Preparing to Acquire the Raw Images

Rather than describe the procedure used for cooled CCD cameras, I will instead detail the process I use for obtaining images with the webcams, which I have now used almost exclusively for the last 18 months. Today it remains a fact that the finest quality images of the planets are now produced with these devices, and currently no cooled CCD alternative to the webcam (that will produce as good results) is available. It is also of great note that color planetary images are much

Figure 7.3. Today's webcams can also produce great results when imaging other objects, such as the Sun. This shot from June 8, 2004, shows Venus in transit across the solar disk. An 80mm Apochromat with Solar filter was used for this shot.

easier to obtain with webcams than the tricolor process of cooled CCDs; however the modern black and white webcams with sensitive CCDs can employ a wide range of filters.

After the telescope has been prepared (having reached ambient temperature, precisely collimated, etc), the observer is ready to begin imaging the target planet. I cannot emphasize enough the importance of critical collimation. A good way to think of the process is to ask the question: "would a pianist continue to use his/her piano without it being properly tuned?" Such is the case for getting the most out of any telescope - it must be properly maintained and collimated for it to produce the best possible results, when the seeing allows.

The collimation process is used when imaging involves the observations of a star at high power (typically at greater than 600x). The concentricity of the diffraction pattern in and out of focus is then observed. The pattern should appear completely concentric with a small bright dot at the center, surrounded by a series of bright and dark rings (see Figure 7.4). The pattern should close itself into

Figure 7.4. The appearance of a perfectly collimated (left) and mis-collimated telescope (right). Even a slight misalignment will result in a loss of contrast.

a symmetric Airy disk at focus and, in good seeing, with no obvious asymmetry. It should be noted at this stage that checking the collimation once a month, or once every few months, is completely unacceptable for high-resolution work (especially if the telescope is transported any great distance by car). Also checking the collimation at low powers will not reveal misalignment - only high-power viewing will do that.

Before imaging can begin, the observer must choose an appropriate focal length to image at. In general, the Nyquist theorem dictates at least two pixels of the CCD must cover the theoretical resolving capability of the telescope. While this is true, the actual resolving capability of the telescope, on a planetary disk, is not the same as the Dawes or Rayleigh values quoted for limiting resolutions. Resolution of planetary detail is dependent on its contrast. For example, the tiny Encke division in Saturn's "A" ring spans just 0.05 arcseconds in angular width but has been imaged with apertures as small as 8 inches (20 cm), which is some 10 times smaller than the Dawes limit for this aperture. This is because the division represents a very dark line on a bright background, while the limit for resolving lower-contrast detail (such as small spots on Jupiter) will be nowhere near as fine as this level but still in excess of the Dawes value under good seeing.

Therefore, for telescopes in the 10-20 cm range, a sampling of around 0.50 to 0.25 arcseconds/pixel is high enough for typical conditions, while under excellent seeing I would recommend nearer 0.1 arcseconds/pixel with larger apertures (25-40 cm). Increasing the focal length is quite simple; I use high-quality Barlow lenses for the task, but eyepiece projection is also quite straightforward. For Jupiter I work at an image sampling between 0.19 and 0.13 arcseconds/pixel, while for Mars and Saturn nearer to 0.1 arcseconds/pixel. The formula you can use to work out the appropriate sampling is:

Pixel size of CCD (microns) / Focal length (mm) x 206 = arcseconds/pixel.

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