Mon, Jul 02, 18

A Guide on Choosing The Right Telescope Eyepieces

A Guide on Choosing The Right Telescope Eyepieces

The optical elements of Eyepieces allow you to focus light collected by a telescope, so you can observe a sharp view of the object or area where the telescope is pointing. It may seem like a small link in the chain, but it has a large effect on your telescope's optical system, and finding suitable eyepieces will greatly enhance its potential. 

With so many options to choose from, selecting the right set of eyepieces for you and your telescope can seem a little tricky. This guide offers some insight and explanations on different eyepiece types, specifications, and how it all ties together to optimize your astronomy and astrophotography sessions!

Focal Length and Magnification

Focal Length is an important specification to consider when determining the magnification, also known as power, of the combination of an eyepiece and the telescope it is being used with. The following formula will help you determine the magnification based on your eyepiece and telescope's specifications:

Magnification  = Telescope Focal Length (mm) / Eyepiece Focal Length (mm)
 
For example:
  • A 20 mm eyepiece on a 2000 mm telescope (2000/20) gives you 100 power (100x). This makes objects appear 100 times closer to you through the telescope than they appear to your unaided eye. 
Note: When using your telescope at different powers, you generally have a choice of a small, sharp, and bright image at lower magnification; or a larger, yet blurred and dim image at higher magnification. The reason being, that the telescope gathers a fixed amount of light, and at higher magnifications, the same amount of light is being spread over a larger area, resulting in a dimmer image.

    Field of View: Apparent and True

    An eyepiece's Apparent Field of View (AFOV) is expressed in degrees (°). It is how much of the sky is seen edge-to-edge through the eyepiece alone. AFOV's range from narrow (25° - 30°) to an extra-wide angle (80° or more).

    An eyepiece's true field of view is the angle of sky seen through the eyepiece when it's attached to the telescope. The true field can be calculated using the following formula:

    True Field = Apparent Field / Magnification

    For example, suppose you have an 8-inch Schmidt-Cassegrain telescope with a 2000 mm focal length, and a 20 mm eyepiece with a 50° apparent field. The magnification would be 2000 mm / 20 mm = 100x. The true field would be 50\100, or 0.5° - about the same apparent diameter as the full moon.



     
    Eye Relief and Corrective Lenses

    Eye Relief refers to the distance between your eye and the eyepiece lens when the image is in focus. Eye relief is traditionally in proportion with focal length: The shorter the focal length, the shorter the eye relief. However, some of the more modern eyepiece designs provide long-eye relief regardless of focal length, which is especially beneficial to those who wear glasses. If you like to keep your glasses on while using a telescope, the eye relief of an eyepiece is an important specification to consider (we recommend looking at long-eye relief eyepieces). 

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    2 mm - 4.9 mm Eyepieces

    These produce very high magnifications and work best on long focal length refractors and Schmidt-Cassegrains. Unless you have very steady seeing conditions, this range more than likely will produce too much magnification for other telescope types.

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    5 mm - 6.9 mm Eyepieces

    These make good planetary detail and double star eyepieces for long focal length telescopes and will work satisfactorily in shorter focal length telescopes with steady seeing conditions.

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    7 mm - 9.9 mm Eyepieces

    Ideal high magnification eyepieces for shorter focal length telescopes, and serve as good planetary, double star and lunar detail units.

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    10 mm - 13.9 mm Eyepieces

    Good to use across all focal lengths and offer great background darkening capabilities for studying planetary nebula, small galaxies, planetary details and lunar details.

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    14 mm - 17.9 mm Eyepieces

    A great mid-range magnification for all focal lengths and helps resolve globular clusters, galaxy details, and spot planetary nebulae.

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    18 mm - 24.9 mm Eyepieces

    Works nicely on long focal length telescopes to show wide field and extended objects. Shorter focal length telescopes will enjoy great mid-range magnification of galaxy clusters and large open clusters.

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    31 mm - 39.9 mm Eyepieces

    Longer focal lengths are good for large nebula and open clusters. Shorter focal lengths, are great for large objects such as the Orion nebula, views of the full lunar disc, large open clusters, and more. It also makes for good "locator" eyepieces in all focal lengths.

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    31 mm - 39.9 mm Eyepieces

    These are well suited for shorter focal length telescopes for extended views and large, starry fields.

