7.3 Binning and Subframing
83
CMOS Versus CCD
As can been seen from the above section, there are pros and cons to both CCD and
CMOS chips. In general, research-grade CCD chips still outperform CMOS-based
cameras for astronomy. They are mostly used for stare mode observations, that is,
long-exposure observations at a single unmoving object of constant brightness. They
have lower bias and dark noise current than CMOS chips and are capable of on-chip
binning, unlike CMOS-based cameras. However, they suffer from CTE, although
CMOS cameras can suffer from edge glow due to the large amount of electrons
needed to power the individual pixels on a CMOS chip. Hence, CMOS pixels are
considered active, while there is no power to a CCD pixel when it is exposed, and
so it is considered inactive. CMOS pixels are therefore considerably more complex
than the pixels within a CCD, and therefore dead and bad pixels are more common
and are likely to multiply faster as the camera ages for reasons other than that there
are more things to go wrong with each pixel.
A CMOS camera, however, is generally less expensive than a CCD-based camera,
permitting you to get a larger sensor for the same money. CMOS cameras have much
higher frame rates that CCD, meaning that they are more suitable for lucky imaging
than CCD-based cameras.
At the time of writing in mid 2019, Sony had decided to stop the development
of their range of CCD chips and eventually will be closing their production line
down. Kodak, which makes the KEF range of CCD chips, have yet to announce any
change, but it is possible that as demand for CCD chips shrinks and the performance
difference between CCD and CMOS diminishes, CCD production may end for all
but a few small manufacturers making CCD chips for very specialist markets.
7.4 The FITS File Format
The Flexible Image Transport System (FITS) is the standard format for astronomical
images and image-derived data such as spectra, and files containing such data may
have either the .fit, .fits, or .fts extension to the file name. The FITS file format is
designed to carry metadata (data about the data), two-dimensional image data, and
three-dimensional data such as radio and millimetre data. The FITS header holds
the metadata in a series of labels known as cards. Each card will have a label stating
what it is, for example EXPTIME for exposure time, and a piece of data, for example
10. This card tells the user that this was a 10 s exposure, but it also tells your software
that it was a 10 s exposure. You can add and change header cards using any number of
astronomical packages. Just keep in mind that if you use something that is a standard
card name, it might cause problems later. Robotic telescopes, as well as taking the
image, run the data through a a series of operations on the image, called a pipeline.
All these operations are noted by the insertion of a card, or cards, in the header. The
addition of these cards allows the user to identify what has happened to the image
and allows the processes to be reversed if needed (which isn’t always possible).
83
CMOS Versus CCD
As can been seen from the above section, there are pros and cons to both CCD and
CMOS chips. In general, research-grade CCD chips still outperform CMOS-based
cameras for astronomy. They are mostly used for stare mode observations, that is,
long-exposure observations at a single unmoving object of constant brightness. They
have lower bias and dark noise current than CMOS chips and are capable of on-chip
binning, unlike CMOS-based cameras. However, they suffer from CTE, although
CMOS cameras can suffer from edge glow due to the large amount of electrons
needed to power the individual pixels on a CMOS chip. Hence, CMOS pixels are
considered active, while there is no power to a CCD pixel when it is exposed, and
so it is considered inactive. CMOS pixels are therefore considerably more complex
than the pixels within a CCD, and therefore dead and bad pixels are more common
and are likely to multiply faster as the camera ages for reasons other than that there
are more things to go wrong with each pixel.
A CMOS camera, however, is generally less expensive than a CCD-based camera,
permitting you to get a larger sensor for the same money. CMOS cameras have much
higher frame rates that CCD, meaning that they are more suitable for lucky imaging
than CCD-based cameras.
At the time of writing in mid 2019, Sony had decided to stop the development
of their range of CCD chips and eventually will be closing their production line
down. Kodak, which makes the KEF range of CCD chips, have yet to announce any
change, but it is possible that as demand for CCD chips shrinks and the performance
difference between CCD and CMOS diminishes, CCD production may end for all
but a few small manufacturers making CCD chips for very specialist markets.
7.4 The FITS File Format
The Flexible Image Transport System (FITS) is the standard format for astronomical
images and image-derived data such as spectra, and files containing such data may
have either the .fit, .fits, or .fts extension to the file name. The FITS file format is
designed to carry metadata (data about the data), two-dimensional image data, and
three-dimensional data such as radio and millimetre data. The FITS header holds
the metadata in a series of labels known as cards. Each card will have a label stating
what it is, for example EXPTIME for exposure time, and a piece of data, for example
10. This card tells the user that this was a 10 s exposure, but it also tells your software
that it was a 10 s exposure. You can add and change header cards using any number of
astronomical packages. Just keep in mind that if you use something that is a standard
card name, it might cause problems later. Robotic telescopes, as well as taking the
image, run the data through a a series of operations on the image, called a pipeline.
All these operations are noted by the insertion of a card, or cards, in the header. The
addition of these cards allows the user to identify what has happened to the image
and allows the processes to be reversed if needed (which isn’t always possible).