Contacter l'auteur / Contact the author

Recherche dans ce site / Search in this site






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.

At left, a black and white Supercircuits PC164C CCD video camera sensitive to 0.0003 lux ! It costs the same price as a webcam, a bit more expensive than the Logitech Quickcam VC webcam displayed at right.

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 la 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

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 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 of sec for camcorders. Usually this range is never used at full because the Moon for example supports exposures times between 1/500th and 1/10th of sec. However, for planets we can go up to 1/10th of sec or less.

About the termal noise, it can not be noticeable on camcorders or videos recorded with compact or DSLR cameras at the speed of 30 fps, the maximum exposure time of most models, as the brain integrates successive images rendering the grainy effect much less obvious.

These systems give excellent results if there is enough light and if we know their limits. In this regard, in planetary imaging, the exposure time is often intantaneous and thermal noise, even if it is low on some models, does not always permit to get images of quality (see examples in links page 5).

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 center, a Pentax MS-5 telescope equipped with a guiding scope and a Vixen color camcorder. The real-time image is displayed on the computer screen and can be saved. Document Astroarts. 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.

For deep sky imaging, conditions worsen because the exposure time can exceed several minutes to reveal all the extent or structure of the object. The noise becomes so visible that the substraction of a dark frame and other bias are practically mandatory as we will explain further.

In these conditions, it is preferable to give up camescopes and DSLRs that are in any case not suited for deep-sky astrophotogaphy. The ideal is to use a CCD or CMOS camera dedicated to astrophotography as those described previoulsy, some models being particularly complete, compact, able to picture in trichromy or to record directly color images.

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

Page 1 - 2 - 3 - 4 - 5 - 6 -

Back to:


Copyright & FAQ