24
3 The Telescope
• The aperture size is the diameter of the telescope and is a measure of its lightgathering power. Several techniques in telescope design may result in the optical
aperture being, in effect, smaller than the physical aperture.
• The focal length (f ) is the distance separating the first optical component and the
first point of focus.
• The field of view (FOV) is the amount of sky visible through the instrument,
measured in degrees or arc minutes. The angular field of view of a telescope (in
radians) is the aperture of the eyepiece (the field stop) or the diameter of the
detector f o divided by the focal length f :
f ov =
f o
f
× 206,265
(3.1)
• Magnification is the ratio between the angular size of an object observed through
the telescope its angular size when it is observed without the telescope. For unresolved objects such as stars, the magnification has little or no effect. For smaller
consumer telescopes, high magnifications are often quoted. In practice, high magnifications are rarely used in astronomy and generally cause more problems than
they are worth. If you are using an eyepiece, the magnification is simply the ratio
of the focal length of the telescope, f , and the focal length of the eyepiece, f e ; if,
however, you have an astronomical camera at the focus, then the nearest approximation to magnification is the image or plate scale. This is merely the focal length
of the telescope, f , over the diagonal diameter of the light-sensitive chip in the
camera (the CCD):
Mag =
f e
f
Plate Scale =
f
f o
.
(3.2)
• Another often quoted feature of telescopes is the f number. This is simply the focal
length f divided by the optical aperture size, A. In general, telescope with low f
numbers are called fast, because they gather light quickly due to their large field
of view. However, the downside is that they have reduced resolution. Additionally,
optics for fast telescopes tend to be very expensive to manufacture in comparison
to those with high f numbers:
F =
f
A
.
(3.3)
• The angular resolution of the telescope (α c ) is the telescope’s ability to separate
two objects; it is given by the average wavelength of the observed spectrum divided
by the aperture of the telescope D 0 . This is the diffraction limit caused by the
formation of the Airy disc, which is produced by the diffraction of light within the
optics:
α
c =
1.22λ
D o
× 206,256.
(3.4)
3 The Telescope
• The aperture size is the diameter of the telescope and is a measure of its lightgathering power. Several techniques in telescope design may result in the optical
aperture being, in effect, smaller than the physical aperture.
• The focal length (f ) is the distance separating the first optical component and the
first point of focus.
• The field of view (FOV) is the amount of sky visible through the instrument,
measured in degrees or arc minutes. The angular field of view of a telescope (in
radians) is the aperture of the eyepiece (the field stop) or the diameter of the
detector f o divided by the focal length f :
f ov =
f o
f
× 206,265
(3.1)
• Magnification is the ratio between the angular size of an object observed through
the telescope its angular size when it is observed without the telescope. For unresolved objects such as stars, the magnification has little or no effect. For smaller
consumer telescopes, high magnifications are often quoted. In practice, high magnifications are rarely used in astronomy and generally cause more problems than
they are worth. If you are using an eyepiece, the magnification is simply the ratio
of the focal length of the telescope, f , and the focal length of the eyepiece, f e ; if,
however, you have an astronomical camera at the focus, then the nearest approximation to magnification is the image or plate scale. This is merely the focal length
of the telescope, f , over the diagonal diameter of the light-sensitive chip in the
camera (the CCD):
Mag =
f e
f
Plate Scale =
f
f o
.
(3.2)
• Another often quoted feature of telescopes is the f number. This is simply the focal
length f divided by the optical aperture size, A. In general, telescope with low f
numbers are called fast, because they gather light quickly due to their large field
of view. However, the downside is that they have reduced resolution. Additionally,
optics for fast telescopes tend to be very expensive to manufacture in comparison
to those with high f numbers:
F =
f
A
.
(3.3)
• The angular resolution of the telescope (α c ) is the telescope’s ability to separate
two objects; it is given by the average wavelength of the observed spectrum divided
by the aperture of the telescope D 0 . This is the diffraction limit caused by the
formation of the Airy disc, which is produced by the diffraction of light within the
optics:
α
c =
1.22λ
D o
× 206,256.
(3.4)