Gerd Waloszek       

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Eyepiece Selection (Focal Length Selection)

Note: For definitions in a small glossary, see page Quick & Dirty Astronomy Glossary.

Introduction: The Question and My Approach...

When you as a beginner purchase an entry-level telescope, this is often supplied with one or two eyepieces. Their quality is, however, often enough insufficient, so that one would like to acquire a set of new eyepieces, either quickly, or in the course of time. One is then faced with the problem of finding eyepieces with focal lengths that make sense and fit together. And one wonders whether there are any general criteria that allow statements about how a meaningful set of three or more eyepieces should look like. In books and on the Internet, I came, more or less accidentally, across the approach of using the size of the exit pupil as a criterion for choosing focal lengths of eyepieces. I found three recommendations and would like to introduce these in the following.

However, because these proposals partly "overlap" and partly complement each other, I found it difficult to deal with this diversity in practice. Therefore, I derived my own "consolidated" recommendations from the three recommendations and present them further down on this page.

Please note that on this page I present my "knowledge acquisition process," which serves as a "justification" for my recommendations and is therefore important, at least or perhaps, only for me. Many readers may, however, only be interested in the final result and its practical application. These readers should skip this page and go directly to the next page, where I briefly repeat my recommendations and apply them exemplarily to my telescopes and - "after the fact" - to my existing eyepieces. Readers should have no trouble applying the recommendations to their own telescopes (and retrospectively to their own eyepieces). All they need is the focal ratio (for calculating the focal lengths of the eyepieces) and the aperture (for calculating the magnifications) of their own telescopes!

In the discussion that follows, I will use a number of terms without going into them further. Definitions and further information about these terms can be found in appendices. If you like, you can read more there about:

• Exit Pupil
• Magnification: Viewfinder Function, Minimum Magnification, Maximum Magnification, High Magnifications, Grading of Focal Lengths of Eyepieces at High Magnifications, Use of Magnifications
• Field of View/Angle of View

Exit Pupil as A Criterion for the Selection of Focal Lengths for Eyepieces

I found four recommendations that suggest, which exit pupil diameter is suitable for which (deep sky) objects / uses. From this data you can calculate the focal length of the eyepiece and the magnification (see the appendix for the formulae). Please note that in the following formulae using the exit pupil, the focal length of the eyepiece just depends on the focal ratio of the telescope, whereas the magnification depends on the aperture of the telescope.

Note: If you do not want to read the recommendations in detail, you can jump directly to the summary or even to my recommendation that I derived from these three recommendations. Even faster you go with jumping to the next page, where I repeat and apply my recommendation!

Criteria According to Stoyan

In his book Deep Sky Reiseführer, Ronald Stoyan provides criteria for the selection and calculation of the focal lengths of eyepieces; these are presented in the table below. Actually, he provides formulae to calculate the magnification, but he does not give a justification for his recommendations and why he uses the exit pupil as a criterion; he even does not mention explicitly that the numbers in the denominators correspond to the exit pupil. I added the column for the calculation of the focal length of the eyepiece, because that is the value that is needed.

*) Added by me

Stoyan emphasizes the importance of including the minimum and maximum magnifications in the range of magnifications that the eyepiece set achieves. This is more or less the case for the minimum magnification if we assume that the number "7" denotes the size of the human exit pupil (valid only for young people). However, according to Stoyan, , the minimum magnifications cannot be realized with small focal ratios (f15 - f8), because there are hardly any eyepieces available with focal lengths of more than 40 mm.

In the literature, different definitions are used for the maximum magnification (see page Magnification, where I try to shed some light on this matter....). These correspond to an exit pupil range between 0.33 and 0.5 mm, so that Stoyan's selection also achieves these values.

Criteria According to penumbra

In a discussion thread, I found similar, but slightly simpler suggestions for the eyepiece selection from poster penumbra (again, without a justification). He specifies only three applications (corresponding to 3 eyepieces), but he provides values ranges for the exit pupil. I added the columns for the calculation of the focal length of the eyepiece and the magnification.

*) Added by me

All in all, these recommendations are somewhat more "cautious" regarding minimum and maximum magnification.

Criteria According to Intercon Spacetec

On the page on eyepiece consultation offered by Intercon Spacetec of the televue.de Website and on a newer page with the same content directly from Intercon Spacetec, I found information about the size of the exit pupil, which can also be transformed into a recommendation on the basis of exit pupil sizes. The pages also contain many useful hints, so that I strongly recommend to visit one of the two original pages (new, old).

