SATURN THROUGH THE GALILEAN TELESCOPE

When Galileo looked at the tiny image of Saturn produced by his telescope, he could distinguish nothing other than a rather odd shape, which looked to him something like a round disk with handles on two sides. He seems to have thought these protuberances (which we now know to be the rings) might have been moons or other structures very close to, perhaps even attached to, the body of the planet and revolving very slowly about or with it, causing them to appear and disappear over a period of several years. Galileo first noticed Saturn's peculiar shape in July 1610, well after the publication of his landmark book Sidereus Nuncius, and the story of how he initially revealed the new discovery to his fellow astronomers by means of an anagram is told in a 1974 article by Albert van Helden. Mention of the discovery in print, albeit an extremely brief description, first appeared in the opening paragraphs of Galileo's 1612 book Discourse on Bodies Floating in Water.

Aside from the mysterious lobes, Galileo apparently failed to detect any indication of satellites orbiting farther from the planet, more like the moons of Jupiter, though he must surely have looked quite carefully.

CCD sensors greatly extend the threshold of what can be seen through a given telescope compared to the limit attainable to the human eye. The following images were taken with an Olympus C5050 digital camera in automatic noise reduction mode. Its ability to clearly show faint images is far superior to that of the Olympus C3000Z with which all the other photographs displayed on this website were obtained. Indeed when used with a long exposure time, its sensitivity is far superior to the human eye. The present photos show objects (the moons of Saturn) which, although most certainly present in the image (as the camera reveals) are (with the possible exception of Titan) much too dim for a human observer looking through the same telescope to see.

To aid in identification of objects present in the photos, this page displays a numerically-coded overlay when you point your mouse at each frame. The overlay should disappear when you move the mouse off the frame. The overlay was prepared using Patrick Chevalley's Cartes du Ciel sky mapping software. The predicted positions of Saturn's brightest moons are indicated in blue, while a number of stars present in the field are coded in red. A couple of faint stars were added based on the Sloan Digital Sky Survey image of this region (RA = 114.6875 deg; Dec = +21.5306 deg). The objects corresponding to each numbered dot are identified in the table given below. A properly scaled and oriented image of Saturn and her rings also appears in the overlay. To give a convenient size for display on this page, the original photos, at a scale of approximately 0.75 arc-seconds per pixel, have been reduced 6.2 times giving a final scale of 4.6 arc-seconds per pixel.

If this page does not display properly in your browser, click here to see the images and transparent overlay separately.

The first image is a normal 1/10th second exposure of Saturn at ISO 200, taken through the 1-inch aperture Galilean telescope on January 24, 2005 at 6:57 pm PST. No moons are visible in this picture which, incidentally, gives a fairly decent impression of how tiny Saturn looks in a 20-power telescope. The second image in the first row is an average of three 16 second exposures taken immediately afterwards, between 7:00 and 7:05 pm PST. Moon 1 (Titan) is clearly visible to Saturn's lower left. The final picture in the first row is the identical image but with the photometric curve artificially adjusted to bring out fainter features. One can now see moons 2 (Rhea) and 4 (Dione) and also star 9.

The second row repeats the same image but with the gain boosted by increasingly large amounts. In the first image moons 5 (Iapetus) has become quite apparent. There are also hints of stars 10, 11 and 12, which become more clear in the middle frame. The slight bulge at the left side of Saturn's image is at about the right angle to be due to moons 6 (Encelades and Mimas), but they are too far in and too faint to possibly be responsible for it. Moon 3 (Tethys) is similarly invisible because it is too far inside the ring of overexposed haze. In the middle image, star 7 is clear at the top of frame, and there may be hints of stars 13 and 14. In the final image star 8 appears to have become visible at the upper right edge of the image. At this point one is seeing elements registering only a very few counts on the CCD, and increasing the gain still further only brightens the random noise. The bright greenish circle visible in the last two photos is the background glow from the night sky. The fact that this circle does not fill the frame demonstrates the limited field of view visible (at any one time) through the Galilean telescope. The edge of this field is not sharply defined (the width of the transition from full illumination to darkness depends on the size of the camera's entrance aperture). Object 8 might be said to be at the edge the field; yet Object 7, although farther out, can still be seen (heavily vignetted) because of its greater intrinsic brightness.

Saturn was near opposition (closest approach to Earth) at the time of these photos, so both it and its moons were near their brightest. In the following table the visual magnitudes of Saturn's moons have been updated using the JPL Horizons ephemeris system. They agree within 0.1 magnitudes with those predicted by the Cartes du Ciel software. From the table it appears that the limiting magnitude for this combination of telescope, detector and exposure time is a little over 11: moon 5 is clearly visible, but only about half the stars fainter than it appear to have been detected, some possibly being confused with random noise (star 12 being particularly iffy, since the suspected spot seems to fall in not quite the correct place). However, it should again be emphasized that this page is in no way meant to imply that a person looking through the present 1-inch aperture Galilean telescope could see these faint stars and moons. An young experienced observer, knowing what to expect, could probably detect Titan, the brightest of Saturn's moons, but Galileo was apparently unable to do so despite claiming to have looked at Saturn through his telescope "thousands" of times.

The limiting magnitude for visual observations depends on many factors including the condition of the observer's eye, the darkness and transparency of the sky, and glare from brighter objects in the field. Judging from his drawing of the Pleiades, Galileo's limiting magnitude at the beginning of his astronomical career appears to have been slightly over 8. Had he looked at the present field, one would guess he might have caught a faint glimpse of Titan, but none of the other faint objects shown here. If he did not seen Titan it would probably have been due to the glare from the parent planet.

Click on the star names to bring up additional information about them, including their identity in numerous other catalogs, via the SIMBAD database, operated by the CDS in Strasbourg, France. Star 8, although in the Tycho catalog, could not be found listed in that database.

Reference :

1. A. Van Helden, "Saturn and his Anses," Journal for the History of Astronomy 5, 105-121 (1974). (freely available from NASA's ADS)

Images (unless otherwise credited) © Tom Pope and Jim Mosher
Last modified: October 7, 2006