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A 2nd hand Takahashi LE50 recently came onto the market for USD200. They are a beautiful eyepiece. 50mm focal length with a 52deg apparent field of view.

My primary visual instrument is a 18" f5.5. The LE50 is totally unsuited to this. At f5.5, the LE50 projects a 9.1mm exit pupil. My 31mm Nagler on the other hand produces a 5.6mm exit pupil, theoretically almost perfectly matched to the maximum dark adapted dilation of my 57 year old eyes.

Back to the LE50. A friend of mine owns a Takahashi 180c 180mm f12. At f12, the exit pupil is 4.2mm and a good match to that instrument. Phil tried one out belonging to a mutual friend of ours and has been pining for one since. The new price was a bit more than the CFO was prepared to approve. 2nd hand LE50's don't come onto the market often in Australia. I saw the ad soon after it was posted but I knew my friend was in hospital that day having an adjustment made to his pacemaker so I pounced and bought it for him confident that he'd be happy to buy it from me, which he was.

Although I frequently use exit pupil to determine suitability of eyepieces etc, I had never really observed the effect of oversize exit pupil. This was a prime opportunity to test it out so I decided to use my 18" scope to satisfy my curiosity. I set it up and pointed it at the Eta Carina nebula.

Both eyepieces yield a true field of view of about 0.9 degrees.

Using the 18" f5.5 This comparison has nothing to do with eyepiece quality. Both eyepieces are definitely premium and both give gorgeous views when matched to the right OTA. This is all about exit/entrance pupil matching. Both eyepieces give approximately the same field of view in any optic because the ratio of the apparent field of views and the focal lengths offset each other.

To the eyepiece! I started with the LE50, lovely sharp image and showed up Eta Carina beautifully. The eyepiece yields fantastic contrast, dark lanes are dark.

I swapped in the Nagler 31 and as I put my eye to the eyepiece, the thing that jumped out at me was that the image indeed was noticeably brighter. It was a stark difference. I swapped back to the LE50 and suddenly, by comparison, the image though still "Tak" sharp looked dull and lifeless by comparison.

So what's happening?
In the dark our pupils dilate. The amount that they dilate varies between individuals but mostly with age. The red line marks average dilation for my age group of around 5.3mm. If the exit pupil of the telescope is greater than the dilated entrance pupil of the eye, that light can't enter the eye and is wasted.
In my case, the (probable) 5.3mm entrance pupil matches the 5.6mm exit pupil with the Nagler 31mm pretty well so nearly 90% of the light is entering my eye. With the LE50, the amount of light entering the eye is the square of (5.3/9.1) or about 33%. So two thirds of the light is wasted and the view through my 18" reflector has the light intensity of a 6 inch reflector. My observation matches this theoretical value quite well.

Yep. The Tak is a wonderful eyepiece but exit pupil rules. I have one but I never use in scopes faster than f7. There are less expensive ways to implement an aperture stop. It really comes into its own f7 to f12.

Amazing!
I never had expected that the waste of light was so easily visible and in fact that much!
It surely underlines the necessity to bring along an eyepiece that exactly matches my eyepupil coming holiday on a dark location, when searching for extended dust- or illuminated- clouds. Thanks for your experiment!

On very small DSO's my zoom-eyepiece will take care of the necessary magnification in order to activate more surface in the retina on that particular object. But that's another story ,related to the human eye and has not to do with the effect of a too large exitpupil in this experiment.

John Baars wrote: ↑Tue May 04, 2021 8:11 am Amazing!
I never had expected that the waste of light was so easily visible and in fact that much!
It surely underlines the necessity to bring along an eyepiece that exactly matches my eyepupil coming holiday on a dark location, when searching for extended dust- or illuminated- clouds. Thanks for your experiment!

On very small DSO's my zoom-eyepiece will take care of the necessary magnification in order to activate more surface in the retina on that particular object. But that's another story ,related to the human eye and has not to do with the effect of a too large exitpupil in this experiment.

Thanks John,

This is a rather extreme example. The combination of my age and probable limited dilation, the moderately short f ratio of my scope compound to make this so obvious a difference. If a much younger observer with a bigger pupil dilation were looking, they'd see a smaller difference in brightness.I also own a GSO Superview 42mm eyepiece. I had noticed that it too gave a darker view than my Nagler31. Because the 42mm yields an exit pupil of 7.6mm, I put the difference down to poor transmission in the GSO, but now I think it might be my reduced dilation. I have to accept my age

Joe

Congratulations Joe on winning the TSS Visual Report of the Day award for your report "Confirming the exit pupil effect."

app.php/article/5-4-2021-tss-visual-report-of-the-day

I wouldn’t put too much down to age, though. The chart of pupil dilation versus age is good only “on the average”. But just as an individual cannot be said to have the average height or weight (unless they actually do ) so it goes for maximum dilation and maximum useful exit pupil. In my own case, the chart has me at about 4.6 based on age. I know that in the field it’s closer to 5.7mm. So use the hex wrench trick to measure or a range of eyepieces. Hopefully one is lucky to beat the average?

Thanks Joe, and congrats on the VROD. I will be 67 shortly and my primary scope (17.5") is f/4.5. My 10 and 12 inch dobs are f/5. In those scopes I try to keep my exit pupil to a max of 5mm if not slightly below. This seems to work well for me. While your experiment may seem a bit extreme, it certainly does highlight the importance of the exit pupil and the idea of trying to match your maximum to suit your individual eye characteristics.

notFritzArgelander wrote: ↑Tue May 04, 2021 1:36 pm I wouldn’t put too much down to age, though. The chart of pupil dilation versus age is good only “on the average”. But just as an individual cannot be said to have the average height or weight (unless they actually do ) so it goes for maximum dilation and maximum useful exit pupil. In my own case, the chart has me at about 4.6 based on age. I know that in the field it’s closer to 5.7mm. So use the hex wrench trick to measure or a range of eyepieces. Hopefully one is lucky to beat the average?

