\section{Introduction}

This lab is intended to introduce you to the concept of digital imaging used
by modern research astronomers, and to show you why astronomers use the
combination of large telescopes and efficient detectors.

The lab is split into 3 stations:

\begin{enumerate}
\item What does a large telescope do for you? You will experiment with using
a small telescope with several different aperture masks to discover the
effect of aperture size on observations.

\item What does a detector do for you? You will use a small digital camera
to demonstrate the advantages of detectors which can integrate the light
arriving over a period of time.

\item What is digital imaging, and how do errors affect what you can measure?
You will experiment with an image simulator to make images of astronomical
objects at a variety of exposure times, sky background levels, and telescope
sizes.

\end{enumerate}

You will work at each station with in small groups of 3-4 people each. 
Each station has associated tasks and questions. You can do the stations
in any order. The third station will probably take the longest amount of
time; as a result there are more of these set up than for the other two
stations.

\begin{itemize}
\item{{\it Goals:} To understand why astronomers use large telescopes and
digital detectors, and to understand some of the limitations of measurements}
\item{{\it Materials:} spotting scope and tripod, masks for scope, hair
dryer, computers, video cameras for computers, light-tight boxes}
\end{itemize}

\section{What does a large telescope do for you?}

\begin{enumerate}
\item Write yourself a message on a piece of paper about 2 inches square with
letters about 1/2 inch high. Tape it to the wall opposite the telescope.

\item You will look at your message with several different masks over the
front of the telescope; each mask has a different sized hole in it. 
\textbf{Before you look through the telescope}, discuss and then write down your
predictions about the differences you expect to see when using the
different masks. You may wish to consider items like how large an area of
your message you'll be able to see, how magnified the letters will look,
how sharp the letters will appear, how bright the message will appear,
and how easy the message will be to read.

\vskip 2in

\item Look at your message through the telescope. You may need to focus the
telescope to get a sharp image. After you have a good image, alternate with
different team members holding the different masks over the front of
the telescope.  Describe all of the differences you see, considering
all of the issues mentioned above. Experiment with moving a given hole
around on the front of the telescope (i.e. put it at the center, towards
an edge, etc.) and describe what you see.

\vskip 2in

\item Hypothesize about why you get the results which you observe.

\vskip 2in

\item Simulate the effects of the earth's atmosphere (winds and looking
through air of different temperature) by looking at a light source
(candle or flashlight bulb) through the telescope and then blowing
hot air from the hair dryer in front of the telescope. Describe
what you see.

\vskip 2in



\end{enumerate}

\section{What does a detector do for you?}

In the olden days, astronomers would sit behind telescopes and look through
them in much same fashion that you have been looking through the campus
observatory telescopes this semester. However, modern astronomers very
rarely look through a telescope using their eyes. Instead,
almost all modern astronomy is done by placing a detector behind telescopes 
and recording images to be analyzed later. Up until about 20 years ago,
the main detector used was simply photographic film, although usually the
film was deposited on large glass plates rather than on the cellulose
which you may be familiar with from normal cameras. In the past several decades,
however, astronomers have switched to using solid-state detectors. In
fact, the same type of devices used by astronomers are used in today's
handheld video cameras.
 
You will be using a small digital camera which is run by a PC to
demonstrate the advantages of using a detector, rather than your eye,
to look at objects. Before you start, note down any ideas you have about
what advantages and disadvantages there might be to using a detector
instead of looking with your eye.

\vskip 2in

\begin{enumerate}
\item If not already started, start the program which controls the camera.
To do so, double-click the icon labelled QuickCamVC with the left mouse
button, or start the program named Connectix QuickCamVC using the Start
menu accessable by clicking the left button on the Start button in the
lower left of the screen. 

\item Take the video camera out of the box and play around with it, 
just for fun. Open up the camera
settings window by clicking on the button in the lower right of the camera
window (or by selecting it from the Settings window). Check the box at the
bottom which says Default Settings to make sure the camera is set up OK.
You can take the camera out of the box and look at videos of yourself,
classroom, etc...

\item Attach the camera onto the bottom of one end of the box using some tape, 
with the threaded hole facing down; point the camera at the opposite side
of the box. Write yourself a message on a piece of paper about 3 inches
square and tape it to the opposite side of the box where the camera is
pointing. Confirm that you can see the message on the computer screen.

\item In the settings window, turn off AutoBrightness, set the brightness
level to the brightest possible and set the exposure time (bottom slider)
to the middle of its range.

\item Tape the box closed. Cover the hole. Confirm that the message disappears
on the computer screen.

\item Gradually open the hole until you can just barely start to see your 
message. You will probably only need a sliver of light.

\item Increase the exposure time by moving the exposure slider towards longer
exposures. What do you see? Comment on both brightness and clarity of
the message. Increase the exposure time more.  What do you see? Imagine
you could increase the exposure time to several hours. What do you think
you would see?

\vskip 2in

\item Uncover the small peephole at the end of the box, and look through it
at your message with the same opening which you used for the camera. 
Can you see it? Stare at it for a longer period of time.
Can you see it any better? Imagine you could stare at it all night. Do you
think you'd see it any better by the end of the night?

\vskip 2in

\item Given your results, why do you think astronomers prefer to use detectors
instead of their eyes?

