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Your Largest Purposeful Exit Pupil


scarp15
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This subject applies to observing under dark skies.

Although my own knowledge on this aspect is a bit sketchy, increasingly I am understanding the advantages for large exit pupil observing in the right circumstance. I would have to assume, as do not know for certain, that my own exit pupil measures at around 5mm. For the past two or three seasons, I have become accustomed for observing using an f4.6 dobsonian, applying on occasion, at low power an exit pupil of 6.7mm (moderating to 5.84mm with a paracorr). It begins to open up, reveal, low contrast nebulae, whether requiring a filter or not (in the case for certain reflection nebulae). Becoming fully dark adapted, the slightly brighter sky background becomes negligible with an aggressive filter, such as a H-beta and useful for gaining subtle details and contrasts on various objects when used without. The low mag wide field assists with gaining expansive objects inside or just immediately outside the field of view. The same low mag gain is applied to my f6 dobsonian:  5.1mm exit pupil. Currently I am considering this factor more so with my f7 refractor, which operates at 4.4mm exit pupil and could potentially go to 5.8mm, such as with a 41mm eyepiece (applied to my f6 dob will increase to 6.8mm). A few years back I used an F4.9 dobsonian and 35mm eyepiece, therefore just over 7mm exit pupil. It was useful but was also aware of leaking light at the edges. I do not recall any issues for seeing the shadow of the secondary though and this period had preceded my interest for effectively using a H-beta filter. 

My particular interest is anticipating roaming through Orion and Auriga with a H-beta filter this winter, hopefully using each of my three scopes in different situations. It would be interesting to get your own viewpoint on this subject and for how effective a large exit pupil is comparing a refractor and dobsonian perhaps, particular large low contrast nebulae that this has either been successful with or planned to attempt and anything else of course. Can only hope that the late autumn through winter is a good one.  

 

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I do think that there are definite benefits to a large exit pupil when under dark enough skies. Usual advice on SGL is to keep to smaller exit pupils to improve the darkness of the sky background, but this is only relevant to the normal suburban observing most of us.

I'm approaching 50, and from what I have read, my dilated pupil size is probably around 6mm. Whilst observing in Wales a few weeks back, I used a 30mm 82 degree ES eyepiece quite successfully in my 14" f4.5 scope ie 6.7mm exit pupil. This was under mag 21.1 skies and using an OIII filter. I think I preferred this over the 20mm with a 4.4mm exit pupil as it gave a brighter view with a larger FOV. I should check my dark adapted pupil size some time out of interest, although I don't think it would change my current kit choices.

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Attending an appointment at the opticians, I asked the optometrist if he could measure my dark adapted pupil size. He said that he was unable to do as they did not have the specific equipment, would be possible only at a private eye clinic. Can only speculate on exit pupil size between 5mm and 6mm perhaps. Even when approaching or in your fifties, sixties, perhaps beyond there are some accounts for observers retaining good dark adapted pupil size. Unclear how many are able to measure their dilated pupil. Under mag 21+ skies particularly with a filter, always a good plan to experiment with varied focal lengths. As you have stated Stu, it was interesting that for much of my most recent dark sky trip I continually used my 31mm Nagler, 6.7mm exit pupil. 

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I would suggest setting up a DSLR on a tripod in a rather dark room, set its ISO as high it can go, sit yourself in front of it and take a few shots of your face using a remote. Then in bright conditions, take a similar shot with a ruler held up to your forehead to give you a measuring scale. I've been meaning to do just this for a while, just not got around to it yet.

My understanding is that your pupil will dilate in dark conditions very quickly, like a matter of seconds. It's your rods' dark adaptation that takes much longer, but you're not measuring that.

Cheers, Magnus

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I found this graph in a paper I found online, name and authors shown in the first image so as to give correct credit, pupil size vs age graph in the second one. It shows the trend line for 50 years old is 6mm in low light (9 cd/m2), but also that there is quite a range from around 4mm to over 7mm. It seems the range is larger at low light levels than in brighter conditions where there is less variation for similar ages.

