Go to Table of Eyepiece Specifications

Note: In addition to the discussion following about eyepiece properties, there is the question of ensuring that the field stop is fully illuminated by the telescope. This very important issue is discussed in a brief addendum at the end of this article. See: Field Stop Illumination

This very simple topic seems to be misunderstood by many. The confusion comes from the rather loose terminology related to eyepieces.  There are two fields of view that are normally described. One field of view is the angular field of view that is seen in the field stop of the eyepiece and is the actual angular field seen through the telescope of a section of the sky. This field of view is determined only by the focal length of the telescope and the diameter of the field stop in the eyepiece. The second field of view is that which the observer sees when looking into the eyepiece. This is usually called the apparent field of view. This has nothing to do with the telescope. It is a property of the eyepiece.

The eyepiece presents to the eye an enlarged view of the field stop located in the eyepiece. This field stop is determined by the desired eyepiece field of view, the magnification of the eyepiece and the focal length of the eyepiece. It is limited only by the size of the mechanical tube holding it and of course the optical characteristics of the eyepiece. Eyepieces come in a great variety of optical types.  They range from very simple Kellner eyepieces which have simple optics to highly complex super-wide types. With the wider type designs, the observer sees a very large apparent field of view. This can be quite impressive, but, it must be emphasized that this large field is only that of the field stop and is only indirectly related to the actual size of the star field observed.

Let us start by looking at a typical set of eyepieces. Shown below, in the top row, are the Meade 56 mm, 40 mm, 32 mm, 26 mm and 24.5 mm.
In the bottom row are the 18 mm, 14 mm, 13 mm, 8.8 mm, 6.7 mm and 4.7 mm.
 
 

Now the actual field of view of the telescope is determined only by the size of the field stop and the focal length of the telescope. The actual angular field of view is the diameter of the field stop divided by the focal length. This measure is in radians and can be converted to degrees by multiplying by 57. This field of view is what is called the actual field of view of the telescope. There is naturally a relationship between the actual field of view, the focal length of the eyepiece, the magnifying power of the eyepiece and the apparent field of view that the observer sees. It is because of all of these factors and the way they are often discussed in books that confusion arises. Let us try to clarify these issues.

The Meade descriptions for these eyepieces are given in the table.  Descriptive terms will vary among manufacturers. (Some get more glitzy for sales appeal)
Basically eyepieces fall into categories of normal, wide and ultra wide. This refers to the APPARENT field of view (APPAR). The wider eyepieces look to the eye like they have a wider field of view.  The actual field of view depends on factors described above and is listed in some detail with examples below. The wider eyepieces generally have more glass in them, are heavier and more expensive. (some excessively so in my opinion)   The ACTUAL field of view (ACTU) is dependent upon the telescope focal length. It is given for a 10" f 10 telescope (fl = 2450 mm)  The Magnification (MAG) is also given for the 10" f10 telescope.

It is also apparent that the actual field of view is also the apparent field of view divided by the magnification. The diameter of the field stop can be measured by placing a mm scale across the bottom of the eyepiece tube and estimating the size of the field stop. The size of the field stop is limited by the size of the tube or course. This is why 2" eyepieces can see more of the sky. They can have larger field stops.  But notice that several of the 2" eyepieces are actually dual use.  That is they can be placed on a 2" or a 1-1/4" tube. Thus the field stop is less than 1-1/4 inches of course.

Keep in mind that the actual field of view of the star field is dependent on the focal length of the telescope and the diameter of the field stop. And that the apparent field of view is dependent only on the design of the eyepiece. But it is not glitzy enough to give the size of the field stop. The eyepiece maker gives the focal length of the eyepiece, which is certainly important, the apparent field of view of the eyepiece and sometimes the magnification (POWER). These are important pieces of information, but now to find the actual field of view one has to go through an obscure and not obvious calculation. A fundamental way to calculate the magnification is to take the focal length of the telescope and divide it by the focal length of the eyepiece to get the magnification of the combination. POWER is an important term from a sales viewpoint.  But it is not a characteristic of the eyepiece alone.

I feel it is important to think about optical elements in the most appropriate physical terms because one can then calculate other things. For example, suppose one has a reticule at the position of the field stop. It will be in focus and will have some pattern on it, often divisions of tenths or hundredths of a mm. (or of an inch)  Suppose we want to know the angular arc subtended on the star field of one of these divisions. We need know nothing except the distance between the divisions and the focal length of the telescope. The distance between the divisions divided by the focal length of the telescope gives the angular measure on the sky in radians. Multiply by 57 to get degrees.

