Astronomy Picture of the Day 11-04-02

Explanation: Cassini, a robot spacecraft launched in 1997 by NASA, is close enough now to resolve many rings and moons of its destination planet: Saturn. The spacecraft has closed to about two Earth-Sun separations from the ringed giant. Last month, Cassini snapped several images during an engineering test. These images have been combined into the contrast-enhanced color composite pictured above. Saturn's rings and cloud-tops are visible on the far right, while Titan, its largest moon, is visible as the speck on the lower left. When arriving at Saturn in July 2004, the Cassini orbiter will begin to circle and study the Saturnian system. Several months later, a probe named Huygens will separate and attempt to land on the surface of Titan.

Astronomy Fun Fact:

Of course, you can't just shoot a spacecraft toward a planet and get the results you want. First, you must know WHERE in its orbit the planet will be at any given time. Second, you have to figure out how to arrive at that point AND match its speed and trajectory in order to accomplish orbital insertion, or even a flyby.

And theory is the easy part; ask any engineer.

I wouldn't even be surprised if many people have forgotten about the Cassini probe, or even never heard of it.

I have posted a simulated position report on Cassini below.

The cameras on board do magnify the image.

Imaging Science Subsystem (ISS)

The Cassini Imaging Science System (ISS) was specifically designed for exploring the Saturn system, and includes spectral filters and imaging capabilities for a multitude of scientific objectives, including capturing lightning, investigating the three dimensional cloud structure and meteorology of the Saturn and Titan atmospheres, imaging the surfaces of its many icy satellites, determining the composition and structure of its enormous ring system, and peering through the hazy Titan atmosphere down its still unexplored surface.

The ISS consists of two framing cameras. The narrow angle camera is a reflecting telescope with a focal length of 2000 mm and a field of view of 0.35 degrees. The wide angle camera is a refractor with a focal length of 200 mm and a field of view of 3.5 degrees. Each camera is outfitted with a large number of spectral filters which, taken together, span the electromagnetic spectrum from 2000 Angstroms to 1.1 microns. At the heart of each camera is a charged coupled device (CCD) detector consisting of a 1024 square array of pixels, each 12 microns on a side. The data system allows many options for data collection, including choices for on-chip summing and data compression.

From the Queen Mary and Westfield College of London webpage:

The ISS Instrument

The ISS instrument comprises a pair of aligned cameras, one wide angle (WAC) and one narrow angle (NAC). These are mounted on the side of the Cassini orbiter spacecraft.

The WAC actually uses an optical system that was originally built as a spare for the Voyager missions, but couples this with the hugely improved detector technology of the 1990's.

The NAC and WAC each have a pair of filter wheels in their optical path. This allows imaging in a large range of spectral bands (i.e. colours) across the wide (ultraviolet to infrared) sensitivity band of the CCD detector.

Each camera has a CCD image sensor of size 1024 x 1024 pixels. The images are digitised with 12 bit resolution.

To allow more images to be stored in the spacecraft memory and to decrease the downlink requirements it is possible to compress the images. Both lossless compression and lossy compression (using a form of JPEG) are supported. Using lossless compression, images can typically be about halved in size and then perfectly reconstructed when received - this is like using a "zip" program to compress computer files. Lossy compression allows images to be shrunk much more (typically to about 10% of their original size), but means that some of the finer detail is discarded and cannot be restored. Many of the images at this and other websites use similar JPEG lossy compression to speed them over the net: these images are fine to look at by eye, but cannot be enhanced much to look for subtle features because the information is just not there any more. By having both options, the Cassini ISS will allow both fast, plentiful imaging and slower, more detailed imaging to be done.

At QMW we investigated what effects lossy compression would have on images of faint rings.

Comparison with Voyager:

The Voyager cameras used 8 bit digitised images from vidicon tubes.The modern cameras have a number of very significant advantages:

Efficiency
CCDs have high quantum efficiency (detect more of the photons that fall on them)
Bigger images
1024 pixels square compared to Voyager 800 pixels square
Dynamic range
The 12 bit digitisation allows 16 times as many greyscale steps as in Voyager images. With the very linear response of CCDs this gives good dynamic range allowing detail to be recorded simultaneously in both dark and bright regions of an image.
Freedom from vidicon distortion effects
Vidicon images suffer distortion as electrons travel along the vidicon tube. This includes pincushion and barrel distortion as seen on imperfectly adjusted TV or monitor screens, and is variable especially when in a magnetic field. To allow this to be compensated for, each Voyager vidicon had a pattern of reseau marks at known positions on its face. Voyager images therefore have a characteristic pattern of square black dots across them which would sometimes be "filled" (smudged out) before pictures were published.

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