OWL Astronomy

OWL Astronomy Products
Eyepieces, Telescopes, Binoculars, Microscopes
Eyepieces - Our Speciality




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About Eyepieces

by Thomas C. Roginski, Ph.D.

There has always been a lot of misunderstanding about what power your telescope can handle and what eyepieces to get. We will try and help answer some of your questions here. First we will answer some basic questions and then we will try to provide a discussion on which series of eyepiece to use with what you are going to observe.

Recommendations for Eyepiece Types

Eyepiece Menu
Eyepiece Types: Pros and Cons*
Eyepiece Type Deep-Sky High Res General Use Eye Relief
  Ultrawide Angle Eyepieces
10
4
4
8
  Enhanced Superwide Angle
7
6
7
8
  Planetary Eyepieces
6
10
7
9
  Super-Plossls (Long/Medium FL)
5
6
9
7
  Super-Plossls (Short Focal Length)
2
5
6`
1
  * Quality ranges from 10 (perfect) down to 1 (bad or unsuitable)

What power do I get? The power of the telescope is calculated by dividing the focal length of the main mirror or lens by the focal length of the eyepiece used. For example, a 8" SCT at f/10 (2000 focal length) will get 200X (200 power) with a 10 mm focal length eyepiece, 80X with a 25 mm eyepiece, or 50X with a 40 mm eyepiece.

What power should I use? People often think that the highest powers possible is the best power to use. Actually the lower power (long focal length) eyepieces are always easier to use than the high power (short focal length) eyepieces. The lowest power that will do the job is the one to use. Remember that power magnifies everything. That includes shakiness in the mount and motion in the air stream. High powers also give dimmer views because the same amount of light is spread out over the larger image.

What is the highest power to use? A lot of people think that the highest power available is the one to use. On any night, however, unsteadiness in the air limits the maximum power you can use. Assuming the air is steady, the telescope is good, and the mount is steady, the highest power that is useful depends on contrast. For the rare high contrast objects like double stars, you can use up to about 100X per inch of telescope under the best of conditions (300X for a 3-inch telescope). Most objects (like the planets and deep-sky), have low contrast details. For most objects about 40X to 50X per inch is a maximum (120X to 150X for a 3-inch telescope). On nights where the atmosphere is very unsteady, it may not be possible to use even 25X per inch. We have lost sales because we have told potential telescope buyers not to use 500X with a 60 mm telescope. Other dealers told these people there was no problem.

The higher powers on low contrast objects lose contrast because the object gets much too dim. The details get less visible rather than more. Dim and big often shows less detail than smaller and brighter. You should try several powers on each object each night to see what works best at that time. On large objects like the North American Nebula, higher powers will lose the entire object because you cannot see the entire nebula against darker sky background. In the same way, details on the moon's seas are lost when the sun is high at the feature. This is because there is no contrast between their background and the brighter regions around the seas.

What is the lowest power to use? The lower the power, the larger the field. For large objects like large star clusters and portions of the Milky Way, wide field, low power views will show the most. But there is a lower limit depending on the size of your eye's pupil in the dark night. If the size of the light leaving the telescope (called the exit pupil) is larger than your eye pupil, you lose light. For a telescope, the exit pupil is about equal to the diameter of the telescope divided by the power. Because your eye pupil at night averages about 7 mm, the exit pupil of this size gives you the most light. This comes out to about 3.5X per inch of telescope.

Which eyepiece type should I use? There is no universal eyepiece for every task. Some people have trouble understanding this considering how awfully much some eyepieces cost. There must always be a compromise with some of the choices. An Ultra-wide angle eyepiece may be the best for wide field views of deep-sky objects, but it is not a good choice at all to see details on the planets - and that includes the very expensive ones. With each purchase of an eyepiece, think about what you are going to view with it. That should help you pick a good eyepiece for a specific purpose.

We have a number of types of eyepieces we stock for different types of astronomical observations. Discussions at the top of the descriptions of each eyepiece series give details on recommended use. The ultrawide eyepiece series is fantastic for deep-sky work. Our planetary series is recommended for cases where resolution of subtle details is required. Our Plossl eyepieces are probably the best general purpose eyepiece. The Enhanced Super-wide series fills a gap between the ultrawide series and high to medium power general purpose eyepieces.

What is the field of view? There are two different fields of view defined. One of these is the apparent field of view (AFOV). This is the angle your eye appears to view when it looks through the eyepiece. Most of the Knight OWL Plossls have an AFOV of about 52°. This is about the maximum for an eyepiece with four-element in two groups. The longest focal length eyepieces in some cases have a smaller AFOV than expected due to the fact that the eyepiece mount restricts the field of view. The 40 mm 1.25 inch eyepiece has about 43° AFOV. The 50 mm 2 inch eyepiece has about a 50° AFOV because of the limit of the 2 inch focuser. Other eyepiece types that we carry have an AFOV of 60°, 65°, 70°, and 80°. These eyepieces make it easier to see more of the larger objects and still have a higher power.

The other field of view is the True Field of View (TFOV). This is the angle of the sky you actually observe. To find the TFOV, divide the AFOV by the power. In other words at 50X with a typical Plossl eyepiece, you are seeing a TFOV of about 1° which is about twice the angle of the moon in the night sky. As mentioned before, the lower the power, the greater the field of view. On some models the AFOV is listed on the eyepiece body. With each of our eyepieces, we list the AFOV.

What is eye relief? The eye relief is the distance from the back lens of the eyepiece to where the image forms. For a standard four-element Plossl design, the eye relief is always a little shorter than the focal length of the eyepiece. For some other types of eyepieces (i.e. Enhanced Superwide and Planetary), it might be much longer. In general a long eye relief can be easier to use especially if you use glasses when you observe. I personally am very uncomfortable with my eye forced down into the eye cup.



Brand New photo of Super Nova in M101.
Taken by Pete Brooks 8/30/2011

Photo by Pete Brooks - Enhanced by Tom Roginski
Original photo had a satellite moving across.



Last Updated September 28, 2011

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