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Eyepiece Calculator:

The following table shows all the club eyepieces available with both the 10 and 12 inch club telescopes. 
However, you can clear that data and enter in your own telescope and eyepiece details to see what they give you.

The 1.6 barlow value below represents the barlowed binoviewer that goes with the 10" scope, you can put your own barlow value in there, common values would be 1.5, 2, 3, no barlow would be a value of 1.

Aperture and focal length should be in the instructions of your scope, the focal length of the eyepiece is usually on a sticker or engraved onto the eyepiece.

The eyepieces apparent field of view (aFOV) is as particular to an eyepeice as its focal length, if its not printed on the eyepiece then it can usually be found on the web, at the bottom of this page is a list of aFOV's for some of the most popular eyepieces. The aFOV is the width in degrees of the field as seen through just the eyepiece alone. If you have two eyepieces with the same focal length, the one with the larger apparent field of view will show more of the sky if inserted into the same telescope. This parameter is determined by the design of the lenses inside an eyepiece.

The exit pupil is the size of the light cone leaving the eyepiece, if it's above 7mm then you're effectively stepping down the aperture of your scope, if its very low (<1mm) then the eyepiece can be difficult to see into. The lenses in an eyepiece form an image that floats in midair just outside the lens closest to your eye. When you observe you place your eye so that it can see this exit pupil image. If all is going as planned, the image size will fit with room to spare within your eye. The size of this image is the exit pupil

The True Field of View (FOV) is measured in degrees, the bigger it is, then the wider the image will be. A small true FOV would  look like youre looking through a long narrow tube, a very wide FOV can give you an almost spacewalk experience. The true FOV is the field of view of the entire telescope system, including the eyepiece 

The maximum magnification given below is the absolute maximum the optics can do, the practical maximum is often alot less then this because of atmospheric seeing, miscollimation and warm optics. Going below the minimum magnification will effectively reduce the aperture of your telescope.

Enter the focal length of your scope in millimeters (FLs)


Enter aperture of scope  (APm)



  Calculated focal ratio         
  Minimun magnification     x
  Maximum magnification   x
  Maximum resolution        arcseconds
Limiting  magnitude          (approx)

Barlow Power

(Enter Barlow Magnification)

Eyepiece Type/Description
(replace with your own)
Eyepiece Focal Length (FLe)
Eyepiece Apparent Field of View  (aFOV) °
Exit Pupil
Eyepiece True Field of View ° (T=aFOV/M)
Barlow Factor

Click to see apparent fields of view for common eyepieces:


Here is a list of definitions of some other words used in connection with eyepieces:

Curvature of field -- good eyepieces provide a field of view which is flat. The focused image should be sharp from edge to edge. Star fields are a tough test of this characteristic. If your eyepiece suffers form this problem, the view through the eyepiece might look like you're viewing the sky through a curved piece fo glass.

Distortion -- good eyepieces also have little distortion, this means if you viewed a piece of lined graph paper that all the lines would be straight and would cross at right angles. Distortion can be a problem for only a small section of the field of view, but curvature generally happens to the entire field of an eyepiece. Pin cushion distortion is a particular form of distortion that can occur in some widefield eyepieces.

Eye relief -- the distance from the eye lens to your eyeball. This value is important to eyeglass wearers. If you need to have your glasses on to view the sky, there must be plenty of eye relief so that your eyeglasses will fit between the eyepiece and your face. Those of us who don't wear glasses to observe generally like some eye relief to avoid the feeling that I am jamming my eye lens against the glass lens of the eyepiece

Focal length -- the apparent distance from the lens to the object being viewed, in this case the image formed by your telescope. Long focal length eyepieces show a large portion of the image being viewed and short focal length eyepieces will allow a small section of the image to be inspected. This is how you choose the magnification of your optical system. Pick out a long focal length eyepiece, say 40mm to 24mm, and the system will give a wide field and low power. Select a short focal length eyepiece, around 8mm to 4mm, and you will get a high power, small field of view look at whatever is in the scope.

Ghost images -- in poorly made eyepieces some of the light from a bright star can reflect about within an eyepiece and form faint images within the field of view. These ghost images can be subdued by multicoating the lenses in the eyepieces. Only the cheapest eyepieces nowadays are not coated to suppress this problem.