Saturn (18 March 2007)

Date: 18 March 2007 U.T.
Time: 3:15 - 3:42 U.T.
Telescope: 13cm refractor (full aperture & stopped down to 30mm)
Magnifications: 66x, 80x, 138x, 218x
Filters: Moon-SkyGlow and unfiltered
Seeing: Good
NELM: 6.5
Temperature: +45 F (+7 C)

This session started with a test. Several years ago I made a series of Saturn sketches using the same telescope stopped down to effective apertures of 10mm, 20mm, 30mm . . . 100mm, 110mm, 120mm. At that time I noted that the presence of Cassini's Division could be detected as a subtle shading near the outermost edges of Saturn's ring system with a 30mm aperture.

Some have doubted that observation, so this evening I attempted to duplicate it. I placed a mask with a 30mm circular opening in front of the telescope. Saturn was then studied at 66x and 80x without the use of any filters. Even though the ring system was opened less now than it was several years ago, I was still able to detect (though not as easily as before) the shading near the outer edges of the rings caused by the presence of Cassini's Division. Furthermore, with the 30mm aperture the ring was visible as a darker line where it passed in front of the planet. It was also possible to discern which part of the ring system Saturn's globe was casting its shadow on.

Saturn at full aperture: The 30mm mask was removed and the planet was studied primarily at 218x. I found that I preferred the view without the Moon-SkyGlow filter, so the MSG filter was removed for most of the session.

Five suspected Saturnian moons were sketched, but later it was discovered that one of the suspects was a star. Titan, Dione, Tethys, and Rhea were all positively identified. Tethys was very close to the south-following limb of the planet. Of these four moons, Titan was the only one noted with the 30mm aperture.

Overall, Saturn looked very much as it did when I observed and sketched it several nights ago using a 30cm Newtonian. The larger telescope resolved the Cassini Division better, particularly near the limb of the planet. Saturn's narrow Equatorial Belt, just south of the ring system in front of the planet was more difficult to see with the 13cm refractor. Saturn's South Equatorial Belt was suspected to be double in the Newtonian; but it appeared single in the refractor. (A blue filter was used with the Newtonian observation.)

One probable error in the earlier Saturn sketch involved a dark line adjacent to the north edge of the ring system in front of the planet. That dark line should probably have been the outermost edge of the A-ring itself. Such was the general appearance this evening with the 13cm refractor.