Q S I 5 0 0 S E R I E S U S E R G U I D E
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because flat fields tend to use fairly short exposures, you can often take a full series of flat
fields and flat-darks in just a few minutes.
The resulting master Flat Field is used to scale the pixel values in the light frame,
eliminating the effects of pixel non-uniformity, optical vignetting and dust on the optical
surfaces.
Bias Frames
A Bias Frame is a zero-length (dark) exposure intended to measure just the difference
between the pixels plus any additional noise added during the process of reading the image
from the CCD and converting it into a digital image file. Because the CCD pixels are
emptied immediately before the image is read from the CCD, only a small amount of dark
current has had a chance to build up, but that rate of accumulation varies slightly for every
pixel. Also, reading an image from a CCD is not instantaneous. Pixels near the bottom of
the CCD are read later than pixels closer to the top of the CCD so pixels toward the bottom
tend to have slightly higher pixel values than pixels closer to the top.
e dark current in any given pixel
,
lly better off taking dark frames that match the exposure
time
One common use of bias frames is for scaling dark frames. By subtracting a bias frame
from a dark frame, you end up with a “thermal frame.” A thermal frame contains pixel
values showing just the effect of dark current. Becaus
accumulates at a constant rate, a thermal frame allows you to predict with reasonable
accuracy how much dark current there would be for different length exposures. However
given the opportunity, you’re genera
s of your light frames.
ame. Note again that this image has been automatically stretched to show
es. MaxIm LE does this automatically when you view an image file. All
Here is an example bias fr
variations in the pixel valu
arn
tml
the pixel values in the original image fall between 181 and 221 out of a possible range of 0-65,535,
meaning that the unstretched image would appear almost perfectly and uniformly black.
Bias frames can also be used to analyze the read noise in a CCD camera. You can le
more about that process on the QSI web site at
http://www.qsimaging.com/ccd_noise.h
.
Taking bias frames is easy and takes only a couple of minutes. When you’re taking your
dark frames and flat fields, also take a series of 16 bias frames. That completes your full set
of calibration frames.