When the image of Cas-A is first displayed, the image is displayed in shades of black and white, and its features are a bit hard to see. But try this:
move the mouse into the window containing the image
press the right mouse button and hold it down
move the mouse around in image with the right mouse button pressed
Notice how the picture changes so that features of the image come in and out of prominence. Changing the contrast & bias like this is one way that astronomers get a better view of their data. You should experiment with this feature until you like the picture that you see. Note that you can change contrast and bias whether the image is in gray scale or in color.
What exactly is Contrast & Bias?
Contrast refers to the rate of change of color with color level. At low contrast, color changes gradually over many intensity levels, while at high contrast it can change rapidly within a few levels. Contrast adjustment works whether the image is in black and white, or in color.
Bias refers to any offset added to the color levels before the color map is applied. In other words, it determines where the color changes start. Changing the bias corresponds to translating the color map with respect to the intensity levels without changing the overall "look" of the map. At low bias, low intensities (i.e., low pixel values) will have non-zero color differences, while at high bias only high pixel values will have non-zero differences. All this is much easier to see than it is to explain, so just experiment with the images, and you will soon see what we are talking about! When you change the contrast and bias, watch the sliding color scale at the bottom of the DS9 window. Soon you'll be an expert!
Changing Contrast: Contrast is adjusted by dragging the right mouse button vertically in the display window.
Changing Bias: Bias is adjusted by dragging the right mouse button
horizontally in the display window
Finessing both Contrast and Bias: Clicking on the 'Color' menu and selecting 'Colormap parameters' will allow you to access both contrast and bias numerically. The default measurement for each is 1.0 and 0.5 respectively. You can increase these measurements incrementally and apply the changes to the image until the view of the data meets your needs.
