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How to select
an eyepiece ?
Introduction (I)
One
of the most common question that arises when an amateur buys a scope is
the choice of eyepieces : what focal lenght, what design, what apparent
field, what brand to choose, knowing that the market offers at least 672 models
as listed in the below file.
My intention
is not to answer to all these questions in a few words,
presenting an universal solution suited for all scopes and subjects.
This task is not realistic, too huge and depending on specific parameters
(budget, interests, seeing, scope f/ratio, subjectivity, etc). However, according
to my own experience and the one of experienced users, I think to be able to draw
the portrait of the typical user, casual or advanced, who want the most of
his (her) material.
I will try to explain
the why of this selection, that I voluntarily enlarged to meet most of your needs,
maybe sometimes ruled by some subjective ideas but mainly on figures and facts.
Each
factor can be disserted at length with many examples and spot diagrams.
But I woudn't like to rewrite what has already been exposed on websites
like ATM and other pages about optics basics. Therefore I have listed at
the end of this report several useful links if you need more information.
Download the list of eyepieces
Excel
sheet
My
objectives
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Select a full range of progressive power and true field from the lowest to
the highest using 4 optics.
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Find a very wide EP suited to circle largest DSO with the highest contrast
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Find a 20-25x per inch (or 1-1.5x per mm) aperture EP for general purposes observations
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Find a short focal EP suited to planetary observations that yields crisp and
contrasted images
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Insert one or more optics to fill the gap between magnifications
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No zoom is used in this configuration, nor any Barlow,
Paracorr or focal reducer (but can be useful of course).
Although eyepieces count by hundreds, manufacturers
supplying excellent optics, what we will try to define, are not numerous.
Among master opticians designing
eyepieces there are Celestron, Meade, Pentax, Takahashi, Vixen and some
others. Their oldest patents are
available online.
Most
of these opticians are under contract with Taiwanese contractors. In the superior
section, there are Leitz, Tele Vue, Zeiss and may be some US or german manufacturers.
For
me as for most amateurs, the brand and the price are the main factors in
the decision process. The first is fully subjective and can finally change
your opinion, even if numbers and facts say the contrary.
So this factor
has to be treated seriously and with care to avoid potential deception
once the bill is paid.
Deciding to
buy such or such optic depends also on your activity and own interests.
If you are only looking at the Moon and planets for example,
you probably will not use many low power eyepieces, but ypu will be
concentrated in the most powerful ones designed to yield a wide field.
DSO's, the Sun, comets and planets, each subject has its own eyepieces set,
a solution optimized for specific seeing conditions, subject size and contrast
according to your scope f/ratio. At last, you may appreciate a brand for the
overall quality of its items but in specific conditions you could be disappointed
by the sensibility of ocular lenses to moisture or, if you had eye surgery or
health problems, you could be discouraged by the small exit pupil or the too
short eye relief of somes models. All these features have to be appreciate case by case.
So
here are some considerations to take into account if you want to make an
objective evaluation of eyepieces.
Technical
considerations
We
will review what determines performances of eyepieces. It is not
enough to buy any model of eyepiece and to say to oneself
"it should be suited for my scope and good enough for my
activites". Several parameters have to be taken into account in
order to not be disappointed by some common mistakes that you could
avoid by doing a short market analysis. Without speaking of the
focal length, here are several parameters we are going to describe in detail :
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1. The barrel size
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2. The true field of view
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3. Aberrations
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4. The eye relief
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5. The exit pupil
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6. The brightness
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7. The contrast
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8. The coating
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9. The size and weight
- 10. The sensitivity to moisture
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11. The price argument
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12. The brand argument
To
watch : Choosing your eyepieces with David Nagler (Televue)
To
close this review, we will answer to two questions : What
prevails, the eyepiece or the scope quality ? and The
best eyepiece does it exist ? At last I will present you
some selections based on all these
considerations to close with appreciations
from advanced observers.
Note
that the eyepiece pictures are not to scale. They are only presented for
illustration purposes.
The
focal length
We
will not discuss much about the focal length per se, because there is not
really many things to say about it. The focal is discrete or variable
(zoom) and it affects the power of your scope throught the relation :
Power
(x) = objective focal length / eyepiece focal length
So,
a 10 mm eyepiece used on a scope of 200 mm f/10, thus of 2000 mm of focal
length, will give a power of 200x, enough to see the moon or planets in
high resolution.
And
that's all ! All other considerations in terms of field of view size,
sharpness, clarity, aberrations, etc, depends on other factors related to
the eyepiece design and its compatibility with the concerned scope
(essentially its focal ratio) as we will explain.
The
focal length of the eyepiece is of course essential to determine the
approximative power that you want to reach. But we will see that in
several circumstances, it is better to use an eyepiece of shorter focal
length if it displays a wider field as it will show you more faint stars.