I put the for me relevant content of the pages it into the following table format, so that it roughly corresponds to the above tables:

*) Added by me; **) the author writes (translated): "Unless the largest possible field of view is to be achieved, I consider an exit pupil of approx. 5 mm to be perfectly sufficient. An exit pupil of more than 5 mm does produce a brighter image. However, this only makes sense when the night sky is absolutely dark, and for the perception of some light-weak objects it only brings a slight increase to the absolute maximum.

Note: The names for the magnifications were taken from the original page. On page Magnification, I try to "sort" the terms and assign them to values of the exit pupil and the resolution.

This author is a bit more cautious (or more realistic) when it comes to the minimum magnification, but he brings in the viewfinder function at the lower end of the magnification. For the maximum magnification, he recommends the typical value for the exit pupil, namely 0.5 mm.

Criteria According to astroshop.de (Marcus Schenk)

On the astroshop.de Website in the new "Magazine" section, I found by accident in January 2020 further recommendations for the selection of eyepieces based on the exit pupil (EP): So finden Sie die richtigen Okulare (in German; how to find the correct eyepieces).

The author's basic advice is to buy three eyepiece focal lengths: one for the minimum (AP = 7), one for the optimum (AP = 0.8) and one for the maximum magnification (AP = 0.5). The magnification values are given by the formula: aperture/EP (as in the table below). However, a detailed table with 8 categories is also provided, showing approximately what is written in a text paragraph for the use of exit pupil values. In detail, however, there are some small "differences" between the previous information and the table... Below I present the "final result", which is offered as a table (supplemented by a column with the statements in the text about the application of certain values for the EP):

f = aperture ratio = (Focal length of telescope)/aperture; *)according th the text (except for planets and planetary nebulae); **) with "deductions" because, according to the author, this value is rarely achieved in practice...

*)Quote (translated): Which eyepieces for which object?
It is important to know what you can observe with a certain focal length. For large-area nebulae small magnifications of 7 - 6 mm are possible, if the nebula is very bright also 4 - 3.5 mm are possible. Open star clusters and galaxies are often observed between 3.5 mm and 1.5 mm. With globular clusters it may be a higher magnification with an AP between 1.5 and 1 mm. Double stars can be magnified really high, between 0.7 and 0.5 mm.

Summary

When looking at the three recommendations, one finds a number of similarities, but also differences. I tried to arrange the four recommendations in a common table so that it is easier to compare them:

In the following, I try to create a table with "personal recommendations" from this table. See my comments there, why I do so...

My Own "Recommendations"

Originally, I had calculated eyepiece focal lengths for each of the recommendations that I had found and also looked at how my existing eyepieces fit. Then, there were just two recommendations, but in the meantime, there are four... Therefore, another approach seems to me to make more sense to me, namely to derive my own "recommendations" from them and to calculate eyepiece focal lengths only for these or to check the existing eyepieces against these. This is done on the following page! Here is, first of all, my "consolidation" of the four recommendations into a version of my own:

Maybe, I should have divided the first item "Minimum Magnification / Maximum/Large Field of View" into the two items "Maximum Field of View" and "Minimum Magnification / Large Field of View," but that would only be "cosmetics." Note that the first item can lead to long focal lengths, which can only be reasonably used with 2" eyepieces (if such eyepieces exist at all...).

The third item "Maximum Magnification / Maximum Resolution" takes into account that the focal lengths of the eyepieces should be finer graded at high magnifications in order to adapt the focal length of the eyepieces to the effects of Seeing (see here as well).

This ends the "theoretical" part of this topic, and I will turn to applying this knowledge on the following page...

What is the Next Step?

Now, all you have to do is multiply the exit pupil values of the recommendation with the focal ratio(s) of your telescope(s) to arrive at the focal lengths of the eyepieces that are suitable for your telescope(s). I will demonstrate this step exemplarily for my own telescopes on the next page!

References

• Ronald Stoyan (2015, 5. improved edition). Deep Sky Reiseführer, Oculum-Verlag: Erlangen (in German).
• Forum posting (penumbra): Mein neues Teleskop (indiana-jones-forum.de/print.php?id=7505&page=1) (in German)
• Eyepiece (Wikipedia): en.wikipedia.org/wiki/Eyepiece
• Okularberatung (Eyepiece consultation; Intercon Spacetec): www.intercon-spacetec.de/beratung/okulare/okularberatung (in German)
• Okularberatung (Eyepiece consultation; Televue-Website by Intercon Spacetec): www.televue.de/beratung-okular (in German; older, but more or less identical version)
• So finden Sie die richtigen Okulare (Marcus Schenk, astroshop.de): www.astroshop.de/magazin/praxis/tipps-und-tricks/so-finden-sie-die-richtigen-okulare/i,1078 (in German)

Appendix: Exit Pupil

Definition

In optical devices for direct visual observation - e.g. telescopes and binoculars - the exit pupil is the diameter of the ray bundle that leaves the eyepiece (according to Wikipedia).

