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7 The Astronomical Detector
at random and tend to leave a jagged trail. This means that the “median of the
neighbouring pixels” process above will not produce a satisfactory result.
7.7.1 Practical 4: Bad Pixels and Cosmic Rays
In this practical, you will use the CCD as a cosmic ray detector. In order for this to be
effective, you need to remove the hot pixels that appear to result from cosmic rays.
This practical is written for Diffraction Ltd.’s MaximDL package, but you could use
any suitable package.
Measuring Cosmic Rays
1. With the dome closed and the lens cap on, connect your control software to your
camera.
2. Ensuring that the camera is down to temperature, that the binning is set to 1 × 1,
and there is no subframing, take two 15 min dark frames and save them as Frames
1 and 2.
3. Subtract one frame from the other.
4. Load the resulting image into DS9 and identify the cosmic rays in the image by
putting a region on them.
5. Using the hot pixel tool, find the number of pixels affected by cosmic rays.
6. Do this for a number of different exposure times and plot cosmic ray count against
dark exposure time.
Analysis
Your analysis should include the following:
1. Your images, including the images with the cosmic rays marked. Also include the
results of the hot pixel tool and your plot.
2. How do cosmic rays differ from hot pixels?
3. Given the size of the array, proportionally how many pixels are affected?
4. How does the exposure time affect the number of cosmic rays? Why do you think
this is?
5. In undertaking precision photometry, the appearance of cosmic rays has to be
addressed. Given that dark frame subtraction is not effective, can you suggest
another way of dealing with cosmic ray strikes on CCD.
7 The Astronomical Detector
at random and tend to leave a jagged trail. This means that the “median of the
neighbouring pixels” process above will not produce a satisfactory result.
7.7.1 Practical 4: Bad Pixels and Cosmic Rays
In this practical, you will use the CCD as a cosmic ray detector. In order for this to be
effective, you need to remove the hot pixels that appear to result from cosmic rays.
This practical is written for Diffraction Ltd.’s MaximDL package, but you could use
any suitable package.
Measuring Cosmic Rays
1. With the dome closed and the lens cap on, connect your control software to your
camera.
2. Ensuring that the camera is down to temperature, that the binning is set to 1 × 1,
and there is no subframing, take two 15 min dark frames and save them as Frames
1 and 2.
3. Subtract one frame from the other.
4. Load the resulting image into DS9 and identify the cosmic rays in the image by
putting a region on them.
5. Using the hot pixel tool, find the number of pixels affected by cosmic rays.
6. Do this for a number of different exposure times and plot cosmic ray count against
dark exposure time.
Analysis
Your analysis should include the following:
1. Your images, including the images with the cosmic rays marked. Also include the
results of the hot pixel tool and your plot.
2. How do cosmic rays differ from hot pixels?
3. Given the size of the array, proportionally how many pixels are affected?
4. How does the exposure time affect the number of cosmic rays? Why do you think
this is?
5. In undertaking precision photometry, the appearance of cosmic rays has to be
addressed. Given that dark frame subtraction is not effective, can you suggest
another way of dealing with cosmic ray strikes on CCD.