How to Choose Eyepieces for Any Telescope

Scott Roberts from Explore Scientific takes you step-by-step on how to choose a good selection of eyepieces for any optical telescope that accepts interchangeable eyepieces that you own. 

Scott covers how to select lowest useful magnification by determining your eye's fully dilated pupil size, and highest useful magnification for the aperture of your telescope and by determining the seeing conditions of the sky. He also discusses intermediate magnifications. Most points that anyone would consider in choosing eyepiece are covered, including: focal length, apparent field of view, true field of view, eye relief, and contrast. 

Although we highlight Explore Scientific eyepieces, the methods described in this video work for any brand or type of eyepieces that you may be considering. 


Transcript of Video: We're at the Explorer scientific
customer service and training center and
we're going to learn how to choose and
use eyepieces modern design of telescope
eyepieces have revolutionised visual
astronomy with breakthroughs and
computer optimization and new optical
glasses today's top eyepiece
manufacturers design high-performance
eyepieces with superior edge performance
with a wide range of apparent fields and
long eye relief so how do you choose
today there are many premium quality
eyepieces to choose from for your
telescopes but with so many focal
lengths in a variety of apparent fields
of view how do you choose what's best
and what fits you best for your specific
telescope here are the eyepiece
selection considerations we're gonna
find out the lowest useful magnification
of your telescope the highest useful
magnification how to choose eyepieces
between lowest and highest useful
magnification that will give you the
medium steps and power with your
telescope selecting a set of eyepieces
that have the apparent fields of view
that you desire that will produce the
true fields of view that you typically
require for the objects that you observe
and then you need to consider a set of
eyepieces that have enough eye relief so
that you can have the eye comfort that
you require this is how telescope works
aperture focal length F ratio the
telescope focal length divided by the
telescope aperture equals F ratio from
here we can determine magnification and
true fields of view so let's discuss
focal length all telescopes and
eyepieces have optics polish to focus at
a certain distance this distance between
the lens and the focused image usually
measured in millimeters is called focal
length a longer focal length lens
produces a narrower field of view and a
shorter focal length lens produces a
wider field of view
so how does magnification work the
focused image from the telescope lens is
being magnified by the eyepiece in this
case the telescope focal length is about
160 millimeters and the eyepiece is only
20 millimeters producing eight times
magnification this is how we calculate
magnification what you do is you take
the telescope focal length usually a
millimeters divided by the eyepiece
focal length also in millimeters and
that will be equal magnification or you
can take the telescope aperture and
millimeters and divide it by the exit
pupil of millimeters and that will also
equal the magnification now we'll
discuss exit pupil every telescope and
eyepiece combination will produce an
exit pupil it's the focus beam of light
where the image is viewed the diameter
of the exit pupil changes with
magnification lower magnifications
produce a larger exit pupil giving a
brighter image higher magnifications
produce a smaller exit people with less
image brightness this is the formula for
calculating exit pupil you take the
eyepiece focal length in millimeters and
you divided by the telescope f ratio and
that will give you the exit pupil number
or you can take the telescope aperture
in millimeters and divide it by
magnification and this will also give
you the exit pupil number both of these
numbers will end up being in millimeters
observing faint objects telescopes are
like gathering devices capable of
focusing images of objects at incredible
distances under dark skies and the right
telescope and eyepiece combination it's
possible to visually observe faint
galaxies that are many millions of
light-years away of course with more
light to your eye you can see the
faintest details of every object you
look at but to see the faintest details
you need to be comfortable and observe
at the lowest useful magnification
in order to see the famous in the most
distant celestial objects possible
through your telescope you need to find
the eyepiece focal length that will
produce an exit pupil that will closely
match the entrance pupil of your fully
dilated eye when dark adapted for young
people it's not unusual for pupils to
dilate to seven millimeters or more
older people may have pupils that can
only dilate to five millimeters so visit
your eye doctor to have your fully
dilated pupils accurately measured with
this information you can choose the
perfect lowest power eyepiece now for
determining lowest useful magnification
power try to determine which eyepiece on
your telescope will produce an exit
people that comes closest to matching
your fully dilated eye if you select an
eyepiece that produces a larger exit
pupil than what your eye can dilate to
then you're not able to experience the
full brightness of the focused image
it's important to calculate the eyepiece
focal length for Lois useful
magnification here's a useful formula
for calculating the eyepiece focal
length for a lowest useful magnification
what you do is you take the telescope
aperture in millimeters and you divided
by the fully dilated eye in millimeters
that will give you lowest useful
magnification then take the telescope
focal length in millimeters and divided
by the lowest useful magnification
number that'll tell you which eyepiece
focal if you need another typical way to
find lowest useful magnification is to
use the rule of thumb a three and a half
power per inch of aperture with 102
millimeter lens a four inch aperture
telescope the lowest useful
magnification is 14 times but what about
high-power observing telescopes are also
detailed resolving devices capable of
focusing highly detailed images of
objects under steady skies called good
or perfect seeing conditions and the
right telescope and eyepiece
it's possible to visually observe
incredibly fine features of bright
objects like the planets and the moon
but there's a limit to how much
magnification you can use and that
limitation will be based on the
atmospheric seeing conditions and the
telescope's aperture having several
eyepiece focal lengths from lowest
useful magnification the highest useful
magnification will maximize your
potential atmospheric seeing and the
Pickering scale the atmosphere can be
very turbulent called bad seeing two
perfectly calm called perfect seeing
turbulence in the air causes stars a
scintillator twinkle viewing conditions
with bad seeing one two four and the
Pickering scale distort images of stars
planets and even the moon
this distortion looks much worse with
larger aperture and higher magnification
viewing conditions with good - perfect
seeing which it would be 7 to 10 on the
Pickering scale allows telescopes to be
