Observing Saturn at Opposition
13/14 January 2005
The 2005 Opposition of Saturn presents the rare opportunity to observe its satellites as the Earth transits the disk of the Sun seen from Saturn. This alignment, which will not occur again until 2020, enables the measurement of the "opposition effect" of the satellites, the dramatic increase in their brightness seen as the solar phase angle decreases from 0.1 to 0.028 degrees (the smallest effective phase angle established by the angular size of the Sun seen from Saturn). The amplitude and width of the opposition effect reveal surface particle properties (such as porosity, grain size, and grain size distribution) that provide important clues to the thermal and weathering history of the satellites.
While 2005 Opposition of Saturn will be observed by professional astronomers at 10 observatories in Europe and the US using 15 telescopes as well as HST, this is an excellent opportunity for amateur astronomers who have equipped their telescopes with CCDs to participate in this worldwide observing campaign. This website is intended to communicate the information essential to participants willing to make calibrated observations of this rare event.
The goal of this observing campaign is to obtain calibrated CCD images of Saturn's airless satellites, not "pretty" pictures of the planet and rings. In order to "see" the satellites, Saturn and the rings must be grossly overexposed (saturated), as shown in the image below. This "family portrait" was obtained using the Vatican Advanced Technology Telescope (VATT) on Mt. Graham in Arizona on 20 December 2004. In this 5-second B filter exposure, shown in false color to enhance the difference in brightness between pixels, Saturn is so overexposed that the saturated pixels actually bleed along the vertical axis of the CCD chip. When satellites are positioned near the poles of the planet, this bleeding prevents us from seeing them. By rotating the CCD, however, we can control the direction of the bleeding and see satellites which would be otherwise lost behind the bright glare. Here we have oriented the CCD such that the angle between the vertical axis of the chip and North was 40 degrees, allowing us to capture all four innermost satellites in one image.
Observers should plan on obtaining calibrated CCD images of the saturnian satellites throughout the nights of 12-14 January 2005 as the solar phase angle, and hence brightness of each satellite, changes rapidly during these nights. The brightness may increase by 20% or more until opposition (see below) and then decrease by the same amount as the phase angle increases afterward.
Note to Observers:
The time-critical nature of this rare opportunity to observe Saturn's moons at true opposition supersedes ANY requirements for photometric observing conditions. If weather conditions allow your telescope dome to be open, YOU SHOULD BE TAKING DATA. We will all be AT LEAST 15 years older the next time we have the chance to observe Saturn's moons at phase angles this small. (Even in 2020, there will be no "central" transit of the Sun by the Earth as there will be on 13/14 January 2005, rather the Earth will trace a chord across the solar disk.)
The following table lists the Universal Times of the onset of the Earth transit for each of Saturn's satellites on 13 January. The transit begins as soon as the phase angle decreases to 0.028 degrees, continues until the phase angle reaches its theoretical minimum (at the time of opposition below), and ends when the phase angle has increased to to 0.028 degrees. The transit is best observed from western Europe, but observers in the US should be able to capture the end of the transit on the evening of 13 January. It is not expected that the satellite brightness will change dramatically while the transit takes place. (Note that the solar phase curves for Europa, Enceladus, and Mimas flatten at phase angles smaller than the solar radius.) However, observations obtained during the Earth transit are expected to provide the true opposition magnitude for each satellite.
Transit and Opposition Times UT
In order to provide absolute calibration and a reference "field" of several ~10th magnitude stars, the following standards should be imaged as well, in all filters used (UBVRI, or any subset thereof):
ALSO: In addition to the observations made 12-14 January, it is also necessary to obtain observations (of the satellites as well as the standards listed above) on at least one other night (later in January or even in February, for example) after opposition at a larger phase angle. This will complete your set of photometric measurements by providing a reference observation for your CCD. Only by observing the satellites at a larger phase angle will you know just how bright they got on 12-14 January!
Please keep accurate records of all images taken which include your location, time of each observation, filters used (e.g. UBVRI), length of each exposure, and the size and scale of your CCD (i.e. the field of view in arcminutes and the pixel size). You may send copies of all log sheets, data images (preferably in fits format), flatfields, bias frames, and calibration images on a CD to:
Please send any questions about observing Saturn at Opposition 2005 to verbiscer[at]virginia.edu.
The solar phase curve for Europa, a bright icy satellite of Jupiter demonstrates a dramatic increase in the "brightness" (I/F on the vertical axis) of different terrains on its surface at very small phase angles. Without the observations in the shaded area (at phase angles smaller than 0.3 degrees), we would have no idea that these terrains get so bright at opposition (phase angle = 0 degrees).
The solar phase curves for Mimas and Enceladus, two icy saturnian satellites, from HST observations, show that the surfaces of these moons get much brighter as the phase angles decrease. Each circle represents the satellite's brightness (I/F) in a single HST observation. Phase angles are on a logarithmic scale to show the curves in detail near opposition. Solid lines are fits to the data; however, we have no observations of Mimas at phase angles less than 0.07 degree and of Enceladus at phase angles less than 0.1 degree. The dashed lines represent model phase curves for surfaces of varying porosities (noted next to the curves). The observations made during the 2005 Saturn Opposition will fill in these portions of the phase curves, enabling us to derive physical characteristics of the surface, such as porosity, grain size, grain size distribution, roughness, and directional scattering behavior.