SBIG ST-8300 Manual
15
Hot pixels
are simply the pixels where the dark current is higher than
the average and thus they show up as white specs against the salt and
pepper background. Again, this is completely normal for CCD images.
The second effect you’ll notice is a brightening on the left of the image,
as if something bright was just outside the field of view. This is called the
readout glow
and is caused by the electronics in the CCD, in particular by a
very small but ever present glow emanating from in the on-chip amplifier
that conditions the weak signals in the CCD so that they may be digitized.
The
readout
(or digitization) of images from CCDs is a sequential
process whereby every pixel is digitized one-after-another until the whole
image has been digitized. The readout starts at pixel (1,1) in the top-left
corner and proceeds to pixel (3326, 2504) in the lower-right hand corner.
As each pixel in a row is readout the pixels to the right of it within the same
row are shifted to the left one position in preparation for the next pixel
readout. The next pixel is then readout and the process repeated until every
pixel in the first row has been digitized.
At this point the whole CCD is shifted up one row and digitization
starts with the left-most pixel of the second row. The
readout glow
on the
left of the images is a buildup of light from a glow from the preamplifier
structures in the upper-left hand corner of the CCD while rows are queuing
up for readout.
The final effect you’ll notice is the salt-and-pepper look of the
background. What you’re seeing here is the ultimate noise floor of the
CCD whereby adjacent pixels have slightly different values due to noise in
the CCD and readout electronics. The noise in dark frames that have a zero
exposure time is referred to as the
read noise
of the camera.
Fortunately for us there are very simple image processing techniques
we can use to eliminate the effects of
dark current
and
readout glow
.
Let’s open another image. Close the first image by clicking the
X
in
the upper-right corner then use the
Open
command in the
File
menu again
but, this time double-click on
Image 2
. Note in the
Image Parameters
dialog that this image had an
Exposure Time
of 10 seconds. Click in the
dialog to close it.
Now this image has a whole lot more hot pixels! That’s because this
exposure was 10 times as long and the pixels built up 10 times the dark
current. It’s hard to even see the background through all the hot pixels.
About this time you’re probably asking yourself “How can I ever take
deep sky images with all these hot pixels?” The answer is simple. Because
the build up of dark current at a given CCD temperature is a repeatable
effect you can remove the effects of dark current by taking two images, one
with the shutter open (
light frame
) and another of equal exposure with the
shutter closed (
dark frame
). You then
subtract
the dark frame from the
light frame, and because the
hot pixels
and the
readout glow
repeat from
one image to another they are removed by the subtraction.
Let’s see how this works. Close
Image 2
and Open
Image 3
, which is
a 10-second
light frame
where you can get a hint of the object but the hot
Содержание ST-8300C
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