With Celestron‘s SkyScout， you can explore outer space without
leaving your backyard. The handheld device gives you
point-and-click convenience to identify thousands of stars，
planets，constellations， and deep-space objects.You don't even need
instructions. Just turn it on and follow the directions on the LCD.
ARM processor power brings the heavens to you.
Adding exterior lighting to streets，parking lots， buildings， and
signage makes our nights safer， but it unknowingly adds to the
growing light pollution problem that may threaten some life on our
planet （see Photo 1）.Light pollution isn‘t obvious unless you like
to stargaze. If you do， you quickly realize its effect of washing
out the nighttime sky. Civil engineers are now required to design
with directional lighting to help minimize this effect. Light
pollution interferes with the way many plants and animals function.
Although we cannot eliminate light pollution， we can reduce its
effects and save energy by using illumination wisely （as we should
every resource）。 Although light pollution interferes with this
month's topic， it‘s not the focus here. However， you are encouraged
to learn more about light pollution and its effects on our planet
by visiting your local library or using your favorite search
engine.
This month， I want to discuss the design of a device that makes it
simple for anyone to enjoy stargazing. Celestron has been making
telescopes since 1960 and it recently released the SkyScout， a
handheld device that gives its users a point-and-click way to
identify thousands of stars， planets，constellations， and deep-space
objects.
The celestial sighting device uses an exterior LCD to give
instructions and information. The device has two basic modes of
use， Identify and Locate. Pressing the Target button automatically
identifies any celestial object that is centered in the sighting
tube. Alternately，you can flip through a database of objects on the
LCD， choose one， and the SkyScout will steer you toward it via
unique pointers in the sight tube.
To enjoy the SkyScout， you don‘t need to know anything about
celestial coordinates，just turn it on and follow the clear
instructions. However， to understand how this device works， you'll
need to know a little about the sky.
EARTH AND SUNThe Earth rotates about its North/South Pole axis once a day （24
hours）。 It also orbits around the sun once in a year while it spins
（365 1/4 rotations or days）。 The Earth‘s axis is not perpendicular
to its orbit but tilted about 23.5°。 （As with the mechanics of most
objects， the tilt isn't constant. But in an effort to make things
less confusing，we‘ll ignore long-term effects.）
The Earth‘s tilt is responsible for the change in seasons. At one
point during the orbital year， the Earth's North Pole will be
closest to the sun（the solstice）。 The northern hemisphere is
midsummer with its longest period of daylight. The southern
hemisphere is mid-winter with its shortest period of daylight.
Three months later， each pole will be equidistant from the sun （the
equinox）.While the northern hemisphere heads toward winter with
shorter periods of daylight， the southern hemisphere looks toward
summer and longer periods of daylight. At the half-year mark，
another solstice occurs. Now the South Pole is closest to the sun
and the southern hemisphere experiences summer，while the northern
hemisphere is midwinter. After another equinox at nine months， the
Earth heads back to its original position and the yearly seasonal
cycle begins again.
The Earth is geographically mapped using latitude and longitude.
Latitude is a north-south measurement. Lines of constant latitude
run east-west around the Earth‘s circumference， parallel to the
equator. Longitude is an east-west measurement， with lines that run
pole-to-pole dividing the Earth into "orange wedges." The prime
meridian is the longitudinal arc between the two poles that passes
through Greenwich， England. It has been assigned 0° （with 180° on
the opposite side of the Earth）。 Longitudinal lines to the east of
the prime meridian are designated 0° to 180°E. To the west of the
prime meridian they are designated 0° to 180°W. Latitudinal lines
are north/south measurements based on the equator. The equator is
designated as 0°。 From the Earth's center，angles measured from the
equator（0°） to either pole （90°） divide each hemisphere into rings
parallel with the equator， either 0° to 90°N （north of the equator）
or 0° to 90°S （south of the equator）。 For instance， the location
0°longitude and 0° latitude defines a point off the shore of
Africa.
It wasn‘t until 1884 that Greenwich was adopted as the universal
prime meridian （0°）。 Imagine the confusion with mapmakers all using
their own designation for the prime meridian！
EARTH AND THE COSMOS
We are so enamored with the sun that we sometimes fail to look at
the bigger picture. Our solar system， the sun， and all nine planets
（yes， I still consider Pluto a planet）， are but a speck in a spiral
arm of the Milky Way galaxy. For a little comparison in scale，
let‘s use the speed of light （the fastest measure we know）。 From
the sun， it takes about 8 min. for light to reach Earth （about 80
min. to reach Pluto）。 This makes the diameter of our solar system
about 160 light minutes，or 2.6 light hours （about 1/10 of a light
day or 1/3，650 of a light year）。 The size of the Milky Way is about
65 million times larger， or more than 18，000 light years！ It
contains hundreds of millions of stars.
