2019 - 11:15 AM

You can hold a series of metric allen wrenches up to your dark adapted eye which blocks

light from a bright star. The size of the wretch that is just big enough to block all the light is

the size of your pupil. The smaller wrenches just block out a portion of the light as there is

room to enter your eye around the edges.



#3 rowdy388

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Posted 21 October 2019 - 11:18 AM

As far as buying eyepieces to stay within your exit pupil limits: good idea. 



#4 martinl

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Posted 21 October 2019 - 11:21 AM

Easiest is to hold increasingly large diameter drill bits vertically in front of a star. When the drill bit completely blocks out the light you know that your pupil size is slightly smaller than the diameter of the drill bit. Or you could take a flash photograph of your dark-adapted eye (turn off the red eye pre-flash) while holding a ruler close to your eye.

It doesn’t have to be perfect. Just plan your longest eyepiece to give a smaller exit pupil than your estimated pupil size. 



#5 TOMDEY

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Posted 21 October 2019 - 11:38 AM

If you occasionally want low power, wide field... it's perfectly fine to overfill your pupil... especially when using a refractor with no central obstruction, but even reflectors and CATs... within reason. You will still get the max possible etendue (and therefore brightness) allowed by your eye. "Try it... you'll like it!"

 

You've gota take ~I have read a lot about~ stuff with a big grain of salt. Most people just don't understand the theory deeply enough or have even tried things... just read and then pass on fake news concoctions of some nth-party ~experts~.

 

Recommendation: Have at least one good wide-field "too low power" eyepiece on hand. Give it a try, on a couple of targets that otherwise get clipped... the views are often comfortable and pleasing!    Tom


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#6 Paul Skee

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Posted 21 October 2019 - 11:55 AM

I am 68 years old and recently had an eye exam. I asked the Doctor to check and he said about 6mm, (surprised me, I thought I'd have lost some of that due to age). There is more to dark adaptation than just pupil size. This article is a real "eye opener", (pun?) https://en.wikipedia...Dark_adaptation

I really like wide field/low magnification for the larger DSO's. I have used a 55mm in my 150mm f/5 refractor, (about 11 mm exit pupil), pleasing, but no match for the 20mm with 4mm exit pupil. Note the 55mm is plossl (50 deg AP), the 20mm is Nagler (82 deg AP). There are lots of variables to consider, target, seeing, optics, and a multitude of opinions.


Edited by Paul Skee, 21 October 2019 - 11:57 AM.

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#7 havasman

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Posted 21 October 2019 - 02:12 PM

I closed myself up in a bathroom with a towel at the bottom of the door sealing light out, waited a while, took a set of calipers set to 7mm and held them with open jaws near my eye and snapped a pic in the mirror to get my exit pupil size. My camera does not use that pre-flash red eye reduction that closes down your pupil before the exposure.

 

My ep set runs 7.4mm (a bit larger than my max pupil) and down in my most-used scopes via ES68 40mm. The 31T5 gives 5.7mm exit pupil. The maxed out exit pupil is used with narrowband filters on faint nebulae like the one behind B33 and old, spread out planetaries. It is certainly NOT a necessary part of my eyepiece kit but it gets used. With the 40mm I get a FOV just over 5 degrees in an NP101, 20 arc' larger than with the 31T5. Again, not at all necessary but having that 5+ degree FOV pleases me.


Edited by havasman, 21 October 2019 - 02:14 PM.


#8 lsfinn

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Posted 21 October 2019 - 02:24 PM

If you occasionally want low power, wide field... it's perfectly fine to overfill your pupil... especially when using a refractor with no central obstruction, but even reflectors and CATs... within reason. You will still get the max possible etendue (and therefore brightness) allowed by your eye. "Try it... you'll like it!"

 

You've gota take ~I have read a lot about~ stuff with a big grain of salt. Most people just don't understand the theory deeply enough or have even tried things... just read and then pass on fake news concoctions of some nth-party ~experts~.

 

Recommendation: Have at least one good wide-field "too low power" eyepiece on hand. Give it a try, on a couple of targets that otherwise get clipped... the views are often comfortable and pleasing!    Tom

I want to echo what @tomdey said. In particular, it's perfectly fine to "overfill" your pupil as long as you don't find yourself looking at the secondary obstruction in a reflecting telescope (Newt, Cat, Mak, etc.). 

 

What do you gain by doing this? The ability to have a larger field of view than if you limit your exit pupil (ep) to the size of your fully dark adapted pupil (dp) [i.e., ep < dp*(f/ratio)]. 

 

What do you lose by doing this? Nothing at all. 


