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Dont' be afraid of CCD Webcams and camcorders (IV) At the time of the electronic integration, small webcams at a few hundreds euros and camcorders have seduced a lot of advanced amateurs yet used to work with devices of another quality. Why such a passion ? First of all, webcams are cheap and display a reasonnable resolution between 18000 and 300000 pixels. Fixed images can be of very good quality, displaying an excellent color balance, contrast and clearness on high ends models, which price exceed 80 euros (Philips ToUcam, Logitech Pro 4000, etc). However, technically speaking the sensitivity of the CCD chip drops quite rapidly in blue light. The best models include several interesting features too (mic, head tracking, speed up to 60 frames/sec).
In view of their low profile and lightness it is also very easy to fix them at the eyepiece of a scope or to build oneself an adapter with second-hand parts, as explain us some fans on Astrocam.org website. But it is still more interesting to know that linked to a computer and an image processing software (IRIS, Photoshop et alike), it is quite easy to convert filmed sequences of an object in a single picture resulting of the stacking of several hundreds to some thousands of individual frames extracted from the best sequences. In other words, if we take a catadioptric scope of 4-5" of aperture, when an individual frame, displaying a low contrast and grainy reaches with difficulties a photographic resolution of 10 arc-sec in eyepiece projection, the stacking of 1500 frames made under good conditions will offer a photographic resolution near 0.5 arc-sec ! Need a webcam with a 31.75 mm barrel ? For technical reasons, due to the file size and the transfert rate between the webcam and the PC, the recording is usually made at speeds between 5 and 10 frames/sec, speed that reduces the file size to some dozen megabytes. Indeed you must known that for a resolution of 640x480 pixels and 24-bit depth per frame, each image is 0.92 MB. Recording a 10 seconds AVI film at 10 frames/sec (thus a 100 frames film) will request a space disk of ... 92 MB ! Avoid also using image compression what should lose image quality and prevent any optimization. All these parameters and many others (focus, gain, luminosity, etc) can be set up via the software driving the webcam.
The same with camcorders that do not suffer from defaults of webcams (low resolution, low speed). Even using a analog camcorder limited to 1/30th sec you can record high resolution images of the Moon of conjunctions for example. Use preferably the video format Super-VHS that offers more detailed images than VHS, otherwhise Betamax. But none of these cameras excels on digital camcorders that will be soon on HDTV format and compatibles with DTV disk recording. Most digital camcorders as well as digicams save their sequences in AVI format that can be read without problem by image processing software like Adobe Photoshop. Being given that the lenses of these camcorders cannot be removed, their usage in astrophotography is limited, and still more if you cannot disable the automatic exposure mode, without control over gain and iris settings. Image rocessing software to download : Like with color CCD cameras, color camcorders tend to have poorer resolution than black-and-white ones because the former require three adjacent red, green and blue photosites to form one color pixel. Finally, if the camcorder is connected to a scope, vibrations from the tape's drive motor may blur the image.
For all these reasons many amateurs prefer using black-and-white camcorders, often use for surveillance, instead of a feature-loaded camcorder. Most have a detachable lens. As they are also cheaper, this is all benefit for the amateurs that we are ! Once the lens removed and the T-ring attached to the camera body you can connect it, depending of your configuration, to the extender, the drawtube, the eyepiece projection adapter or to the Barlow lens as you should do using a 35 mm reflex camera. Optionally you can buy a "video enhancer" which performs the analog equivalent of unsharp masking, producing a sharper image. Video sequences processing As all astronomy images recorded in low light conditions, individual frames will appear grainy due to electronic noise from the uncooled CCD chip; this effect is more apparent in the low light levels. It can however not be noticeable at the speed of 30 images per second, maximum exposure time of most off-the-shelf camcorders, as the brain integrates successive images rendering the grainy effect much less obvious.
At last, once your film recorded you have to transfer the analog signal into your computer; this is done using a video digitizer or "frame grabber" that costs about 200 euros. Now your film can be read by any good image processing software and you can apply to it the entire range of image-enhancement techniques to improve its quality. The avantage of webcams and camcorders is the fact that since one usually records large number of video frames in a short while (1800 images are taping in one minute at a rate of 30 images per second), you can easily extract the best sequences, displaying the less turbulence and well sharp, brought them into register, and averaged. This procedure both reduces noise (residual noise is inversely proportional to the square root of the number of framed averaged) and averages out images shifts due to seeing. The effect is amazing as we can see in both pictures of Jupiter and Mars displayed above. This is still truer in color. Indeed, as for an LRGB composite, it is not important that images extracted from RGB channels are a bit blur or shifted (the lesser the better of course) because this is first of all the luminance that will give its contrast to the result, RGB images reducing only the electronic noise and averaging the seeing. To read : VIDEOASTRO Discussion group and resources Professional CCDs Speaking so about CCD cameras, I cannot lack of showing you pictures of the latest technology used by professionals. 8k and 12k CCDs At Hawaii Observatories, David Jewitt and his colleagues use several CCD models. A 8192x8192 pixels device usually attached at the prime focus of the 2.2m telescope. It was used to discover the first trans-neptunian asteroid.
