Thinking About Your Eyepiece Options

In choosing eyepieces I highly recommend speaking to fellow astronomers, or posing questions in an astronomy forum such as Cloudy Nights, where you will also find a Reviews section for all manner of telescope accessories.

Understanding the many different types of eyepieces, and when they should be used can be very tricky, and you often end up with paralysis-by-analysis! It's better to do just a little research and then actually bite the bullet and buy something, even second-hand bargains can be picked up from the Classifieds and Auctions sections of sites such as And if you do make a mistake you can sell the eyepiece again. Eyepieces generally keep their value reasonably well provided they are maintained properly. Dry and store them properly, use their protective lens caps, keep dirty fingers off them (observe with clean hands), and if cleaning is really necessary, do it infrequently and learn the right way to clean eyepieces. Then you will not have any problem selling on if the EP was a bad choice for your scope.

While you are beginning astronomy, by all means research before buying, or join an astronomy club so you can view through different eyepieces one evening at a star-party to see what views they give. Unless you get new EP's and try them out you will not gain any experience. You will just be bogged down by critique and theory, and won't be able to "see" what to actually look for to better understand what makes an eyepiece good or bad.

Probably the first item I recommend you get to complement the two Orion EP's that come with the XT10, is a Barlow Lens of some kind, because this will instantly increase the magnitude and options of the optics you already have. Barlow lenses are defined by their power increase, e.g. commonly 1.5x, 2x, 2.5x or 3x. I decided to get the Orion Shorty-Plus 3 Element 2x Barlow.

The EP's Orion provide are basic but of quite nice quality 1.25" barrel Sirius Plossl's. Magnification is calculated as Telescope Focal Length (mm) divided by Eyepiece Focal Length (mm). The XT10 has 1200mm focal length.

With the scope you get:-

  • 1 x 25mm eyepiece, giving 48x magnification (1200 / 25 = 48).
  • 1 x 10mm eyepiece, giving 120x magnification (1200 / 10 = 120).

So if you buy a 2x Barlow you will effectively have 96x and 240x views at your disposal as well.

Plossl EyepiecesSirius Plossl Eyepieces

The 10mm and 25mm Sirius Plossl EP's are shown to the right (the basic plastic collimation cap is also shown). They come complete with top and bottom protective lens caps.

They are ideal for all types of telescopes: reflector, refractor, and catadioptric, and offer a 52 degree apparent field of view for clear, sharp images of impressively high contrast. The optics are fully anti-reflection coated with magnesium fluoride on all air-to-glass surfaces. The Plossl 4-element design is renowned for outstanding performance.

  • The 10mm (120x) is good for high-power planetary and lunar observation, although celestial objects will move quickly across the field of view, especially when Barlowed (e.g. 240x). The viewing lens of the eyepiece is a bit small and more care is required to line up your eye comfortably. One of the first things you will want to consider for your next high-power eyepiece is a wider angle field of view, because it will provide a more comfortable viewing field and increased viewing time before having to move the telescope again. Remember the Orion XT's are push-to dobsonians, which are very hands on. No motors to track the view. Just you chasing the planets!

  • The 25mm (48x) eyepiece excels for moderately low power scanning of open star clusters and viewing of extended nebulas and larger galaxies. This is a nice eyepiece to begin with, and gives a good middle-range view. I constantly use this even after purchasing other eyepieces.

Lens edges are blackened to reduce scattering of stray light and to maximize contrast. The lenses are mounted in black-anodized aluminum housings with a 1.25"-diameter chrome-plated brass barrel for durability and precision fit. They are such a snug fit in my Barlow that I can feel the air pressure cushioning as the eyepiece is pushed in. The eyepiece barrel is internally threaded to accept standard filters.

The built-in rubber eyeguard enhances viewing by helping to block out ambient light and to position the eye at the correct Exit Pupil distance.

When not in use during an observing session the eyepieces can be stored in the metal eyepiece holder bracket fixed to the base of the dobsonian mount, which places them very conveniently at hand ready for use. After a nights viewing I always store my eyepieces in a padded equipment bag, and keep them indoors. If the night was particularly cold or damp resulting in dew on the eyepieces, then I leave them for an hour or so without the lens caps on so that any moisture can dry off before storing them.

Next section: Orion XT10i Finder Scope

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