Why are power ranges represented as exit pupil sizes?

[Chipela]

When searching for information about telescopes, I noticed that sometimes the different power ranges (low, medium, high etc) are expressed as sizes of the exit pupil. Why is that?

For me it would be more natural to speak about magnification ranges. Or is the concept of 'power' something completely different from the magnification?

Thanks for any info!

[ChrisLX200]

Well that would only apply when referring to a specific objective lens diameter, but it does give you an estimate of 'relative brightness' of the image. Take a 10x50 binocular (for e.g.,) the 50mm objective divided by the 10x power eyepiece gives a 5mm exit pupil. If you used a 50x eyepiece with that same 50mm objective the exit pupil would be only 1mm - comparitively dim. This may be a useful metric for spotting scopes where you're trying to gauge whether the combination would work well for low-light applications.

ChrisH

[Stu]

I don't think exit pupil is related to power in any way, power and magnification are much more normally treated as one and the same.

As Chris mentions, the exit pupil gives an indication of the brightness of the image, but it's quite a complex subject. There is a balance between a large enough exit pupil to give enough brightness vs keeping it small enough not to wash out the sky background and lose contrast.

It is a very useful measure to understand though. For instance x200 magnification in a 100mm refractor gives a 0.5mm exit pupil, pretty good as a max mag for planetary observing, but too small for DSOs.

Try x200 in an 18" (450 ish mm), and the exit pupil is 2.25mm, pretty good for hunting small faint galaxies under dark skies.

So, understanding power in terms of both the magnification plus the exit pupil created by a particular scope/eyepiece combination is important in predicting the views you will get.

[jetstream]

Different exit pupils will affect the eye/brain in certain, predictable ways ie larger ones will will be optimum for use with filters, espc OIII,Hb and smaller exit pupils allow the eye/brain to give the best resolution on targets like planets and the moon.

A 2mm exit pupil is a good starting place for faint, small ( in the EP galaxies).

This concept is a very good thing to learn IMHO.

[YKSE]

Excellent explanations of above posts. I'd like to add a few words, hopefully not confuse you.

As we know, exit pupil, scope aperture and magnification are related in this way

exit pupil= aperture/magnification

So knowing two of the parameters is enough to know the third, abd higher magnifcation means dimmer objects for the same aperture.

Same exit pupil meaning the same brightness, and same magnification meaning the same size of an extended DSO.

For our eyes to see an extend DSO in darkness, the DSO needs to be magnified to a certain size, also to have certain brightness, therefore there's fine balance between exit pupil and magnification.

[Chipela]

Thanks for all the answers! I guess understanding the math behind telescopes is not crucial for the hobby (if the hobby is not the math behind telescopes   ) but I just like to immerse myself also to that area.

[jetstream]

Understanding this will rapidly improve your views and especially if seeing and how truly dark skies improve views are also understood.

[Don Pensack]

Some exit pupil considerations:

4.5-7mm: the lowest power you can use that fills the pupil of your eye, and yields the brightest field of view possible in your scope (varies according to age).

2-2.5mm: the point of best visual acuity for your eye.  Small enough to avoid your eye's astigmatism, and large enough to avoid floaters.

1mm: the point at which the Airy Disc becomes an "extended object" and above which no improved resolution results.

0.5mm: the maximum usable magnification in the scope and a point where floaters in the eye can really interfere with observing details.

Those do correspond to magnifications: 

4-6x/inch of aperture--very low power

10-12.5x/inch of aperture--low medium power

25x/inch of aperture--high power

50x/inch of aperture--very high power

But, like you, I prefer to speak in terms of magnification, from a basic set of 3 in 1X, 2X, 3X where X is a figure related to the aperture of the scope (for instance, a 10" scope with x=60)

to a more complete set with 5x/inch, 10x/inch, 15x/inch, 20x/inch, 30x/inch and a good barlow to hit 40x/inch and 60x/inch if conditions ever warrant.

Currently, I vary from 7x/inch to 40x/inch but I also have a 318mm aperture.

There are many ways to figure out a set of eyepieces.  One other is a shrinking of the true field from one eyepiece to the next, by area, or a set percentage.  That makes a lot of sense if all your eyepieces have the same apparent fields.

[Pig]

And here is me just simply placing an eyepiece in the barrel and looking through it  

[jetstream]

5 hours ago, Pig said:

And here is me just simply placing an eyepiece in the barrel and looking through it  

                                                              

WHAT?  you know this is not allowed