The LTVi and LTMi_j keywords give a linear mapping from a reference pixel coordinate system to the current pixel coordinate system of an image. The reference coordinate system is detector pixels for the CCD, and it is low-res detector pixels for the MAMA. In IRAF notation, the first pixel is number one, and the pixel number is an integer at the center of the pixel. The first pixel runs from 0.5 to 1.5. LTMi_i is the reference pixel size in units of the current pixel size. LTVi is the location, in the current coordinate system, of pixel zero of the reference coordinate system. Note that "pixel zero" does not mean the first pixel using zero indexing; it means the location one pixel to the left of (or below) the first pixel in the image. The situation is straightforward for MAMA data or for unbinned CCD data, and the values of LTVi and LTMi_i are given in the following table. For STIS data, LTM1_2 and LTM2_1 are zero (no rotation or skew). detector bin axis lengths LTV1 LTV2 LTM1_1 LTM2_2 -------- --- ------------ ---- ---- ------ ------ MAMA high 2048 x 2048 -0.5 -0.5 2. 2. MAMA low 1024 x 1024 0. 0. 1. 1. MAMA 2x2 512 x 512 0.25 0.25 0.5 0.5 MAMA 4x4 256 x 256 0.375 0.375 0.25 0.25 MAMA 8x8 128 x 128 0.4375 0.4375 0.125 0.125 CCD none 1024 x 1024 0. 0. 1. 1. Reference images can have the above values of LTVi and LTMi_i. Even in the case that binned CCD images are used to create the reference images (e.g. bias), it seems reasonable to me to expand the data to 1024 x 1024. The complication for binned CCD observations is that the physical overscan region is 19 pixels, so when binning there is a pixel near each end of the line that consists of part overscan and part illuminated. This pixel is removed with the overscan when subtracting the bias level, so the first pixel in the output image does not begin at the beginning of the illuminated portion. This fractional pixel results in some peculiar values of LTVi, which can depend on which amplifier was used for readout. Here are specific values of LTVi and LTMi_i for the CCD detector including the overscan: No binning; axis lengths 1062 x 1044; LTM1_1 = LTM2_2 = 1.; the line contains 19 overscan, 1024 illuminated, 19 overscan. amp LTV1 LTV2 --- ---- ---- A 19. 0. B 19. 0. C 19. 20. D 19. 20. Bin 2 x 2; axis lengths 532 x 522; LTM1_1 = LTM2_2 = 0.5; the line contains 9 overscan, 1 mixed, 511 illuminated, 1 mixed, 10 overscan pixels. amp LTV1 LTV2 --- ---- ---- A 9.75 0.25 B 10.75 0.25 C 9.75 10.25 D 10.75 10.25 Bin 4 x 4; axis lengths 271 x 266; LTM1_1 = LTM2_2 = 0.25; the line contains 4 overscan, 1 mixed, 255 illuminated, 1 mixed, 10 overscan pixels. amp LTV1 LTV2 --- ---- ---- A 5.125 0.375 B 10.625 0.375 C 5.125 10.375 D 10.625 10.375 Bin 8 x 8; axis lengths 140 x 138; LTM1_1 = LTM2_2 = 0.125; the line contains 2 overscan, 1 mixed, 127 illuminated, 1 mixed, 9 overscan pixels. amp LTV1 LTV2 --- ---- ---- A 2.8125 0.4375 B 10.0625 0.4375 C 2.8125 10.4375 D 10.0625 10.4375 After stripping off the serial and parallel overscans we have: detector amp bin axis lengths LTV1 LTV2 LTM1_1 LTM2_2 -------- --- --- ------------ ---- ---- ------ ------ CCD any none 1024 x 1024 0. 0. 1. 1. CCD any 2x2 511 x 512 -0.25 0.25 0.5 0.5 CCD A or C 4x4 255 x 256 0.125 0.375 0.25 0.25 CCD B or D 4x4 255 x 256 -0.375 0.375 0.25 0.25 CCD A or C 8x8 127 x 128 -0.1875 0.4375 0.125 0.125 CCD B or D 8x8 127 x 128 0.0625 0.4375 0.125 0.125 As a specific example, consider CCD data binned 4 x 4. The length of the first axis is 1044 / 4 + 10 = 271 pixels, which break down as follows for amplifier A: four overscan pixels, one mixed overscan/illuminated, 255 illuminated pixels, one mixed, and ten overscan. Graphically this is **** **** **** **** ***0 0000 ... 0000 000* **** **** **** ... **** **** 1 2 3 4 5 6 260 261 262 263 264 270 271 where * represents an unbinned overscan pixel, and 0 represents an unbinned illuminated pixel. **** is then a binned pure overscan pixel, 0000 is a binned illuminated pixel, and ***0 and 000* are the mixed pixels. The numbers below the * and 0 are pixel numbers in binned coordinates. Let's work out the value of LTV1 for amp A. The first pixel in the reference coordinate system is the first unbinned illuminated pixel, which is the fourth quarter of the fifth binned pixel: pixel 4 pixel 5 | | | | | | binned pixels | | | | | | | | | | | | | * | * | * | * | * | * | * | 0 | unbinned pixels | | | | | | | | | ^ | reference pixel zero The "*" and "0" symbols are used in the figure above to indicate overscan and illuminated (unbinned) pixels. The "0" flags the first pixel in the reference coordinate system. From the figure it can be seen that pixel zero in the reference coordinate system is located 1/8 binned pixel to the right of pixel five in binned coordinates; therefore, LTV1 is 5.125. When the overscan is subtracted, five pixels will be removed from the beginning of the line (four pure overscan and one mixed), so LTV1 for the output image will be 5.125 - 5.0 = 0.125.