Much astronomical data is in the form of 2D images. It is critical to understand how to display such data and be able to see all of the information it contains. This is complicated by the fact that the dynamic range of most computer displays is smaller than that of most astronomical detectors. Most image displays provide only 8-bits of display range, giving only 256 possible colors. Any image with more dynamic range must somehow be compressed into 8-bits before it can be displayed. This can be done either by sampling the true image coarsely, which allows viewing of the whole dynamic range but can lead to the apparent loss of intensity detail, or by fully sampling only a part of the true image range, which leads to the loss of ability to view detail outside the chosen range. Beware of packages which make this choice automatically; note that this ``convenience'' can lead to loss of apparent information, and also that manual choice of scaling levels assures that the user knows values of numbers in his/her image: something you should know to make sure that values are somewhere around levels that you expect! Image scaling parameters are generally specified by a low and a high data value (or a low value and a range) which give the limits in the true data which will be scaled into 8-bits.
Note you can also use a nonlinear scaling to sample a larger (or smaller) range. Example: logarithmic or square root scaling.
Once an intensity subsection is chosen, it can be displayed with any choice of ``color map'', which specifies the colors to be assigned to each of the display levels. These can be various shades of grey (greyscale) or some other color, or some arbitrary color scheme (pseudo-color). Note that most packages allow the user to manipulate the color table, allowing users to change the contrast and brightness of a displayed subsection; for this reason, it is usually reasonable to chose a range with a significantly larger range than 256 data values.
Most packages will allow the user to inspect individual data values based on a cursor location. Beware, however, of packages which give data readout based on scaling parameter and 8-bit display number only: these are unable to give correct values outside of the scaled region of the image.
Other useful display tools include zoom, blink, interactive image analysis (peak, valley, fwhm, etc), marking of objects, etc.