INTRODUCTION TO IMAGE PROCESSING
and
BROWSER’S GUIDE TO THE UNIVERSE
Reading: article from Sky and Telescope: Sky on a Chip: the Fabulous CCD
OPENING AN IMAGE in HOU
·
The
Hands On Universe image processing program and files are accessible from any
networked school computer. After
logging on, click on Start, mouse-over Academic Applications, Astronomy, then
select Hands On Universe
·
Under
File, select Open, (the directory “images” should come up), then double click
on Images-High School, then on 2browsers_guide_to_the_universe, then on
browser5
·
To maximize
the image space, first click the rectangle in the upper right corner (this
maximizes the window on the screen); then drag the browser 5 image’s blue bar
up as high as it will go in the dark gray area; then click on the square in the
upper right corner of the browser 5 frame
·
The
screen should display a black-and-white image of an astronomical object.
Investigate the options under the View menu (i.e., Tool Bar, StatusBar,
Color Palette Bar, and Display Controls Bar; click these on and
off to see what each name refers to.
After experimentation, you will want to have the ToolBar, StatusBar,
Color Palette Bar, Display Controls Bar “on” and the others “off.”
You may want to make some notes about what the bars show.
·
By
this time you know something about CCDs and understand the concepts of pixels
and digital imaging. Each pixel is
“colored” the appropriate shade of gray according to the “Counts” measured for
that pixel; the translation table from “Counts” to “color” is the palette at
the right of the screen. Experiment
with the zoom by increasing the zoom factor to 2, 4, 8, 16, all the
while looking at the object’s center.
[To keep track of where the object’s center is, note that it continues
to have the same x-y pixel coordinates at the bottom right of the screen (as
long as the Status Bar Option has been selected). Have you also noticed that the x-y coordinates and the “Counts”
number change as you move the mouse cursor around the screen?] At what zoom factor can you first begin to
see the actual pixels?
·
Note
that you can also zoom on only part of the image by using the Zoom box
command under Data Tools. The cursor symbol changes to a square of 4
squares. Click and drag the mouse to
create a rectangle in the image that you would like to zoom in on. A new pop-up
window appears with the zoomed area.
Did you notice the zoom factor for the zoom box? Can you zoom in on the zoomed area?
(You can remove any red marks on the image by clicking on the “broom and
bucket” icon just below the menu bar.)
·
Return
to Zoom 1 on the original Browser 5 image, maximize the image, and center the
object in your window. The
black-and-white image is not very interesting because of the lack of
detail. Can you tell what type of
object it is?
How much of the image frame would you say the object occupies?
Astronomers use the idea of “false coloring” to bring out contrast and detail.
·
Under
the File Menu Bar, click on Load Color Palette.... select rain.pal
I think it’s the most interesting; you can experiment with other palettes later
if you want.
·
Under
the View Menu Bar, click on Color Palette Bar so that it is displayed on
the right hand part of the screen... move the mouse/cursor around on the screen
over different color areas of the image.... does the color match what the “Counts”
number (in the lower right of the screen) and Color Palette Bar predict?
At some point, I will ask you to describe in a
quantitative manner how false color digital imaging works; i.e., what does the
computer actually do (given the minimum and maximum counts recorded on the
image) in order to display the false-colored image…
·
Your
goal is to fiddle with the false coloring so that the object’s detail is
brought out; you do this by changing the min and max in the boxes under the
menu bar (and on top and bottom of the color palette). Upon opening the image, the min was probably
set to 215 and the max to 459). Some
hints for setting the min/max follow:
a) To find
a good min,
move the mouse around over the black
parts of the image and notice the Counts reading; select a min value that is
not the absolute minimum, but is a little bit (5 or 10?) above most of the
very smallest values of the Counts
(which are the Counts values in the black areas)
Record your chosen min, and adjust the
min to your selected value. There are
three ways to adjust the min:
(1)
click in the box next to the word Min and then type in the desired value,
(2)
drag the red bar left or right to decrease or increase the min,
(3)
click on the left-pointing arrow box....
