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When looking into a space telescope, the eye is placed up to the eyepiece, which helps determine the magnification of a lens. Telescope eyepieces can alter the focal length of the scope as well as determine the apparent field of view, the width of what you can see, when looking through the space telescope. 

As a general rule, the focal length of the space telescope, divided by the space telescope eyepieces’ focal length will give you the power of the lens. For example, a space telescope with a focal length of 500 used with a 10mm eyepiece, would result in a space telescope to make objects appear 50 times closer. A 25 mm focal length on an eyepiece will drop the power to 20X and a 2.5mm, one of the smallest commercially available eyepiece sizes would render a 200X space telescope. 

With space telescope power, bigger is not always better as the field of view is also affected by the eyepiece, as well as the eye relief measurement to how well you can see through the space telescope. The field of view indicates the amount of sky that is visible when you look through space telecsope eyepieces. The stronger the lens, the closer the stars may appear, but the less sky you can include in the view. For example, a space telescope with a 100-power lens and a field of view of about 50 degrees will fit a typical full moon in the lens. 

Barrel Diameter Affects Field Of View 

Another feature of space telecope eyepeices is the diameter and while this measurement will have no affect of the magnification power, it will affect the field of view. If the lens has a long focal length and small barrel size, the view may be restricted to the point of finding images in the sky difficult. To better understand this concept try looking through a straw with the opposite eye closed and try to find a specific spot on the wall while standing about 10 feet away. 

Now, move to within a foot of the wall and look for the same spot. The field of view gets smaller the closer you stand to the wall. The same theory works when choosing space telescope eyepieces for your scope. Another important measurement is referred to as eye relief. Simply put, it is the distance from the eyepiece glass and your eye. If it is too close, your eyelashes may streak the glass and if too far away, your vision may not be able to focus properly on the lens.