Because ORDERG operates by bootstrapping positions of previously located orders, a potential problem in finding the first order can occur. The SWP camera circumvents this problem by ignoring the first order (m=66), as it describes a sinuous path on the raw image. As a result, ORDERG begins with order 67 for the SWP. The situation is more complex for the long-wavelength cameras because the gradient in camera sensitivity with increasing wavelength near the camera edge causes the identification of the first order to be sensitive to exposure level. If this identification is wrong, the window limits will be misassigned and large errors introduced into the mean global shift. This problem has been addressed for the long-wavelength cameras by starting the search at the (normally) third visible order.
A second type of problem involving order misidentification occurs for images having broad P-Cygni features with weak continuum (e.g., Wolf-Rayet spectra). If such broad features are common and distributed throughout the image, the true continuum orders may be too weak for ORDERG to use for registration. If the search for several such orders in a row is unsuccessful, errors in the search windows for new orders may become large enough to cause a misregistration of the next orders by an order. This problem was addressed by specifying that the search window positions be computed from the average of the positions of the previous three orders (per Step 1 discussion). A very large number of test images for these problems were checked, but there is no guarantee that the solutions for every image in the NEWSIPS archive will be accurate.