Don't be afraid of CCD
Webcams
and other video cameras (II)
At
the time of the electronic integration, small webcams at a few hundreds
euros and digital astronomy cameras have seduced a lot of advanced amateurs yet used to
work with devices of another quality. Why such a passion ?
Honour
to pioneers, the webcam
was invented in 1993 in England, at the Cambridge University Computer Science department.
In 1994, Jeff Schwartz and Dan Wong then students at San Francisco State University (SFU)
did the same discovery and developed the "fogcam".
The
first commercial webcam was sold in 1994, it was the QuickCam
manufactured by the company Connectix which products were bought in 1998
by Logitech.
Pros and cons
First
of all, webcams are cheap and display a wide choice of definitions ranging between
0.76 kpixels (320x240 pixels) and more than 50 Mpixels, so as much as the last generation
of DSLR. They support images in VGA or full HD format and video formats AVI, some WMN or MOV.
Their price increases with their performances but remains very low (20-80 €).
Able
to record between 5 and 60 fps depending on the definition and their performances,
individual images can display a very good quality, an excellent color balance,
contrast, clearness and a sharp image on models like Philips ToUcam or Logitech Pro 9000.
In view of their low profile and lightness it is also very easy to fix
them at the eyepiece of a scope using a simple adapter
or to build oneself an adapter with second-hand parts, as explain French-speaking
fans on Astrocam
Yahoo usergroup.
However, technically speaking the sensitivity of the CCD chip drops quite
rapidly in blue light but offer a good efficiency up to the near infrared.
If webcams
benefit of a low price, a light weight and are simple to use, they require a
direct connexion by USB to a computer on the observation site.
Astronomy cameras
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CCD
camera i.Nova PLA MX 310kp |
Beside
webcams and classic still CCD cameras, some manufacturers (ImagingSource, i.Nova,
Lumenera, ZWO, etc) provide high
definition CCD or CMOS cameras able to record images of 2.8 Mpixels at 53 fps
to Lumenera, and in best cases images of 6.4 Mpixels at 164 fps or 239.8 fps at
low definition (320x240 pixels) for ZWO cameras. They also use a high speed link to the computer and are more
flexible and performing than most traditional models due to their new technology.
More expensive
(300-1500 €) and a bit heavier (400 g in average for cooled models) than webcams, some show
a low profile and are not more cumbersome and even sometimes smaller than
classic CCD cameras. They can be thus be fixed on small quality scopes from
60 to 130 mm in diameter fixed on stable and sturdy mounts (these scopes
belong usually to mid- and high-end categories).
In
addition to their excellent image quality in monochrome or color, these astronomy
cameras are equipped with an USB 2.0 or 3.0 port, Firewire (IEEE1394) or Gigabit
Ethernet (GigE). The high-rate connexion is required due to the high definition
and size of image files, too large and thus too slow to be transfered via a standard
serial link that would ask hours to download hundred or thousand images of several megabytes each.
At
last, these cameras of new generation support most image formats RAW, BMP,
JPEG, PNG, FITS and TIFF as well as video formats AVI and SER.
Camcorders,
DSLRs and compact cameras with video capabilities
HD camcorders, DSLRs and compact cameras with video capabilities are
autonomous, versatiles, relatively light (200-800 g) and mid-end like
high-end models are not more expensive than a CCD astronomy cameras at
30 fps but do not include all their functionalities (except DSLRs and some
hybrids, the lens cannot be removed, they are not cooled, have no built-on guide
chip, no binning mode nor anti-blooming among other functions).
In
general, these systems support the AVCHD video format (MPEG-4 Internet and sometimes MOV
while digicams (compacts) usually support AVCHD Lite and Motion JPEG (M-JPEG)
in low resolution.
Exposure
times are generally ranging between 1/10000th
to 60 minutes for an ImagingSource camera, from 1/8000th
to Bulb for a DSLR and from 1/2000th
to 1/2 s for camcorders. Usually this range is never used at full
because the Moon for example supports exposures times between 1/500th
and 1/10th
s. However, for planets we can go up to 1/10th
s or even slower.
Regarding
the electronic noise that comes from different sources including thermal agitation,
this parasitic noise can go unnoticed on camcorders and videos taken by DSLR because
at the rate of 30 fps, maximum shutter time of a camcorder ordinary, the brain integrates
the successive images and makes the graininess much less apparent.
These
video systems give excellent results if the luminosity is sufficient and if one knows their
limits. Precisely, in planetary imaging, the exposure time is often instantaneous and the
thermal noise it causes, even if it is low on certain models, does not always allow high
quality images to be obtained (see the examples in the links on page 6).
For
deep sky imaging, conditions worsen because the exposure time can exceed several
minutes up to several tens of hours in total to reveal all the extent or structure of the
subject. The noise becomes so visible that the substraction of a dark frame and other bias
are practically mandatory as we will explain further.
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At
left, M20 recorded by Xavier
Ambs at prime focus of a William Optics FLT 132 apochromat fixed on a Losmandy G11 mount and equipped
with a Canon EOS 350D DSLR modified (without hot mirror) and equipped with a Baader
BCF H-alpha filter. It is a stacking of 75 color frames exposed 2 minutes each. At right, Saturn pictured
on Feb 12, 2002 by David Hanon using an Astro-Physics 180 mm f/9 EDT
refractor equipped with an 11 mm eyepiece. This image stacking result of 46 frames recorded with a camcorder MiniDV,
zoom full extended. Images have been postprocessed in MaxImDL.
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Under
these conditions, it is preferable to give up camcorders and DSLR which for the most part
are not suitable for deep sky astrophotography. The ideal is to use a CCD camera dedicated
to astrophotography as those described previoulsy, some models being particularly complete,
compact, suitable for trichromy or directly recording color images. Reserve only the camcorders
and the video mode of the DSLRs for planetary imaging but know that CCD cameras are more efficient.
Among the few DSLR suitable for astrophotography, let's mention the Canon EOS 1DX, Canon EOS Ra
or better still the Sigma fp hybrid camera.
Duration
and size of video recordings
For
technical reasons, due to the file size and the low transfer rate between
the camera and the computer (few cameras have a Firewire interface at 50 or 100 MB/s),
the recording is usually made at rates between 5-10 fps, rate limiting the
size of files to some tens of megabytes. Indeed one must known that for a
definition of 640x480 pixels and 24-bit depth per frame, each image is 0.92 MB.
Recording a 10 seconds AVI film at 10 fps (thus a 100 frames film) will request a space disk
of 92 MB. Avoid also using a too high image compression what should lose image quality
and prevent any later optimization. All these parameters and many others (focus, gain,
luminosity, etc) can be set up via the software driving the camera.
For
DSLR and compacts with video capabilities, it is a bit simpler and settings
are usually limited to the selection of the format and definition, the
other settings being set automatically (white balance, sensitivity, etc.)
At
last, if you work with an analog camera, you can digitize the film using a video digitizer or "frame
grabber". Matrox
among other manufacturers provides various performing interfaces (1750$ for Matrox
Radient eCL). Now your film can be read by any good video processing software
(e.g. Adobe Lightroom, After Effect) and you can apply to it the entire range
of image-enhancement techniques to improve its quality and even convert it in other formats.
Next chapter
Guiding systems
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