Joe, I had to run to the store to get the turkey so I didn't get a chance to reply to your question. My book "A Practical Guide to CCD Imaging" that I recently acquired says "The flat field takes into account the gain irregularies of the array's different pixels and the optical transmission irregularities, in particular, vignetting and the presence of DUST in the light path. We must therefore produce a flat field for each optical assembly and, if possible, at each observation session, since dust can appear, disappear, or move from one day to another. Since it is imperative not to change the optical configuration between the flat field and the image acquisitions, we can work with a maximum of two optical configurations per night: one at the night's start, for which we produce flat fields in the evening, the other at the end of the night, for which we produce the morning flat fields before sunrise (which requires some self-sacrifice, ie sleep). The flat field must be produced after focusing, in order for the light path to be the same during the flat field and the nocturnal sky image. Thus, the focusing is done on the stars, and the flat field must be finished before the first stars appear! The solution to this paradox consists in keeping the camera mounted from one night to another, or to accurately find the position that corresponds to the focus. In all cases, the first night's flat field, produced on an approximate adjustment, will not necessarily be very satisfying." This is all fine and dandy if you have an observatory or set up at a remote site so you don't have to worry about equipment being stolen. I don't feel confortable leaving my camera or telescope set up all night. Is it possible to focus on Venus or Jupiter before it gets dark? Then take the flats and download them directly to the computer. I know SBIG recommends putting a white T-shirt over the optical system but will there be enough light available once you get your camera focused? I didn't get a chance to try this Sunday night because I filled up my memory card. Hope this helps to clarify flat fields. I'm just trying to figure out the best way to do it with my portable setup. Debbie
From: Joe Bauman <bau@desnews.com> Date: 2005/11/22 Tue AM 10:53:59 MST To: Utah Astronomy <utah-astronomy@mailman.xmission.com> Subject: Re: [Utah-astronomy] Flat Field Frames
Many thanks!
-- Black skies! -- Joe
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