100
7 The Astronomical Detector
this feature, it is worth having, as it means you can control the camera without
touching it, thereby avoiding moving the telescope in the process.
In standard, nonastronomical, photography, there is what is known as the exposure
triangle, which consists of the ISO, the aperture, and the exposure time. In the case
of astronomical photography, you have a fixed aperture, so that cannot be changed.
Most photographers assume that the ISO is the same as the old film speed and
represents the sensitivity of the sensor. Such is not the case. Changing the ISO on a
DSLR just changes the preamplification from each pixel, which improves low light
performance at the cost of increased noise. Obviously, this is not a desirable side
effect. As a result, you should keep the ISO of your camera set to its base level,
which is normally 100 or 200. You will not be using white balance, so either turn
that off or set it to daylight. If your camera has an exposure compensation setting,
that should be set to zero. The camera should be set to manual mode. Every DSLR
camera has a maximum exposure time; the author’s own camera is limited to 30 s.
For exposures beyond this, you need to set the camera to bulb mode. In bulb mode,
the shutter stays open for as long as required. It is not determined by the camera.
There are a number of problems with using a DSLR rather than an astronomical camera. The first is that commercial DSLRs are much noisier than astronomical
cameras, which are built to be cooled and to have low noise. DSLRs are also nonphotometric, due to the fact that each pixel is filtered by the manufacturer. Lastly,
the processor controlling a DSLR may perform some image processing such as dark
and bias removal before the image is downloaded. It might be possible to turn this
off. If you have a noise reduction option, this should be turned off, since although it
might be reducing the noise, you do not know how or what else it is removing.
Lastly, if you are using a DSLR, ensure that it is saving the files as RAW, which
might also be called NEF or ORF depending on the manufacturer. This produces a file
that is uncompressed and has all the image data in it. RAW files are large, especially
for large sensor cameras, and they may seem very large compared to JPEG, but you
should always use RAW.
7.11 File Formats
When a digital image is downloaded from an imaging device, the data is stored in
a file format. The file format describes how the data is encoded and how it may be
converted into an image on a screen. Broadly speaking, there are three file formats
types. Uncompressed formats such as FITS store data as a matrix, with each data point
(or several data points) in the matrix representing a pixel in the image. Uncompressed
file formats tend to result in large file sizes. A 640 × 480 pixel image in such a format
would result in a 900 KB file size.
Compressed formats use a compression algorithm to reduce the file size, and a
single data point may represent many pixels in the image. The compressed format
may be lossless or lossy. A lossless compression format can restore an image to its
original format, while a lossy format can produce a smaller file size but with a loss
7 The Astronomical Detector
this feature, it is worth having, as it means you can control the camera without
touching it, thereby avoiding moving the telescope in the process.
In standard, nonastronomical, photography, there is what is known as the exposure
triangle, which consists of the ISO, the aperture, and the exposure time. In the case
of astronomical photography, you have a fixed aperture, so that cannot be changed.
Most photographers assume that the ISO is the same as the old film speed and
represents the sensitivity of the sensor. Such is not the case. Changing the ISO on a
DSLR just changes the preamplification from each pixel, which improves low light
performance at the cost of increased noise. Obviously, this is not a desirable side
effect. As a result, you should keep the ISO of your camera set to its base level,
which is normally 100 or 200. You will not be using white balance, so either turn
that off or set it to daylight. If your camera has an exposure compensation setting,
that should be set to zero. The camera should be set to manual mode. Every DSLR
camera has a maximum exposure time; the author’s own camera is limited to 30 s.
For exposures beyond this, you need to set the camera to bulb mode. In bulb mode,
the shutter stays open for as long as required. It is not determined by the camera.
There are a number of problems with using a DSLR rather than an astronomical camera. The first is that commercial DSLRs are much noisier than astronomical
cameras, which are built to be cooled and to have low noise. DSLRs are also nonphotometric, due to the fact that each pixel is filtered by the manufacturer. Lastly,
the processor controlling a DSLR may perform some image processing such as dark
and bias removal before the image is downloaded. It might be possible to turn this
off. If you have a noise reduction option, this should be turned off, since although it
might be reducing the noise, you do not know how or what else it is removing.
Lastly, if you are using a DSLR, ensure that it is saving the files as RAW, which
might also be called NEF or ORF depending on the manufacturer. This produces a file
that is uncompressed and has all the image data in it. RAW files are large, especially
for large sensor cameras, and they may seem very large compared to JPEG, but you
should always use RAW.
7.11 File Formats
When a digital image is downloaded from an imaging device, the data is stored in
a file format. The file format describes how the data is encoded and how it may be
converted into an image on a screen. Broadly speaking, there are three file formats
types. Uncompressed formats such as FITS store data as a matrix, with each data point
(or several data points) in the matrix representing a pixel in the image. Uncompressed
file formats tend to result in large file sizes. A 640 × 480 pixel image in such a format
would result in a 900 KB file size.
Compressed formats use a compression algorithm to reduce the file size, and a
single data point may represent many pixels in the image. The compressed format
may be lossless or lossy. A lossless compression format can restore an image to its
original format, while a lossy format can produce a smaller file size but with a loss