Preprocessing of the HRC CCD data included overscan trimming and overscan bias level subtraction. No preprocessing of the SBC images was required. The same method was used for determining Y positions for slope computation using a 31 row by 13 column region of the image. Each measured position was corrected for image motion as determined from the filter observations. A constant rate of motion was assumed between the filter observations.
Pixel position is in the dispersion direction and starts at zero for the first pixel.
For the HRC CCD, the pixel position is the position after trimming the overscan regions of the image. Initial guesses for the dispersion coefficients were obtained by trying hundreds of randomly chosen initial guesses and picking the one giving the final best RMS residual. The wavelength calibration results for PR110L are shown in Table 3 . Image positions were computed using cross correlation with the F115LP observation number 8831 whose centroid was computed at pixel 499.18. Image motion was corrected for by using the F115LP observation number 8846 taken at the end of the sequence of PR110L observations.