While reducing some December 1995 GRIM data, I noticed that the bias pattern shows non-repeatable structure in the y direction, with typical levels a few 10s of ADU and spatial scales of about 10-20 pixels. This is most easily seen by subtracting two dark current frames of equal exposure time. The result shows clearly visible horizontal stripes perhaps 1/3 of the time. These are best examined by projecting the difference frame along the x axis to get the mean of the 256 columns. The pattern amplitude is usually largest near the bottom of each quadrant, but exceptions occur. Similar patterns can sometimes be seen in the differences of sky frames. The differences of sky frames, he pattern appears to me to repeat, i.e., the structure in rows 129-256 is the same as that in rows 1-128. I confirmed that the effect was still present in late April. The potential effect on photometry of faint, diffuse objects is substantial, although less than the old quandrant-to-quadrant fluctuations in the bias level. To "destripe" my data, I have written a fortran program that (in its simplest form) subtracts from rows j and j+128 the median pixel value in rows j and j+128, for all j in 0<j<129. Pixels within some circular region of the frame may be excluded from the medians (to avoid subtracting out diffuse sources), and a model image may be subtracted from each data frame before de-striping (though I would not recommend this under most cuircumstances). The code uses the IRAF Imfort image i/o routines, and can be compiled using the IRAF "fc" task. This method seems to reduce the problem substantially, but of course it will not work well for objects larger than the GRIM field of view. Subtraction of two dark frames also often reveals a low-level diagonal striping in the lower right hand quadrant. This looks to me like it is nicely periodic and could probably be removed in the Fourier domain, but so far I have not tried to do so. -James Rhoads rhoads@astro.princeton.edu APO APO APO APO APO Apache Point Observatory 3.5m APO APO APO APO APO This is message 14 in the apo35-grim archive. You can find APO the archive on http://astro.princeton.edu:82/apo35-grim/INDEX.html APO To join/leave the list, send mail to apo35-request@astro.princeton.edu APO To post a message, mail it to apo35-grim@astro.princeton.edu APO APO APO APO APO APO APO APO APO APO APO APO APO APO APO APO APO