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XMM-Newton Science Analysis System


omgsource (omgsource-1.2.8) [xmmsas_20040318_1831-6.0.0]

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The spectrum-selection window

This window is shown in Figure 2.

Figure 2: Spectrum-selection window

When the cursor is moved across the image (using the left-mouse button) a profile, from the cursor-position upwards to the height of the spectrum extraction-region height, is drawn at the bottom of the image. A new spectrum is selected by centering the cursor over the approximate centre of the corresponding zeroth-order feature and clicking the left-mouse button. The extraction region is plotted below the image and its position and dimensions are stored in the table in the upper-left corner of the window. A selected extraction region can be discarded by clicking the right-mouse button within the spectrum-extraction region.

The user can change the width and height of the extraction rectangle and the widths and offsets of the left and right background-extraction rectangles (within certain limits). The left and right background extraction offsets are the distances (pixels) from the left and right sides of the inner spectrum extraction-region, respectively.

The computation of a spectrum is done using the SAS task omgrism, and omgsource produces a source-list file containing two sources, the zero and first order spectra, for input to omgrism. The program allows the approximate y cursor-coordinate of the zeroth-order spectrum to be either fixed at the cursor-position (y Zero-order button unchecked, or refined using a centroiding algorithm (yZero-order button checked. Since the wavelength scale of the first-order spectrum is set using the y-coordinate of the zeroth-order spectrum, the accuracy of the scale is therefore determined by the accuracy of the y-coordinate. The user should use the automatic centroiding option, unless he/she has good reason to believe that it is inaccurate.

Likewise, the approximate x cursor-coordinate of the first-order spectrum can be either fixed at the cursor-position (x First-order button unchecked, or refined using a centroiding algorithm (x First-order button checked. The reason why the first-order x coordinate is used instead of the zeroth-order coordinate, is that there could be a shift of several pixels between the two positions.

NOTE THAT WHEN OMGRISM ANALYSES A SOURCE-LIST PRODUCED BY OMGSOURCE, NO CHECKING IS DONE ON THE SOURCE PARAMETERS- IT IS UP TO THE USER TO CHECK THE VALIDITY OF ANY SPECTRUM PRODUCED