The simspec task computes a simulated spectral image for STIS long-slit and echelle modes, given the observing configuration and the object description table. The task parameters cenwave and cenorder specify the wavelength and order that fall on the image center, and the dispersion relation is adjusted accordingly. Two ancillary tasks have been added as well. The simbackgd task permits the construction of a customized background image, which may be used instead of or in addition to the background computed by simimg or simspec . The calculated background has contributions due to zodiacal light, scattered earthlight, and thermal background.
Zodiacal light is a function of the relative position of the sun and the telescope pointing.
The earthlight background is calculated from task parameter earthtab , which specifies the maximum earthlight spectrum, and task parameter eshine , which specifies a fraction of the maximum earthlight. The thermal background is calculated from thermtab , which specifies the spectrum of the thermal background. The simnoise task permits the incorporation of customized noise into an image, optionally following a user-specified prescription. The distribution is sampled and the random noise minus its mean is added to each pixel. The noise expression can contain constants and the three variables t , n , and x , which represent the exposure time, the number of reads, and the pixel flux, respectively.