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3.3 Flat-Field Frame Calibration
Each pixel in the camera has a slightly different sensitivity to light. These
sensitivity differences add another noise component to the image (known as
flat-fielding error) unless steps are taken to compensate. While flat-fielding
correction is important for achieving high quality images, it is absolutely
essential for accurate photometric measurements.
Pixel-to-pixel variations in light sensitivity are imprinted into an image, with
the more sensitive pixels showing up as brighter dots. When long exposures
are used to image extremely faint subjects, such as in astronomical
applications, the ultimate sensitivity limit is determined by how precisely the
flat-fielding error can be removed.
There are several common sources of flat-fielding variations. Typical sensors
have pixel-to-pixel variations on the order of 1%. Vignetting in the optical
system can reduce the light flux at the corners of the sensor. Dust on optical
surfaces near the sensor can cast shadows (often called "dust donuts" due to
their appearance in centrally-obstructed optical systems). Compressed air can
help reduce dust donuts, but it is often difficult to completely eliminate them.
To create a flat-field frame, the optical system must be illuminated by a
uniform light source and an exposure is taken. To avoid non-linearity at the
top and noise at the bottom of the camera's range, the exposure is usually
chosen to get an average value of 30% to 50% of the saturation level. The flat-
field is then renormalized by dividing each pixel into the average value in the
array. Any pixel that is more sensitive is assigned a number slightly below 1
and any pixel that is less sensitive is assigned a number slightly above 1.
When this frame is multiplied by a raw image, it removes the sensitivity
variations.
Methods for acquiring Flat-Fields
Flat-fielding is by far the most troublesome calibration method. The entire
aperture of the optical system must be evenly illuminated with light – if this is
not done very carefully, then the flat-field will be wrong. Light leaks will ruin
the calibration by adding unfocussed light that did not pass through the
optical system. In addition, once calibrated, the camera cannot be moved or
even refocused. Finally, some sensors have significant flat-field variation as a
function of wavelength (color), and it can be difficult to create a reasonable
facsimile of the normal illumination spectrum.
Given these problems, a good flat-field can be very difficult to achieve for
certain types of optical equipment, and so this calibration step is sometimes
skipped. However, if there is no vignetting and dust donuts are not an issue,
calibrating the camera alone may be sufficient. Cover the end of a roughly six-
inch long opaque tube with a translucent material (a few layers of white
photocopy paper will do in a pinch). Place this over the front of the camera,
gently illuminate the assembly with white light (natural or incandescent, not
fluorescent or LED), and take an exposure with a duration that produces a
brightness level of roughly 30% of full scale. The resulting images can be used
to flat-field the camera, regardless of the optics used. Note that the camera's
window must be very clean (that is, no dust spots) for this to work properly.
SBIG STF SERIES - CAMERA USER'S MANUAL
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