OF THE ROGER N. CLARK

The Lumicon H-Beta Filter (Figure 3.7)
has a narrow band pass designed for use on
objects that emit most of their light at 4861
angstroms. But when light passes through
this filter only slightly off-axis, the band pass
wavelength becomes too short to transmit
much of this light (dashed line). This filter is
only good for use between the eyepiece and
the objective and should not be used even for
casual observations between the eyepiece and
eye.
The Daystar Nebular Filter (Figure 3.8)
acts similarly to the Edmund filter, but the
bandpass is at a slightly longer wavelength,
the transmittance is higher at the center of
the band pass, and the off-axis blocking is as
good as on-axis. These factors are probably
why some observers declare this filter better
than the Lumicon UHC filter in actual
observing practice, though the author believes
them to be very close.
The manufacture of nebula filters is so new
that different ones may be available by the
time this book is published. The factors discussed
here, however, will help in evaluating
any of them. Finally, observers about to make
a purchase should remember that the difference
between even the best nebula filter and
none at all is usually rather subtle.
USING THE TELESCOPE TO FIND
OBJECTS
SO far this book has discussed observing techniques.
But the best deep-sky techniques are
useless unless the objects can first be found.
In order to find anything in the sky, you
must become familiar with the bright stars
and constellations - just as when you drive
into a new city you get oriented by major
landmarks. Once you're in the right celestial
neighborhood, a detailed map is required for
locating small, faint objects. Constellation
guides and star atlases are listed in Appendix
A.
Celestial coordinates
The stars appear fixed on an imaginary
celestial sphere, which is always centered on the
observer. The celestial sphere appears to
rotate once every day because we observe it
from the rotating Earth. Lines projected from
the center of the earth through the north and
VISUAL ASTRONOMY OF THE DEEP SKY
south poles extend to the north and
poles of the celestial sphere. The stars
to rotate around these poles, as was
ancient times.
Astronomers have set up a
system for describing positions on
celestial sphere similar to latitude
longitude on Earth. The Earth's latitude
are projected outward from our globe
the sky to become the lines of u,-,uu· larln
The celestial equator, straight overhead
the Earth's equator, defines 0° declination
the celestial sphere. The north celestial
is at +90° declination, the south celestial
at _90° declination.
"Longitude" on the celestial sphere
called Right Ascension. Longitude lines
little more difficult than latitude to
onto the sky because the stars are v'''0 LallL
rotating across the longitude lines on
Earth. For the 0° point astronomers
chosen the place at which the Sun crosses
celestial equator from south to north on
yearly journey around the sky. This point
called the First Point of Aries (though it
now in Pisces because of a slow movement
the coordinate system over the
known as precession).
Imagine aiming a telescope at
Point of Aries and keeping the telescope
with respect to the earth (on a
mount). As the Earth turns, the
ascension of stars crossing the view ·
Right ascension is commonly measured
hours, minutes, and seconds of time
of degrees, minutes and seconds of arc.
are 24 hours of right ascension just as
are 360 degrees of longitude. (Therefore,
minute or second of right ascension is not
same as an angular minute or second of
Each is 360/24 or 15 times greater than
corresponding unit of longitude.)
Note that as declination increases
tive) or decreases (negative) from the
tial equator, the right ascension lines
closer together. They converge at the
Since the Earth's axis is tilted with
to its path around the Sun, the
track of the Sun on the sky is north
celestial equator for half the year and
for half. The sun's apparent track on
celestial sphere is called the ecliptic, and
tilted 23.5° to the celestial equator.
planets generally follow the ecliptic too
---
h orbits of all except Pluto are inause
t e
ehne on .
THE EYE AND THE TELESCOPE
r: h I f
e . cl ly a few degrees lrom t e pane 0
h Earth'S orbit.
t
he line on the celestial sphere that passes
h the pole and directly overhead, and
throug
h · h
. s
is called the men lan. . e pOint h
hOrIZon,
·d· Th .
cl to the north and sout pOints on t e
cl· tly overhead is called the zemt .
Iree . h .
Th Sun is at 6 hours ng t ascenSIOn at t e
. e .ng of summer in the Northern
begmm . 8 h ( h
Hemisphere, so right ascensIO 1 ours t e
. te side of the sky) Will be on the
OppOSI . .
meridian at mlmht.
At the begInmng of autumn In the
h
Northern Hemisphere, right ascension 0
hours is on the meridian at midnight. At the
beginning of winter, 6 hours R.A. is on the
meridian, and at the beginning of spring, 12
hours.
With these facts, and a little study of the
approximate right ascensions of the
constellations, you should be able to figure
out what constellations are up at night any
time of the year.
For example, say it is 9 p.m. on August 20;
what constellations are on the meridian
Consider that one month later is the
beginning of fall, when 0 hours R.A. is on the
Figure 3.9. The celestial sphere. The observer is aligning the equatorial mount's polar axis by
making it parallel to the Earth's axis. (Courtesy of Sky & Telescope.)
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