82
7 The Astronomical Detector
or flat. However, as we have discussed earlier, there are a number of sources of error
that are structural in nature, such as variability in QE across the CCD or CMOS and
the CTE in the case of a CCD. In addition, there is structural noise within the optics
that contributes to the overall flat noise problem. These problems are addressed by
flat fielding, which is discussed in Sect. 7.5. However, even the best flat field cannot
entirely deal with this structural noise, and for some instruments, such as space
telescopes, flat field noise can be the dominant noise source.
7.3 Binning and Subframing
An alternative to on-chip binning is off-chip binning (or post-download binning). Offchip binning is a software action that is performed after download, so the benefits
of reduced download time and the associated noise reduction are lost. However, the
noise gain from averaging the noise over some pixels is still obtained, and for a
CCD that does not support on-chip binning or for a CMOS based camera that cannot
on-chip bin, off-chip binning is a suitable alternative.
When on-chip binning is performed, the calibration frames should have the same
binning. Binning is not always a linear process, so you cannot just turn an unbinned
calibration frame into a binned one using the software. Likewise, in performing
off-chip binning, the same process must be applied to the calibration frames before
subtracting them.
An alternative to binning is subframing. Subframing is a process whereby only
part of the CCD is read. Subframing is a software-driven process and is not always
supported. It involves taking an image of a target and selecting a region within the
downloaded frame. Once that region is selected, only those pixels in the selected
region are downloaded in future exposures. Hence with subframing, no resolution
is lost, but you do not gain the noise reduction associated with binning. It is used
mostly when download speed is needed, such as in focusing. The University of
Hertfordshire’s observatory uses a fast CCD camera to observe asteroidal occultations
of stars. These are very transient events often only a few seconds in length. A typical
astronomical CCD has a download time of a few seconds, so it is unsuitable for
precision timing of such events. If we consider that the occulted star may fill only
100 pixels, by subframing, so that we download only the pixels with light from the
target, we can significantly improve the download time and hence the cycle time of
the camera. In fact, this camera has been run at a thousand frames per second for
such observations.
A word of caution. Leaving the camera in subframing mode is one of the most
common errors the author sees. It is not always entirely obvious to observers that
they are taking subframed images. So make sure you turn subframing off when you
have finished using the camera or when you are no longer subframing.
7 The Astronomical Detector
or flat. However, as we have discussed earlier, there are a number of sources of error
that are structural in nature, such as variability in QE across the CCD or CMOS and
the CTE in the case of a CCD. In addition, there is structural noise within the optics
that contributes to the overall flat noise problem. These problems are addressed by
flat fielding, which is discussed in Sect. 7.5. However, even the best flat field cannot
entirely deal with this structural noise, and for some instruments, such as space
telescopes, flat field noise can be the dominant noise source.
7.3 Binning and Subframing
An alternative to on-chip binning is off-chip binning (or post-download binning). Offchip binning is a software action that is performed after download, so the benefits
of reduced download time and the associated noise reduction are lost. However, the
noise gain from averaging the noise over some pixels is still obtained, and for a
CCD that does not support on-chip binning or for a CMOS based camera that cannot
on-chip bin, off-chip binning is a suitable alternative.
When on-chip binning is performed, the calibration frames should have the same
binning. Binning is not always a linear process, so you cannot just turn an unbinned
calibration frame into a binned one using the software. Likewise, in performing
off-chip binning, the same process must be applied to the calibration frames before
subtracting them.
An alternative to binning is subframing. Subframing is a process whereby only
part of the CCD is read. Subframing is a software-driven process and is not always
supported. It involves taking an image of a target and selecting a region within the
downloaded frame. Once that region is selected, only those pixels in the selected
region are downloaded in future exposures. Hence with subframing, no resolution
is lost, but you do not gain the noise reduction associated with binning. It is used
mostly when download speed is needed, such as in focusing. The University of
Hertfordshire’s observatory uses a fast CCD camera to observe asteroidal occultations
of stars. These are very transient events often only a few seconds in length. A typical
astronomical CCD has a download time of a few seconds, so it is unsuitable for
precision timing of such events. If we consider that the occulted star may fill only
100 pixels, by subframing, so that we download only the pixels with light from the
target, we can significantly improve the download time and hence the cycle time of
the camera. In fact, this camera has been run at a thousand frames per second for
such observations.
A word of caution. Leaving the camera in subframing mode is one of the most
common errors the author sees. It is not always entirely obvious to observers that
they are taking subframed images. So make sure you turn subframing off when you
have finished using the camera or when you are no longer subframing.