Calibrating Your Digital Camera
1. Set up a piece of white paper so that it is
equally illuminated by indirect natural light. Choose a place where the light
is bright. The most important thing is that the paper is illuminated evenly. Choose
a time and place during which it is unlikely there will be any changes in how
much light is present.
2. Set up your digital camera so that the white
paper fills the entire field of view. The distance from the camera to the paper
is not important.
3. Put the camera in manual mode and make the
a. Adjust the focus so that the
camera is focused on the white piece of paper. Once you set the focus, do not change
b. Set the camera’s sensitivity to
c. Set the aperture to f/2.8.
d. Set the image resolution to a
e. If your camera has a self-timer
feature, set it so that the camera shutter opens a few seconds after you take a
picture. This minimizes shake.
4. Now that your camera is focused on the paper,
with the settings adjusted correctly, take a series of photos at different
shutter speeds, changing by a factor of 2. (I used: 30, 15, 8, 4, 2, 1, 1/2,
1/4, 1/8, 1/15, 1/30, 1/60, 1/125, 1/250, 1/500, and 1/1,000 sec shutter
5. Repeat step 4. This will give you two sets of
calibration photos. You will compare these later in this procedure.
6. Download all of the calibration photos onto your
7. Make two data tables in your lab notebook to keep
track of the pixel value for each calibration photo. One data table will be for
the first set of calibration photos, the other for the second.
8. Measure the average pixel intensity of each photo
a. Start ImageJ.
b. Open the first photo using “File/Open…”
c. Click on “Analyze” and
d. A histogram of the pixel values
in the photo will open in its own window. You will use the histogram to measure
the average pixel gray value in each photo.
e. Record the Mean, StDev, Min,
Max, and Mode in your data tables. StDev is short for “standard
deviation”. Min and Max are short for minimum and maximum, respectively.
Mean is another name for the average. The mean of this histogram is the average
f. Click on “File” and
select “Open Next” to open the next file.
Repeat steps 3.d.–3.f. until you
have analyzed all images in both sets of calibration photos.
9. Make a calibration curve by graphing the average
pixel value (the mean of the histogram) on the x-axis and exposure time (in
seconds) on the y-axis. Your graph will have two data series, one for each set
of calibration photos.
a. This is a “semi-log”
10. Look at your graph and check the calibration
curves from each set. The two curves should cross or only be a little bit apart.
If there is a large amount of space between the two calibration curves or if
one of the curves has a drastically different shape from the other, you will
need to repeat steps the whole process, making sure that the lighting
conditions are the same for both sets.
Taking Skyglow Photos
Now that you have finished calibration, you are ready
to measure skyglow. Pick three or four places where you would like to measure
skyglow. Choose places you think will have different amounts of skyglow.
a. Make sure it is around the same
time of night and that there are no clouds.
2. Travel to your first location with all of your
supplies. Pick the one you think will have the most skyglow. In your lab
notebook, write down the address of your first location. Include a brief
3. Set up your camera to take skyglow photos. Make
sure the camera is in full manual mode. It is necessary to use the same camera
settings at each site you visit. These are the same settings you used for
a. Set the camera’s sensitivity to
b. Set the aperture to f/2.8.
c. Set the image resolution to a
d. If your camera has a self-timer
feature, set it so that the camera shutter opens a few seconds after you press
the button to take a picture. This minimizes shake.
3. Lay your towel or rag on the ground, then lay the
camera down on it, with the lens pointing toward the sky. If you have a tripod,
you can mount the camera on the tripod and then point the camera toward the
4. Double-check that your camera’s field of view
does not include the Moon, street lamps, or house lights.
5. Take skyglow photos.
6. Repeat steps 2–5 for each of the rest of the places
you plan to measure skyglow.
Using Your Calibration to Measure Skyglow
1. Use the ImageJ software to measure the average pixel
value in each skyglow photo by doing the same thing in step 8 of the
calibration portion. Make a data table in your lab notebook, and record the
mean, standard deviation, minimum, maximum, and mode of each pixel value
2. Because all of your skyglow images were taken
with the same camera settings and exposure time, you can use the calibration
curve to find a “equivalent exposure time” (EET) for each skyglow
photo. The EET is how long the exposure time would have to have been under
calibration conditions to reach the same average pixel value as measured in the
3. Convert the average pixel value in each skyglow
image to an EET. Record the EET for each image in your lab notebook.
4. By converting the average pixel values of each
skyglow image into an EET, you can determine how much brighter or darker one
location is compared to another.
5. Determine which of your skyglow locations had the
smallest EET. This is the location with the darkest skyglow.