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    40 mm Eyepieces

    These are exclusively the domain of shorter focal length telescopes. This magnification range is superb for showing large, starry vistas as well as extended nebulae with star fields and etc.

    How Exit Pupil Relates to Power

    Exit pupil refers to the size of the bundle of light rays coming out of the eyepiece. Exit pupil size (in inches) can be calculated by:

    Exit pupil size (mm) = Telescope aperture (mm) / Telescope magnification
    or
    Exit pupil size (mm) = Eyepiece focal length (mm) / Telescope f-ratio

     

    In order for all the light rays to enter your pupil, the exit pupil must be smaller than the pupil of your eye. A young person's fully dark-adapted eyes may have 7 mm-wide pupils. As you age, the maximum pupil diameter decreases. For middle-aged adults, the practical maximum is closer to 5 mm.
    At the other end of the scale, at magnifications that yield an exit pupil in the range of 0.5 mm to 1.0 mm, empty magnification begins to set in, depending on the quality of your telescope and your eyes. In other words, this much magnification really starts to degrade the image you see.

    How Many Eyepieces Do I Really Need

    Although there is no specific number of eyepieces you should own, with a few different telescope eyepieces, you have a better chance of hitting the optimal power for the particular object you are observing, given the sky conditions at the time. Usually, you'll want to start with low power (i.e., long eyepiece focal length, such as 25 mm or 30 mm) to get the object in the field of view of the telescope. Then you might want to try a slightly higher-power (shorter focal length, maybe 18 mm or 15 mm) eyepiece and see if the view looks any better. If it does, swap in an even higher-power eyepiece and etc., until you hit that "sweet spot" where image brightness, image scale, and the amount of visible detail combine to form the most pleasing view. 

    What about Barlow Lenses?

    You can also choose a long focal length eyepiece with comfortable eye relief and use image amplifiers to increase power, such as a Barlow lens. A Barlow increases the effective focal length of an objective lens, increasing the magnification. The idea is that 2 eyepieces and a Barlow will give you the flexibility of magnification of 4 eyepieces, and will give higher magnifications with less powerful eyepieces.

    Using different eyepieces can profoundly increase the versatility and functionality of any telescope. While shopping for eyepieces, remember these basic tips:

    • Consider the focal length of your telescope, or telescopes, to make sure the eyepiece will provide an appropriate magnification to suit your needs.

    • If you wear eyeglasses while using a telescope, pay attention to the eye relief specification of different eyepieces, as ample eye relief can improve comfort and ease-of-use while wearing corrective lenses.

    • Depending on your observing goals, consider the apparent field of view of your eyepiece choices.

    • If versatility is paramount, consider a zoom eyepiece or Barlow lens to increase the number of possible magnifications to use.


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    9 comments

    • Del Johnson

      Eyepiece focal lengths are selected by telescope f/ratio, not by telescope focal length. For a given telescope aperture, 40mm eyepieces would be used by telescopes with longer focal lengths, not shorter. Your advice for ultra short forcal length eyepieces is also reversed, and their use is not tied to telescope design as you described. Ultra short focal length eyepieces are definitely not tied to the common f/10 SCT.

    • Alan Sanchez (OPT)

      Hello Jim,
      For your first question regarding the “Corrective Lens” section, that is referring to the eyepiece focal length.

      For the second question, eyepiece focal lengths and diagonals essentially only function as connecting pieces. Nearly all diagonals these days are of excellent quality and you can rest easy know they won’t negatively affect your viewing. And by upgrading to 2-inch you’ll open up access to much larger focal length eyepieces!

      As for choosing the right one for you, I highly recommend getting in contact with one of our sales experts at internetsales@optcorp.com or opening a chat box on the bottom right of your screen here. They will set you up for success, I promise you that!

      Thank you for your question!

    • Jim

      Another question, how do the eyepiece focal lengths and diagonals work with each other. I’m considering upgrading to a 2-inch diagonal…I wear glasses and want to get a couple nice eyepieces that won’t break the bank. Also don’t want to waste money on things that I’m not going to be happy with.

    • Jim

      Nice article. Under the “Corrective Lens” Section, are those lengths (accompanied by example photos) referring to the eye relief distance or the eyepiece focal length?

    • Jason

      Thank you for braking the different types of eyepieces down so someone new (like me) to telescopes know what to look for in eyepieces.

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