OF THE ROGER N. CLARK

and a low voltage source such as a car
battery. (Do not use the car's own battery
because after heating all night it might not
have enough power to start the car.)
SUMMARY
The purpose of a telescope is to gather light
and focus it to form an image. An eyepiece
magnifies the image and directs the light to
the eye, where it is refocused on the retina.
Two basic parameters govern a telescope's
ability to show an object: the amount of light
gathered (determined by the size of the primary
mirror or lens) and the total magnification
at the eye.
The magnification and the eyepiece's
apparent field of view determine the true field
of view on the sky. A short-focus telescope
(i.e. f/4.5) and a long focus instrument (such
as fl 15) of the same aperture will give essentially
identical views of deep-sky objects
when used at the same magnification with
eyepieces having the same field of view. The
short-focus telescope has an advantage in
that it can achieve low powers and wide fields
with standard eyepieces. The long-focus telescope
has - the advantage when working at
high powers, as they can be achieved with
eyepieces having reasonable focal lengths and
comfortable viewing positions for the eye.
Optical aberrations are also reduced.
The differences among telescope types are
not as important for viewing deep-sky objects
as for planetary or double-star observing,
since most of the limitations for deep-sky
work are in the human eye.
Even in perfect seeing conditions, an upper
limit to the useful magnification is set by the
resolution of both the telescope and the eye.
The eye can resolve about one arc-minute
when an object is bright, but resolution decreases
as the object becomes fainter, and at
the limit of detection, the eye's resolution is
only about 0.5°. In this situation very high
powers are necessary to magnify the finest
details a telescope itself resolves so that they
become visible to the eye.
For bright subjects, the magnification limit
VISUAL ASTRONOMY OF THE DEEP SKY
48
is often said to be about 60x per inch
telescope's objective diameter. (A
of
I
telescope is said to have a limit of600
However, for faint objects the limit is
much as 330 X per inch. The lesson to
member is to use however high a ..,.",,,,,,,,,
tion seems to work best.
Special filters can partially reject
nebula filters work by increasing con
tween certain objects and the sky
natural and manmade light pollution.
ground. Even though the filters decrease
light of all objects, if the background is
duced even further, the improved
may more than compensate, and the
will actually appear brighter to the
Nebula filters work best on emission
planetary nebulae, rather than on stars,
xies, and reflection nebulae. The filters
ally should be used between the eyepiece
the telescope objective, because many
filters do not work well if the light is
than a few degrees off-axis. The ",,"CUIV .. no
of filters from different manufacturers
considerably, so try to use several in
observing sessions, and choose the best
purchase.
For finding objects in the first place,
best method for the amateur to learn is
hopping. First the brightest stars must
learned with the aid of an all-sky map
planisphere. Then, using a detailed star
the telescope's finder is pointed to a
naked-eye star. By using the patterns on
star chart, the finder telescope, along with
main telescope, is moved step by step to
object of interest.
The finder should have an aperture
least 35 mm for main telescopes of 3
aperture or less, and at least 50 mm (2
ches) for telescopes with apertures over 3
ches. The finder magnification should be
less than one-tenth the magnification
the main telescope. Otherwise the main
scope cannot be pointed accurately.
telescopes larger than about 8 inches
have two finders, the second with an
and magnification midway between the
finder and the main telescope.
4
The faintest star visible in a
telescope
INTRODUCTION
Th faintest point of light detectable by the
:ided eye was derived in Chapter 2. The
un ,s fundamental limit is around 50 to 150
eye . h . , I
photons of green hg t arnv d
lg over a severa -
second period, correspon lr:g to a star . as
faint as magnitude 8.5. Seemg such a famt
star requires perfect conditions and dark
adaptation as . well s exclusion of a!1 extraneous
light, mcludmg all other stars m the
sky. . .
Thus it's not surpnsmg that no one sees
8.5-magnitude stars with the naked eye in
real life. A more typical limit is 7 or 7.5 for a
skilled observer in excellent country skies.
Stars and other small objects form a special
case for detection by the eye in the telescope.
As we have seen, any object less than about
0.5° across as presented to the eye can be
considered a point source if it is so dim it's at
the threshold of detection. A star image is
actually a diffraction disk, but it is so small
that, if faint, the disk is a point to the eye at
any reasonable magnification at all.
Magnification does not change the brightness
of a point source in the telescope, but it
does decrease the surface brightness of the
background and reduces the field of view so
other stars do not interfere. Therefore, the
fundamental limits of the eye can be reached
when a telescope is used. Here it really is
possible to see the equivalent of 8.5-
magnitude stars naked-eye.
MAGNIFICATION
The fundamental magnitude limit Mt of a
telescope is given by
M, '=
M e + 2.5 loglO(D2 t I De 2)
where M e
(equation 4.1)
is the eye limiting magnitude (8.5
49
for the ideal case), D is the telescope diameter,
De is is the eye diameter, taken to be
7.5 millimeters, and t is the telescope transmission
factor (which is usually about 0.7).
This equation reduces to
(equation 4.2)
where D is expressed in millimeters. This
formula was used to list the ideal limiting
magnitudes for telescopes of various apertures
in Table 4. 1.
The surface brightness Mb of the sky or an
extended object (in magnitudes) is darkened
by the telescope magnification and transmission
factor as follows:
Mb = -2.5 log 10 (D2t Im2De2) (equation 4.3)
where m is the magnification. This is why
magnification helps to detect faint stars when
the sky is bright, or even under dark country
skies compared to when low power is used.
High magnification also increases the apparent
angle between field stars.
Consider a dark country sky with a surface
brightness of 24 magnitudes per square arcsecond.
At the minimum usable magnification,
mm, computed from the equation
(equation 4.4)
which is 27 X for an 8-inch telescope, the sky
brightness is reduced from its naked-eye level
by
Mm = -2.510glO(t), (equation 4.5)
or if t is 0.7, 0.39 magnitudes I sq. arc-second.
Examining Figure 4.1, which shows the
faintest star detectable by the eye, we find
that at a sky background of 24.4 mag./sq.
arc-sec., the naked-eye limit is 7.6. Using this
value for Me in equation 4.1, we find a limiting
magnitude of 14.4 for an 8-inch telescope
at its lowest usable power of 27 x.
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