Eyepieces & Accessories

All eyepieces are marked with their focal length in millimeters (mm).  The focal length of the eyepiece, along with the focal length of your telescope, determines the magnification of the image.

To figure out the magnification of any given eyepiece, just divide the focal length of your telescope in mm by the focal length of the eyepiece, also in mm.  For instance, a 10 mm eyepiece used in a telescope with a 1,000 mm focal length will yield 100X magnification. 

Choose a selection of eyepieces that will cover a variety of magnifications and fields of view (see below) for the types of objects you like to view best, and don’t forget to count the eyepiece you received with your telescope.  Most astronomers have from 3 to 5 eyepieces and a Barlow lens in their collection.  Read our Telescope Eyepiece Guide for a lot more info on focal length and magnification!

It would be nice if we could magnify an object enough to see, say, the US flag on the Moon, but Earth-based telescopes run into a little barrier when attempting to achieve clear views at extreme magnifications, and that little barrier is our atmosphere.  The Earth’s atmosphere allows us to breathe, which is a good thing, but it also causes distortion.  The level of distortion, or seeing conditions, varies all the time depending on the current weather, air currents and the geographical location you are observing from. In typical seeing conditions, therefore, the highest magnification eyepiece in your viewing arsenal should produce no more than 40X to 50X per inch of your telescope’s aperture, up to about 300X.  That means if you are looking for a high power planetary eyepiece for a telescope with 4.5” (114 mm) of aperture, the eyepiece should not produce magnifications higher than 225X if you calculate using the guideline of 50X per inch of aperture.  At 40X per inch, that number would be 180X magnification. Learn more about Seeing Conditions!

The eyepiece field of view is often listed on the eyepiece itself, but not always.  Still, you can easily find it in any eyepiece’s specifications.  This is the measurement most eyepiece manufacturers use when listing an eyepiece field of view.  It describes, in degrees, the calculated angular diameter of light you can see through the eyepiece before it is inserted into the telescope. The simplest designs available today, such as Plossl eyepieces, have a narrower field of view that is well suited for use with the planets or the Moon.  Eyepieces with a wider field, like many Celestron eyepieces, Tele Vue eyepieces, or Explore Scientific eyepieces, are perfect for large deep-sky objects such as galaxies or nebulae.

An eyepiece barrel is the chrome portion of the eyepiece that slides into the telescope focuser or diagonal.  When you see a reference to a 1.25” eyepiece, it is referring to the barrel size.  The standard, and most popular size eyepiece, has a 1.25” diameter barrel.  These eyepieces fit in 1.25” focusers and 1.25” diagonals, or in larger focusers or 2” diagonals when used with a 1.25 inch adapter. 

2” telescope eyepieces work with 2” focusers and 2” diagonals.  Eyepieces with 2” barrels are generally designed for low magnification views of faint deep-sky objects.  Their larger barrel and low magnification allow the most light possible to reach your eye; a good thing if you’re trying to see a galaxy or other celestial object at the far reaches of our universe. 

Lastly, there are telescope eyepieces available that have a hybrid 1.25 and 2 inch barrel, which means they will fit into either size focuser or diagonal.  You may also run across .965” eyepieces for sale from time to time, but they are old-school for the most part and will not fit into the majority of current telescopes unless you have the proper adapter.

Decide which eyepiece barrel size you need before you begin shopping for a new eyepiece.  If you are not sure,  you can find and read the specifications in your telescope instruction manual or online.  It is also easy to do a quick measurement of the eyepiece that came standard with your scope by turning it upside down and pulling a tape measure across the diameter of the opening in the barrel.  Odds are, it will be 1.25”.

Eye relief refers to how far your eye needs to be from the eyepiece in order for an image to come to focus.  Eye relief is measured in millimeters (mm).  Long eye relief eyepieces make observing more comfortable to all viewers but is especially important for those who wear glasses.  We recommend an eye relief of at least 10 mm for all viewers and at least 15-20 mm for those who want to, or need to, wear glasses.  People who have an astigmatism should keep their glasses on while viewing, but other folks can decide which works best for them…on or off.