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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6-minute Dark Frame
Above is a 6-minute dark frame taken during the same imaging session with the above
image of M78. Notice the brighter pixels scattered randomly around the image.
Note:
The pixel values in this image have been stretched significantly to show
the variations in the dark frame. In reality this image is almost completely black
with perhaps a few hundred “hot” pixels. This is completely normal and a
natural consequence of how CCDs are manufactured.
MaxIm LE automatically scales the visible range of pixels to match the underlying data. In
the dark frame shown above the average pixel value is just 203 out of a ossible 16-bit
dynamic range of 0-65,535. Seeing an automatically scaled dark frame r bias frame can
subtracted from a light frame to remove the dark current from the image.
l
t a
r of dark frames you collect by using a set of bias frames but, in general,
p
o
be a bit disconcerting for a new imager. Fear not, this “noise” will be almost completely
eliminated by subtracting a dark frame from your images.
Dark frames are
This subtraction removes the slight differences in dark current build-up from pixel to pixe
along with the larger variations caused by hot or dark pixels.
In general you’ll want to take at least 5 dark frames at each exposure used for your light
frames. If all your light frames were taken with 5-minute exposures, you’ll need to collec
set of 5-minute dark frames. If you took both 5-minute and 10-minute light frames, you’ll
need a set of 5-minute dark frames and a set of 10 minute dark frames. There is a way to
reduce the numbe
you’ll achieve the best results taking dark frames with the same exposure as your light
frames.