EYEPIECES : Basic Knowledge.


     Eyepieces usually consist of several lenses or elements combined to produce a clear, sharp field of view. The arrangement and number of these elements can vary with the design or type of eyepiece. Higher end eyepieces generally produce better overall views in terms of sharpness and contrast. Some of the types of eyepieces you may encounter are Plössl, Orthoscopic, and Nagler.


Barrel Size

     The bottom part of an eyepiece that fits into a telescope’s focuser is called the barrel. It is usually silver in color and they come in three standard sizes of .965”, 1.24”, and 2”. The .965” barrel is usually found on older telescope models and is generally not in use today. Be sure to check the barrel size of a new eyepiece to confirm that your telescope’s focuser will accept it.




- You generally get what you pay for when it comes to purchasing eyepieces. Choose fully multi-coated eyepieces for better light transmission.


- Eye glass wearers should look for long eye relief. Telescopes can compensate for near-sighted or far-sighted vision simply by focusing.


- It’s ok to go the budget route as long as you understand that there may be better options available. (Don’t get turned off by perceived crappy views.)


- Eyepieces usually consist of multiple lenses or optical elements. The types of eyepieces vary based on the internal arrangement of these elements.


- Eyepiece barrel diameters come in three standard sizes of .965”, 1.25”, and 2”. The most common in use today are 1.25” and 2”.


- The ‘mm’ number on an eyepiece is the eyepiece focal length in millimeters. The smaller the focal length, the higher the magnification.


- Magnification is given by dividing the telescope focal length by the eyepiece focal length.


- A Barlow lens will double(2X) or triple(3X) the magnification of an eyepiece. There is a limit to how much magnification can be useful.


- Keep a range of eyepieces in your eyepiece collection. Low power 25-30mm, medium power 10-15mm, and high power 3-4mm.


- ‘Try before you buy’ at star parties by talking to telescope owners and looking through their eyepieces.


EYEPIECES : What should I buy?


     The kind of eyepiece you choose can have a baring on the quality of the image you see through your telescope. This is why its so important to think about investing a little money in your eyepiece collection when it comes to adding to it. Expect to spend anywhere from $25US for a low end eyepiece to upwards of $1000US for a very high end eyepiece. The price is usually based on the overall quality, which may include the design of an eyepiece’s internal optical elements, the detail that goes into it’s manufacture, and performance of the eyepiece. You don’t necessarily need a $1000 eyepiece to enjoy the views through your telescope. Seeing conditions are the great equalizer. No matter how much money you spend on an eyepiece air movement between you and an object can null any benefit. Having said this there are still valid reasons to spend a little extra on quality.


Amateur Astronomy for Beginners

Multi-Coated Surfaces

     Its important to look for the words FULLY MULTI-COATED when choosing an eyepiece. Fully multi-coated eyepieces mean that all of the surface to air parts of each element within the eyepiece have multiple coatings to make the transmission of light as smooth as possible with little reflection. Some eyepieces are only multi-coated on one surface to air part with single coatings on all other surfaces. These eyepieces may say multi-coated but are not considered FULLY multi-coated. If you find that a fully multi-coated eyepiece is not in your price range, its ok to go the budget route as long as you understand there are better options available.


Observers Wearing Glasses

     Observers wearing glasses should look for an eyepiece with a long eye relief. A longer eye relief means that you don’t have to put your eye as close to the eyepiece to get the full view. If your vision is near-sighted or far sighted you can remove your glasses and simply focus the telescope to compensate. Focusing the telescope cannot compensate for astigmatism.


Try Before You Buy

     There is a way you can ‘try before you buy’. Look for a star party to attend at a local astronomy club near you. The larger the event the better. You can ask the attendees with telescopes about their equipment and even snag a view through their telescope. This way you can get first hand experience with a range of eyepieces and telescopes before you think about buying one yourself.



     Eyepiece focal length is expressed in millimeters and can usually be found printed on the side of the eyepiece. A smaller focal length number indicates a shorter focal length and a higher magnification. A larger focal length number indicates a longer focal length and lower magnification. You can use the eyepiece focal length to find magnification if you know the focal length of the telescope you are using it with. Simply divide the telescope’s focal length by the eyepiece focal length and you will get the magnification.


Telescope Focal Length (F) = 1200mm

     Eyepiece Focal Length =   25mm

1200 ÷ 25 = 48X


Barlow Lenses

     Barlow lenses can extend the magnification of a given eyepiece by inserting them between the focuser and eyepiece. They can come in a 2X or 3X version. Go for the odd number 3X Barlow to avoid duplicating magnifications in your eyepiece collection with a 2X Barlow.


Telescope Focal Length (F) = 1200mm

     Eyepiece Focal Length =   25mm

               Barlow Lens =    2X

1200 ÷ 25 = 48X


48 X 2 = 96X


Useful Magnification

     There is a limit to how much USEFUL MAGNIFICATION a telescope can provide. Magnifying an object also reduces its brightness. Most deep space objects are generally best viewed with low power in small telescopes since they are inherently dim. Bright planets however can take a fair amount of magnification, upwards of 300X in small scopes. After a certain point however, there is just not enough resolution to justify more magnification. Anything more will simply magnify a blob. Larger telescope apertures can increase useful magnification but just the same astronomical seeing conditions can limit it.



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 Amateur Astronomy    Focal Length

 Astronomical Seeing  Magnification

 Barlow Lens          Optical Coating

 Eye Relief           Star Diagonal

 Eyepiece             Star Party

 Exit Pupil           Vignetting



      Barlow lenses can be great when it comes to low budgets and they can work well. However my personal recommendation is that you work towards keeping a range of eyepieces in your collection. Using a budget Barlow with a quality eyepiece can magnify the image but it can also take away from the quality of view.


LOW        MEDIUM        HIGH

25-30mm     10-15mm       3-4mm