Biomedical Engineering Reference
In-Depth Information
eyepiece on bottom). (When using this formula, you can keep the minus sign in it
or just remember that an uncompensated Keplerian telescope creates an inverted
image.)
The Keplerian telescope in Figure 12-7B is referred to as
an astronomical
telescope
.
13
Such a telescope would be inappropriate for clinical applications due
to the inverted image. With the incorporation of prisms or mirrors, however, an
astronomical telescope can be converted into a
terrestrial telescope
that produces
an erect image. An advantage of Keplerian over Galilean telescopes is that they may
provide the patient with a wider field of view, thereby allowing the patient to see a
greater expanse of the visual world (for a given amount of magnification).
An Alternative Formula to Determine
a Telescope's Angular Magnification
The distance between the lenses of a telescope—
the tube length
—can be used
in combination with the power of the objective to calculate a telescope's angular
magnification. From Figure 12-7 we can see that tube length, which we'll call
d
, is
calculated as follows
14
:
1
F
1
+
1
F
2
=
f
1
′
−
d
f
2
=
By substituting into the formula for angular magnification that we derived previ-
ously and rearranging, we have:
1
M
ang
=
1
−
dF
1
Later in this chapter, we'll see how this formula can be useful for understanding
magnification when a telescope is used to correct for ametropia.
A Convenient Clinical Method to Determine
a Telescope's Angular Magnification
The amount of light that enters an optical system is limited by the size of its
entrance pupil
. Light exits the system through its
exit pupil
. For most telescopes,
the entrance pupil is the objective lens. The exit pupil is the image of the objective
lens as seen from the eyepiece side of the telescope. A telescope's exit pupil is also
called a
Ramsden circle
. Entrance and exit pupils are discussed in more detail in
Appendix A.
13. Galilean telescopes may also be used in astronomy.
14. Keep in mind that for the negative eyepiece in a Galilean telescope
f
2
has a positive value, while for
the plus eyepiece of a Keplerian telescope
f
2
has a minus value.