After you make a change in the min,
the HOU software should immediately refresh the image to your new standards
b) To find
a good max,
start with the max given; what
happens to the image as you decrease the max?
what
are trade are you making as you decrease the max?
c) The
image will probably still not look very good. Now click on the box for Log
Scaling. The
result should be
impressive. You want to do some
additional fine tuning with min and max.
At
some later point, I will ask you to describe how log scaling works. For the moment,
notice (and record) what has
happened to the Color Palette bar during the switch from linear
to log scaling.
Record your “best” values for min
and max.
Can you trace the galaxy's spiral
arms to the edge of the image window?
How much of the image frame would you say the object occupies? Can you see the knots of bright hydrogen in
the spiral arms?
·
Now
open browser1. I would suggest that
you revert to the grey palette. What is
this object?
How large do you think the large crater is?
How deep?
Did you notice anything inside the crater?
What is it? Any idea where it
came from?
·
Now
open browser2. Can you tell what
it is?
How many moons can you find?
Can you find a min/max combination that allows you to see both the atmospheric
bands on the planet and all three moons?
·
Open browser3. What’s this object? A number of rows on the left side of the
image have been saturated (i.e., the maximum number of electrons possible on a
pixel has been reached, and electrons have spilled over onto adjacent rows of
pixels… this is called “blooming”)
You’ll now use the Slice command under Data Tools to measure the sun’s
diameter in pixels. After clicking on
Slice, click and drag the mouse from one side of the sun to the other. Don’t worry about starting and ending on the
exact edge; in fact, start somewhere outside the edge and end somewhere outside
the edge on the other side. Do try to
have your Slice go through the full diameter of the sun though. A new pop-up window shows a graph of
brightness (in counts) versus distance along the Slice (in pixels).
Place your cursor somewhere on the actual curve in the graph. Click and drag
the mouse along the curve… notice the red dot moving along the Slice? Drag along your Slice to measure the
diameter of the sun in pixels by noting the starting and ending pixel distances
showing in the pop-up window as you drag.
What do we call the bright spots over the limb of the sun?
·
Open browser4. Often, there is information about the object
in the image and where, when, and how the image was taken in the Image info
box (accessed under Data Tools).
name of the object?
exposure time?
time and date of exposure?
where was the image taken? (hint: look at the telescope name and the time)
Can you find objects similar to it
in the Stellar
Evolution summary? Do you see the
white dwarf at the center?
·
Open browser7. Can you find this object in the Stellar Evolution
summary?
This supernova remnant is one of the very few that contains a visible pulsar (or
neutron star) at its center.
To find out which of the central stars is the pulsar go here.
There are a number of ways to manipulate FITS images found under Manipulation
on the Tool Bar. Use a combination of Flip,
Rotate, and/or Transpose to make the browser7 HOU image look like
the one with the arrow pointing at the pulsar.
(There is more than one correct answer.)
A better image for use with HOU software can be downloaded here.
·
Open browser6. Use the rain color palette.
Suppose that you wanted to measure the length of the minor axis (which is
perpendicular to the longest axis of the galaxy through its center). Use Slice and then the same technique that
you used for the sun. As you drag the
mouse along the graph, is there any obvious jump in the counts number when you cross
what appears to be the boundary of the galaxy in the image? If you know why not (and also understand why
there is no well-defined “boundary” to the galaxy), you have learned an
important lesson
Some random questions I ask my students about the
browser images:
Which image cannot be duplicated fairly accurately by
any telescope in the world at any time during this semester? Why not?
Which image has a July 4th
connection? Why?
If we could travel 6 billion years into the future
and then take an image of the sun, which image would it look like?
In which image could you (if the image had much
better resolution) see active volcanoes?
In which image (if the image had much better
resolution) could we see a liquid water ocean?
In which image is the bulk of what you see solid or
liquid (as opposed to gas)?
What is the name of the large lunar crater?
Who is it named after? Why was he famous?
(You might think that (some
of these) questions are impossible to answer!
But all the answers to the questions about the Browser images in this
activity are contained in a single web page.
Your goal, should you want to answer the above questions, is to use a
good web search engine with good search techniques, to find that page.)
