Calculating The Exit Pupil; Sky Conditions; Selecting An Observing Site - Dörr Danubia ATLAS 2000 Manual De Instrucciones

10.2 CALCULATING THE FIELD OF VIEW
The size of the view that you see through your telescope is called the true (or actual) field of view and it is
determined by the design of the Eyepiece. Every eyepiece has a value, called the apparent field of view, which
is supplied by the manufacturer. Field of view is usually measured in degrees and/or arc-minutes (there are
60 arc-minutes in a degree). The true field of view produced by your telescope is calculated by dividing the
eyepiece's apparent field of view by the magnification that you previously calculated for the combination.
Using the figures in the previous magnification example, if your 10mm eyepiece has an apparent field of view
of 52 degrees, then the true field of view is 0.65 degrees or 39 arc-minutes.
Apparent Field of View
True Field of View =
Magnification
To put this in perspective, the moon is about 0.5° or 30 arc-minutes in diameter, so this combination would be
fine for viewing the whole moon with a little room to spare. Remember, too much magnification and too small
a field of view can make it very hard to find things. It is usually best to start at a lower magnification with its
wider field and then increase the magnification when you have found what you are looking for. First find the
moon then look at the shadows in the craters!
10.3

CALCULATING THE EXIT PUPIL

The Exit Pupil is the diameter (in mm) of the narrowest point of the cone of light leaving your telescope. Kno-
wing 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 de-
creases as you get older. To determine an exit pupil, you divide the diameter of the primary of your telescope
(in mm) by the magnification.
Diameter of Primary mirror in mm
Exit Pupil =
Magnification
For example, a 200mm f/5 telescope with a 40mm eyepiece produces a magnification of 25x and an exit pupil
of 8mm. 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 magnification of about 31x and an exit pupil of
6.4mm which should be fine for most dark adapted eyes. In contrast, a 200mm f/10 telescope with the 40mm
eyepiece gives a magnification of 50x and an exit pupil of 4mm, which is fine for everyone.
11 | OBSERVING THE SKY
11.1

SKY CONDITIONS

Sky conditions are usually defined 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.
11.2

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 flow 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 affects images. Astronomy is an outdoor activity.
52°
e.g. = 0,65°
80x
200 mm
e.g. = 6,4 mm
32 mm
25