How to select
an eyepiece ?
exit pupil (III)
exit pupil of a scope or a binocular represents the all set of rays passing
completely through the field stop. This is the diameter, in millimeters,
of the light cone coming out the eyepiece from a focused pinpoint object.
is expressed by exit pupil. The larger the exit pupil, the more light
reaches your eyes. This definition is expressed as the ratio between the
diameter of the objective and the magnification. Another way to figure the
exit pupil is to divide the scope f/ratio by the eyepiece focal length.
For a 200 mm scope using a magnification of 100 or a f/7 scope using a 14 mm
eyepiece the exit pupil is 2 mm. For highest magnifications the exit pupil
could not be smaller than around 0.5 mm, the smallest size of the eye
pupil fully contracted (equivalent to an eyepiece providing a power
of 50x per inch of aperture or 2x per mm). That can be a good lesson as at
such high magnifications the image is often too dim to be comfortable. On
the other hand for a specific eyepiece whatever the scope aperture the
exit pupil is a constant. It only changes with the aperture ratio,
becoming larger as the f/ratio decreases (with a faster scope).
exit pupil view from the eyepiece and the objective side in
a 60 mm f/23 Stanley brass refractor. Document Stanley
small bright circular image you can see at some distance of the eyepiece
or from the film plane if you do astrophotography is defined as the exit
pupil. If you image is not fully circular your optical system is defect,
uncollimated or you probably use a cheap prism diagonal.
exit pupil is also related to the eye's pupil diameter. It is easy to
understand that at night while your eye pupil dilates more light reaches
your retina with a maximum opening of ~7 mm for "youngs" between
20-50 years old. When the exit pupil of your eyes matches the one of your
eyepiece, topla, you get the maximum brightness under your current
this drawing the exit pupil is smaller that the eye pupil of
the observer in order to avoid blackouts and vignetting
browsing the field. Drawing by T.Lombry.
explain why childrens below 21 or observers older than ~65 cannot take
advantage of very low eyepieces over ~40 mm, providing an exit pupil
larger than 5 mm, as they can't yet or no more accomodate their eye pupil
over 5.5 mm. On the contrary an observer of 30 observing at night a faint
DSO has his/her eye pupils fully dilated and can take advantage of very fast scopes or
very low focalseyepieces having an exit pupil as large as their own pupil. Unfortunately
their power is so low that too few stars are visible. Binoculars exit
pupils work the same way and this is a non-sense of using for example a 7x35
binocular having a 5mm exit pupil (35/7) when your eye dilates to
7 mm. Select rather a 10x60 or larger to get the most of it if you want to use them at night.
eye pupil pictured in the dark with a flash. The red area is the
image of the retina lighted by the flash, hence the expression the
best way to calculate your eye pupil is placing a millimeter ruler near
your eye and picture your face in the dark with a flash - sorry for the
super dazzle... Then ask to the photographer an enlargement of the
negative showing your eye. Once you will have your prints back,
you could measure the maximum size of your eye pupil and buy your low
power eyepieces consequently.
you are interested in planetaries observations which includes not only
planets, but also the Sun and Moon, you need high power eyepieces (focals
below ~10 mm) having a 0.5 to 1 mm exit pupil. For fainter objects like
small galaxies or nebulosities, a 4 or 5mm exit pupil is recommended
(eyepieces lower than ~30 mm).
that under urban light-polluted sky, large exit pupils are useless as the
sky background is bright, showing green or orange emissions lines under
which you could never get a 7 mm or larger eye pupil, excepting using nebulae filters
(UHC, OIII, etc) which darken the background
in isolating lines in which these nebulae irradiate the most.
the exit pupil is a excellent parameter to evaluate both the brightness
your scope can support and the eyepieces to buy to fit with your favorites
subjects. If you want to hunt faint galaxies for example that count by
thousands or comets recently discovered you need a very large exit pupil
to catch the maximum of light. This
is because changing from a medium or high magnification eyepiece to a
lower power one, the same amount of light fills into a smaller image,
increasing the light per surface unit and its contrast against the sky background.
DSO and all faint objects will appear smaller, but brighter. By using the
next example you can find this way the eyepiece you should buy by
selecting the one giving you an exit pupil of the same size of your
largest eye pupil at dark (5 or 7 mm depending your age). This is the only
way to optimize your choice.
