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7 The Astronomical Detector
7.6.1 Practical 3: Measuring CCD Linearity
In this practical, you will be investigating how the properties of telescope design and
the CCD detector can affect the accuracy of the results you obtain from your astronomical imaging observations. This practical can be undertaken either with the dome
closed or by imaging a star. The choice depends on weather conditions. The point
of loss of linearity of a CCD is one of the most import components of photometry,
the measuring of a star’s brightness, and it is important that you understand why it
happens, where it happens, and how to avoid it.
Taking the Data
1. Turn on the dome lights. If you can dim them, turn them down enough that you
can still just see the control computer’s keyboard.
2. Rotate the dome so that the flat field board (or if there isn’t one just the inside of
the dome) is facing the telescope. You may have to move the telescope in Alt to
get it directly facing the flat field board. Rather than power the telescope down,
turn its tracking off. If your telescope has a flat field diffuser, put it in place.
3. Power on the camera, connect the control software, put the cooler on, and set
the temperature. While you are waiting for the camera to get down to operating
temperature, make sure the binning is either off or set to 1 × 1 and that the
subframing and any autocalibration are off.
4. Select the narrowest filter. In most cases, this will be either OIII, Hydrogen α, or
an SII Filter. If you do not have any of these in the filter wheel, use an R or B (but
not a V).
5. Take a 1 s exposure, save it, and open it in DS9. Scale the image to 99.5%. Create
a circular region. It needs to be fairly small, say 100 pixels in area, and locate it
near the centre of the field. You will need to save the region so you can load it
into future frames. Look at the mean count within that region. If it is over 30,000,
your exposure is too long or your dome lights too bright. Try to reduce them until
you have 5,000 counts or less.
6. Take a succession of light frames, increasing the exposure time with each. I suggest
2, 5, 10, 15, 30, 60, 90, 120, 180, and 240 s exposures. After you save each one,
open it in DS9, load the region you saved in step 5, and make a note of the mean
count. You can stop when the mean count hits about 64,000 (for a 16-bit camera),
at which point the CCD is, of course, saturated.
7. Plot the mean count against exposure time. You may wish to try fitting a line to
your data. If you have undertaken practicals 1 and 2, you will have some idea of
the size of your error bars, but remember that you are plotting the mean count,
not the actual count.
7 The Astronomical Detector
7.6.1 Practical 3: Measuring CCD Linearity
In this practical, you will be investigating how the properties of telescope design and
the CCD detector can affect the accuracy of the results you obtain from your astronomical imaging observations. This practical can be undertaken either with the dome
closed or by imaging a star. The choice depends on weather conditions. The point
of loss of linearity of a CCD is one of the most import components of photometry,
the measuring of a star’s brightness, and it is important that you understand why it
happens, where it happens, and how to avoid it.
Taking the Data
1. Turn on the dome lights. If you can dim them, turn them down enough that you
can still just see the control computer’s keyboard.
2. Rotate the dome so that the flat field board (or if there isn’t one just the inside of
the dome) is facing the telescope. You may have to move the telescope in Alt to
get it directly facing the flat field board. Rather than power the telescope down,
turn its tracking off. If your telescope has a flat field diffuser, put it in place.
3. Power on the camera, connect the control software, put the cooler on, and set
the temperature. While you are waiting for the camera to get down to operating
temperature, make sure the binning is either off or set to 1 × 1 and that the
subframing and any autocalibration are off.
4. Select the narrowest filter. In most cases, this will be either OIII, Hydrogen α, or
an SII Filter. If you do not have any of these in the filter wheel, use an R or B (but
not a V).
5. Take a 1 s exposure, save it, and open it in DS9. Scale the image to 99.5%. Create
a circular region. It needs to be fairly small, say 100 pixels in area, and locate it
near the centre of the field. You will need to save the region so you can load it
into future frames. Look at the mean count within that region. If it is over 30,000,
your exposure is too long or your dome lights too bright. Try to reduce them until
you have 5,000 counts or less.
6. Take a succession of light frames, increasing the exposure time with each. I suggest
2, 5, 10, 15, 30, 60, 90, 120, 180, and 240 s exposures. After you save each one,
open it in DS9, load the region you saved in step 5, and make a note of the mean
count. You can stop when the mean count hits about 64,000 (for a 16-bit camera),
at which point the CCD is, of course, saturated.
7. Plot the mean count against exposure time. You may wish to try fitting a line to
your data. If you have undertaken practicals 1 and 2, you will have some idea of
the size of your error bars, but remember that you are plotting the mean count,
not the actual count.