• Eypiece 101
  • Add on Aug. 2, 2017, 3:55 p.m.



Eyepieces are the most critical factor concerning the performance of your telescope.

The eyepieces creates the image your eyes see, and the right ones are the ones that will make your astronomy experience more rewarding. under are some questions people have asked and we think would help answer some beginners questions

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    ★What is an eyepiece?

An eyepiece is a magnifier, much like a high power magnifying glass. When placed at the real image made by the lens or mirror of a telescope, the eyepiece projects a virtual image into your eye, enabling you to see the target.


   ★How to choose the correct eyepieces for scopes?

Even the best instrument will never achieve it's full potential visually with poor oculars (aka eyepiece). Since most manufacturers sell their telescopes with inexpensive eyepieces, borrow as many eyepiece as you can and try them out before purchasing; for every object there will be an eyepiece that works best with your particular telescope.


    ★Which eyepiece should I choose?

If brightness is not a factor, choose the eyepiece that will encompass the object, then allow for a suitable backdrop. If you want to know the actual field on view the eyepiece will give (True Field), this can be calculated as:


    ★How many eye pieces should I have?

You'll probably be satisfied with 5-8 good eyepieces; and you will be able to use your telescope to it's maximum potential with a good one.


    ★Which eyepiece design is best?

This often asked question is quite irrelevant, as different design's performance varies with different telescopes. Different eyepiece designs have various characteristics. For example, and expensive widefield design is not required for planetary viewing, where the only important thing is maximum contrast. A Plossl or Orthoscopic would probably be best, but almost all design s are good performers on-axis for any f/ratio. Telescopes with F/ratios>10 are quite tolerant of simple low element eyepieces up to 55 deg. A.F., but telescopes <6 are a different matter. Off-axis performance requires powerful correction to properly image the highly convergent beam. Each eyepiece and telescope performs as a system, and their image can only be evaluated as much.


    ★The image with my low power eyepiece is clear, but my high power is fuzzy. What's wrong with it?

There's probably nothing wrong with the eyepiece: you have probably exceeded the resolving power of your telescope. A television set looks clear 10 metres away, but up close you can see the imperfections.


   ★Why are some eyepiece more expensive than others?

When you pay more for an eyepiece you are usually paying for: Field of view: Eyepieces that have many lenses to correct for the five major aberration (these aberrations give increasingly worse, the lower the focal ratio of the telescope) have obviously higher costs in lenses and coatings. Eye relief: Using larger, more expensive elements in eyepieces allows for a greater distance between the eyes and eyepiece. Coatings: 2-layer multicoatings on both faces of all lenses will typically add 25% to the cost of an eyepiece, but this is absolutely necessary to preserve the contrast of the image when the light has to go through 7-9 lenses. Advertising: Those ads aren't free.


    ★What is the black spot I see in a low powered eyepiece in my reflector during daylight?

A low powered eyepiece in a reflector produces a large exit pupil with a large image of the secondary mirror obstruction. During the day, when the pupil of the eye is small, if the size of the secondary obstruction image approaches the size of the pupil, it will appear as a darkened region in the center of the field. At night, when the pupil of the eye is large, the darkened region is not noticed.


    ★How do I calculate FOV (Field of View)?

Apparent Field of View (Usually given by the manufacturer) ÷ Magnification Power = Actual Field of View


    ★What's the difference between Super eyepiece and Super plössl eyepiece?

Plossl eye pieces gives a wider apprant field of view
Super are the standard eyepieces that usually comes with market sold telescope sets.


    ★What are wide angle eyepieces? | What are W eyepiece?

The Skywatcher Panorama Eyepiece set, W6~20mm are ultra wide angle eyepieces with apparent field of view more than 60°.


    ★Which works better? An eyepiece or a Barlow+eyepiece giving the same magnification?

The only time the eyepiece alone may perform as well, is on-axis, in a high-contrast application, as the extra optics of the barlow may cause a slight depreciation. Optically, for all other sues, the eyepice+barlow outperforms the eyepiece working alone. The reason? Most of the aberrations caused by positive spherical lenses (Coma, Astigmatism, Curvature of Field and Spherical Aberration) can be reduced and sometimes almost eliminated by introducing a negative system (barlow) which has the same aberrations in negative quantities! Spherical aberration of the system is reduced as the positive spherical aberration of the eyepiece is cancelled by the negative spherical aberration of the barlow. The other aberrations cancel in a similar way! This is one of the eyepiece designer's most powerful weapons, and it is used in most of the shorter focal length ultra-wide designs. Another great benefit of this idea is that the longer eye relief of the longer f.l. eyepiece used with the barlow is retained.


    ★Will a telescope work without an eyepiece??

Not for visual purposes, as the eye cannot process the real image made by the objective. The telescope may be used without an eyepiece for camera and other instruments.


   ★How important it is to get a parfocal series?

Parfocal eyepiece sets reduce the amount of refocusing when changing powers, but it is rare when no refocusing is required. Parfocallizing of eyepiece sets is a non-performance factor when choosing oculars.