In the end, only the size of the exit pupil determines how bright the image of a certain object, for example the moon, appears in the eyepiece. If the exit pupil is the same, the object always appears equally bright, regardless of the telescope, aperture, and magnification.

Calculation of the Exit Pupil and of the Focal Length of the Eyepiece from the Exit Pupil

The exit pupil of an eyepiece is calculated from:

• (Exit pupil) = (Focal length of the eyepiece) / (Focal ratio of the telescope)

This leads to a formula for the focal length of the eyepiece, which is what we actually need when selecting eyepieces:

• Focal length of the eyepiece = (Focal ratio of the telescope) * (Exit pupil)

In practice, this means that the corresponding focal lengths for eyepieces can be calculated from values for the exit pupil, as they are typically given in the recommendations for the focal lengths of eyepieces AND from the focal ratio of the telescope at hand. And not just for one telescope, but for all telescopes having the same focal ratio, no matter how large or small they are.

Alternatively, the exit pupil diameter is given by aperture of a telescope divided by its magnification (see below for details):

• (Exit pupil) = Aperture / Magnification

Exit Pupil and Magnification

The exit pupil determines (in the direction of the eye) the minimum as well as the maximum usable magnification of an optical instrument, that is, of a telescope (if you disregard other effects). Within this range, the magnification can be calculated from / depends on the size of the exit pupil and the telescope's aperture (see formula above).

The minimum magnification is achieved with an exit pupil equal to the human pupil (often 7 mm are assumed; but see below), the maximum magnification, depending on its definition, with an exit pupil between 0.33 and 0.5 mm. According to Stoyan, this range should be exploited as much as possible when selecting eyepieces. More about this below!

Size and Age of Observer

The exit pupil of a telescope should be matched to the pupil, better to the entrance pupil, of the human eye. After dark adaptation, the pupil is about 7 mm wide for young people and about 6.4 mm wide for adults; it gets smaller with growing age. An exit pupil of 6 mm is, according to the German Intercon Spacetec -Website, however, still valid for observers that are 70 years old.

General Rules for the Exit Pupil

• If the exit pupil of an eyepiece is larger than the observer's pupil (> 6 / 6.4 / 6.5 / 7 mm depending on the observer's age), light is wasted that falls outside of the human pupil. But there may be cases where this is not an issue and just the larger field of view is what is wanted.
• If the exit pupil of an eyepiece is too small, objects become too dim:
  • below 1 mm for deep sky objects,
  • below 0.7 mm for planets,
  • below 0.5 mm for the moon).

Playing Around with Formulae for the Exit Pupil

Note: This more technical section can be easily skipped...

After I had created an Excel table with magnifications for my telescopes and, using these magnifications, in a second table the focal lengths of the eyepieces for my telescopes, I realized that this was a somewhat cumbersome approach. After some transformations of the formula for the exit pupil, I found that the focal ratio is actually the "critical" telescope parameter for determining the focal lengths of eyepieces (if it is unknown).

I start from the formula for the exit pupil::

• Exit pupil = (Diameter of primary mirror*) * (Focal length of the eyepiece) / (Focal length of the telescope) = (Focal length of the eyepiece) / (Focal ratio)

If the formula is solved according to the focal length of the eyepiece, you get:

• Focal length of the eyepiece = (Exit pupil) * (Focal ratio)

You get the magnification by inserting the focal length of the eyepiece into to following formula:

• Magnification = (Focal length of the telescope) / (Focal length of the eyepiece)

Stoyan and other author use a different approach. For illustration purpose, I start from the same formula as above and insert the formula for the magnification:

• Exit pupil = (Diameter of primary mirror*) * (Focal length of the eyepiece) / (Focal length of the telescope) = (Diameter of primary mirror*) / Magnification

Solving the formula for the magnification, results in Stoyan's formula:

• Magnification = (Diameter of primary mirror*) / (Exit pupil)

For determining the focal length of the eyepiece, you can insert the formula for the magnification and solve the formula for the focal length of the eyepiece:

• Focal length of the eyepiece = (Focal length of the telescope) / Magnification

Ultimately, both paths lead to the same end, because, in addition to the focal length of the eyepiece, one needs to know the magnification to be able to determine whether an eyepiece's focal length makes sense (i.e., whether the magnification lies within the range of minimum and maximum usable magnification).