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#9 André Heijkoop

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Posted 21 October 2019 - 02:34 PM

I want to echo what @tomdey said. In particular, it's perfectly fine to "overfill" your pupil as long as you don't find yourself looking at the secondary obstruction in a reflecting telescope (Newt, Cat, Mak, etc.). 

 

What do you gain by doing this? The ability to have a larger field of view than if you limit your exit pupil (ep) to the size of your fully dark adapted pupil (dp) [i.e., ep < dp*(f/ratio)]. 

 

What do you lose by doing this? Nothing at all. 

+1

 

I use my XW40 in my f3.7 Dobson a lot and with lots of joy.


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#10 mogur

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Posted 21 October 2019 - 03:15 PM

I just measured with a small mm ruler held as close as I could to my eye. This was done at night in a room with only a dim red light. I did it in the morning before sunrise when I woke up for work. I got about 5mm. It doesn't need to be exact. smirk.gif



#11 Jon Isaacs

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Posted 21 October 2019 - 03:28 PM

I measure my dark adapted pupil with a camera. It was a slow process because It was necessary to use a flash and getting the focus right was an experimental process.

 

The basic procedure:  dark adapt in a closet for ~ 60 seconds. Hold standard width stick next eye and photograph eye region.  I transfered the image to a computer screen and measured the width of standard and the diameter of my pupil using digital calipers. The diameter of my pupil was the ratio of the screen diameter to the screen standard times the actual standard width.

 

The result was ~7.7 mm, abnormally large for a senior citizen. However the reason I decided to measure it was that I had noticed that under dark skies with narrow band filters, faint nebulae appeared brighter with an 8mm exit pupil than with a 7 mm exit pupil.

 

The camera must be in fully manual mode to avoid any preflash. Getting focus correct required several shorts, each with a blinding flash that required some time to recover.

 

I like the photographic method because it's a direct measurement. Methods that depend on blocking the pupil are difficult for precise measurements since its a null and not only are closely spaced standards needed but they must be positioned very precisely.

 

An eye doctor's measurement may have been done with an artificially dilated pupil or insufficient dark adaptation. 

 

Tom's point about using an eyepiece with an oversized exit pupil is a good one. My saga began when I purchased a 41 mm Panoptic to maximize the field of view of my 25 inch F/5. With the Paracorr, the 31mm Nagler provided a 0.66 degree field, without the Paracorr, the 41 mm Panoptic providedva 0.83 degree field. I figured I had aperture to burn.. 

 

What was surprising was that the 41 mm Panoptic with 8.2 mm exit pupil was brighter under extreme conditions than the 35 mm with its 7 mm exit pupil. 

 

I recommend measuring your dark adapted pupil. I figured I was a normal Joe but discovered differently. In one study, thirty people in their 60's had an average dark adapted pupil of 5.6 mm but they ranged from 3.5 mm to 7.5 mm. You could anywhere in the middle or even outside that range like me.

 

Jon



#12 TOMDEY

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Posted 21 October 2019 - 03:38 PM

And then... there's these eyes >>> optimized for these custom  16-inch binos >>>

 

I asked the surgeons to spread my eyes apart farther... to "at least seventy millimeters" and to pry my pupils open "to at least eight millimeters." They exceeded both goals, and threw in a free tummy-tuck and other cosmetic and structural enhancements. In parallel, JMI was building my chartered 16-inch binos, using the custom matched mirrors that Normand made and Cary coated. Then I sketched the Horsehead, using mostly oils and emulating the style of van Gogh... best I could. I'm still adding some remembered detail and vibrant colors to the final rendition, which will eventually find its way to the vaulted ceiling of the Sistine Chapel. At least that's the way I see it. That's my story, and I'm stickin by it!    Tom, delusional Tom

 

1st pic >>> entitled "The eyes have it" >>>

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#13 TOMDEY

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Posted 21 October 2019 - 03:39 PM

2nd pic JMI 16-inch Custom Binos >>>

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#14 Ken Watts

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Posted 21 October 2019 - 06:46 PM

Thank you all for the responses! 



#15 JimmyTrom

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Posted 21 October 2019 - 08:35 PM

Attached File  eyepiece calc.xlsx   16.64KB   10 downloadsI have been putzing around making a Excel spreadsheet to assist me in eyepiece

selection as well.  I'm 67 and never had night vision to speak of so the effects are really

felt with decreasing pupular size.

Take a look and correct my calcs if need be.

I highlighted the eyepieces I own and tried to match the eyepiece with the scope that

is within its capabilities and mine as well.

