7.6 Linearity of CCD
95
Analysis
As previously, you need to write your report up in standard laboratory style. You
should include your table of exposure times, mean counts, and your plot as well as
the standard notes about the camera and software used in addition to any problems
you may have encountered. In particular, your report should include the following:
1. In what circumstances is the detector response of a CCD preferred to that of
photographic film?
2. Describe the shape of your plot, in particular how the gradient changes with
exposure time if it does.
3. Try to explain the significance of any change in the gradient and its impact on
performing precision measurements such as those needed to detect exoplanets
using the transit technique.
4. Your plot may not be straight through the whole length. Can you suggest a process
that might cause this effect?
7.7 Bad Pixels and Cosmic Rays
It is unlikely that any astronomical camera will have every pixel working, and more
pixels will be lost as the instrument ages. The most common fault is a dead pixel,
a pixel that will either not read or is no longer sensitive to light. Dead pixels will
read as having either zero counts or a count near the bias level and are difficult to
see in an image. Hot pixels are pixels that read artificially high, either at or close
to the maximum value (65,536 for a 16-bit camera) and appear as bright isolated
points in the image. You may also see dead columns. These can be caused by a faulty
pixel preventing the read process in that column. These faults may cause problems
when you try to extract data from the image, so you need to be aware of them.
Most astronomical imaging software has a function that attempts to deal with hot
and cold pixels, although no process, besides physically avoiding that part of the
CCD, is totally satisfactory. The most common method works by identifying pixels
with values under or over a set value and replacing the faulty pixel with the mean or
median value of its neighbours.
A CCD cannot distinguish between photons and other high-energy particles such
as those emitted naturally by radioactive decay from the surrounding environment.
Given that the CCD is effectively shielded, local events rarely have enough energy to
penetrate the casing and register. However, cosmic rays, very high energy particles,
many thought to be from supernova explosions, stream into the Earth’s atmosphere,
causing cascades of high-energy particles that pass through the Cassegrain aperture of
the mirror and dump their energy into the CCD. This causes a trail of high registering
pixels that appear hot when read, which of course they are not. Cosmic ray detections
on CCD images are difficult to remove using standard methods, as they appear
95
Analysis
As previously, you need to write your report up in standard laboratory style. You
should include your table of exposure times, mean counts, and your plot as well as
the standard notes about the camera and software used in addition to any problems
you may have encountered. In particular, your report should include the following:
1. In what circumstances is the detector response of a CCD preferred to that of
photographic film?
2. Describe the shape of your plot, in particular how the gradient changes with
exposure time if it does.
3. Try to explain the significance of any change in the gradient and its impact on
performing precision measurements such as those needed to detect exoplanets
using the transit technique.
4. Your plot may not be straight through the whole length. Can you suggest a process
that might cause this effect?
7.7 Bad Pixels and Cosmic Rays
It is unlikely that any astronomical camera will have every pixel working, and more
pixels will be lost as the instrument ages. The most common fault is a dead pixel,
a pixel that will either not read or is no longer sensitive to light. Dead pixels will
read as having either zero counts or a count near the bias level and are difficult to
see in an image. Hot pixels are pixels that read artificially high, either at or close
to the maximum value (65,536 for a 16-bit camera) and appear as bright isolated
points in the image. You may also see dead columns. These can be caused by a faulty
pixel preventing the read process in that column. These faults may cause problems
when you try to extract data from the image, so you need to be aware of them.
Most astronomical imaging software has a function that attempts to deal with hot
and cold pixels, although no process, besides physically avoiding that part of the
CCD, is totally satisfactory. The most common method works by identifying pixels
with values under or over a set value and replacing the faulty pixel with the mean or
median value of its neighbours.
A CCD cannot distinguish between photons and other high-energy particles such
as those emitted naturally by radioactive decay from the surrounding environment.
Given that the CCD is effectively shielded, local events rarely have enough energy to
penetrate the casing and register. However, cosmic rays, very high energy particles,
many thought to be from supernova explosions, stream into the Earth’s atmosphere,
causing cascades of high-energy particles that pass through the Cassegrain aperture of
the mirror and dump their energy into the CCD. This causes a trail of high registering
pixels that appear hot when read, which of course they are not. Cosmic ray detections
on CCD images are difficult to remove using standard methods, as they appear