There has always been a lot of misunderstanding about what power your telescope can handle. This brief introduction will try and help answer some of your questions.

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 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.

What is the highest power to use? On any night, unsteadiness in the air might limit 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 a high contrast object like a double star, you can use up to about 100X per inch of telescope under the best of conditions. For low contrast details like the planets, about 40X to 50X per inch is a maximum. On nights where the atmosphere is very unsteady, it may not be possible to use even 25X per inch.

The higher powers on low contrast objects lose contrast because the object gets too dim. 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 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 usually a little shorter than the focal length of the eyepiece. In general a long eye relief can be easier to use especially if you use glasses when you observe. When you use a very long focal length eyepiece, such as a 40 mm focal length, you might have to get adjusted to the long eye relief. The 40mm has about a 32 mm eye relief which is about 1-1/4". Your eye must be about 1-1/4" behind the top of the eyepiece to see the image. For very short focal length eyepieces such as the 4 mm Plossl, the eye relief can be too short to use glasses with the eyepieces. For any Plossl eyepiece, regardless of the brand or price, the eye relief is about equal to 3/4 of the focal length - 3 mm for 4 mm Plossl. Try taking the rubber eyecup off in these cases. It almost feels that your eye must be inside the eyepiece with these high power eyepieces. This is another problem with using higher powers than are necessary. The wide field eyepieces we carry also have great eye relief at the higher powers. This is another major advantage with their use.

If you have any other questions, give us a call. What you will see is not as much a factor of the size of the telescope as much as it is the amount of experience you get as an observer.