*) Or simply "aperture" as Stoyan puts it

Appendix: Magnification

The magnification of a telescope can be calculated as follows:

• Magnification = (Focal length of the telescope) / (Focal length of the eyepiece)

For a given telescope with a specific focal length, magnification depends only on the focal length of the eyepiece in use and can be easily calculated in your head.

The formula above suggests that any magnification can be achieved. In fact, however, there are limits at the top and bottom of the range, which I discuss in more detail on page Magnification. If one disregards certain influencing variables, such as the quality of the sky or the practical availability of certain eyepiece focal lengths, and focuses on the exit pupil criterion, one can say that the exit pupil to the eye limits both the minimum and maximum usable magnification of a telescope (I mentioned this already when discussing the exit pupil). Stoyan therefore explicitly points out in his recommendations that the eyepiece set should fully exploit this range. Below the minimum magnification, however, the so-called "search function" can be useful.

In the following, some remarks about the viewfinder function, minimum magnification, maximum magnification, high magnifications, eyepiece gradation at high magnifications, and about the use of magnifications in practice!

Viewfinder Function - Maximum Field of View

The viewfinder function uses a magnification smaller than the minimum magnification and refers to eyepieces with an exit pupil of 6 (or 7) to 10 mm, that is, eyepieces with an exit pupil in which light is wasted because the light beam leaving the eyepiece is larger than the eye pupil. Correspondingly, the field of view is large, and the magnification is low. In this function, the eyepiece is consciously used to locate objects and not to observe them, and light losses are no or only a minor problem. (According to Intercon Spacetec -Website)

Examples

• Focal ratio F/10 (e.g. Celestron C8-Series), exit pupil 10 mm >> focal length 100 mm; with an exit pupil of 8 mm, it is still 80 mm, with one of 7 mm, it is only 70 mm.
  The magnification depends on the focal length of the telescope! For the C8 with 2032 mm focal length, we get magnifications of 20.3 x, 25.4 x and 29.0 x for the above values.
• Omegon PS72/432 with focal ratio F/6, exit pupil 10 mm >> focal length 60 mm; with an exit pupil of 8 mm, it is still 48 mm, with one of 7 mm, it is only 42 mm.
  The magnification depends on the focal length of the telescope! For the PS72/432 with 432 mm focal length we get magnifications of 7.2 x, 9 x and 10.3 x for the above values.

Stoyan's note on minimum magnification applies also to the viewfinder function (see next section), namely that there are hardly any eyepieces with a focal length of more than 40 mm. This is especially true for 1.25" eyepieces, but there are not many 2" eyepieces with focal lengths greater than 40 mm as well.

Minimum Magnification

At the minimum magnification, Stoyan's above cited requirement is automatically met because it is calculated directly from the aperture ratio of the telescope and the maximum possible exit pupil. It should be noted, however, that the pupil value of 7 mm assumed by Stoyan is only valid for young people; for adults I found a value of 6.4 mm, and it should decrease further with age. However, according to the Intercon Spacetec-Website, active observers, even at the age of 70, should not consider themselves below 6 mm (according to the Gahberg study). On the other hand, the author considers a pupil value of 5 mm to be quite sufficient if the largest possible field of view is not to be achieved. In any case, the range of 4.5 to 6 mm that penumbra indicates appears to be more appropriate in reality than a value of 7 mm.

Stoyan points out that for telescopes with a large aperture ratio (F/8 ... F/15), the calculated minimum magnification can no longer be achieved, because there are hardly any eyepieces with a focal length of more than 40 mm. This is particularly true for 1.25" eyepieces, but there are even few 2" eyepieces with a focal length greater than 40 mm. This can perhaps be "adapted" a little by assuming a smaller exit pupil (6.4...4), but only marginally...

Another problem is that with 1.25" eyepieces having a focal length of 32 mm or more, viewing is always difficult. I made this experience with a 40 mm Plössl eyepiece, a starfriend also reported this to me, and a dealer confirmed this as well. In this case, it is good if you have a telescope for 2" eyepieces at your disposal, and to use it with 2" wide field eyepieces (for example, I own a 2" eyepiece with a focal length of 35 mm).

