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7 The Astronomical Detector
The FITS header may also contain information about the translation between pixel
location and astronomical coordinate systems such as Galactic. The translational
system is known as the World Coordinate System (WCS). If the application you are
using has the ability to display pixel WCS position but does not do so for the image
you are displaying, the WCS has likely not been calculated for that image. This is
easy to resolve as we will show later, but it is not always necessary.
Unlike other image formats such as JPEG and PNG, FITS files are not compressed,
so they tend to be quite large; however, they contain all the data taken by the camera,
unlike more traditional imaging formats. If you are familiar with digital photography,
you may have noticed that your DSLR camera can save images in RAW format; when
you select RAW, the number of images you can store on your memory card is much
reduced. This is because RAW is the terrestrial version of FITS. Most professional
photographers will take images in RAW, as they can then be easily manipulated.
For the same reason, professional astronomers use FITS, as should you. If you need
to make a JPEG image from a FITS, it is fairly straightforward, and most FITS
display applications have a “Save As” option. You should, however, always keep your
scientific data in FITS. FITS is supported by a wide range of computer languages
with libraries available in C, C++, Perl, R, and Python, as well as many others. Using
code to manipulate FITS files is very straightforward and very powerful.
7.5 Calibration Frames
Read noise can be easily addressed by taking a bias frame. A bias frame is an
exposure of zero seconds, with the shutter closed. Hence, it forces the camera to
go through a read cycle while not exposing it to any light. Therefore, any electrons
registered have not come from photoelectric interaction in the CCD, but are due to
random fluctuations introduced by the reading process, electrical components in and
near the camera, and to a very small extent cosmic rays (see Sect. 7.7). When you take
an image of your target, a light frame, the bias frame should be subtracted from your
light frame. This subtraction process normally happens automatically with modern
camera control software. When using a previously generated bias frame (and there
is nothing wrong with doing this, as the variation of the bias over time is very small),
it has to be taken at the same camera temperature as the light frame. Subtracting the
bias frame removes any read noise, as well as the pedestal that was added during the
reading process. Although pixel values within a bias may be quite high, depending
on the camera and the software you are using, the standard deviation across the array
should be very low, perhaps only a few counts. If it is not, then you might have a
reading problem or a faulty piece of equipment, for example a bad camera cooling
fan. In general, a good bias frame is the median of a very large number of bias frames.
Taking the median reduces the amount of random noise associated with individual
frames and to some extent addresses cosmic rays.
With so many pixels on a modern camera—the camera on the Gaia space telescope
has over a billion—clearly there will be some random noise in the sensor. To address
7 The Astronomical Detector
The FITS header may also contain information about the translation between pixel
location and astronomical coordinate systems such as Galactic. The translational
system is known as the World Coordinate System (WCS). If the application you are
using has the ability to display pixel WCS position but does not do so for the image
you are displaying, the WCS has likely not been calculated for that image. This is
easy to resolve as we will show later, but it is not always necessary.
Unlike other image formats such as JPEG and PNG, FITS files are not compressed,
so they tend to be quite large; however, they contain all the data taken by the camera,
unlike more traditional imaging formats. If you are familiar with digital photography,
you may have noticed that your DSLR camera can save images in RAW format; when
you select RAW, the number of images you can store on your memory card is much
reduced. This is because RAW is the terrestrial version of FITS. Most professional
photographers will take images in RAW, as they can then be easily manipulated.
For the same reason, professional astronomers use FITS, as should you. If you need
to make a JPEG image from a FITS, it is fairly straightforward, and most FITS
display applications have a “Save As” option. You should, however, always keep your
scientific data in FITS. FITS is supported by a wide range of computer languages
with libraries available in C, C++, Perl, R, and Python, as well as many others. Using
code to manipulate FITS files is very straightforward and very powerful.
7.5 Calibration Frames
Read noise can be easily addressed by taking a bias frame. A bias frame is an
exposure of zero seconds, with the shutter closed. Hence, it forces the camera to
go through a read cycle while not exposing it to any light. Therefore, any electrons
registered have not come from photoelectric interaction in the CCD, but are due to
random fluctuations introduced by the reading process, electrical components in and
near the camera, and to a very small extent cosmic rays (see Sect. 7.7). When you take
an image of your target, a light frame, the bias frame should be subtracted from your
light frame. This subtraction process normally happens automatically with modern
camera control software. When using a previously generated bias frame (and there
is nothing wrong with doing this, as the variation of the bias over time is very small),
it has to be taken at the same camera temperature as the light frame. Subtracting the
bias frame removes any read noise, as well as the pedestal that was added during the
reading process. Although pixel values within a bias may be quite high, depending
on the camera and the software you are using, the standard deviation across the array
should be very low, perhaps only a few counts. If it is not, then you might have a
reading problem or a faulty piece of equipment, for example a bad camera cooling
fan. In general, a good bias frame is the median of a very large number of bias frames.
Taking the median reduces the amount of random noise associated with individual
frames and to some extent addresses cosmic rays.
With so many pixels on a modern camera—the camera on the Gaia space telescope
has over a billion—clearly there will be some random noise in the sensor. To address