2 Photoelectric observations of a mutual occultation of Saturnian satellites made at CL SAI

The observatory of CL SAI is located in Nauchnyi settlement at an altitude of 600 m. Its geographic coordinates are: east longitude $2^{\rm h} 16^{\rm m} 04^{\rm s}$, north latitude $44^{\circ} 43^{\prime} 37^{\prime\prime}$.

The observations were made with a single-channel WBVR photoelectric photometer in a photon-counting mode. The satellite images were projected into the photomultiplier. The photoelectric photometer was mounted on a Zeiss-600 reflector with an aperture of 600 mm and focal length of 7500 mm. An interface counter mounted on an IMB PC compatible was used to count the pulses coming from the photoelectric photometer during the adopted exposure time. The number of these pulses is proportional to the incident flux on the cathode of the photomultiplier. The flux measured during the adopted exposure time was then read by a special interface software and stored in the computer memory. The interface and the corresponding software were developed by V.G. Kornilov (SAI).

The light curve was visualized on a display monitor in real-time mode thereby substantially facilitating the tracking of the event under study. The time signals were transmitted from a synchronometer, which we manually referred to the UTC scale to an accuracy of 0.1 seconds using radio time signals.

We began the photometry of satellite pair beforehand so that the duration of the event proper would be about 20% of the total observing time.

When observing Saturnian satellites the background contamination due to scattered light from both the Saturn and its ring poses a considerable challenge. The background brightness depends on the satellite position and relative fluctuations of this background due to imperfect telescope tracking can reach several tenths of magnitude. Thus, background measurements made with a $20^{\prime\prime}$diaphragm in the immediate vicinity of rings show quasiperiodic flux fluctuations with an amplitude of about 0.1 of the flux itself. As a result, satellite light variations during mutual events observed at angular separations less $15^{\prime\prime}$from the planetary disk are simply lost in the background fluctuations. Photometric data for mutual events in a pair of satellites at a sufficiently large separation from the Saturn's limb (where the ring background can be neglected) show a uniform trend on the light curve. Guiding attempts result in abrupt changes of the level of this trend. At the Crimean laboratory of Sternberg Astronomical Institute we performed photoelectric photometry of ten mutual occultations and eclipses of Saturnian satellites and only for two of these events the data proved to be free from unaccountable scattered light fluctuations. In this paper we present the results only for one completely successful photometric observing set.

This observing set had the following characteristics:

The exposure was 0.128 second resulting in a large number of data points for each mutual event. Our analysis showed that photometric data points can be combined into 1-second intervals without loss of accuracy. We actually averaged each eight consecutive counts. We considered the time of each such combined measurement to be the midtime of the fourth and fifth count. It is well to bear in mind that during such observations we measure the combined flux of both satellites, sky background, and the light scattered in the optical elements of the telescope.

The results of the set of observations described above can be found in file t0806o54.pmr. Each record in this file has a form of a simple sequence of integers equal to the numbers of pulses counted corresponding to the average number of photons crossing the photometer diaphragm a 0.128-second time interval. The time of the first count is $21^{\rm h} 31^{\rm m} 40\hbox{$.\!\!^{\rm s}$}766$ (UTC scale) and the following counts are spaced at 1.024-s intervals. Figure 1 shows the resulting light curve with the flux given in relative units.

  

Figure 1: Photometric observations of occultation of Diona by Rhea performed from CL SAI on August 6, 1995. The total flux, S, is given in relative units. The line is a preliminary ephemeride

This observing set has been reduced in order to extract astrometric information [12, (Emel'yanov et al. 1997]). In particular, the apparent angular coordinate differences of satellites x₀, y₀ have been derived for a certain instant t₀ of time within the occultation observed. The measured quantity E as a function of time t is determined by the formula

$$E(t)=K \;S(t, x_0, y_0) + P + L\;(t-t_0),$$ (1)

where the empirical parameters K, P, L were derived from the observations themselves, while the function S(t, x₀, y₀) is the total light flux of the satellite pair normalized to the light of the two satellites outside the event. The adopted dependence of S on t, x₀, y₀ is described in [9, (Emel'yanov 1995]; [12, Emel'yanov et al. 1997]). This observing set allowed us to determine the satellite coordinate differences with an accuracy of $0\hbox{$.\!\!^{\prime\prime}$}003$ and $0\hbox{$.\!\!^{\prime\prime}$}005$ in right ascension and declination, respectively. The values of the refined parameters are published in [12, (Emel'yanov et al. 1997]). In Fig. 2 we compare observations with the theoretical satellite light curve computed after adjusting the above parameters. The flux is normalized to the combined light of the two satellites outside the event.

  

Figure 2: The calculated (line) and observed (dots) total brightness curves of the satellites for the occultation of Diona by Rhea performed from CL SAI on August 6, 1995. The calculated curve is based on adjusted event parameters. The observations have been corrected for scale, zero point and secular trend effects

  

Figure 3: Photometric observations of occultation and simultaneous eclipse of Enceladus by Tethys performed from Assy on September 14, 1995. The total flux, S, is given in relative units. The line is a preliminary ephemeris

  

Figure 4: The calculated (line) and observed (dots) total brightness curves of the satellites for the occultation and simultaneous eclipse of Enceladus by Tethys performed from Assy on September 14, 1995. The calculated curve is based on adjusted event parameters. The observations have been corrected for scale, zero point and secular trend effects