Biomedical Engineering Reference
In-Depth Information
TABLE 12-3.
COMPARISON OF GALILEAN AND KEPLERIAN TELESCOPES
Characteristic
Galilean
Keplerian
Objective
Positive
Positive
Eyepiece
Negative
Positive
Image orientation
Erect
Inverted or erect*
Location of exit pupil
Within tube
Outside of tube
Field of view
Less
Greater
Tube length
Shorter
Longer
Shape
Straight
May be bent*
Weight
Generally lighter
Generally heavier
*Image-erecting systems (e.g., prisms or mirrors) are included in Keplerian systems to create the terrestrial systems
that are used for clinical purposes. This may result in a bent tube.
Telescope Use in Ametropia
When using a telescope for distance viewing, a person with spherical ametropia has
basically two choices. She can wear her distance prescription or she can adjust the
tube length so that the vergence emerging from the telescope is equal to her far-
point vergence. When wearing her distance prescription, the angular magnification
is equal to that of the telescope. If, for instance, a patient with 10.00 D of myopia
wears her spectacles when looking through a Galilean telescope that consists of a
+
10.00 DS objective and
−
50.00 DS eyepiece, she experiences an angular magnifi-
cation of
+
5
×
.
Suppose the patient prefers to look through the telescope without her spectacles
and to adjust the tube length instead. What would be the tube length? What
magnification would she experience?
The easiest way to answer this question is to divide the eyepiece into two compo-
nents: one that corrects the patient's ametropia and the remainder that contributes
to the telescope magnification. Since the patient has 10.00 D of myopia,
10.00 D of
the eyepiece's power is used to correct this ametropia and the remaining
−
−
40.00 D
contributes to the telescope.
F
2
F
1
M
ang
= −
40.00 D)
(10.00 D)
We can also solve this problem using the alternative formula for telescope
magnification. Since the tube length must be shortened from 8.00 cm (i.e.,
(
−
M
ang
= −
= +
4.0
×
