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Calibration : Part I - Pre-processing
Calibration frames are used to correct for optical defects in the imaging train such as: hot pixels, cold pixels, dead columns, vignetting, and dust motes. This process is indispensable in applications such as asteroid photometry. These frames are classified into three separate categories:
Bias frames :
With the cap on the lens or telescope, we take the shortest exposure possible. This tells Prism that there is no light coming in and all the noise present is inherent to the system's electronics themselves. Bias noise is also available in dark and flats frames and needs to be taken out.
Dark Frames :
Also taken with the cap on the lens of the telescope, dark Frames differ from Bias Frames in that the exposure length needs to match that of your light frames (example: 5 minute exposures of galaxies or nebulae). It is important to remain consistent between the light and darks when it comes to binning (CCD and CMOS cameras), ISO (DSLRS), exposure length, and temperature. Prism can use your bias frames to compensate for differences between you lights and darks in case they differ slightly.
Flats Frames :
Just like the other types of calibration frames, Flats are required to correct for optical issues that reduced the light throughput like: dust most, vignetting, etc. A good calibration frame can even out the light in the light frames and produce a flat field free of dust doughnuts and light fall-off if the corners.
Importance of calibration frames
The importance of calibration frames becomes obvious when dealing with cosmetic and scientific imaging. With the latter, it is imperative to remove a maximum of the polluting factors (hot and cold pixels, motes, gradients) inherent to astronomical imaging systems. This becomes quite evident in photometric measurements which we will demonstrate in later tutorials.
Both pre and post processing tools are available in all versions of Prism, allowing calibration of data immediately after you acquire it. This negates the need for multiple software packages and reduces the learning curve. Prism is designed around power, compactness, and ease of use.