Jim



#16 Ken Watts

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Posted 21 October 2019 - 09:06 PM

JimmyTrom, I also made a spreadsheet but only with my collection of eyepieces and my scopes.  Based on all the responses, I think I am going to stick with the eyepieces I have. 

 

Once again, thank you for sharing your spreadsheet!

 

Clear and steady skies,

 

Ken W 



#17 Dick Jacobson

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Posted 22 October 2019 - 07:00 AM

I want to echo what @tomdey said. In particular, it's perfectly fine to "overfill" your pupil as long as you don't find yourself looking at the secondary obstruction in a reflecting telescope (Newt, Cat, Mak, etc.). 

 

What do you gain by doing this? The ability to have a larger field of view than if you limit your exit pupil (ep) to the size of your fully dark adapted pupil (dp) [i.e., ep < dp*(f/ratio)]. 

 

What do you lose by doing this? Nothing at all. 

I disagree. You do lose brightness of individual objects in the field, since the light from the objective is spread over a larger area than your pupil can admit. Despite this, I agree that it is useful to have an eyepiece with as large a field as possible despite losing star brightness. I also like to have an eyepiece whose exit pupil exactly matches my eyes, so the stars are as bright as possible and the field is as wide as possible within the limitations of pupil size.



#18 airbleeder

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Posted 22 October 2019 - 12:16 PM

<p>&nbsp; &nbsp; Would the stars' brightness not be determined by the exit pupil of the eyepiece while at the same time, how it appears to the observer be determined by the their pupil size?&nbsp;</p>
<p>&nbsp; &nbsp; I haven't measured my dark adapted pupil, but chances are it is smaller than Jon's, so I would think that if we were standing side by side looking at the night sky, the stars would appear brighter to him than myself. I'm a blue collar guy with nothing to back it up, so that is just how I see it.&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; Anybody please feel free to enlighten if I have it all wrong.</p>

   EDIT: 

   What the heck happened to my post??


Edited by airbleeder, 22 October 2019 - 02:16 PM.


#19 Starman1

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Posted 22 October 2019 - 12:34 PM

I have read a lot about matching eyepiece exit pupil size to the dark adapted pupil of the eye size when looking for faint objects.  My question is: How do I determine my fully dark adapted pupil size?  The reason I ask this is if I have a maximum pupil size of 5mm, why would I purchase an eyepeice with a 6mm exit pupil?

 

I recently purchased an XT8 withe a 2-in focuser and all my current eyepieces are 1.25 in.  

 

Any education you can provide would be appreciated!

 

Clear and steady skies,

 

Ken W

OK, when you use an eyepiece whose exit pupil matches the pupil of your eye, you achieve the brightest image that scope can give you.

If you use an eyepiece that yields a larger exit pupil, some of the light doesn't go in the eye, but the magnification is also lower.

The light loss perfectly matches, mathematically, the brightening that comes from a lower magnification.

Hence, the image is exactly as bright as it was using the eyepiece whose exit pupil matched the eye.

 

So, what are possible reasons to not go with larger exit pupils?

1) You may not need a lower power or larger true field.

2) The aberrations of your own eye might overwhelm the image quality

3) coma in the wider field might appear worse (if a newtonian used without a coma corrector).

4) the shadow of the secondary might become obtrusive.

 

The last reason is a big one, since the percentage of the field covered by the secondary shadow in your eye increases as the exit pupil gets larger.

since your pupil does not get larger, the secondary shadow might become a noticeable problem in the field as the magnification goes down.

 

This also points out that, because there is no secondary obstruction n a refractor, there is also no lower limit to magnification or size of exit pupil.

My dark adapted pupil is 4.5mm, yet I have used a 41mm eyepiece in my f/7 refractor to gain the widest possible field: 3.7° at 17x, with a 5.9mm exit pupil.


Edited by Starman1, 22 October 2019 - 12:35 PM.

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#20 airbleeder

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Posted 22 October 2019 - 02:21 PM

OK, when you use an eyepiece whose exit pupil matches the pupil of your eye, you achieve the brightest image that scope can give you.

If you use an eyepiece that yields a larger exit pupil, some of the light doesn't go in the eye, but the magnification is also lower.

The light loss perfectly matches, mathematically, the brightening that comes from a lower magnification.

Hence, the image is exactly as bright as it was using the eyepiece whose exit pupil matched the eye.

 

So, what are possible reasons to not go with larger exit pupils?

1) You may not need a lower power or larger true field.

2) The aberrations of your own eye might overwhelm the image quality

3) coma in the wider field might appear worse (if a newtonian used without a coma corrector).

