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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 more than 620 models as displayed in the below list.

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

- Select a full range of progressive power and true field from the lowest to the highest using 4 optics.

- Find a very wide EP suited to circle largest DSO with the highest contrast

- Find a 20-25x per inch (or 1-1.5x per mm) aperture EP for general purposes observations

- Find a short focal EP suited to planetary observations that yields crisp and contrasted images

- Insert one or more optics to fill the gap between magnifications

- 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, Lichtenecker Optics, 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 :

-   1. The barrel size

-   2. The true field of view

-   3. Aberrations

-   4. The eye relief

-   5. The exit pupil

-   6. The brightness

-   7. The contrast

-   8. The coating

-   9. The size and weight

- 10. The sensitivity to moisture

- 11. The price argument

- 12. The brand argument

Be careful about the size of eyepieces : high-ends models suited to 2" holder are sometimes... huge and their weight can exceed 1.1 kg (without to mention their value) ! Documents from manufacturers.

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 200mm 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.

12 mm Nagler type 4 Tele Vue

1. The barrel size

What could be the benefits of having a 2" (50mm) eyepiece over a 1.25" (31.75mm) ? At first sight the apparent field of view (FOV) should be the same using a 2" or a 1.25" barrel. In reality both eyepieces are not exactly the same. But several cases have to be considered. Sometimes manufacturers sell a same eyepiece in the two diameters, like the Axiom 19mm 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.

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.

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.

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.

Celestron Axiom 50 and 15 mm, Tele Vue 55 mm Plössl and Meade SWA Plössl's 4000 serie 56 to 9.7 mm. Documents from manufacturers.

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.

Next chapter

The true field of view

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