Given that the reduction in pupil size is due to the muscles in the iris losing their strength, I do wonder if regular observing at dark sites might help maintain this function a little? Probably not but just a crazy thought 🤣.

20190917_123446.jpg

20190917_123101-01.jpeg

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50 minutes ago, Stu said:

Given that the reduction in pupil size is due to the muscles in the iris losing their strength

I heard recently (can't remember where - that's another age problem) that age related focusing problems are due to the lens becoming stiffer rather than the muscles attached to the lens becoming weaker.

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More from Mel...

 

"Exit pupil thoughts

How to test for exit pupil without ruining dark adaption and causing the pupil to shrink? I test by slightly defocusing the star image so that it forms a disc. Since I can see the mirror's edge, I know that my eye's pupil is not truncating the mirror's aperture. I must place my eye exactly at the correct location centered and above the eyepiece using the rubber eyeguard. I also use the 17mm Ethos with its 3+ degree field of view at slightly higher power where my eye doesn't need to stay exactly centered.
The 21mm Ethos eyepiece with this scope gives an exit pupil of 6.4mm. My eye opens to this size. But what would be the consequences if my exit pupil were smaller?
If my eye opened only to 6mm then that's equivalent to stopping down the aperture to 5.6 inches, or a drop of about 10% illumination or about 0.1 magnitude. And keep in mind that both the object and the sky background are equally affected, leaving the ratio between the two or the contrast the same. I've not been able to observe this difference.
Aperture is an important consideration seting the overall size and weight of the telescope. Field of view is equally important. Try considering field of view first. With this approach you determine the aperture based on the lowest power eyepiece in combination with your eye's pupil, giving the widest possible field of view. This makes the eye the limitation, not aperture, not field of view. That's as good as it gets."

 

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Just to remember....posted this 9 months ago - a proven method to assess eye pupil diameter:

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.

Stephan

 

Edited by Nyctimene
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On 17/09/2019 at 09:25, Nyctimene said:

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

Stephan, have you been observing for a long time?

Curious, because as Stu says astronomy might help keep the pupils opening nice and wide. Interesting actually.

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Interesting question, Gerry.

I've been observing with binos and scopes for almost exactly fifty years now, starting with comet Bennett in spring 1970. But there have been many years with only few observations, up to ten/year. The frequency increased with my first Dob, the 13.1" Odyssey, that I bought 1987, having moved a few years before to a very dark location in the deepest Odenwald forests. Numbers climbed to 30 - 40 observations. After retirement, I was able to observe a lot more - up to three digit numbers in 2017 and 2018 (this year, 59 so far). I'll try to get some medical/physiological information about the subject; never heard of it before.

Stephan

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On 17/09/2019 at 11:18, scarp15 said:

Attending an appointment at the opticians, I asked the optometrist if he could measure my dark adapted pupil size. He said that he was unable to do as they did not have the specific equipment, would be possible only at a private eye clinic. Can only speculate on exit pupil size between 5mm and 6mm perhaps. Even when approaching or in your fifties, sixties, perhaps beyond there are some accounts for observers retaining good dark adapted pupil size. Unclear how many are able to measure their dilated pupil. Under mag 21+ skies particularly with a filter, always a good plan to experiment with varied focal lengths. As you have stated Stu, it was interesting that for much of my most recent dark sky trip I continually used my 31mm Nagler, 6.7mm exit pupil. 

Here is a scientific paper that might help give you an idea of your pupil size based on age. Might not be perfect but should give you a good idea.

https://www.ncbi.nlm.nih.gov/pubmed/20506961

Steve 

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I can only go my the experiences that I have had at the eyepiece because I've not had my max exit pupil measured or tried to measure it myself.

Most (98%) of my observing is done from my back garden which suffers from some light pollution from major urban areas around 12-20 miles distant as well as locally produced LP.

