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Exit pupil.

Daz Type-R

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Hi guys and gals.


I`m not too hot on exit pupils, I (think) I understand them but not 100% sure.


If I was to get an ep with an exit pupil of 2.2mm, would that be a problem?


I believe at my age my exit pupil fully dark adapted is approx 6mm (I think), so any thing over 6mm is sort of a waste but can you go too small?


Over to you.

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Guest Tweedledum

Hi Darren,


That should be ok, its when you waste it and have a greater than your eye can see there is an issue, You can also go very low but it will be like looking into a cardboard tube... Aslo for fun see here :---- http://www.stargazing.net/naa/scopemath.htm


Good article below.




Exit pupils: A second way to choose eyepieces (other than power per inch
of aperture) is to match the eyepiece exit pupil to the type of observing you
want to do. To find the eyepiece exit pupil – the diameter of the beam of light
coming out of the eyepiece – divide the eyepiece focal length by the telescope
focal ratio. Thus, while a 10mm eyepiece has a 1mm exit pupil with an f/10 scope
(10 , 10), the same eyepiece has a 2mm exit pupil with an f/5 scope (10 , 5).
The higher the power, the smaller the exit pupil.

The brightness of extended objects (galaxies and nebulas) is proportional to the square of the
exit pupil. Therefore, a low power 4mm exit pupil (42 = 16) is four times as
bright on galaxies and nebulas as a medium power 2mm exit pupil (22 = 4). To put
it another way, twice the power results in one-fourth the brightness on the
faint fuzzies outside our solar system.

On the other hand, the brightness of a point of light (a star) is a function of the aperture of your
scope – not the exit pupil. The bigger the aperture, the fainter the star you
can see. Stars do not get dimmer as a scope’s power increases and the exit pupil
gets smaller. Extended objects do, however, and the sky (the most extended
object you’ll ever see through your scope) becomes progressively darker as the
power goes up. The result is that faint stars are usually more visible at higher
powers, as the contrast between the unchanging star brightness and the
progressively darker sky background increases.

From dark sky sites, a 5mm to 7mm exit pupil is best for observing Milky Way star clouds, open
clusters, large nebulas such as the Veil, etc. From mildly light-polluted
suburban sites, a 3mm to 4mm exit pupil improves the contrast of these
large-scale objects by darkening the light-polluted skies somewhat without
overly dimming the objects themselves.

A 2mm exit pupil most closely matches the area of highest resolution in your eye and gives you good detail for
planetary, lunar, and globular cluster observing. The sharpness of those details
is likewise improved by a 2mm exit pupil, as a smaller exit pupil minimizes
astigmatism at the edges of your dark-adapted eye. Also, the visibilty of small
galaxies and planetary nebulas is often enhanced by the darkening of the sky
background with a 2mm exit pupil.

A 1mm exit pupil gives you maximum planetary detail and is excellent for splitting binary stars. A 0.5mm exit pupil
is useful for splitting close double stars, but only during very good seeing.

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WOW - info overload.


I think I have got that.


So does the ep F.O.V make much difference in the calculation?


For example, the higher the F.O.V, you could get away with such a small exit pupil?


Or are exit pupils and F.O.V completely separate when deciding on an ep?

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