Education Activities To Accompany Chandra Data Analysis Software Cas A
Activity 3: How big is it?
Go back to the image of Cas A, and zoom it back out so you can see the
entire object in the DS9 window. Place the cursor on the very top of
the SNR, and note the physical Y value of the position. Sweep the
cursor down to the very bottom of the remnant, trying to keep the X
value constant. This is a crude "eyeball" measurement of the size of
the remnant in the sky. Do the same by sweeping across the remnant,
keeping the Y value constant this time.
Note: You will see that in the Photo Album image of Cas A the
scale (size of the image) is given as 6 arc minutes, or 360 arc seconds
which is 720 physical pixels Your result will be somewhat different
depending on where you select the "edge" of the remnant to be.
Moreover, different observations will yield slightly different results
if their exposure times are different. (If the exposure is longer, for
example, you might see fainter features beyond where you detected the
edge to be here). In any event, the values you derive should be in the
vicinity of the answers given below. Please be aware that since the
data can be binned (i.e. several adjacent pixels can be added and
displayed as one image pixel), you must always use "physical" pixels
when calculating the extent of any object, since these values will
always reflect the native 0.5 arc-sec resolution of the satellite. This
point will is discussed more fully in the activities chapter for
"galaxies and and clusters of galaxies".
How wide is the remnant (in pixels)? How high is the remnant?
(You can also answer this by placing a region down, centered on the
pulsar, selecting it, and dragging the handles to make the size roughly
encompass the entire remnant. Then you can go to the region menu in DS9, click on Get Info... and read off the radius directly).
