The extraction of high-dispersion spectral and background fluxes requires a precise knowledge of the placement of the echelle orders. Target centering errors and camera temperature (THDA) variations can shift an image potentially by several pixels. The ORDERG module in RAW_SCREEN computes an estimated shift that corresponds to the spatial direction in the high-dispersion resampled image (SI) geometry. Because ORDERG reckons this shift from the raw image, it uses raw DN values as the unit of flux. In addition, the computations are made from a simple rotation of the raw image (identical to the one described in Chapter 4.6) which is meant to approximate the high-dispersion SI geometry.
Studies of high-dispersion echellograms show that the order locations for individual images can differ not only by simple translational offsets but also by an expansion and contraction term (differential order shifts) as well. Because attempts to correlate such distortions with instrumental variables have been unsuccessful, ORDERG was designed to determine both the mean global shift of each image and, if possible, the differential order shifts, which can be as large as 0.6 pixels. Note that similar shifts in the dispersion (spectral) direction cannot be corrected for in this manner because they are indistinguishable from wavelength shifts due to target centering errors or the radial velocity of the source. Shifts in the dispersion direction are addressed in Chapter 8 , which deals with the wavelength calibration.