Alpy 600 - user guide
2.4.1
Offset image
To avoid negative signal digital conversion, CCD
cameras have a fixed offset on the analog signal which
is called Offset. Any data readout from the CCD chip
includes this offset.
Offset images are also called Bias in some
documents. In this User Guide, we made the
choice to use Offset, but consider it is equivalent
to Bias.
This offset is fairly stable and once you have a mas-
ter offset frame, you can keep it for long time. You
can make one again once per year approximately. It is
also not too much temperature dependant. Because it
is very stable, you can improve your data by taking a
long series of offset frame (around 100 exposures)
and perform a median average to create a smooth
master offset frame for your camera. A median av-
erage is better than a simple average as it will totally
remove out of the ordinary defects on some of your
images, such as cosmic rays for example.
To get an offset image, put a cap on the Alpy 600
light entrance to ensure that no light comes in. Take
an exposure of 0 second, binning 1x1.
Offset images are sometime called Bias im-
ages.
You can measure the average level in the image (draw
a rectangle, and right-click to get the statistics in the
window):
2.4.2
Dark frames
CCD chips have a tendency to build up electrons
over time, particularly at high temperature. This tem-
perature dependant effect is called « thermal noise ».
The thermal noise is included in dark frames which
are exposures taken with the camera shutter closed;
the offset is also included in dark frames.
Hot pixels are individual cells that are particularly
sensitive to this thermal noise and tend to saturate
quickly during long exposure. These hot pixels have
to be processed separately (usually replaced by the
average of the surrounding pixels). An interesting prop-
erty of thermal noise is that it is linear with exposure
time. For example, the thermal noise in a 2sec expo-
sure is twice the thermal noise in a 1 sec exposure.
This means that you can extrapolate any shorter ther-
mal noise frame from a master thermal noise frame
with long exposure – assuming you have mapped the
hot pixels properly. This is done by doing a median av-
erage of a long exposure dark frame series (at least 7
frames, if possible more). Most astronomical software
require both offset and dark frames to pre-process
your image. The thermal noise is of course embeded
in the dark frame. Be very careful as the thermal noise
(thus the dark frame) is very sensitive to temperature.
Make sure to use a dark frame taken at the same tem-
perature as your spectra. If your camera doesn’t have
temperature regulation, you should take dark frames
close to the time you expose your spectra. Some soft-
ware can still extrapolate a proper thermal noise map
based on the area of your chip with no signal, which
is very easy in the case of Alpy 600 spectra. You do not
have to rebuild your dark frame every night; you can
create your library of dark frames taken at different
temperatures and use it for several months.
A dark frame is taken with the same conditions
as the target spectrum (exposure time, temperature),
but with no light coming into the spectroscope.
For instance, if your star spectra are made with
300s exposure, dark frames must be taken with the
same duration.
Here is a typical dark frame of 300s (binnng 1x1,
of course):
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Summary of Contents for Alpy 600
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