We will come back on this very important notion.
1. The barrel size
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12
mm Nagler type 4 Tele Vue |
What may be the benefit of using
an eyepiece offering a 2" (50 mm) barrel over a 1.25" (31.75 mm)
model ? In fact, the question must be formulated differently: what is the
barrel of the eyepieces adapted to my optical instrument ? In summary, the
barrel size of an eyepiece depends on the focal ratio of the optics: the smaller the aperture ratio
f/D (the brighter and "faster" the optic is), the more the beam or cone of light
entering in the eyepiece and the apparent field are wide. Therefore, if we do
not want to block the light beam at the edge of the field, we must use a larger
barrel, 2" or 50.8 mm in the case of amateur instruments.
Take
some examples. Sometimes manufacturers sell
a same eyepiece in the two diameters, like the Axiom 19 mm from
Celestron which exists in 2" and 1.25". Tele Vue sells some
hybrid eyepieces like the 12 mm Nagler type 4 and the 22 mm Panoptic which barrel
supports both 2" and 1.25", Vixen sells a the 22 mm LVW Lanthanum
Superwide using both barrels as well as Meade that sells a 40 mm in the two diameters
but their optics are different. The 2" is a superwide eyepiece (SWA Plössl 4000)
providing an apparent field of view of 67° while the 1.25" is an
ordinary Plössl (Super Plössl 4000) that yields an apparent field of view
44° wide only.
For
a same focal and design we can say that in a 1.25" barrel the light
cone is also narrower than in a 2". The apparent field of view of the
first is thus reduced and restricted by design compared to the second
eyepiece which can benefit of the supplemental space available in the
larger barrel and field stop.
Technically speaking when comparing eyepiece specifications we can extract some
constants. At any focal length for example there is a maximum field of
view. Empirically, it appears that the apparent FOV
* Focal Length is always less than 1600 rad (or 1600/57.3 = 27.9 mm)
for 1.25" eyepieces below 40 mm except Siebert's, and less than 2800
rad (48 mm) for 2" eyepieces, even the lowest.
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Without
speaking of the design and f/ratio, the field stop works
like a fixed size diaphragm defining the wide of the light
cone and the size of the apparent field of view. The field
stop is also used to avoid the vignetting. |
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In fact our critical parameter has the same value
as the field stop diameter which limits the size of the true field of view of eyepieces.
So if you buy an hybrid 2-1.25" eyepiece there are some chance the field
stop is internal and the measurement of the true field will probably be
not easy to calculate. In that case, in absence of relay lens the
1.25" restriction would cleary dominate, and you will loose the
advantage of using a 2" barrel and its larger field stop. In
the field the result is obvious : in a 4" f/9 refractor a 40 mm
eyepiece gives a magnification of 23x wathever the design. But things
change when we compare the true field of oculars.
The Meade SWA Plössl
yields a true field of 2°54' wide when its Super Plössl reaches with difficulties
1°55'. The larger barrel provides a deeper feeling of "flying between stars"
than using the shorter barrel. With experience looking back through the 1.25" eyepiece give
the feeling of looking at the stars through a straw, as we are constrained
by the eyepiece field stop ! However all scopes do not support both barrels.
They are often reserve to very expensive and high quality scopes or the largest ones.
A NexStar 5"
for example is designed to use 1.25" eyepieces and accessories but
nothing larger. A large barrel can increase some atmospheric problems too. In some countries
a low focal eyepiece like the 32 mm super Plössl 4000 from Meade for example (52° AFOV) is the only one
low power to get and we can forget all other 40 or 55 mm which can be of
high quality. If your location is polluted by light and if you cannot get
a dark sky where mag 6 stars are visible regularly, an eyepiece presenting
a focal a bit shorter will darker the sky background, giving a better
contrast of objects. In the same time your true field of view can be quite
near your lower eyepiece but you will gain more faint stars due to the
higher magnification and you will quickly appreciate this more starry
field.
So
the benefit of using a 2" barrel over the 1.25" depends
first on the capacity of your instrument to support it and on the size of
the true field of view, which depends on your optical characteristics. The
other criterion to take into account is the magnification vs the field size.
As we said, it is more useful to close (or open) a bit the field of view
if you gain more faint stars. If these criteria are mandatory other considerations
graft on them, reducing the choice of your eyepieces.
About
the thread of eyepieces
The
thread of a 31.75 mm eyepiece is M28.5 x 0.5 (that is to say with a pitch
of 0.5 mm) or M28.5 x 0.625 (1.125" x 40 TPI), that of a 50 mm eyepiece
is M48 x 0.60 or M48 x 0.75. As for the thread of the barrels of telescopes,
it varies from one manufacturer to another as explained in this
documentation
in French.
Next chapter
The
true field of view
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