98
7 The Astronomical Detector
5. Taking each pair of images, subtract one of the pair from the other to make a
variance image.
6. For each of your five variance images, open them in FITS Liberator and find the
mean value and the sigma value (σ ).
7. Plot (in Excel, for example) σ
2
/2 against the mean and then fit a straight line to
your points.
Analysis
When writing up your report in the standard format, you should include the following:
1. What feature of your plot do you think indicates the gain? How does it compare
to the gain reported by the manufacturer?
2. How does the measured gain compare to the maximum gain?
3. As you subtract the pairs of flats to create your variance images, do you think it
makes a significant difference which is subtracted from the other? Why?
7.9 Saturation, Blooming, and Other Effects
Each pixel can take a limited number of electrons before it becomes full. This is
known as saturation and should be avoided as much as possible. When a saturated
pixel is read, the reading process causes electrons to flow into the pixels above and
below the saturated pixel, often more in one direction than the other. In extreme
cases, this causes a spike to appear across your star, a feature known as blooming.
Once an image has a bloom in it, the area around the affected part of the image is
effectively a loss as far as science is concerned. Some cameras have anti-blooming
gates, which act as capacitors between the pixels, and thereby stop stray electrons. In
general, for scientific applications, the disadvantages of cameras with anti-blooming
outweigh the advantages. If you have a camera with this feature and you intend to
take scientifically useful frames (as opposed to just nice images), then turn the antiblooming off if you can and keep the maximum pixel value well below the saturation
point. In press images from large telescopes, you often see stars with crosses and
halos around them. These are diffraction halos and diffraction spikes caused by
light being diffracted around the secondary and the secondary supports. You should
probably avoid these becoming too bright when doing science frames, as they will
affect your measurements. You should also be aware that these are sometimes added
after the image has been processed for artistic reasons.
Another problem that may be encountered with long exposures is residual bulk
image (RBI), another effect of saturation. If a pixel becomes saturated, further photon
strikes can cause electrons to move out of the pixel and into the substrate on which
the CCD is mounted. When you move to another target, these electrons leak back
into the pixel from whence they originated and create a ghost image of the previously
7 The Astronomical Detector
5. Taking each pair of images, subtract one of the pair from the other to make a
variance image.
6. For each of your five variance images, open them in FITS Liberator and find the
mean value and the sigma value (σ ).
7. Plot (in Excel, for example) σ
2
/2 against the mean and then fit a straight line to
your points.
Analysis
When writing up your report in the standard format, you should include the following:
1. What feature of your plot do you think indicates the gain? How does it compare
to the gain reported by the manufacturer?
2. How does the measured gain compare to the maximum gain?
3. As you subtract the pairs of flats to create your variance images, do you think it
makes a significant difference which is subtracted from the other? Why?
7.9 Saturation, Blooming, and Other Effects
Each pixel can take a limited number of electrons before it becomes full. This is
known as saturation and should be avoided as much as possible. When a saturated
pixel is read, the reading process causes electrons to flow into the pixels above and
below the saturated pixel, often more in one direction than the other. In extreme
cases, this causes a spike to appear across your star, a feature known as blooming.
Once an image has a bloom in it, the area around the affected part of the image is
effectively a loss as far as science is concerned. Some cameras have anti-blooming
gates, which act as capacitors between the pixels, and thereby stop stray electrons. In
general, for scientific applications, the disadvantages of cameras with anti-blooming
outweigh the advantages. If you have a camera with this feature and you intend to
take scientifically useful frames (as opposed to just nice images), then turn the antiblooming off if you can and keep the maximum pixel value well below the saturation
point. In press images from large telescopes, you often see stars with crosses and
halos around them. These are diffraction halos and diffraction spikes caused by
light being diffracted around the secondary and the secondary supports. You should
probably avoid these becoming too bright when doing science frames, as they will
affect your measurements. You should also be aware that these are sometimes added
after the image has been processed for artistic reasons.
Another problem that may be encountered with long exposures is residual bulk
image (RBI), another effect of saturation. If a pixel becomes saturated, further photon
strikes can cause electrons to move out of the pixel and into the substrate on which
the CCD is mounted. When you move to another target, these electrons leak back
into the pixel from whence they originated and create a ghost image of the previously