116
8 Imaging
Fig. 8.6 Inverted B-band image of the open cluster M26. (Image courtesy University of Hertfordshire)
improve the detection of saturated pixels, but it does have an important application
that will be discussed later.
How is saturation detected? The simplest method is to do a surface plot or a
slice of some of the brighter stars in the field. Most astronomical imaging or image
processing applications should be able to perform this function, including MaxIm
DL, APT, and SalsaJ. To illustrate, Fig. 8.6 shows a B-band image of the open cluster
M26. The image is inverted, i.e., black and white are reversed, in order to make it
clearer. From visual inspection, none of the stars appear saturated. However, when
we perform a surface plot of a bright star within the field, Fig. 8.7, we see that it
does not have the characteristic Gaussian shape we would expect; rather, it is domed.
This indicates that the exposure was long enough for the pixel’s response to become
nonlinear. Note that if the top of the peak were flat, that would indicate saturation,
although of course, the pixels would have gone nonlinear before that happened. The
smaller peak in Fig. 8.7 is another, dimmer, star, caught within the plot region. As
can be seen, this is broadly Gaussian and therefore acceptable. Figure 8.8 shows the
same region as Fig. 8.7, but in this case, it is a slice instead of a region plot. However,
we can see the nonlinear nature of the bright star within the plot and the Gaussian
nature of the dimmer star also intersected by the slice.
Ideally, you want the brightest star in the image to be just inside the linear range
of the CCD. If you have a saturated star in the field, you may wish to retake the
light frame with reduced exposure time. If you know the count and the magnitude
of an unsaturated star (see the chapter on photometry for how to measure the count)
as well as the magnitude of the saturated star, you can determine the count of the
saturated star if it was unsaturated using (8.1), where M ref and M i are the magnitude
8 Imaging
Fig. 8.6 Inverted B-band image of the open cluster M26. (Image courtesy University of Hertfordshire)
improve the detection of saturated pixels, but it does have an important application
that will be discussed later.
How is saturation detected? The simplest method is to do a surface plot or a
slice of some of the brighter stars in the field. Most astronomical imaging or image
processing applications should be able to perform this function, including MaxIm
DL, APT, and SalsaJ. To illustrate, Fig. 8.6 shows a B-band image of the open cluster
M26. The image is inverted, i.e., black and white are reversed, in order to make it
clearer. From visual inspection, none of the stars appear saturated. However, when
we perform a surface plot of a bright star within the field, Fig. 8.7, we see that it
does not have the characteristic Gaussian shape we would expect; rather, it is domed.
This indicates that the exposure was long enough for the pixel’s response to become
nonlinear. Note that if the top of the peak were flat, that would indicate saturation,
although of course, the pixels would have gone nonlinear before that happened. The
smaller peak in Fig. 8.7 is another, dimmer, star, caught within the plot region. As
can be seen, this is broadly Gaussian and therefore acceptable. Figure 8.8 shows the
same region as Fig. 8.7, but in this case, it is a slice instead of a region plot. However,
we can see the nonlinear nature of the bright star within the plot and the Gaussian
nature of the dimmer star also intersected by the slice.
Ideally, you want the brightest star in the image to be just inside the linear range
of the CCD. If you have a saturated star in the field, you may wish to retake the
light frame with reduced exposure time. If you know the count and the magnitude
of an unsaturated star (see the chapter on photometry for how to measure the count)
as well as the magnitude of the saturated star, you can determine the count of the
saturated star if it was unsaturated using (8.1), where M ref and M i are the magnitude