Imagine you buy a 31 mm Nagler
VI and want to use it with a f/10 scope. 31 / 10 = 3.1 mm. This is
unsuited to search for the faintest objects as you must reach an exit pupil of
~7 mm. If you buy a focal reducer 0.63x, you reach 4.9 mm, not enough yet excepting
under ligh-polluted skies. The best to do is buying a fast scope, open at
f/4.5 for example as 31/ 4.5 = 6.9 mm. But you know that using such focal
ratio aberrations appear at the edges of the field and that such a scope
is very sensible to collimation. What to do ?
You have to know that using
a medium to fast scope (f/4.5-6) can not acccomodate of cheap eyepieces,
showing residual aberrations. To get a wide field free of aberrations with
such scopes you must use eyepieces offering the best correction all over
the field. Those eyepieces cost over $100 each. So if you think to make a
good deal buying a cheap russian scope or a dobsonian for example you will
be disappointed when buying your eyepieces as only the most expensives
will be suited to its fast f/ ratio.
Forget your Kellner and similar as
you should have to invest in eyepieces using 5 to 8 lenses, all fully
multicoated and corrected at a few degrees off-axis too.
optimized solution should be to select a f/6 scope for example which
adapts to standard quality eyepieces like Plössl's. You win on both
sides, on the scope and eyepieces prices without to mention the high
contrast and crisp image that provides such a design. In the worst case you
could always buy a Paracorr to correct the coma or using a Powermate to
"slow down" a bit the scope without observing the defaults of barlow's.
that some observers don't care of the image quality off-axis and prefer
moving their scope to center the object.. For these people a mid-range
eyepiece is probably most that suited even if it displays some aberrations
at a few degrees off-axis. Not all amateurs think this way.
an eyepiece giving a exit pupil larger that the maximum opening of your
eye pupil, to say 7 mm on a large scope is like observing the sky not
using the full aperture of that scope.
that you look through a 300 mm scope f/4 so open and fast it gives an exit
pupil 10 mm wide using a 40 mm eyepiece. You can only catch 7/10 = 70% of
the light reaching this scope. That means you are practically using a 210
mm scope ! The fast scope will display very wide stars fields but it’s a
shame to not taking advantage of the its full aperture.
Among the effects afecting the exit pupil there is the one caused by the
central obstruction of the secondary mirror of telescopes. The percentage
of obstruction gives also the size of the obstruction visible in the exit
pupil. A scope presenting an obstruction of 35% by area, typical of
Schmidt-Cassegrain designs for example, and using an eyepiece providing a
20 mm of exit pupil, will present an unilluminated central area of 33% or
as large as 7 mm.
is also the vignetting that occurs when the exit pupil matches the eye
pupil. In these conditions the slightest movement of your head of your eye
to look at an object off axis will causes a vignetting; the edges of the
field dim and look like cut off. You can prevent this effect by using an
eyepiece that yields an exit pupil larger or smaller than the eye pupil,
the smaller being the best choice.
At last, the exit pupil or rather the brightness of
the image is affected by the Kidney bean effect, a peculiar spherical
aberration independent of the image quality. One of the
downsides of using a very large exit pupil is that the eye pupil needs to
be in the right place to intercept the light cone.
kidney bean effect occurs when the exit pupil is very large and close to the size of eye
pupil. Looking at the edges of the field your eye pupil cuts
off a part of the light cone; the medium field becomes
dimmer with occasionally blackouts.
you observe the edges of the field or the
center of the image, the exit pupil of the eyepiece stays in the center of
the field. Most eyepieces have an eyecup that will help you
position your eye properly in the axis of the light one. But
when the exit pupil is very large and close to the size of eye pupil,
looking at the edge of the field your eye pupil will cut off a part of the
light cone in the medium field region of the eyepiece. That medium field
become dimmer while the center of the field of view is unaffected. You see blackout areas
in the field When the exit pupil is small, the effect is less apparent as your eye pupil can
more easily move inside the light cone, this last having more place to
browse your eye pupil. The "blackout" effect mainly arises with
eyepieces of large eye relief and exit pupil or barlowing a low focal eyepiece as a
brightness and the limit magnitude