OBSERVING
THE
SKY
SKY
CONDITIONS
Sky conditions can significantly affect the performance
of
your telescope
in
three ways.
· Steadiness
of
the
air:
On
windy days, images ofthe
moon and planets will appear
to
wave or jump around
in
the eyepiece; as if you are looking t them through moving
water. Nights where winds are calm will offer the best
higher magnification views ofthe planets and the moon.
The best way to judge the stability ofthe atmosphere is
to
look at bright stars with the naked
eye.
If they are "twin-
kling" or rapidly changing colors, the air is unstable and you
are better off using lower powers and looking for deep sky
objects. If the stars are sharp and not twinkling, the air is
stable and should offer great high magnification planetary
views.
· Transparency:
How
clear is the air you are looking
through? If there is a high amount
of
humidity
in
the
air,
the
faint light from galaxies and nebulae can be scattered and
diffused before reaching your telescope, causing a loss
of
brightness
in
your image. Debris
in
the air from local forest
fires
or
even distant volcanic eruptions can contribute
to
large loss of brightness. Sometimes this humidrrJ
or
debris
can help stabilize the
air,
making for good planetary and
lunar images, but the loss of light would make it difficult
to
see fainter deep-sky objects.
·
Sky
Brightness: The amount of ambient light
in
the atmo-
sphere can also effect deep-sky observing.
How
dark the
sky is can depend on your local surroundings.
In
the middle
of
a city, sky-glow caused by city lights being reflected
back
to
earth from the sky can overpower the faint light
from distant galaxies. Getting away from the bright lights of
a major city can make the difference between seeing a faint
deep-sky object and missing it altogether. Planets and the
moon are plenty bright enough on their own so the effect
on observing them is minimal.
SELECTING
AN
OBSERVING
SITE
If you are going
to
be observing deep-sky objects, such
as galaxies and nebulae, you should consider traveling
to
a dark sky site that is reasonably accessible. It should be
away from city lights, a relatively unobstructed view
of
the
horizon, and upwind
of
any major source
of
air pollution.
Always choose as high
an
elevation as possible as this
can lowerthe effects of atmospheric instability and can
ensure that you are above any ground fog. While it can be
desirable
to
take your telescope
to
a dark sky site, it is not
always necessary. Ifyou plan
to
view the planets, the moon
or
even some of the brighter deep-sky objects, you can
do
this from any location, such as your own backyard.
Try
to
setup the
scope
in
a location that is out
of
the direct path
of
streetlights
or
house lights
to
help protect your night
vision.
Try
to
avoid observing anything that lies within 5
to
10 degrees over the roof
of
a building. Roofs absorb heat
during the day and radiate this heat qut at night. This can
cause a layer of turbulent air directly over the building that
can degrade your image. It is best ifyou set your telescope
up
directly on a dirt
or
grassy surface. Setting
up
on any
raised platform such as a wooden
deck
or
a hard surface
like concrete
or
a sidewalk, movements you make may
cause the telescope
to
vibrate.
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.
CHOOSING
THE
BEST
TIME
TO
OBSERVE
Try
not
to
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 ob-
serving time is often
in
the early morning hours before dawn.
Objects are best observed as they cross the meridian, the
imaginary line that runs from north
to
south through a point
directly over your head. This is the point at which objects
reach their highest points
in
the sky and your telescope is
looking through the least amount
of
atmosphere possible.
Objects that are rising
or
setting near the horizon will suffer
more atmospheric turbulence since you are looking through a
much longer column
of
air.
It is not always necessary
to
have
cloud-free skies if you are looking at planets
or
the moon.
Often broken cloud conditions provide excellent seeing.
COOLING
THE
TELESCOPE
Telescopes require at least 10 minutes
to
cool down
to
outside air temperature. This may take longer if there is a big
difference between the temperature ofthe telescope and
the outside
air.
This minimizes heat wave distortion inside
telescope tube (tube currents).
ADAPTING
YOUR
EVES
Ifyou are planning to observe deep sky objects at a dark
sky site, it is best ifyou allow your eyes to fully adapt to the
dark by avoiding exposure
to
white light sources such as
flashlights, car headlights, streetlights, etc. It will take your
pupils about
30
minutes
to
expand to their maximum diam-
eter and build up the levels
of
optical pigments
to
help your
eyes see the faint light from a distant target. Ifyou need light
to help setup your telescope
in
the dark, try using a red LED
flashlight at as low a brightness setting as possible and avoid
looking straight at the light source. This will give you the best
chance
of
capturing those faint deep sky objects.
When observing, it is important
to
observe with both eyes
open. This avoids eye fatigue at the eyepiece. Ifyou find
this too distracting, cover the unused eye with your hand
or
an
eye patch. The center
of
your eye works well
in
bright
daylight, but is the least sensitive part ofthe eye when trying
to
see subtle detail at
low
light levels. When looking
in
the
eyepiece for a faint target, don't look directly at it. Instead
look toward the edge ofthe field
of
view and the object will
appear brighter.
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