Jerry, Thanks for filling in the huge blanks I left. You wrote: >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 a matter of fact that is exactly what I have been reading. Very good book. I have worked out the math for most of their formulas for my specific camera, in the early chapters. I was also reading Jerry Lodrigus’s CD book, “A Guide to Astrophotography with Digital SLR Cameras” on the same subject.
From my reading I am looking forward to making a Dark library about 5 to 10 degrees apart. I am not sure if I want to do scaling yet. Most of my shots will be at ISO 400, 800 and 1600 so when the sky is too lousy to image I can create darks. Jerry Lodrigus makes about 64 darks per ISO and temperature (he goes 10 degree apart) then make a master from that. Too bad Flats are so light train dependant or I would do flats too. But at least with the Dark library I can cut down on the time that they take and spend more time imaging deep sky objects. Flats should be rather fast. From what I read I should take 16 flats, followed by 16 darks for the flats, followed by 16 bias frames, then create a master flat from all that. That will take some discipline because I know when I get to where I can start imaging, it is hard to give up image time for flats. But in the end it should be worth it. Jim
--- On Wed, 3/18/09, Jerry Foote <jfoote@scopecraft.com> wrote: From: Jerry Foote <jfoote@scopecraft.com> Subject: [Utah-astronomy] Bias & Flats To: utah-astronomy@mailman.xmission.com Date: Wednesday, March 18, 2009, 11:06 AM 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 _______________________________________________ Utah-Astronomy mailing list Utah-Astronomy@mailman.xmission.com http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy Visit the Photo Gallery: http://gallery.utahastronomy.com Visit the Wiki: http://www.utahastronomy.com