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8 Imaging
seeing becomes a major limiting factor. As we have seen, the turbulent motion of
the atmosphere limits the resolution of the telescope, which becomes very obvious
with solar system objects. Again we could reduce our exposure time to below the
turbulent time scale, which can be smaller than a hundredth of a second, far shorter
than most astronomical CCDs can image.
8.8.1 Lucky Imaging
There is, however, a simple solution to these problems that was first implemented
by amateur astronomers but is now being picked up by professionals; the use of
webcams. Although insensitive and noisy compared to astronomical cameras, their
speed and comparatively low cost led to their modification by amateurs into planetary
imaging cameras. However, in recent years, low noise astronomical webcams, both
colour and black and white, have started to appear on the market, and these are a
cost-effective solution to the problems with planetary imaging using a conventional
astronomical camera. Professional ultra-high-speed cameras, which may be cooled
and have noise and sensitivity levels close to the standard astronomical cameras
as well as frame rates as high as a thousand frames a second, are also becoming
available, but at considerable cost.
The advantage of using a high-speed, but relatively insensitive, camera is that the
frame rate is typically faster than the turbulence time scale. Imaging software, such
as Registax, is capable of processing the video file that comes off these cameras
frame by frame. By picking only the best-aligned frames, where turbulence is not
present, and stacking those frames, ultra-high-resolution imagery is possible, well
below the typical seeing and often limited by the diffraction limit of the telescope or
the pixel size of the camera. Resolutions of 0.2 arcsec using this technique, known as
lucky imaging, are common. This is typical of the resolutions achieved at very high
altitude observatories such as those in Hawaii and Chile, or with adaptive optics.
There is yet another use for lucky imagers, as these devices have become known.
Their very high frame rate allows the observation and precise timing of very transient
astronomical phenomena that are too short for normal CCD-based astronomical
cameras to capture. Examples of these are asteroid occultations, which may be a few
seconds long, and lunar meteoroid impacts on the Moon.
Given the very light nature of most lucky imagers, the time taken in removing and
then replacing an astronomical camera, the fact that a guide camera is not needed for
planetary imaging, and that most guide cameras are mounted on refractors, which
are ideal for planetary imaging, there is an advantage in using the guide telescope
for planetary imaging. If you have a choice, and you might not, we would suggest
going down this route for most planetary imaging.
8 Imaging
seeing becomes a major limiting factor. As we have seen, the turbulent motion of
the atmosphere limits the resolution of the telescope, which becomes very obvious
with solar system objects. Again we could reduce our exposure time to below the
turbulent time scale, which can be smaller than a hundredth of a second, far shorter
than most astronomical CCDs can image.
8.8.1 Lucky Imaging
There is, however, a simple solution to these problems that was first implemented
by amateur astronomers but is now being picked up by professionals; the use of
webcams. Although insensitive and noisy compared to astronomical cameras, their
speed and comparatively low cost led to their modification by amateurs into planetary
imaging cameras. However, in recent years, low noise astronomical webcams, both
colour and black and white, have started to appear on the market, and these are a
cost-effective solution to the problems with planetary imaging using a conventional
astronomical camera. Professional ultra-high-speed cameras, which may be cooled
and have noise and sensitivity levels close to the standard astronomical cameras
as well as frame rates as high as a thousand frames a second, are also becoming
available, but at considerable cost.
The advantage of using a high-speed, but relatively insensitive, camera is that the
frame rate is typically faster than the turbulence time scale. Imaging software, such
as Registax, is capable of processing the video file that comes off these cameras
frame by frame. By picking only the best-aligned frames, where turbulence is not
present, and stacking those frames, ultra-high-resolution imagery is possible, well
below the typical seeing and often limited by the diffraction limit of the telescope or
the pixel size of the camera. Resolutions of 0.2 arcsec using this technique, known as
lucky imaging, are common. This is typical of the resolutions achieved at very high
altitude observatories such as those in Hawaii and Chile, or with adaptive optics.
There is yet another use for lucky imagers, as these devices have become known.
Their very high frame rate allows the observation and precise timing of very transient
astronomical phenomena that are too short for normal CCD-based astronomical
cameras to capture. Examples of these are asteroid occultations, which may be a few
seconds long, and lunar meteoroid impacts on the Moon.
Given the very light nature of most lucky imagers, the time taken in removing and
then replacing an astronomical camera, the fact that a guide camera is not needed for
planetary imaging, and that most guide cameras are mounted on refractors, which
are ideal for planetary imaging, there is an advantage in using the guide telescope
for planetary imaging. If you have a choice, and you might not, we would suggest
going down this route for most planetary imaging.