For example, for the familiar Meade 9 mm eyepiece and with a 1600 mm telescope focal length, the actual angular view of the outside circle is 83", that of the inner circle is 37" and that between the parallel lines 8.2". If the concept of actual field of view were more commonly used, it would be clearer to observers why they can't see M31 no matter how high the POWER of their telescope. Meade gives a nice table in their operating manual showing their eyepieces, their focal lengths, their apparent fields of view and the actual fields of view when used with a variety of telescopes.

A better idea about the issues of the relative sizes of the glass elements and the field stops can be had by looking at the following photographs.

Below are the field stop side of the 56 mm, 40 mm, 32 mm, 26 mm and 24.5 mm. All of these eyepieces have field stops about as large as is possible for the tube size in which they are mounted. In order to have a larger actual angular field, the tube simply has to be larger. Amateur telescopes typically have a largest tube of 2". A few have a tube of 3".  On the other hand, 4" diameter eyepieces are not uncommon on professional telescopes. Such eyepieces get very large, heavy and expensive.
 
 

Below are shown the field stop side of the 40 mm, 24.5 mm and the 8.8 mm eyepieces. The difference in the sizes of the field stops are quite apparent.
 

Finally, below are shown the field stop side of the 26 mm, 18 mm, 13.8 mm 6.7 mm and 4.7 mm eyepieces.
 

A very nice book, which describes a great variety of optical features and simple calculations for telescope systems is "Astrophotography II" by Patrick Martinez. It is published by Willmann Bell (www.willbell.com)  It has a wonderful variety of useful information for anyone interested in astrophotography.

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Field Stop Illumination Limits Caused by the Telescope

The above analysis helps the user select an eyepiece that has a large enough field stop so as to attain an actual field of view that is desired.   Another closely related issue is that of the ability of the telescope to fill the field stop with a real image of the sky that is both sharp over the field stop and also uniformly illuminated. For refractor telescopes this is usually not a problem since the objective lens throws a cone of light directly down the tube and is limited only by the size of the eyepiece holder and focuser mechanism. If the telescope will take a 2" eyepiece, the focusing mechanism will generally be open enough to accept the necessary cone of light.

The problem is not so simply with a more complex, folded optical system like the SCT. In the SCT, there is a secondary which projects a cone of light down a rather restrictive tube to the final opening in the back plate of the telescope. It is easily possible for this tube and the size of the opening in the back plate to limit the cone of light.  With smaller SCTs it is almost certain that the cone of light will show some vignetting at diameters will under 2". Thus, with the larger diameter eyepieces, the field stop may not be fully illuminated. For example, a 2" 56 mm eyepiece has a field stop diameter of 49 mm.   That is as large as a field stop can possible be in a 2" tube. Note however, that if the 2" eyepiece is connected to the back plate of a SCT with an adapter of the type that carries a Schmidt thread, the inside diameter of the opening in the adapter will be only 38 mm. It is clear that this type of adapter will vignette the cone of illumination at the edges of the field stop in a 2" eyepiece. In order to get full illumination with a field stop of 49 mm one should use an adapter with a full 2" diameter opening. This size is provided, for example, by the JMI focuser, which has a clear opening of 50.6 mm when it is used with the special adapter plate that they provide. I must recommend that when a 2" eyepiece is used, care be given to opening the connecting tube to the full 2" with appropriately large adapter tubes. JMI, Optec, and Lumicon provide adapters which take these matters into consideration in their designs. (Remember that the Schmidt Thread was designed for small SCTs and is quite marginally sized for 10" and larger telescopes.)
For a discussion of back plate opening sizes, see elsewhere on this website:Telescope Back Plate Aperture Sizes

This comment and warning is given because it is too bad to spend a large amount of money for a 2" eyepiece when the field stop cannot be fully illuminated by the telescope. The use of a field reducer is especially problematical since they compress the cone of illumination even more. The un-vignetted light cone for the 0.63 reducer is only about 26 mm. This circle of illumination is suitable for a 1-1/4 " eyepiece only. In such a case, the longer focal length eyepiece will decrease the magnification without increasing the actual field of view.
 

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