\vskip 2in

\end{enumerate}

\section{What is digital imaging, and how do errors affect what you can measure?}

Astronomers use detectors that record images as a large number of brightness
measurements at different location. At each different location in an image,
the digital detector records the brightness at that location as a number.
Regions which are brighter have a larger number associated with them than
regions which are fainter. Every astronomical picture taken with a digital
detector is actually just a collection of thousands or millions of individual
numbers, each recording the brightness at a different location. In the
language of the trade, each individual location in a picture is called
a \textit{picture element}, or \textit{pixel} for short.

To make a color picture, three separate images are recorded: one is
what the image you see of the object if you only look at the red light
coming from it, one with only the green light, and one with only the blue
light. Thus, to make a color picture, astronomers take three separate
images. They isolate the red, green, or blue light by putting a colored
filter in front of the detector, using a different color filter for each
different observation.

Because the pictures which astronomers take are a collection of numbers, it
is possible to make quantitative measurements, for example, of brightnesses
of objects. It is also possible to use \textit{image processing} to enhance
certain features in an image, to calibrate images, and to combine images to
bring out important physical information.

To investigate the concepts of digital images and some inherent limitations
which nature places on our ability to enhance the images, you will experiment
with an image simulator which will let you create simulated images of
a galaxy using different exposure times and different sky brightnesses.

\begin{enumerate}

\item The \htmladdnormallink{image simulator}{../TELESCOPE/simulate.htm}
should be running when you get to the computer.
If not, consult the instructor.

\item To simulate an image, answer the questions. You will need to choose
an exposure time in seconds; remember that an hour is 3600 seconds, which
is a typical exposure time used by astronomers. You will also need to select
whether you want your image to be simulated as it would appear from a
bright site or a dark site. This option is designed to show you the effects
of light pollution on astronomical observations.

\item Once you have simulated an image, you can demonstrate the digital nature 
of your image by using the \$ key in the display window to zoom in and see 
individual pixel values. Note that pixels which appear bright have
bigger numbers. Use the mouse buttons in the display window to zoom in and
out of your image.

\item Note that while the image is a collection of numbers, you can alter
the \textit{appearance} of the image by choosing different values to match
with the displayed black and white colors. Experiment with changing the
black and white values to change the apperance of the image, while at the
same time, use the \$ key or look at the pixel values in the lower left
of the display window to confirm that the pixel values remain unchanged.
Adjusting both the black and white value up or down by the same amount is
called changing the brightness; can you see why? Adjusting the levels so
that the black and white values get closer to each other is called increasing
the contrast; can you see why? These are the same adjustments you can make
on most TVs and video cameras.

\item Experiment with observing the object with several different exposure
times, using the same sky brightness.

\item Since the images are just collections of numbers, one can easily just
multiply all of the numbers by a constant value. For example, one could
take a short exposure and then multiply all the numbers by 10. Is this the
same thing as taking an exposure which is 10 times longer? Experiment with
the image simulator to look for qualitative differences between a scaled up
short exposure and a long exposure:
\begin{enumerate}
  \item Simulate a short exposure (e.g. 100 seconds, dark sky)
  \item Use the scaling option (S) to multiply this by 10. Note the appearance
        of the image.
  \item Simulate a longer exposure (e.g. 1000 seconds, dark sky). 
        Compare this exposure with the scaled short exposure.
  \item Describe the differences and comment on the need to take longer 
        exposures. How can the same argument be applied to the need to
        build bigger telescopes?

\vskip 2in

\end{enumerate}

\item Experiment with observing the object at the same exposure time, but
with different sky brightnesses.

\item Again, since the pictures are just collections of numbers,
one can easily subtract a number from an image.
Can you use this ability to compensate for the effects of observing from
a location where the sky is bright? Experiment with the simulator to look
for qualitative differences between an exposure taken in a dark site with
the background subtracted, and one taken in a bright site with the
background subtracted:
\begin{enumerate}
  \item Simulate an exposure from a dark site (e.g. 1000 seconds)
  \item Use the backgroun option (B) to subtract off a background value. Move
        the cursor around in the image, and look at the readout in the lower
        left corner to determine a reasonable background. Note the appearance
        of the image after background subtraction.
  \item Simulate the same length exposure from a bright site. Again, subtract
        off a background value. Compare this exposure with the background
        subtracted exposure from the dark site.
  \item Describe the differences and comment on the need to make astronomical
        observations from dark locations. Comment on any personal experiences
        you might have about looking at the sky from a dark location vs.
        from a bright location. Do you think the location of the campus
        observatory is optimal?

\vskip 2in
\end{enumerate}
\end{enumerate}

The key point to get from these experiments is to realize that when
light is detected, it is not measured with perfect accuracy. When the light
level is lower, the accuracy is lower; when it is higher, the accuracy
is higher. The inaccuracy in measuring light is an intrinsic property of
light itself and cannot be eliminated by any sort of technology. The
inaccuracy of the measurement of light is often called \textit{noise} by
astronomers. This is a general concept which applies to almost all measurements
made in any science: when one makes a measurement, it is almost never made
with perfect accuracy. Understanding the level of inaccuracy is often as
important as understanding the quantity which is measured.


\section{Summary}

\begin{enumerate}
\item Summarize in a few sentences what you have learned from this lab.

\vskip 2in

\item How did you like this lab compared with the other labs of the semester?
Do you have any suggestions for improvements?

\end{enumerate}