My low power eyepieces comprise the 21mm Ethos, 31mm Nagler and 40mm Aero ED. My largest aperture and fastest scope is my 12 inch F/5.3 dobsonian. With that scope, the 21mm Ethos delivers a 3.96mm exit pupil and the Nagler 31 and Aero ED 40 5.85mm and 7.55mm respectively.

I consistently find that the 21mm Ethos provides the most contrasty views of faint DSO's, with and without a narrowband or line filter in use. I do use the 31mm Nagler and 40mm Aero ED from time to time to get a different perspective on the view but invariably find myself going back to the 21mm Ethos.

So, to answer the question posed, for me and my usual observing location, 4mm is the most effective exit pupil.

I'm nearly a decade older than Stu, which is probably relevant.

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I am probably at 57, a similar age to yourself John, I can understand the 4mm exit pupil factor from your quite good location. Situated at perhaps a darker location, enabling more fully dark adaption, I can really open up the potential with the VX14 and 6.8mm exit pupil. Unfiltered I would use this for much of the session (interacting back and forth to a 21mm ethos). It could well be beyond my max pupil dilation, but it worked within the context of the targets I was observing. I would be curious to open up a little more. The age related assessments are interesting, yet in the scenario of this thread discussion do not necessarily apply to a skilful seasoned observer, when fully dark adapted. 

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11 minutes ago, scarp15 said:

I am probably at 57, a similar age to yourself John, I can understand the 4mm exit pupil factor from your quite good location. Situated at perhaps a darker location, enabling more fully dark adaption, I can really open up the potential with the VX14 and 6.8mm exit pupil. Unfiltered I would use this for much of the session (interacting back and forth to a 21mm ethos). It could well be beyond my max pupil dilation, but it worked within the context of the targets I was observing. I would be curious to open up a little more. The age related assessments are interesting, yet in the scenario of this thread discussion do not necessarily apply to a skilful seasoned observer, when fully dark adapted. 

I agree with this Iain, observing from a mag 21+ sky is a different case, and the sky background issues you see at say mag 20 or 20.5 are much reduced. I thought the contrast (filtered) was better with my 30mm ES, although obviously there is better image scale at 20mm. I really must try my 25mm Zeiss ortho in the Sumerian at a dark site one day, transmission is terrific in that.

Must also measure my dilated pupil size. Good excuse to buy some new drill bits 🤣🤣

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18 hours ago, Nyctimene said:

I'll try to get some medical/physiological information about the subject; never heard of it before

Thanks Stephan any info is appreciated to satisfy my curiousity on the subject. At 56 I see well in the dark and when younger my eyes were really good in the dark, being able to drive boats through remote lakes in the pitch black etc. Passengers used to wonder how I could do it- it was easy- I could see lol!

The eye Doctor said my blue eyes might have something to do with it and has an interest in astronomy.

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33 minutes ago, John said:

My mum used to say that eating carrots would help night vision, or at least help you see in the dark.

I suspect she was just trying to get my brother and I to eat up all our vegetables :rolleyes2:

 

Wasn't that a ruse to put the Germans off the scent of radar? Must have resulted in alot of carrots being eaten!

 

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10 minutes ago, Stu said:

Wasn't that a ruse to put the Germans off the scent of radar? Must have resulted in alot of carrots being eaten!

 

So the tale goes. Lot of disinformation around at the time.

Strangely enough the Germans were the first to use naval radar....in 1916. Worked but had one rather big problem . Firing the guns shattered all the super fragile tubes in the radar gear so they gave up on it.

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After using the 2499mm fl scope at f4.1 it appears to be very flexible in many regards. It seems that I can use large exit pupils without diminishing some DSO's views, in part because the focal length gives enough image size so there is no degraded views at low power. In the 15" some objects can become small enough to impair views at this level...

Up until now I thought the effect was due to the increase in exit pupil, but now believe object size (eyepiece) plays a role.

Blackwell etc al apply this effect to thresh hold objects- I think visual object size also has an effect in other circumstances. Just a thought...

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