In 2001, researchers ordered a 12k-pixels CCD of 15 microns. Its field reaches 42x28 arc-min at f/4 what means that is covers the entire Moon ! In this configuration its resolution is 0.206 arc-sec per pixel. In others words, even without doing high resolution, if astronomers enlarge the image they can detect features as small as 1 km wide on the Moon surface ! This is fine you will tell me, but where is the benefit ? This CCD camera is in fact the most powerful we have ever made in term of resolution and exceeds by a factor 10 the most sophisticated amateur CCD camera. Take an example comparing it to the SpectraSource Lynxx 2000. This latter uses a TC255 chip measuring 336x243 pixels of 10 microns each. Although its size is modest its offers an enviable resolution of 0.25 arc-sec per pixel on a 12" f/39 scope, near the one of the 12k-CCD. Only drawback, at this focal ratio the field does not exceed 87x63 arc-seconds, while at f/4 its resolution drop to... 1.8 arc-seconds per pixel, 9 times lower that the one of the 12k-CCD ! If the Lynxx creates images of 160 KB in size containing 4096 brightness levels, the 12k-CCD requires 200 MB of disk space to store each image ! Hard to beat professionals... Megacam In 2003, engineers built the MegaCam : 40 CCDs of 2048 x 4612 pixels each for a total of 340 megapixels ! This CCD covers a field 1 x 1° wide (the field of 4 full Moon) with a spatial resolution of 0.187"/pixel in order to sample correctly the signal when seeing is at best (CFHT reaches an average resolution of 0.7"). Drawback, the weight of the system that contains the MegaCam is 375 kg and measures 1.7m of height for 1.2m in diameter !
Gigacam En 2007, IfA astronomers in collaboration with MIT Lincoln Laboratory engineers pushed the feat still further with the Gigacam GPC1 : 4096 CCDs offering a total resolution of 1.4 gigapixels ! The image is 40 cm in diameter, the CCD opening measuring 56 cm of diameter ! The system is constituted of an array of 64 x 64 CCDs offering each a resolution of 600x600 pixels. CCDs are mounted in 8x8 array on a silicon chip called "orthogonal transfer array" (OTA) of about 5 cm2. Each CCD of each OTA can be controlled and read individually. In total, 66 OTAs are placed at the telescope focal plane (the 4 corners are omitted because they are too far of the optical axis). The system can record objects to magnitude +24 in 60 seconds of integration, or 10 million times dimmer than the weakest objects visible naked eye !
The GPC1 camera has been mounted in august 2007 on the first of four 1.8m PS1 (Pan-STARRS-1) telescopes actually installed on top of Haleakala on Maui island, Hawaii. The system has a 3° field of view with a spatial resolution of about 0.3"/pixel. PS1 can survey 6000 deg2 per night. The whole available sky as seen from Hawaii will be observed 3 times during the dark time in each lunar cycle. Four identical telescopes will be equipped with Gigacam and will have for task to monitor trajectories of potential hazardous asteroids. They will mainly work in visible between 500 and 800 nm as well as in standard photometric g, r, and i bands, including z-band, in which works SDSS, and in infrared in y-band at 1 micron, where by nature CCDs are always very sensitive. But where will they stop ? Know-how Voilà in a few words all the process to follow in order to get good CCD pictures. The apparent difficulty of the first contact should not put off any amateur interested in the subject. Of course sit down behind your computer screen there are a lot of experiments to do, the first being the selection of your image processing software and to familiarize with digital functions. With this reading you have probably realized that one does not act as "CCD imagery guru" from one day to another. If you can master a digicam or a digital camcorder in a few hours, to get high resolution images using a CCD camera or achieving a quality stacking requires experience that one can only acquire in the field. If you want to do rigorous work you have to improve your raws CCD images. I still see too many amateurs acquiring CCD images and stopping once the picture taken. Stars are huge fuzzy blops, some display artifacts of saturated pixels. (L)RGB composites are grainy or often all but balanced, the sharpness pushed too high or detail lost in the compression. Of course this a long-run job but which results can compete vs professional imaging. That also is the challenge ! Saying this I suggest you to read the other files listed in the Digital Darkroom section to learn more about color composites and image processing techniques as well as to read the French page, heavily illustrated, about the martian oppositions and titled Attirantes oppositions de Mars in which I have presented some among the best (L)RGB pictures of the Red planet, before closing the reading with my Image Gallery in which I present you some of the best astronomy pictures ever made by amateurs. For more information About CCD VIDEOASTRO, forum Perseu, sells webcams modified for the astrophotography Steve Chambers's CCD and webcam Astrophotography CCD's manufacturers technical documentation (see letter M) Ron Wodaski's Magic CCD Calculator Sky Tonight - Astrophotography and CCD imaging (Sky & Telescope) Articles about Image processing Basic Concepts in Digital Image Processing, Microscopy primer (java applets) Software Imaging software reviews (on this site) Anti-Blooming Filter Software, by K.Takana IRIS, by C.Buil Book and magazines The New CCD Astronomy, Ron Wodaski Photoshop for Astrophotographers, Jerry Lodriguss CCD Astronomy, C.Buil, Willmann-Bell, 1991 CCD Astronomy (magazine from Sky Publishing Corp.) |
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