Planetary Webcams and Their Alternatives

Imaging technology is a very fast moving area in the early 21st century. Once millions of dollars are invested into any form of technology, fueled by public consumption of a product, remarkable advances can occur. In 1989 the very first amateur CCD cameras started to appear on the market. A decade later, the first decent digital cameras and webcams started to be mass produced. In 2004/2005, Canon's Digital Rebel/300D Digital SLR put a real SLR camera in consumers' hands at a very affordable price. I mention all of this because it is likely that even in the few months it takes to print this book, more advances will be made. However, I think it very likely that a webcam-type product will still dominate planetary imaging for many years to come. It is all a question of how good and affordable that product will be. While the standard ToUcam Pro imager and its equivalents (with the same CCD chip) are fine for planetary imaging in 2005, manufacturers' product lines change with time and it is always good to be ahead of the game. Many of the world's most advanced planetary observers use custom astronomy webcams to get the best results, either so that they can use LRGB techniques with a sensitive monochrome camera, or so they can use multi-second exposures for IR and UV work. Some of these custom cameras can also be used for low-cost deep sky imaging too, killing two birds with one stone.

The prime consideration in planetary imaging is to have a sensitive, fast-download detector at the heart of the system and as little noise in the detector electronics as possible. The Sony CCDs in the best webcams already have chips with good quantum efficiency (QE) and low noise, but 100% QE must be the eventual aim. Once this is achieved it is important to reduce the readout noise and thermal noise as far as possible. Cooling the detector is used to achieve this latter requirement in long exposures, but it seems unlikely that mass-produced commercial webcams will go down this route, especially as readout, not thermal noise, dominates in short exposures. However, there are already air-cooled astronomy webcams on the market, although, frankly, the cooling does not make much difference for planetary imaging; indeed, I know amateurs who have cut the fan leads and sealed the back to reduce dust ingress. Download speed is another area where improvements can be made. Remember the Planetary Imager's Mission Statement: "To record as many high quality frames before the planet has rotated more than the resolution of the telescope." In current USB 1.1 webcams, minimal image compression takes place at 5 and 10 frames per second and exposures of 1/10th of a second are needed to get good signal-to-noise images, especially with Saturn. So, with exposures of 1/10th or 1/5th of a second being needed, frame rates higher than 10 frames per second are unachievable. The current webcams do fine. Another issue here is hard drive usage and address ability. With USB 2.0 webcams, far more data can be transferred from webcam to hard disk, but over several minutes an entire hard disk can be eaten up! Also, the frame stacking software Registax can (in 2005) only address a maximum of about 10,000 frames or 2 Gigabytes at any one time (although multiple AVIs can now be opened in parallel). Older PCs (such as used in the observatory) have operating systems that will only allow the addressing of half this amount of memory, so your webcam AVIs may have to be restricted to 2,000 frames and processed separately once you start collecting multi Gigabytes of data. These are vital considerations that need to be considered before upgrading to a USB 2.0 or Firewire download camera. Would you ever need such a system at all? Well, the speed of USB 2.0 does allow a completely uncompressed deeper bit-depth (luminance and color) transfer of data and may well be useful on very bright targets (like Venus in the UV, the Moon, Mars, or the Sun) where freezing the seeing with ultrashort exposures is of great use. But, when stacking hundreds or thousands of frames, USB 1.1 webcams do a great job with a basic PC and at an affordable price. At the time of writing, the Philips ToUcam Pro, despite being an off-the-shelf $100 webcam, designed for the public, not astronomers, is still the best color planetary imaging device and it is affordable. With a telescope adapter, UV-IR blocking filter, and Registax software you are in business!

Sony CCD chips lie at the heart of the most light-sensitive webcams. These are technically known as Interline CCDs, which means they are of the type used widely in domestic video devices and feature circuitry on the chip surface that download the charge from the pixels and to the A/D converter. The presence of electronics on the top of the photosites used to mean that Interline devices were far less sensitive: the pipe work was blocking photons. However, in recent years, Sony has perfected the use of microlenses above the pixel surface. These microlenses collect the light before the "piping" intervenes. Thus, the current generation of Interline chips are both sensitive and low-priced and therefore ideal for webcams and cheap astro-cams. The alternative Progressive Scan/Frame Transfer devices such as the Kodak KAF chips used in SBIG's cameras are far more expensive. The Sony chips that are most widely used in astronomer's webcams have the prefix ICX. There are a number of different subtypes, namely, the ICX098 series (suffixed with AK, BQ, AL, and BL) and the ICX424 series (suffixed with AQ and AL). Other Sony chips in the ICX series have been used by astro-enthusiasts to build their own modified webcams, too, namely those suffixed 254, 255, and 414. One astro-webcam manufacturer, Astro-Meccanica, uses the ICX 409AL chip in its KC381 planetary camera. All of the L suffix cameras are monochrome units with unfiltered pixels. The sensitivity of these monochrome CCDs (i.e., the voltage produced for a specific illumination) is, not surprisingly, much better than the filtered CCDs of the same type (typically, about 2.5x times better). But, of course, once R, G, and B filters are used it is back to square one (almost).

A summary of Sony's "planetary-friendly" CCD chips, i.e., those most used by amateur astronomers and astro-webcam manufacturers, is shown in Table 6.1.

While mentioning CCD webcams, I should, perhaps, mention Meade's LPI Imager/Autostar Suite, which appeared in 2003 and was marketed as a lunar and planetary imager and autoguider combined with a planetarium and telescope control package. While the package was incredible value for money, the CMOS detector in the LPI imager was noisy and insensitive compared to a ToUcam Pro and the download time was sluggish when compared to any webcam. No serious planetary imagers that I know use this detector. However, as a cheap and cheerful bundled package for complete imaging beginners, it worked well.

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