How To Begin Your Observations
There are many excellent astronomy resources available. The references made are not all inclusive nor
are they an endorsement by the USAF Academy.
Making observations with a telescope requires a lot of careful planning. In particular, new users
of the FTN telescopes need to consider a few specific questions:
1) Is your target (or targets) visible at all from an FTN site?
2) What time of year and what time of night is best for observing your target (or targets)?
3) Is your target (or targets) well matched to the capabilities of the telescope’s camera?
Below is some information to help you address these one by one.
1) Target Visibility From Your Location
(One good resource to read more about this topic is
here: http://www.astronomynotes.com/nakedeye/s4.htm)
To begin this discussion, it is helpful to have a globe of the Earth handy. In particular, there are
special globes, called "Celestial Spheres", that are even more helpful. A Celestial Sphere shows
the globe of the Earth encased in a see-through globe that has stars in the sky printed on
it. Here's an example from National Geographic:
What a globe like that helps you picture is that from one location
on Earth, you can only ever see half of the sky -- the half that is
above your horizon. For example, the North Star, Polaris, is
located almost exactly above the North Pole of the Earth. If you
are anywhere north of the equator on Earth, you can see that
star. If you picture living in Australia, though, the North Star
would be below your horizon.
So the first thing you have to figure out is, given the location of
the telescope (for example, La Junta, Colorado), would your
object be above the horizon?
This can be done mathematically, so more sophisticated students may want to research the
"equatorial coordinate system" of right ascension and declination. This system is similar in
many ways to our system of longitude and latitude on the Earth. The declination of a star (or
other astronomical object) is measured in degrees north or south of the celestial equator. So,
again for example, Polaris has a declination close to +90 degrees (the positive sign means
north). Your horizon can also be defined as having a minimum declination. For State College,
PA, at close to 41 degrees north latitude, our horizon stretches to -49 degrees in declination(the
negative sign means south).
So in principle, you can observe any object in State College, PA, that has a declination from -49
to +90. There are a few issues, though. The first is that observing anything close to your horizon
is more difficult, so in reality our view is limited to more like -40 than -49. The other issue is
that the time of year is also important. That will be discussed more in section 2.
2) Best Time of Year and Time of Night to Observe
If you are used to watching the Sun or the Moon during one day or night, you probably know
that both objects rise in the east, get higher and higher on the sky, and then set in the west. Every
object in the sky appears to rise and set, too, because this effect is caused by the rotation of the
Earth. So, during one night, you usually have multiple hours to observe your object in the sky
from the time it rises to the time it sets. The best time to observe any object is when it is highest
in the sky, and astronomers refer to that point on its path as when it is "transiting the
meridian". Even if you've never heard that terminology before, you use it even if you aren't
aware you're using it! Noon is defined by astronomers as the time when the Sun is at its highest
point (transiting the meridian), so it is morning or AM (ante meridian) when the Sun hasn't
reached the meridian and it is afternoon or PM (post meridian) when the Sun has passed the
meridian. So you want to find the time when your object is close to the meridian from your
location.
Another issue you have to deal with is the time of the year. I'm sure you've noticed that you can
only see stars at night, and not during the day. So on any given night, we can again only see half
of the sky -- the half of the sky that is in the direction opposite that of the Sun. The half of the
sky in the direction of the Sun is the daytime sky, so any stars in that half of the sky will be "up"
(above our horizon) during the daytime, so you can't see them. So you need to be able to figure
out what stars are up only at night from your location at your chosen time of year. As the Earth
orbits the Sun, which half of the sky is visible changes. For example, the stars visible in the
summer are those on the opposite side of the sky from those visible during the winter.
You can often find star maps that are labeled with the season -- you might know that Orion is a
winter constellation (group of stars) and Scorpius is a summer constellation. From State College,
PA, the constellation Orion is visible somewhere in our sky at 9:30pm from early November to
late April.
To figure out what time of year and what time of night to observe your object, you can use a star
map to figure out if your object is a fall, spring, winter, or summer object, or again, you can use
the equatorial coordinate system. This time, the right ascension coordinate is important. When
your chosen astronomical object is transiting the meridian, it will appear due south of you. On
any given date and at any given time, you can find out what right ascension corresponds to the
direction due south. For example, at midnight on March 21, due south has a right ascension of
12 hours. The star Denebola in the constellation of Leo has a right ascension of 11 hours 50
minutes, so it is very close to the meridian at midnight on March 21. An object with a right
ascension of 0 hours is opposite in the sky from Denebola, so it would not be visible at night
time on March 21st.
Picking the best time and date to observe your object is a pretty complicated topic, so there are
lots of tools to help you. For example, you can use a planisphere, like the one below from the
Astronomical Society of the Pacific to show you what part of the sky is visible on a given date at
a given time. There are also on-line resources. You can consult Sky Maps like those offered by
Astronomy or Sky & Telescope magazines, the interactive sky chart at heavens-above.com,
computer software like Stellarium (free) or Starry
Night (not free), or smartphone apps like Star Walk or
Sky Safari.
3) The FTN Cameras
After you have discovered what part of the sky is
visible from your site at your time of year and your
time of night, you will want to take a picture of that
object. Just like a camera you might use to take a
picture of your friends, though, you want the subject of
your picture to fit well in the camera. That is, if you
are taking a picture of your whole class, you don't
want to be too close, or you'll only get a few people in
the picture, or too far away or you won't be able to see
everyone's face.
In our case with astronomy, we can't control how far away we are, so objects will appear to be
some "angular size" as seen from Earth. For example, the Sun and the Moon are about 1/2 a
degree on the sky (the distance from horizon, overhead to the opposite horizon is 180
degrees). In every degree, there are 60 arcminutes, so the Sun appears to be about 30 arcminutes
on our sky.
The camera attached to the FTN telescopes has a field of view of about 5 arcminutes. That
means if you pointed it at the Moon, it would only capture about 5/30 or 1/6 of the entire Moon's
face.
For every astronomical object, you should be able to find its angular size expressed in degrees,
arcminutes, or arcseconds (1 arcsecond is 1/60th of an arcminute). If your chosen object is larger
than 5 arcminutes, it will be too big, and if it is only a few arcseconds, it will appear too small.
The cameras also have a limit for how bright or how faint of an object they can detect. The Sun
and the Moon are too bright -- they will make the camera record too much light, and you won't
be able to see anything. Some objects that are incredibly far away, like distant galaxies, are so
faint that you need much bigger telescopes to observe them. However, if you find an object that
fits the angular size of the FTN camera, it will probably be the correct brightness as well.