It is enough to know that the Milky Way galaxy is only a speck in
the universe. The number of galaxies in the universe is almost
beyond comprehension. All that is out there is in constant motion.
To the average Joe on Earth， what he can see from Earth （by the
unaided eye） defines his playing field. From our vantage point， all
objects outside our solar system appear to be motionless. If the
first stargazers were able to take photographs of the
constellations thousands of years ago and we compared them to what
we see today， there would be differences. That is because most
stars in a constellation are at different distances from us. Each
moves at its own rate， changing the general shape of the
constellation's pattern by very small amounts.
Imagine for now that we had no sun and it was night 24 hours a day.
This means that we would always be able to see the stars. In 24
hours，we would see a complete tour of the constellations as the
Earth makes one complete rotation. If we picked out a star， such as
Betelgeuse （in the constellation Orion）， and timed how long it
takes the Earth to rotate bringing Betelgeuse back to the same
position in the sky， it would not be 24 hours（24：00：00）， but
23：56：04. Hmm.
Because the rotating Earth moves in an orbit around the sun， it
needs to rotate past 360° to get pointing back to the sun （see
Figure 1）。 So， when we say the sun rotates 360° every day， this is
in reference to the sun （a minor player）。This definition of a solar
day is 24：00：00 in length. From the perspective of the cosmos，what
we take for granted is all wrong. The celestial or sidereal day is
00：03：56 shorter.
With the sun back in place， things don‘t really change. It's just
our perception that changes because we base our lives on the solar
day and not the sidereal day. With the sun back in the picture，we
see the stars only at night， and our view of them changes by
00：03：56 each night. It takes a full year to see all that the
heavens has to offer because we get to"see" the stars only when we
face away from the sun.
The mapping system of the heavens is similar to our geographical
coordinate system on Earth. Imagine a clear sphere surrounding the
Earth that is able to stay fixed with the universe so the Earth
rotates within it. The Earth‘s axis extends through the sphere，
effectively giving this new sphere poles. The sphere has
longitudinal wedges that are marked by time（like time zones）。
Whereas the geographical coordinate system uses the prime
meridian's intersection with the equator as the starting reference
point，the celestial coordinate system uses the arc between its
poles that passes through the first star in the constellation
Pisces as a reference of time zero （00：00：00）。
Twenty-four wedges create a longitudinal line every 15 degrees， or
one hour（360°/24 hours） （see Figure 2）。 Thus， time is used to
indicate a "right ascension" to the west in hours， minutes， and
seconds（the amount of time it takes the Earth to rotate from the
reference to the object of desire， 00：00：00 to 24：00：00）。
Just like latitude lines in the geographic coordinate system，
declination（DEC） is given in degrees north and south of the
equator. Only in the celestial coordinate system， the degrees are
designated by using （+）instead of （N） and （-） instead of （S）。
I like to think of the radius of this clear sphere to be variable
and just smaller than any object of interest. This way， any
parallax error （the difference in viewing angle between the center
of the Earth-the center of the sphere-and a viewer on the Earth‘s
surface） will be insignificant.
The path the stars seem to take （while you spin on the Earth
beneath them）will change based on where you view them from. If you
lie down at the equator and view the movement of the sky over the
period of the night， the stars will rise， move overhead， and set.
If you fly to one of the poles and spend the night watching the
stars， they will rotate in a circular pattern about the pole.
SkyScoutThe small， lightweight SkyScout can be used at length without
fatigue. Any product‘s success will depend on many factors， but the
user interface is the most important. If the product is intuitive，
it is more likely to be successful. The SkyScout has a friendly
appearance with all controls， except for one， mounted on one face
with a backlit （red to preserve night vision）LCD （see Photo 2）。
Each of the nine large rubber buttons （with a construction similar
to a remote control） is clearly marked with its function
（i.e.，Locate）。 A Target button is positioned for easy access
beneath the forefinger while viewing an object through the sighting
tube. Unlike a telescope or binoculars， there is no magnification
through the sighting tube. When you look at an object through the
Sky- Scout‘s sighting tube， the Target button provides a snapshot
of the device's position. This is used for identifying objects. In
Locate mode， a database of objects is displayed via the LCD. Once
an object is selected， eight LEDs around the inside of the sighting
tube direct you to move toward the object until it is directly in
the center of the sighting tube. The simplicity of coaxing you to
move using a ring of directional LEDs disguises the complicated
sensors and math computations associated with the action. This is
the basis of good product design.
