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7 The Astronomical Detector
7.5.1 Practical 1: The Bias and Dark Frames
In this, the first of the practical sessions, you will learn how to take dark and bias
frames. This practical can be undertaken in daylight and in poor weather, as the
dome does not need to be opened nor the lens cap removed. It will give you a first
introduction to your camera control software and the FITS Liberator package.
Leaving the bias and read noise aside for the moment, as we take progressively
longer exposures even with the shutter closed, we find a mysterious build-up of counts
within the pixels. These are thermal electrons produced by the CCD itself entering
the pixel and are one of the main sources of errors in CCD-based observations. In this
practical, you will investigate both the dark and bias frames in order to understand
their impact on uncertainties in CCD-based observations.
Taking the Frames
1. Set up the telescope, ensuring that no light is entering the camera by placing
the lens cap on the telescope. The telescope does not need to be powered up
and does not need to be tracking. Connect it to the camera using your camera
control software.
2. Using the camera control software, make sure you have the binning set to 1 × 1
and that you have subframing turned off. Turn off the cooler and let the CCD
warm up to ambient temperature. If your control software has a warmup option,
you may use this to save time.
3. Set the camera temperature just under ambient, so if the uncooled camera is at
12
◦ C set it to 10
◦ C. If your camera control software has the ability to select
between frame types, then select bias. If your software can do autocalibration,
make sure this is also turned off. Once your CCD is down to temperature and is
stable at that temperature, take five zero-second exposures, saving each frame.
It would help if you saved each frame with the temperature as part of the name.
Your camera control software should put the temperature in the header, but it
might not.
4. Once you have taken your first set of bias frames, reduce the temperature by
five degrees. Again wait until the temperature is stable and then take a further
five frames. Continue this process until the camera reaches either its normal
operating temperature or the point where the cooler is running at 100% in order
to maintain the temperature.
5. You will now need to take your dark frames. Warm the CCD up to ambient
temperature, and as before, cool it down to just a few degrees below ambient. If
you have the ability to set the software to dark frame, do so.
6. Once the temperature is stable take five dark frames with an exposure time of 1 s.
7. Increases the exposure time to 5 s and take and save a further five darks. Continue
this process for 10, 30, 60, 120, and 240 s. Once this is done, cool the camera
7 The Astronomical Detector
7.5.1 Practical 1: The Bias and Dark Frames
In this, the first of the practical sessions, you will learn how to take dark and bias
frames. This practical can be undertaken in daylight and in poor weather, as the
dome does not need to be opened nor the lens cap removed. It will give you a first
introduction to your camera control software and the FITS Liberator package.
Leaving the bias and read noise aside for the moment, as we take progressively
longer exposures even with the shutter closed, we find a mysterious build-up of counts
within the pixels. These are thermal electrons produced by the CCD itself entering
the pixel and are one of the main sources of errors in CCD-based observations. In this
practical, you will investigate both the dark and bias frames in order to understand
their impact on uncertainties in CCD-based observations.
Taking the Frames
1. Set up the telescope, ensuring that no light is entering the camera by placing
the lens cap on the telescope. The telescope does not need to be powered up
and does not need to be tracking. Connect it to the camera using your camera
control software.
2. Using the camera control software, make sure you have the binning set to 1 × 1
and that you have subframing turned off. Turn off the cooler and let the CCD
warm up to ambient temperature. If your control software has a warmup option,
you may use this to save time.
3. Set the camera temperature just under ambient, so if the uncooled camera is at
12
◦ C set it to 10
◦ C. If your camera control software has the ability to select
between frame types, then select bias. If your software can do autocalibration,
make sure this is also turned off. Once your CCD is down to temperature and is
stable at that temperature, take five zero-second exposures, saving each frame.
It would help if you saved each frame with the temperature as part of the name.
Your camera control software should put the temperature in the header, but it
might not.
4. Once you have taken your first set of bias frames, reduce the temperature by
five degrees. Again wait until the temperature is stable and then take a further
five frames. Continue this process until the camera reaches either its normal
operating temperature or the point where the cooler is running at 100% in order
to maintain the temperature.
5. You will now need to take your dark frames. Warm the CCD up to ambient
temperature, and as before, cool it down to just a few degrees below ambient. If
you have the ability to set the software to dark frame, do so.
6. Once the temperature is stable take five dark frames with an exposure time of 1 s.
7. Increases the exposure time to 5 s and take and save a further five darks. Continue
this process for 10, 30, 60, 120, and 240 s. Once this is done, cool the camera