A bias frame is the result of the read-out of a CCD with zero exposure time. Such a frame is non-zero due to both the properties of the read-out amplifier and the detailed properties of the detector itself. Formally, such a bias frame is thus the combination of three things: Read Noise; a constant (or "DC-offset") term; and a 2-D pattern term. The Read Noise is introduced by the amplifier. For our device, the quoted Read Noise is
RN = 15 e-
The Read Noise defines the absolute floor for the total noise in any exposure. The value of the DC-offset is a function of many things, most crucially of temperature, and will vary from exposure to exposure. The 2-D pattern term is also a function of temperature, and should be assumed to vary from night to night, but can be determined for a given temperature, on given night, by taking the median of a series of individual bias frames.
More detailed technical background on bias frames, and methods for bias correction in general can be found in a number of sources. Among such sources available at our library are Berry & Burnell (2000), Howell (2000), Ratledge (1997), and Wodaski (2002).
Mean Bias Properties 2-D Bias Properties Medianing of Bias Frames Guide Bias Properties
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