Review of the Literature
For most of Earth’s history, our universe
of stars and galaxies has been visible in the darkness of the night sky. From
our earliest beginnings, the display arrayed across the dark sky has inspired
questions about our universe and our relation to it. The history of scientific
discovery, art, literature, astronomy, navigation, exploration, philosophy, and
even human curiosity itself would be diminished without our view of the stars.
But today, the increasing number of people living on earth and the
corresponding increase in inappropriate and unshielded outdoor lighting has
resulted in light pollution—a brightening night sky that has obliterated the
stars for much of the world’s population. Most people must travel far from
home, away from the glow of artificial lighting, to experience the
awe-inspiring expanse of the Milky Way as our ancestors once knew it.
Light pollution is light that is
not being efficiently or completely utilized and is often pointed outwards or
upwards and not downwards. Also known as skyglow, light pollution occurs from
both natural and human-made sources. The natural component of sky glow has five
sources: sunlight reflected off the moon and earth, faint air glow in the upper
atmosphere, which results in a permanent low-grade aurora, sunlight reflected
off interplanetary dust(also known as zodiacal light), starlight scattered in
the atmosphere, and background light from faint, unresolved stars and nebulae,
which are celestial objects or diffused masses of interstellar dust and gas
that appear as hazy smudges of light. Natural sky glow is well quantified.
However, in the discussion of sky glow it is mainly human-made sources that are
lighting also increases night sky brightness and is the human-made source of
sky glow. Light that is either emitted directly upward by luminaires or
reflected from the ground is scattered by dust and gas molecules in the
atmosphere, producing a luminous background. It has the effect of reducing
one’s ability to view the stars. Sky glow is highly variable depending on the
immediate weather conditions, the quantity of dust and gas in the atmosphere,
the amount of light directed skyward, and the direction from which it is viewed.
In worse weather conditions, more particles are in the atmosphere to scatter
the light, so the sky glow becomes a very clear effect of wasted light and
glow, while it is a problem for almost everyone, is mostly problematic to
astronomers since it lessens their ability to view the night sky. Sky glow
increases the illumination of the dark areas of the sky, which reduces the difference
of stars or other objects against the dark sky background. Astronomers
typically like very dry, unclouded, dark nights for observing. An average suburban
sky is 5 to 10 times brighter at the top than the natural sky. The top being
the angle that points upwards, or 180°, from the observation area. In city
centers, the top may be 25 or 50 times lighter than the natural background.
are three types of skyglow, each falling on a spectrum. Technically speaking,
three main types of light pollution include glare, light trespass and skyglow (along
with over-illumination and clutter). Glare, produced by uncontained lighting is
a risk, for older people. Glare scattering in the eye blinds you temporarily
and leads to dangerous driving conditions, for instance. Light trespass occurs
when undesired light enters one’s home, for example, by shining undesired light
into a bedroom window of someone trying to sleep.
pollution doesn’t just affect astronomers.
The negative effects of the loss of the night sky might seem insubstantial,
however evidence links the brightening night sky directly to negative impacts
on human health and immune function, on hostile behavioral changes in insect
and animal populations, and on a fall of both quality and safety in our
nighttime environment. Astronomers the first to discover the negative impacts
of wasted lighting on scientific research, but for all of us, the negative
economic and environmental impacts of wasted energy are obvious in everything
from the electric bill to global warming.
are ways of measuring sky glow. This is not easy, because many factors are
involved in sky glow. One must not only consider the lighting, but also the dispersal
of the light emitted from the source of that light, the light reflected from
the ground and its dispersal, as well as humidity and the interaction of light
with aerosols (particles in the atmosphere that may be caused by manufactured
pollutants, fire, volcanic eruptions, etc.), all of which can change in an
are multiple ways to help the cause. Aside from houses, street lamps are one of
the most prominent causes of light pollution. The worst ones are the lamps with
a strong blue radiation, like Metal Halide and white LEDs. Change from the now
widely used sodium lamps to white lamps (MH and LEDs) could produce an increase
of pollution. This increase will worsen known and unknown effects of light
pollution on human health, environment and on perception of the night sky by
is an important topic to bring to attention. There are epidemiological evidence
of increased breast and colon cancer risk in workers from light pollution. An
inhibition of the pineal gland function with exposure to the constant light
(LL) regimen promoted carcinogenesis whereas the lack of light impedes the
are many different ways to help decrease the amount of light pollution. The
most important thing to do is to reduce light leaving your home and to direct
it down, not up. These aren’t the hardest things to do and can even help to money.
Using dimmer light switches also can help. Light pollution is a serious issue;
it is widespread and has many negative side effects. Additionally it doesn’t just harm scientists;
it causes many diseases and conditions. It also affects the sleep pattern of nocturnal