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Effect of exit pupil on colors of stars and DSO

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#1 Guest_**DONOTDELETE**_*

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Posted 02 March 2004 - 06:47 PM

The general consensus among binocular observers seems to be that when viewing at low powers under a sky which is not fully dark there are not a great number of benefits in increasing exit pupil size. However, one thing more light will do is allow eyes to easier sense color. The eyes color receptors are more heavily used when more light is available, thus the result of a larger exit pupil is a more colorful view. Another way of increasing color sensitivity seems to be to observe from a site that will not allow the eye to fully dark adapt- I have read that people can see a green color in M42 easier in their light polluted back yard than at a fully dark site.

Please share any information you have relating to this. I'm curious about what kind of differences there are in binoculars with different sized exit pupils at equal magnifications and what effects different levels of light in the observing site have on the view. What kind of differences have you seen in star colors or even very bright deep space objects under different conditions?

Take the Fujinon 25x150mm binoculars as an example and compare them to a pair of 25x100mm binoculars of equal quality. Assuming an observer has an eye pupil of 6mm or greater, but is observing under a heavily light polluted sky, what kind of differences would he/she observe between the two?

#2 KennyJ


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Posted 02 March 2004 - 07:50 PM

Daniel asserts :

< However, one thing more light will do is allow eyes to easier sense color. The eyes color receptors are more heavily used when more light is available, thus the result of a larger exit pupil is a more colorful view >

I'm not certain this is true of all kinds of binocular observing , but if it is true at all, it certainly supports my long -standing viewpoint ( no pun intended ) that there is much to be said that has has hitherto been not fully explained ( to me at any rate ), about using binoculars in daylight that have exit -pupils larger than is "theoretically perfect" given the entrance pupil / exit pupil ratio and "conventional optical theory"

You continue to ask some very interesting questions Daniel.

Regards and Clear Skies -- Kenny.

#3 Guest_**DONOTDELETE**_*

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Posted 02 March 2004 - 08:32 PM

Thanks Kenny.

I've found your findings on the performance increase observed at equal magnifications when using a larger aperture then should be necessary very interesting. Unfortunately I don’t yet own a pair of binoculars with a large exit pupil, so I was unable to adequately mimic your tests. Even so, I have come up with some fun explanations I’d like to share with you.

The most obvious explanation would be that you are in fact making use of the larger exit pupil. Something I think would be interesting and add more certainty to your findings would be to create a circular central obstruction the same size as the aperture of the smaller pair of binoculars, and put it over the larger pair. This way the smaller pair has an exit pupil the same size as the dark center of the exit pupil in the larger pair. Look through the larger pair, and make sure that you can’t see any light at all to be sure the outer light of the exit pupil isn’t being used.

If you find that you still can not see any light this way, then the only explanation I can come up with is that quality of the larger pair must be higher. One reason this could be is that your smaller eye pupil is making the larger pair’s effective focal ratio smaller. If you’re using two F/5 binoculars, and in one you make full use of the exit pupil it will still be at F/5, however if you only use part of the exit pupil in the larger pair it might effectively be at F/7.

#4 EdZ


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Posted 02 March 2004 - 09:34 PM

One thing you need to keep in mind is that contrast between objects and dark sky background will improve views. It's true that more light allows you to see more, but if the means to achieve a brighter image are to keep aperture the same and reduce magnification (OR keep magnification the same and increase aperture) to result in a larger exit pupil, you will be taking in a lot more light from not only the object but also the bright sky background. This will reduce contrast making it more difficult to see.

It requires greater contrast and darker skies to see the red end of the spectrum than it does to see the middle yellow spectrum and the blue end of the spectrumappears brighter. There can be almost a 2 magnitude difference in the appearance of equally bright blue stars and red stars. A larger exit pupil unless the aperture is really large, will have a much more difficult time seeing red.

You could get to the point with aperture where the light gathering gains outweigh all other factors, including contrast losses from a too large exit pupil. Obviously a 6mm exit pupil thru a 150mm binocular is going to deliver a lot more light than a 6mm exit pupil thru a 100mm binocular.


#5 Guest_**DONOTDELETE**_*

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Posted 02 March 2004 - 11:26 PM

Wow, that's very interesting, I didn't know red was harder to see than other colors, although it does make sense given that red lights tend to work best when trying to preserve dark adaptation.

Here's another question. Which would work best for seeing the color of a red star? Not necessarily seeing fainter red stars, but getting the most pronounced color of red.

At a dark site:

An eye that has had over an hour in the dark and is fully dark adapted.
An eye with a pupil that has only just expanded to a large size and is not yet dark adapted?

Is the eyes ability to perceive color increased or diminished in this situation? I’m hoping it is increased as the eye’s color sensors are used more.

At a heavily light polluted site:

An eye that is as dark adapted as possible.
An eye that has just had a bright flashlight shone at it and has only now expanded to its full size.

In this example suppose the magnification is not high enough to create a dark background, even so, because the eye is not yet fully dark adapted it would appear to be dark. In this situation would the color be more pronounced because of the apparent dark background? Would there still be an advantage in increasing the power even though the sky would look almost totally dark?

Basically I’m asking if it would be possible to increase the eyes ability to perceive color without increasing magnification. I don’t know enough about how the eye works to figure this out on my own, but it seems that ruining the eyes dark adaptation when observing with a bright sky would have the same effect as increasing magnification when ONLY trying to see color. If so, it would be possible to have a large exit pupil in a binocular and a dark background all at a heavily light polluted site. Perhaps this would even be a better way to see color in DSO than going to a dark site(unless of course you did the same thing there). I know this is not good for most observing, but is this a way of increasing sensitivity to color?

#6 Guest_**DONOTDELETE**_*

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Posted 03 March 2004 - 08:36 AM

The ability to see some colors is mostly dictated by the natural sensitivity of our eyes and not so much having to do with the exit pupil size. Human eyes are most sensitive to the color green and much less to the color red (rods and cones). For example, this is why hydrogen[I] emission nebula are nearly impossible to observe with just your eyes...like the Horse Head Nebula. But M42, which possess hydrogen[I], has a moderate concentration of oxygen[III] which emits green light. Thus you can see the ghostly green component of M42's total emission.

#7 wilash


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Posted 04 March 2004 - 03:01 AM

The perception of color has to do with the amount of light received by the eye - not the exit pupil of the optical system. The eye controls brightness in a number of ways. The pupil is the most obvious. But the sensitivity of the retina can be controlled chemically. The eye's pupil cannot by itself control the difference between a bright sunny day on the beach and looking for an eyepiece at night. This is why when you come in from observing the room lights may appear so bright that you have to squint - your eyes are more sensitive to light and your pupils can't become small enough to control the level of illumination.

There is a pchycological element to vision. Objects you know to be white will appear white regardless of the strength or color of the illumination. We expect stars to be white so we see them as white. With practice you can see the color of stars and wonder why you never saw it before.

Image brightness cannot simply be linked to exit pupil. Two systems with the same exit pupil will give images with the same apparent brightness, but other factors influence our judgement of the image - magnifiction and resolutiion as well as object brightness and contrast.

There is no cut and dry statement that can be made about an optical system in regards to peak performance and a specfic specification. The optical system needs to be adjusted based on what you want to acheive with it. This is why there are different types of binoculars as well as the large selection of eyepieces for telescopes.

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