Examples

• Focal ratio F/10 (e.g. Celestron C8-Series), exit pupil 7 mm >> focal length 70 mm; with an exit pupil of 6 mm, it is still 60 mm, with one of 4 mm, it is only 40 mm.
  The magnification depends on the focal length of the telescope! For the C8 with 2000 mm focal length, we get magnifications of 29.0 x, 33.9 x and 50.8 x for the above values.
• Omegon PS72/432 with focal ratio F/6, exit pupil 7 mm >> focal length 42 mm; with an exit pupil of 6 mm, it is still 36 mm, with one of 4 mm, it is only 24 mm.
  The magnification depends on the focal length of the telescope! For the PS72/432 with 432 mm focal length we get magnifications of 10.3 x, 12 x and 18 x.

Maximum Magnification

On the page Magnification, I discuss the various definitions of magnification, including terms such as normal magnification, beneficial magnification, maximum useful magnification, and maximum magnification. It is difficult for me to understand all the different definitions by different sources, but all of them can be expressed as multiples of the telescope aperture in mm. The maximum magnification defined by Stoyan is 3 x the aperture in mm, the maximum usable magnification, which also goes under different names, is 2 x the aperture in mm, and the beneficial magnification amounts to 1.5 x the aperture in mm. The Stoyan's maximum magnification is therefore twice the beneficial magnification!

Since it is easy to determine the values for these magnification types for a given telescope, you can do this quickly and determine the focal lengths of the eyepieces. Then you can at least see where you are... I will do this exemplarily for my telescopes on another page.

Examples

• Celestron C8 with 200 mm aperture (203 mm to be exact): the three above-mentioned types of magnification amount to 300 x, 400 x, and 600 x.
  For an exit pupil of 0.4 we get the result: 200 mm / 0.4 = 500 x; this is still below Stoyan's maximum magnification!
• Omegon PS72/432 with 72 mm aperture: the three above-mentioned types of magnification amount to 108 x, 144 x, and 288 x.
  For an exit pupil of 0.4 we get the result: 72 mm / 0.4 = 185 x; this is still below Stoyan's maximum magnification!

High Magnifications, Grading of Focal Lengths of Eyepieces at High Magnifications

The beneficial magnification is defined as the magnification above which further magnification does not provide any new information. This is also referred to as "empty" magnification. Nevertheless, can it, according to Stoyan, make sense to go beyond the beneficial magnification to the maximum magnification. This applies in particular to structures (e.g. on the moon) for which the standard resolution formulae (Dawes, Rayleigh) do not apply. According to my experience, some structures only become recognizable when they have reached a certain size, no matter whether this is done with empty magnification or not...

The use of high magnifications of course depends very much on the quality of the sky (seeing), and often it is not possible to use high magnifications. As a rule of thumb, I have read that on most days in Central Europe, a magnification beyond 200 x does not make any sense. With my telescopes, the limit often seems to be reached already at 100 x...

A starfriend wrote me that particularly in the range of high magnifications, it is important to have several finer graduated eyepiece focal lengths available. Depending on the seeing, on certain days, a slightly longer focal length eyepiece (e.g. 12 mm) can already mark the "boundary", whereas on other days it may be possible to use shorter focal lengths (e.g. 8 mm or even 4 mm). He often switches between the eyepieces, until he has found the best one for the day. In his opinion, having only three eyepieces for one telescope is not enough: He needs one for the overview, a medium magnification, and several ones for fine tuning, depending on the seeing, for the higher magnifications.

Use of Magnifications

In practice, it is, of course, important to know, which magnification fits which purpose. I found (and added to... source regrettably unknown...) the following recommendations for the use of magnifications:

• Very low (10-20 x): Search for objects, extended objects (large-area nebulae, Andromeda galaxy, ...) (added by me)
• Low (30-50 x): Star clusters, galaxies, and nebulae
• Medium (80-100 x): Craters and valleys on the moon, Saturn rings, Jupiter and moons
• High (150-200 x): Mountain peaks and fine details on the moon, surface details on Mars, separation of close double stars

In principle, these magnification values might be used to select the appropriate focal lengths of eyepieces for a particular telescope according to the following formula (transformation of the above formula):

• Focal length of the eyepiece = (Focal length of the telescope) / Magnification

Apparently, however, it is more common to use the exit pupil as a criterion for selecting focal lengths of eyepieces, because the brightness of the image is also taken into account. For example, a starfriend told me that he does not like the approach of using just the magnification that much. One should always consider the exit pupil and be aware that besides the appropriate magnification, also the brightness of the image has a decisive part in the observational success.