4) the shadow of the secondary might become obtrusive.

 

The last reason is a big one, since the percentage of the field covered by the secondary shadow in your eye increases as the exit pupil gets larger.

since your pupil does not get larger, the secondary shadow might become a noticeable problem in the field as the magnification goes down.

 

This also points out that, because there is no secondary obstruction n a refractor, there is also no lower limit to magnification or size of exit pupil.

My dark adapted pupil is 4.5mm, yet I have used a 41mm eyepiece in my f/7 refractor to gain the widest possible field: 3.7° at 17x, with a 5.9mm exit pupil.

    +1^


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#21 Jon Isaacs

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Posted 22 October 2019 - 03:31 PM

The light loss perfectly matches, mathematically, the brightening that comes from a lower magnification.

 

 

Don:

 

An excellent post, as usual. 

 

I just wanted to address this point.  There is a slight reduction in brightness if the scope has an secondary obstruction because the shadow of the CO is not reduced.As an example, for a scope with a 25% CO, if someone with a 6 mm dark adapted pupil uses an eyepiece that produces an 8 mm exit pupil, the shadow of the 25% CO is 2 mm and so the effective CO is now 2mm/6mm = 33%.

 

I consider this an extreme case but I calculate the loss of brightness at 0.06 magnitudes. I consider this insignificant.

 

This is for extended objects. Stars will be dimmed in proportion to the area of the exit pupil because stars are points of light and do not change size with magnification. In the above example, this will be significant, about 0.6 magnitudes. 

 

- Large dark adapted pupils are rarely an advantage. Most often magnifying an object, dimming it but making it larger, increases the detail visible. With stars, increasing the magnification at low and mid range magnifications increases the contrast since the stars brightness is unchanged but the background sky dims.

 

I think the only situation where a large dark adapted pupil can be a significant advantage is for larger, very dim low contrast objects. 

 

Jon


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#22 Starman1

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Posted 22 October 2019 - 03:48 PM

Jon,

You're right, but as far as the field brightness per unit area is concerned, you won't be able to notice any dimming with increased exit pupil in a newtonian

not only because the brightness reduction is small, but more noticeably because the brightness of the background sky will seem

a lot brighter to the eye when looking through the scope due to the lower magnification.

Perception isn't always the same as mathematics.

For instance, I rarely use a truly low power eyepiece (with exit pupil larger than my pupil) here at my home because the background sky is just too bright.

But at a dark site, those low power eyepieces don't make the background unacceptably bright.

If they do, it's a sign of poor transparency, usually.


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#23 Tony Flanders

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Posted 22 October 2019 - 07:23 PM

For the record, the exit pupil has to get a mighty lot bigger than your eye's pupil before the shadow of the secondary obstruction becomes a problem. My own pupils open to not much more than 5 mm, and I routinely use exit pupils of 8 mm in my f/5 telescopes with no ill effects whatsoever.

 

Now if I try to use the same scope and eyepiece during the day or to observe the Moon -- that's a whole 'nother story. Daylight or moonlight shrink my pupils right down to 2 mm, at which point the shadow of the secondary becomes a very big problem indeed.


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#24 NinePlanets

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Posted 22 October 2019 - 08:18 PM

Here is simple device you can make to measure your eye's pupil. You can leave it in your eyepiece case or your wallet:

 

1. Take a piece of paper card stock (about 1 x 5 inches) and draw two slightly diverging lines on it. Then use a needle or safety pin to put several pairs of holes along the lines, about ½ inch apart. With care, most pairs will turn out to be of differing separations varying from about 8 or 9 millimeters, down to about 2 or 3 in roughly half-millimeter steps.

 

2. Measure the separation of the holes, not from center to center, but from inner edge to inner edge. The use of a good ruler and a magnifier is required to measure them accurately. Label the pairs with their separation distances.

 

3. To use the thing, just hold it up to your eye and peer through the holes. Select the pair that appears as if the holes merge to the point that their inner edges just touch but don't overlap. Your pupillary diameter is then read from your pre-marked labels.

 

From a dark site the sky should provide just enough light to do the trick. If not, look through the device at a faintly illuminated object like a white car or the side of a building.

 

No allen wrenches or drill bit set needed.


Edited by NinePlanets, 22 October 2019 - 08:19 PM.

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#25 airbleeder

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Posted 22 October 2019 - 08:20 PM

    A 2" 40mm konig ep was included with my first scope, a 8"f5 reflector. When I first saw the central obstruction, I didn't know what it was but I had an idea it wasn't supposed to be there.


Edited by airbleeder, 22 October 2019 - 08:26 PM.



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