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Next: Step 2: Differential Up: 4.9.1 Order Registration Process Previous: 4.9.1 Order Registration Process Step 1: Global Shifts

This step generates a global spatial shift, which is an average of the individual shifts for each order that can be located successfully. The calculations are based on an 11-pixel-wide swath in the spatial direction of the crudely rotated raw image. Initially, the maximum and minimum DN values are computed in a predefined search window, which is expected to include the first (long-wavelength) order. These maxima and minima are assumed to be the ``peak'' flux of the order and the local ``background'' level, respectively. The local background (interpolated from pixels on either side of the order) is subtracted from the spatial profile, resulting in a ``net-flux'' profile. If the peak of the net-flux profile exceeds 5 DN, the algorithm proceeds to compute the centroid position of the order.

The centroid position of the order is computed using a least-squares gaussian fit to the net-flux profile within the preselected search window limits. Following the determination of an order centroid, ORDERG steps to the search window for the next order. This window is computed from the found positions of the preceding three orders (except for the first two orders). If an order does not have sufficient net flux for explicit centroid-finding, ORDERG steps to the estimated position for the next order, and an attempt is made to find that order. This process continues from the long-wavelength orders to the short-wavelength orders.

Following the determination of the order centroid positions, relative weights are assigned to each position according to the peak net flux. Found-minus-expected centroid position differences are computed for these orders and compared to the corresponding differences computed for orders from a fiducial image unique to each camera (LWP06316, LWR14996, and SWP13589). The order positions for these special images were calculated during the initial development of the ORDERG algorithm. A weighted least-squares solution of these pixel differences computes both a mean global shift and an rms statistic. The mean shift is the final value output from Step 1 and is the value applied to all lines in the image if any test in Step 2 fails (see below). However, two tests must first be passed before Step 1 is completed. The first is that a minimum number of orders (five for SWP, three for LWP/LWR) must be found with sufficient flux for order-centroiding. The second is that the rms statistic referred to above must be below a threshold (1.5 pixels). If either of these tests fails, a default value is adopted for the global spatial shift based on statistical predictions using time- and THDA-dependent spatial motions. Also, for either of these failure conditions, the noncontinuum keyword for the appropriate aperture is also set to ``YES" in the HISTORY portion of the FITS header. Note that the setting of this keyword to ``NO" (i.e., all tests in Step 1 are passed) denotes that the image is considered to have continuum flux and is treated as such by the background-extraction algorithm described in Chapter 10.

next up previous contents
Next: Step 2: Differential Up: 4.9.1 Order Registration Process Previous: 4.9.1 Order Registration Process
Karen Levay