Hi nFA,
I agree it is only an average however, in my case, I think I may be average
I will try the hex wrench trick to determine it.
Joe

Hi Joe. A superb report on the exit pupil effect. And, this is not only true with telescopes and various eyepieces, but also in the magnification and power of binoculars. Too large an exit pupil (or light cone), especially with older users, results in a lot of wasted light (and details) not seen in the view. Thanks for the great report Joe, and congratulations on winning the TSS VROD Award today.

OzEclipse wrote: ↑Tue May 04, 2021 9:54 pm

notFritzArgelander wrote: ↑Tue May 04, 2021 1:36 pm I wouldn’t put too much down to age, though. The chart of pupil dilation versus age is good only “on the average”. But just as an individual cannot be said to have the average height or weight (unless they actually do ) so it goes for maximum dilation and maximum useful exit pupil. In my own case, the chart has me at about 4.6 based on age. I know that in the field it’s closer to 5.7mm. So use the hex wrench trick to measure or a range of eyepieces. Hopefully one is lucky to beat the average?

Hi nFA,
I agree it is only an average however, in my case, I think I may be average
I will try the hex wrench trick to determine it.
Joe

Well that might be the case, of course. Hopefully not. Cheers.

Makuser wrote: ↑Tue May 04, 2021 10:20 pm Hi Joe. A superb report on the exit pupil effect. And, this is not only true with telescopes and various eyepieces, but also in the magnification and power of binoculars. Too large an exit pupil (or light cone), especially with older users, results in a lot of wasted light (and details) not seen in the view. Thanks for the great report Joe, and congratulations on winning the TSS VROD Award today.

Hi Marshall,
Thanks for your comments and I agree with your comment on binoculars.

One product which always has me scratching my head are the 2x42 Vixen and other brand similar binoculars The resulting exit pupil is 21mm?? The exact same view could be achieved with a 15mm objective yielding a 7mm exit pupil. This would also make the binoculars much lighter but from a marketing viewpoint, probably unable to attract the big price tag.

Joe

This is a nice illustration of exit pupil effect. I should add that even my eyes can accommodate 5-6mm EP, my sky does not. It is far from true dark sky even at my "dark" site. So, with 5-6mm EP the background sky is too bright for my liking and contrast is low. I don't go above 3-4mm PP (unless using filters) and from this perspective can't understand how some people could love 7mm EP in binos.

Bigzmey wrote: ↑Wed May 05, 2021 12:01 am This is a nice illustration of exit pupil effect. I should add that even my eyes can accommodate 5-6mm EP, my sky does not. It is far from true dark sky even at my "dark" site. So, with 5-6mm EP the background sky is too bright for my liking and contrast is low. I don't go above 3-4mm PP (unless using filters) and from this perspective can't understand how some people could love 7mm EP in binos.

Hi Andrey,

I cut my teeth observing with my 6" newtonian from my parents suburban backyard in Brisbane in the late 70's and early 80's. Back then, the population was just under a million, light pollution was moderate, probably Bortle 5-6 and from our backyard so I could just faintly see the brightest parts of the Milky Way core fighting through the skyglow. I know exactly what you mean. My 40mm eyepiece yielded a 6mm exit pupil but I gained much better contrast on DSO's with a 25mm eyepiece with a 3.5mm exit pupil.

As you would know, observing from really dark skies is a completely different ballgame and while some objects benefit from magnification, for other objects, the bigger the exit pupil the better the view. One example is galaxy clusters. While observing an individual galaxy at magnification is rewarding, I get a lot of pleasure sucking multiple galaxies into one wide field view in the 18" or even my 6". Leo Triplet, Markarian's Chain, Grus Quartet, just to name a few. An interesting perspective on our universe. I also find that novices really love seeing multiple galaxies. In a really dark sky, the view of the expansive nebulae, Tarantula, M42, M8, Eta Carina are amazing and immersive through the 18" with the Nagler 31. M42 doesn't just have the side wings but you can see the full closed loop of nebulosity.

In my late teens, I had a wonderful astronomy mentor, who gave me regular access to observe from dark skies at his observatory. He died just over 10 years ago but in a conversation with his daughter just a few weeks ago, I mentioned to her that the mix of paid workshops, plus free mentoring and access to stay overs at my property that I am giving to some individuals is in large part influenced by the example set and the generosity he extended to me and others at his observatory.

cheers

Joe

Congratulations on the VROD!

The bino you refer to is of the Galilean type. The magnification of 2 X is responsible for a limited use of the objective. As far as light grasp is concerned the owner of a 6 mm eyepupil will only use a 12mm entrance area of the lens.

The only reason for a big aperture is the fact that in a Galilean telescope the edge of the objective is the edge of the real field of view. One literally sees the edge of the objective as the edge of the real field in the sky. In this case 24-28 degrees. If it were a 50mm objective lens the field of view would be even more extended.

It is useful in urban area's, one can overlook whole constellations; and stars approximately one and half magnitude fainter than the naked eye can see suddenly become visible. Without it they stay obscure. On dark locations this has no use, you will see the whole constellation anyway. From the city it makes more sense. From dark locations the Milky way however is very beautiful with this instrument, but that is the only reason for use from dark locations.

I have one and I admit that for starhopping in the city a 4 or 5 degree field with an exitpupil of 2.5mm is more useful. So it is of limited use.

Nice work Joe.
Congratulations on the VROD !