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Deep Sky Photography: Image Calibration. Signal vs Noise: Image Calibration. First… some terminology: Light Frame : The individual pictures you take of your target. Dark Frame : An image taken with no light hitting the sensor.
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Deep Sky Photography: Image Calibration Signal vs Noise: Image Calibration • First… some terminology: • Light Frame: The individual pictures you take of your target. • Dark Frame: An image taken with no light hitting the sensor. • Flat Frame: A half saturated white light picture used to correct for optical path problems • Flat Dark: Same as a Dark frame but at the exposure time of the Flat (it is applied to the Flat). • Bias Frame: A minimum time exposure dark frame taken to account for CCD internal baseline charge. (can be safely ignored unless doing advanced imaging & cal). • ADU or PV: Anolog Digital Units or Pixel Values. The “count” of how many photons hit a given pixel.
Deep Sky Photography: Image Calibration Image Calibration: • Images (Light frames) are “calibrated” to remove noise from the Sensor and Optics chain. • Dark Frames • Taken with the scope/camera covered and the sensor at the same temp as the Light frames to be taken. • The Dark frames exposure length should be the same as the Light frames. Several frames (5 to 8) should be taken and stacked/averaged to make a “master dark”. • The Master Dark frame is subtracted from the Light Frames. • This removes the noise added by hot pixels, dark current, and bias noise in the CCD itself.
Deep Sky Photography: Image Calibration • Flat frames • Used to correct for imperfections (dust, vignette, etc) in the optical train. • Taken at focus, under a uniform, white, dim source, at an exposure time long enough to about half saturate the CCD pixels. • Taking flats doesn’t take much time. Typical exposure lengths are 0.3 to 2 seconds each. • Like the darks, many frames should be taken, stacked and averaged. • After the Flat is stacked. It is normalized (each pixel value divided by the average PV of the frame). • The Flat is applied by dividing it into the Light frames. • The process of Applying the flats is typically accomplished by your calibrating/stacking software • Flat Darks • Flat Darks are taken at the same exposure time as the Flat frames. • Used just like regular dark frames against the individual Flat frames to corrects the Flat frames for CCD noise.
