National Geographic STAR APP50 Instruction Manual page 15

Eyepiece Lens
Eye Relief
Eye relief is all about a comfortable viewing experience because it is the distance at which you
need to position your eye from the eyepiece's outermost surface to enjoy the full field of view. This
characteristic is of special concern to observers who wear glasses to correct an astigmatism, because a
long enough eye relief is necessary to allow room
for glasses.
Huygenian Eyepiece
2 Lens Elements
Eyepiece Lens
Light
Huygenian Eyepieces:
A Huygenian eyepiece uses two plano-convex lenses separated by an air gap. They have a fairly narrow
apparent field of view.
Kellner Eyepieces:
A Kellner eyepiece uses three lens elements - two of which are paired together in an achromatic
doublet design to minimize chromatic aberrations. They typically produce an apparent field of view
around 45°.
Plössl Eyepieces:
A Plossl eyepiece uses two doublets (a pairing of lens) for a total of four lens elements. This eyepiece
design delivers sharp views and an apparent field of view of approximately 50°, which works well for
both planetary and deep sky viewing.
Barlow Lens:
A Barlow lens effectively increases the focal length of a telescope. It is inserted between the eyepiece
and the focuser/diagonal (depending on the optical setup) and multiplies the magnification power of
the eyepiece.
For example, a 2x Barlow will double the magnification of a particular eyepiece.
Short Eye Relief Distance
Long Eye Relief Distance
Kellner Eyepiece
3 Lens Elements
Eye Lens
Field Lens
Light
Eyepiece Lens
Long Eye Relief Distance
Eyepiece Lens
Plössl Eyepiece
4 Lens Elements
Light
15
Exit Pupil (mm)
Eyepiece Lens
Barlow Lens
2 Lens Elements
Eye Lens
Field Lens
Field Lens
Light