SBIG
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SBIG ST-4/0489
Page 15
The Baseline and Gain Parameters
As previously mentioned, long exposures will
reduce the dynamic range of images with the
background level due to dark current taking up a
good portion of the overall dynamic range
available. Again, this is because with the dark
current background using say 191 counts, only 64
remain for the image data itself. This can be
avoided using a technique where one first takes a
dark exposure of the intended duration, sets the
electronic readout baseline level (referred to as
the
baseline
in the host software) slightly lower
than the background level value in the dark
image, and then boosts the
gain
by 2, 3 or 4 times
to reduce the effects of digitizing the data. The
light frame and dark frame are then captured and
subtracted as before.
This technique is also useful for short
exposures of faint objects, which do not begin to
saturate the CCD (levels of 255). Set the
baseline
to the dark sky level, and boost the
gain
by 4 if
short exposures are unavoidable due to drive
errors, etc. The noise of the CCD is greater at
higher pixel values; a boost of four is useful for
images in which the CCD pixel does not reach
one quarter of saturation, while a boost of two is
all that is useful at pixel levels near saturation.
Note
:
The CCD has occasional "hot" pixels which
saturate first in a long exposure. These hot
pixels can show up in a subtracted image as a
black pixel, since the saturated value is the
same in light and dark exposures. Keeping
the maximum exposure below that required
to fill the CCD pixels 50% of the way will
effectively eliminate this problem.
Automatic Dark Frame Subtraction
As mentioned previously, the ST-4 has memory
for two image buffers, a light buffer and a dark
buffer. The host software allows you to capture a
dark frame and retain it in the ST-4, then on
subsequent light images, you can have the ST-4
subtract that dark frame from the light images
immediately after the image is readout from the
CCD and prior to it being sent to the host.
As always, you must manually cover the
telescope when you ask the host software to take
the dark frame. This is handy for searching an
area for an object of interest, but for your final
images (the ones you really want to keep) you
will want to capture and download both light
and dark frames and let the host software do the
subtraction. This is so the time difference
between the light and dark frames will be as
small as possible and hence any possible change
in the CCD's temperature and dark current will
be minimized.
Focussing and Special Imaging Modes
To facilitate focussing, imaging planets using the
half frame mode alluded to above, taking a series
of images and selecting the best, etc., the host
software supports the following image sub-
modes in a grouping under a mode of operation
called the Focus Mode:
Full Frame Mode
In this mode the host software continually
takes full frame images, and downloads them
from the ST-4 and displays them.
Half Frame Mode
This mode allows using the upper half of the
CCD as an image buffer for bright objects as
described in the "Caveats of CCD
Readout"section before. Because only half of
the image is downloaded from the ST-4, the
data transmission time is only half as long as
in the full frame case. Finally note that the
only way you can take Half Frame mode
images is through the Focus Mode.
Quarter Frame Mode
In Quarter Frame Mode, the host software
first takes and downloads a full frame image.
The user then specifies an area of the full
image equal to a quarter frame by positioning
a square on the full frame image. The host
software then takes full frame images but
only downloads the pixels within the quarter
frame specified by the user, resulting in a
transmission time which is 1/4th as long as
for full frame images.
Low-Res Mode
This mode, also called one of four mode, is
also faster that the full frame mode at the cost
of image resolution. After each image is
taken, the ST-4 reduces the image to 1/4th the