arc-sec - Abbreviation for an arc-second, a measure of angular
distance on the sky. An arc-second is 1/60th of an arc-minute. An
arc-minute is 1/60th of a degree. The moon is about 1/2 a degree in
diameter, or 1800 arc-seconds.
CCD - Charge-Coupled Device. This is the instrument of choice for
collecting nearly all optical modern-day astronomical data.
Originally developed for military purposes, a CCD is a silicon-based
device that is very sensitive to incoming photons. These instruments
revolutionized the field of astronomy for several reasons that I won't
go into right now.
FWHM - Abbreviation for the Full-Width at Half-Maximum. This is a
measure of the width of a Gaussian (bell-shaped) curve. Draw a
horizontal line across the curve, halfway from the base to the peak.
Now measure the distance from the left side to the right side of the
curve along the line. This is the Full-Width at Half-Maximum.
magnitude - An astronomical convention for describing the
brightness of a star. The larger the number, the fainter the star.
Your eye can see to 5th magnitude in a very dark sky. Each factor of
100 decrease in a star's brightness is equivalent to a 5 magnitude
increase. Thus, a 20th magnitude object is a million times fainter
than the faintest star you can see by eye.
seeing - a term that describes the quality of the data (smaller is
better). It is a measure of how large a typical star's image is in
the data. If your star's light is all spread out, it becomes
difficult to distinguish the star from the sky background (even at
night, the sky glows a little bit). The largest contributor to poor
seeing is the turbulence in the atmosphere (turbulence is responsible
for a star's twinkling). The quality of your telescope also affects
seeing (if your telescope is wobbling all over the place, a star's
image is going to get blurred and spread out).
signal-to-noise - Abbreviation for the ratio of signal to noise.
It is a useful scientific measure of the quality of your data. If
your signal (the thing your studying) is three times larger than the
noise (random events that tend to mask the signal - like the amount of
static you hear when trying to tune in a weak radio station), then you
have a minimally significant detection of your signal.