used at higher magnifications which will
reveal fine details on every object you
observe the Pickering scale shows one is
the worst seen with five arc seconds are
more star blur at the sharpest focus and
10 is perfect with 0.5 arc seconds or
less average nights of seeing would
range from 5 to 7 on the Pickering scale
to determine highest useful
magnification on any given telescope
determine the telescope's aperture in
millimeters and multiply that number by
3 with this 102 millimeters refractor
the result is 306 power so how do you
calculate the eyepiece focal length for
highest useful magnification so this
formulas for calculating the eyepiece
focal length for the highest useful
magnification what you do is you take
the telescope aperture and millimeters
multiply by 3 and that will give you the
highest useful magnification number or a
more conservative number can be brought
about by taking the telescope aperture
and inches multiplying it by 60 or if
you're seeing conditions are not that
great maybe by 50 and that would give
you the new highest useful magnification
number then you take the telescope focal
length the millimeters and you divide by
a highest useful magnification number
that will give you the eyepiece focal
length that you need to get
what's aperture masking and how can it
improve your high-power observing during
times a porta bad seeing images of the
moon planets and double stars observed
at higher power can be improved by
reducing the entrance aperture of your
telescope mass can be made from various
sizes from cardboard with less aperture
there is less resolving power but images
will look sharper now that you
understand lowest useful magnification
and highest useful magnification you
need to calculate the medium steps
empowered so that you can get the best
observing potential possible many
astronomers own several eyepieces that
range from lowest to highest power for
their telescope medium power eyepieces
are also selected by magnification a
parent field of view and eye relief many
astronomers begin their observations at
low or lowest power and then gradually
increase power until the object is best
seen what our eyepiece multipliers
increasing magnification of your lower
power eyepieces can also be done by
adding focal extenders or a Barlow lens
to your eyepieces the advantage is that
these devices can increase the
magnification of all your eyepieces
but the eye relief will remain
unaffected this is especially useful if
you need long eye relief of your long
focal length eyepiece but you want more
magnification many people want to know
what's the difference between a parent
field of view and true field of view
true field of view is the actual degrees
of sky coverage that you're looking at
the chunk of sky that you see but a
parent field of view is the angle of
degrees from your eye to the edge of the
field stop in the eyepiece when you're
looking through an eyepiece that has
wider apparent field of view you not
only get more true field of view but you
get this effect that you're seeing
through a wide picture window
more apparent field of view gives you
more true field of view the Orion Nebula
m42 is 60 arc minutes by 66 arc minutes
an angular size or twice the size of the
full moon in the sky this is in 42 with
the telescope of 714 millimeters with
focal length and a 68 degree 20
millimeter eyepiece this gives you 35
point 7 power and it will also produce a
1.9 degree true field of view then in
this other image
we've got M 42 with a telescope of the
same focal length 714 millimeters and
the same focal length of eyepiece of 20
millimetres so we're getting the same
magnification of 35.7 power but with a
hundred degree apparent field of view
we're getting two point eight degrees
true field of view you actually see more
area around the object but at the same
magnification so how do you calculate
true field of view you take the eyepiece
apparent field of view and you divide it
by magnification and that will equal
true field of view and degrees or
another way of doing this is take the
eyepiece field stop you can get that
from the manufacturer you divided by the
telescope focal length in millimeters
and you multiply it by 57 point three
and that will also give you the true
field of view number and degrees you
have to relax to see more and this is
the importance of eye relief eye relief
is the distance from the last surface of
an eyepiece of which the observers eye
can see the full viewing angle if the
observers eye is outside of this
distance then the observer will not see
the full viewing angle long eye really
five pieces give more eye comfort and if
long enough 18 millimeters or more will
allow observers to wear glasses when
observing that's usually needed for
someone that has astigmatism in their
generally speaking short focal length
higher-power eyepieces have less eye
relief than long focal length
lower-power eyepieces eye relief
distance information is provided by the
you'll love to observe deep sky objects
but what about light pollution just
because you're not into the best dark
sky sight doesn't mean that you can't
still observe some bright galaxies star
clusters in nebula improving your skills
an astronomer has a lot to do with how
often you observe even if that means you
do most of your observing near city
lights to improve the view of deep sky
objects under light polluted skies you
can use moderate increases in
magnification which will make the
background sky look darker there are
also special filters that you can use
with your eyepieces that can increase
contrast to judge the quality of your
site you can use the bordel scale you
can have the darkest skies possible but
also not seem very many stars and that
has to do with atmospheric transparency
the bordel scale is a 9 level numeric
scale that measures the night skies
brightness of a particular location it
quantifies the astronomical
observability of celestial objects and
the interference caused by light
pollution class 9 on the bordel scale is
the most light polluted sky where you
can barely see any stars in the sky at
all when you go all the way down to
class 1 you're now under perfectly dark
sky conditions where the sky is black to
the horizon but there's another aspect
that's very important to astronomers and
that is sky transparency and atmospheric
extinction it's possible to be under a
dark sky but not to be able to see many
stars due to poor sky transparency or
atmospheric extinction
generally speaking the very best dark
sky sites are far away from city lights
you're up high in elevation with smooth
terrain and the sky has very little
water vapour or aerosols in the
atmosphere you can learn more about sky
transparency an atmospheric extinction
at this website
so to sum it up by choosing the right
you're going to have the most relaxing
most detailed observations you can
possibly make the more comfortable you
are and the longer you look the more you
see long I really find pieces with large
apparent field characteristics allow you
to relax more at the eyepiece by letting
you back away from the optics while
allowing your ID to comfortably scan the
field of view in order to use a
vertiginous I observe at any
that's not staring at the object you can
see fainter details and finer structure
than a strained I so happy observing