Hi Jim, I bias frame is an image of the noise that is contained in the readout register of the chip as well as the noise contained in the readout electronics plus any electronic offset built into the readout electronics. Ideally it should be taken with zero exposure time but on a practical side your fast exposure is so close to zero that it doesn't make any difference. The reason for a zero exposure time is to make sure that there isn't any thermal contribution to the bias frame. Every time an image is taken the contribution to the image from bias is included. This includes darks and flats as well as the actual image. In addition to the artifacts you mentioned in flats there is the individual photo site sensitivity of each pixel. Think of it as each pixel having a slightly different ASA speed rating. This comes about from uneven polishing of the silicon chip before fabrication of the imager and slight variations in the gate structures for each pixel. Remember that a flat image when ready to apply to an actual image has been normalized: the individual pixel values have been divided by the average pixel values in the central portion of the frame. The actual image is then divided on a pixel by pixel basis by this normalized flat frame. When making flat frames, be sure to take dark frames of the same exposure and temperature to subtract out from the flat frame. This also subtracts out the bias from the flat image. One additional thing about flats is that anytime the optical train is changed new flats must be taken. This includes focusing, filters, cleaning the optics or re-collimation. One additional item is that all frames taken with cameras, DSLR, CMOS or CCD contain noise and the only way to reduce that noise is to use take multiple darks and flats and then averaging, or median combining them. This reduces the noise content by a factor of the square root of the number of the frames. Thus if ten frames are combined the noise is reduced by over 3 times. If you want to get into this in depth I would recommend Richard Berry and James Burnell's book "The Handbook of Astronomical Image processing". As it also includes some very powerful software it is real value. Check out Willman-Bell. Jerry Foote ScopeCraft, Inc. 4175 E. Red Cliffs Dr. Kanab, UT 84741 435-899-1255 jfoote@scopecraft.com