Cell phones， navigation aids， the LoJack， and other products take
advantage of the system of satellites in geosynchronous orbit to
gather positional information. The SkyScout uses an internal GPS
module to calculate the precise position of its user. It requires
sufficient signal from three satellites to triangulate the user‘s
position on Earth （longitude， latitude， and elevation）。 It may
attempt to receive signals from up to 12 satellites. Once the
SkyScout has acquired a fix， you can review the identified
satellites and their associated strength along with your present
calculated position. GPS technology is the first step in making
this product user-friendly. It eliminates the need for a user to
determine longitude，latitude， and time of day and then have to
enter them manually.
The remaining sensor technology（physical position of the SkyScout）
is used to locate or identify objects in the sky. The order of the
next two procedures depends on whether you are locating or
identifying. When using the locate function， the database is used
to determine the celestial position of the object. Then the
positioning function directs the SkyScout toward the object‘s
position relative to the user's position. When the identify
function is chosen， the positioning functions determine the
celestial position of the viewing tube and then check the database
for an object that matches that celestial position.
DATABASE OF OBJECTS
Objects outside of our solar system require a minimum of data.
These objects can be considered standing still in reference to our
lifetime. While only RA and DEC are necessary to fix its position
on the celestial sphere， other data provides plenty of educational
fodder. The scientific data displayed for Polaris is shown in Table
1.
Objects within the solar system have a life of their own. Because
they are （relatively）
close by and have periodic orbits like that of Earth，their
positions relative to the Earth are always changing. Here a fixed
RA and DEC won‘t work. The RA and DEC must be calculated. This is
based on where the Earth is within its orbit and where Jupiter （in
this case） is within its orbit. A two-line element （TLE）is a data
record that describes an orbit‘s features. Date and time
information from the GPS enables the SkyScout to calculate orbit
positions based on a TLE. The two calculated orbital positions （in
this case Earth and Jupiter） can be used to determine an RA and DEC
for locating Jupiter. The scientific data displayed for Jupiter is
shown in Table 2.
GPSThe Global Positioning System（GPS） is a network of satellites， set
in geostationary orbit around the Earth，which transmits a
time-synchronized signal. The National Marine Electronics
Association （NMEA） developed the specification that defines the
data issued by GPS receivers. One such sentence （ASCII data） is
shown below：
$GPRMC，123519，A，4807.038，N，01
131.000，E，022.4，084.4，230394，003.1，W*6A
"RMC" is the recommended minimum sentence C data. "123519" is the
time of the Fix， 12：35：19 UTC. "A" is the Status （A = active or V =
void）。"4807.038，N" is the latitude 48°07.038′N. "01131.000，E" is
the longitude 11° 31.000′E. "022.4" is the Speed over the ground in
knots."084.4" is the Track angle in degrees True. "230394" is the
Date， 23rd March 1994. "003.1，W" is the Magnetic Variation."*6A" is
the checksum data，which always begins with an asterisk.
You might want to visit the NMEA web site or read one of my a
previous columns for more information on using GPS data （"Where‘s
Waldo？： Pinpointing Location by Interfacing with a GPS Receiver，"
Circuit Cellar 126，2001）。 The more satellites a GPS receiver has to
work with， the more accurately it can calculate its location. You
can see from the selected sentence that date and time information
is also provided by the GPS.
The SkyScout uses the GPS to locate the user on the surface of the
Earth（where）， and also to define a snapshot in time （when）。 This
information is used to define a point in time when the user
geographic coordinate system is referenced to the celestial
coordinate system， as if the Earth has stopped rotating in a known
position in reference to all of the objects in the sky.
WHICH WAY IS UP？At this point， all we‘ve done is stop the Earth. （We've taken a
snapshot in time.） We still need to know where to look to find an
object or where we are looking to identify an object. The SkyScout
uses magnetic and accelerometer sensors to determine its
orientation based on the Earth‘s magnetic field and gravity.
Several months ago， I explained how to use an accelerometer to
determine slope （"What's the Slope？： Use an Accelerometer to
Measure Slope，" Circuit Cellar 202，2007）。 The Earth‘s gravity looks
like a 1-G acceleration toward the center of the planet. Once the
GPS has stopped and the Earth and the sky are in a fixed position，
the accelerometer will provide a reference vector from the center
of the Earth， through your position， and extend directly overhead
into the universe. This vector enables the SkyScout to point
directly toward the point in the celestial sphere that the GPS＊s
data has been able to calculate.
