Calculating The Exit Pupil; Observing The Sky; Sky Conditions; Selecting An Observing Site - Dörr DANUBIA DELTA 20 Notice D'utilisation

08.3

CALCULATING THE EXIT PUPIL

The Exit Pupil is the diameter (in mm) of the narrowest point of the cone of light leaving your telescope. Know-
ing this value for a telescope-eyepiece combination tells you whether your eye is receiving all of the light that
your primary lens or mirror is providing. The average person has a fully dilated pupil diameter of about 7mm.
This value varies a bit from person to person, is less until your eyes become fully dark adapted and decreases
as you get older. To determine an exit pupil, you divide the diameter of the primary of your telescope (in mm)
by the magnifi cation.
Diameter of Primary mirror in mm
Exit Pupil =
Magnifi cation
For example, a 200 mm f/5 telescope with a 40mm eyepiece produces a magnifi cation of 25x and an exit pupil
of 8 mm. This combination can probably be used by a young person but would not be of much value to a senior
citizen. The same telescope used with a 32mm eyepiece gives a magnifi cation of about 31x and an exit pupilof
6.4 mm which should be fi ne for most dark adapted eyes. In contrast, a 200 mm f/10 telescope with the 40mm
eyepiece gives a magnifi cation of 50x and an exit pupil of 4 mm, which is fi ne for everyone.

09 | OBSERVING THE SKY

09.1

SKY CONDITIONS

Sky conditions are usually defi ned by two atmospheric characteristics, seeing, or the steadiness of the air,
and transparency, light scattering due to the amount of water vapour and particulate material in the air. When
you observe the Moon and the planets, and they appear as though water is running over them, you probably
have bad "seeing" because you are observing through turbulent air. In conditions of good "seeing", the stars
appear steady, without twinkling, when you look at them with unassisted eyes (without a telescope). Ideal
"transparency" is when the sky is inky black and the air is unpolluted.
09.2

SELECTING AN OBSERVING SITE

Travel to the best site that is reasonably accessible. It should be away from city lights, and upwind from any
source of air pollution. Always choose as high an elevation as possible; this will get you above some of the
lights and pollution and will ensure that you aren't in any ground fog. Sometimes low fog banks help to block
light pollution if you get above them. Try to have a dark, unobstructed view of the horizon, especially the
southern horizon if you are in the Northern Hemisphere and vice versa. However, remember that the darkest
sky is usually at the "Zenith", directly above your head. It is the shortest path through the atmosphere. Do not
try to observe any object when the light path passes near any protrusion on the ground. Even extremely light
winds can cause major air turbulence as they fl ow over the top of a building or wall.
Observing through a window is not recommended because the window glass will distort images considerably.
And an open window can be even worse, because warmer indoor air will escape out the window, causing
turbulence which also a ects images. Astronomy is an outdoor activity.
09.3

CHOOSING THE BEST TIME TO OBSERVE

The best conditions will have still air, and obviously, a clear view of the sky. It is not necessary that the sky
be cloud-free. Often broken cloud conditions provide excellent seeing. Do not view immediately after sunset.
After the sun goes down, the Earth is still cooling, causing air turbulence. As the night goes on, not only will
seeing improve, but air pollution and ground lights will often diminish. Some of the best observing time is often
in the early morning hours. Objects are best observed as they cross the meridian, which is an imaginary line
that runs through the Zenith, due North-South. This is the point at which objects reach their highest points in
the sky. Observing at this time reduces bad atmospheric e ects. When observing near the horizon, you look
through lots of atmosphere, complete with turbulence, dust particles and increased light pollution.
09.4 COOLING THE TELESCOPE & ADAPTING YOUR EYES
Telescopes require at least 10 to 30 minutes to cool down to outside air temperature. This may take longer if
there is a big di erence between the temperature of the telescope and the outside air. This minimizes heat
wave distortion inside telescope tube (tube currents). Allow a longer cooling time for larger optics. If you are
using an equatorial mount, use this time for polar alignment. Do not expose your eyes to anything except red
light for 30 minutes prior to observing. This allows your pupils to expand to their maximum diameter and build
up the levels of optical pigments, which are rapidly lost if exposed to bright light. It is important to observe
with both eyes open. This avoids fatigue at the eyepiece. If you fi nd this too distracting, cover the non-used
eye with your hand or an eye patch. Use averted vision on faint objects: The center of your eye is the least
sensitive to low light levels. When viewing a faint object, don't look directly at it. Instead, look slightly to the
side, and the object will appear brighter.
200 mm
e.g. = 6,4 mm
32 mm
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