Appendix: Field of View/Angle of View

Apparent Field of View

The apparent field of view (apparent viewing angle) determines the angle that is shown by an eyepiece as a section of the sky. It depends on the type of the eyepiece an is usually given by the manufacturer of the eyepiece. Here are some examples:

• Kellner eyepiece: 42°
• Orthoscopic eyepiece (after Abbé, König): 40° (modern variants can achieve larger fields of view)
• Plössl eyepiece: 52-55°
• Planetary eyepiece (HR Planetary): 58 - 60°
• Eyepiece with "optimal" viewing angle (wide-angle eyepieces, Erfle eyepieces): 68 - 70° (according to the advertisements...)
• Super/Ultra wide-angle eyepieces, Nagler eyepieces: 80 - 82°
• Ethos and similar eyepieces: 100 - 110°

Eyepieces with a field of view up to 55° are often characterized as providing "tunnel vision." Eyepieces with a field of view of 80° and more are often advertised as that you are "floating in front of objects in space." Eyepieces with a viewing angle around 70° are considered as "ideal for the human eye" and as "optimal for observation" - with larger fields of view you have to "look around the corner" to overlook the whole field of view. More about eyepiece types can be found on Wikipedia and on page Some Information About Eyepieces.

True Field of View

The true field of view (true angle of view/vision, visual angle) determines the size of an object as it can be observed in a telescope:

• True field of view = (Apparent field of view) / Magnification

It is thus, obtained by dividing the apparent viewing angle by the magnification (e.g., 70 degrees / 100 = 0.7 degrees).

Why is the true visual angle important? The size of objects in the sky is indicated in angular degrees, that is, as viewing angles (for example, the sun and the moon both have a visual angle of 0.5 ° or 30'). Such data can be found, for example, in books about deep sky objects.

Two unfavorable situations may arise when observing sky objects:

• You cannot find the object you are looking for, although the telescope is correctly aligned (this is what happened to me with the ring nebula M57 in constellation Lyra and the Heritage 100P): Probably the magnification (that is, the viewing angle) is too low for being able to identify the object.
• The object (for example, a large-area nebula or an open star cluster, but also the Andromeda galaxy M 31) is too large to be captured completely with the eyepiece. Its true viewing angle is therefore too small.

Thus, when choosing eyepieces, it is good to know how the visual angle of sky objects and the field of view (the true visual angle) of an eyepiece relate to each other.

Deciding between Eyepieces...

Above, I mentioned already that older Kellner and orthoscopic eyepieces have an apparent viewing angle of 40 degrees, Plössl eyepieces one of 50 degrees or a little more, and above them, the wide-angle, super- and ultra-wide-angle eyepieces start, which have become increasingly popular in recent years. According to the criteria discussed above, there is no difference between the different types of eyepieces: all have the same magnification and exit pupil. In practice, however, according to advertising, this means a difference between "tunnel vision" and "space walk" - which is where the angle of view of the eyepiece comes into play... To put it simply, the larger the apparent viewing angle, the larger the true viewing angle and thus, the section of the sky that can be observed in the eyepiece.

By the way, eyepieces with an apparent viewing angle of 70 degrees are regarded as "optimal" for our eyes, because at this viewing angle you do not have to wander around with your eyes and you can see everything at a glance.

If you have to decide between a "normal" eyepiece and a (U)WA eyepiece, you are faced with the following alternatives:

• Same focal length: The (U)WA eyepiece shows a larger section of the sky; magnification and exit pupil, i.e. brightness, are the same. The advantage of the (U)WA is a larger sky section, so you do not have to "push" your Dobsonian or other "hand-operated" telescope as often.
• Same viewing angle: The (U)WA eyepiece shows the same section of the sky; the magnification is higher, so you may see details better, but the exit pupil, and thus the brightness, is lower. Everyone has to decide for himself and try it out (calculating is easy...), to what extent the loss of brightness is disturbing him or her.

When choosing a (U)WA eyepiece, you have to decide how large the viewing angle should be. I already find a viewing angle of 60 degrees quite pleasant, 70 degrees even more so, and with my 80 degrees eyepieces, I have to "wander around" a bit with my eyes, which I do not find pleasant at all... In addition, eyepieces with a viewing angle of 100 to 110 degrees are heavy and very expensive. I feel that the 80s are still "just affordable"... Since I have never looked through a 100-110 degree eyepiece, I cannot tell you anything about the "space walk" experience. But there are starfriends who do not want to miss this experience anymore...