Remember that the geographical coordinate and celestial coordinate
systems have the N/S pole axis in common. To move to other
locations on the celestial sphere， we need to know which direction
is north. The SkyScout uses sensitive magnetic sensors to
accomplish this.（The SkyScout is so sensitive to magnetic fields
that it provides mu-metal shields to block any magnetic field
interference from the AA batteries.） Most can tell you that a
compass needle will point north. Some know that this is not really
north. Few know where on Earth it actually points because it is
constantly in motion. Presently， it is located in northern Canada
and it is moving northwest by miles a year. （It is interesting to
note that charged particles from the sun cause it to wander in a
daily elliptical perturbance.） Mapmakers note the difference
between true north and magnetic north for a given map by declaring
the magnetic declination for the area. As you pass from the East to
the West Coast in the U.S.， this declination changes from about
-20 to 20 （see Figure 3）。 You can see that the declination is
important to accuracy and its value is determined by where on Earth
you are located. You may have noticed that the GPS output string
includes a magnetic variation element.
Now that we know which way is north， we can move to any other
coordinate in the celestial coordinate system from the known point
directly overhead. But how does the SkyScout lead the user toward
an object of interest？The sighting tube has eight LEDs located
around its interior circumference. Real-time calculations， based on
the celestial map and position sensors（up and north）， determine the
shortest route and guide your movement toward the object of
interest. One or more of the LEDs are illuminated to gently direct
the user toward the destination （see Figure 4）。 Because the
SkyScout senses magnetism and acceleration in all three axes， the
processor can continue to calculate the correct direction
independently of how you are holding the Sky- Scout. Object
identification and location will work night or day and even through
the Earth， as if the sun and Earth don‘t exist. Although we can't
"see" objects during daylight or on the opposite side of the Earth，
the fact that the SkyScout points there reinforces the fact that
just because we can‘t see it doesn't mean it‘s not there.
To provide the SkyScout with the necessary computational power， the
design was built around Samsung's S3C24310，an ARM9 series 32-bit
processor. The device supports plenty of goodies， including USB，
LCD， and SD interfacing （see Figure 5）。 The total design is based
on three PCBs， GPS， an LED directional ring， and the main processor
board. Photo 3 shows the LED directional ring（removed from its slot
in the sight tube）and the GPS board （with the GPS patch antenna
showing）。 The main board holds most of the other components with
the opposite side saved for the LCD and membrane push buttons （see
Photo 4）。 A USB and headphone jack can be seen on the left and an
SD card slot to the right.
EMBEDDED EDUCATIONAL
The SkyScout‘s ARM processor puts its power to good use in
providing realtime calculations as its sensor arrays indicate a
continually changing position. But its expertise doesn't stop
there. Integrated SD card slot and stereo DAC interfaces add to the
design features. The SkyScout‘s internal field guide provides text
of astronomical facts （about astronomers， man-made objects，
comets，and asteroids） and a glossary. The guide includes a six-part
audio guide to astronomy and narrations or text descriptions and
scientific data of approximately 200 objects. The SD card interface
permits expanding its knowledge bank.
One of the typical improvements to newer embedded designs is the
ability to update the operation system without major surgery. The
processor‘s integrated slave USB port supports updates to the
SkyScout.
FRIENDLY SKIES
Except for picking out "the big dipper，"most of us must plead
ignorance to much of what‘s out there. While location information
of celestial objects is well documented， it requires a clock and
calendar （time and date）， a map of the world （location）， a compass
（direction of north）， and a protractor （for azimuth） to calculate
where and when to look. This complexity scares off most people who
might otherwise enjoy knowing more about the night sky. Our success
as designers depends on our ability to take sensor technology and
provide a simple solution to a complicated problem. The SkyScout is
a perfect example of how individual advances can be combined to
provide a solution that was once unthinkable.
While combining the proper electronic components in the right
packaging provides a solution，a convoluted user interface can ruin
an otherwise great product. The ultimate interface requires no
"user‘s manual." As designers， we need to pay as much attention to
the basic concepts of the design as we do to coding the algorithms
within. Can you say，"iPhone"？
Author's note: Special thanks to Mike Lemp, the architect and
father of the SkyScout.
Jeff Bachiochi （pronounced BAH-key- AH-key） has been writing for
Circuit Cellar since 1988. His background includes product design
and manufacturing. He may be reached through the magazine
（jeff.bachiochi@circuitcellar. com） or his web site（www.imaginethat
now.com）.
RESOURCESJ. Bachiochi, "Car 54, Where (Exactly)Are You?: Adding E-Mail
Capabilities to
Your Project," Circuit Cellar 127, 2001.
---, "Magneto-Inductive Direction Finding," Circuit Cellar 80,
1997.
---, "What's the Slope?: Use an Accelerometer to Measure
Slope,"Circuit Cellar 202, 2007.
---, "Where's Waldo?: Pinpointing Location by Interfacing with a
GPSReceiver," Circuit Cellar 126, 2001.
National Marine Electronics Association (NMEA), www.nmea.org.
SOURCESSkyScout
Celestron
www.celestron.com
S3C24310 Evaluation board
Samsung
www.samsung.com
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