The recent occultation of a star by Saturn was preceeded with much discussion about how observable the event would be. It is known from the occultation of 28 Sagittarii (SAO 187255) on 1989 July 3 that an occultation of a mag 5.8 star is readily visible as the star traverses the ring system. The following is a tentative conclusion about the visibility of the occultation of a mag. 7.9 star by Saturn, with inferences for future events. Dave Herald Canberra, Australia 29 Jan 2006 ************************************************************************** Summary of observability - Occultation of HIP 42705 (SAO 98054) by Saturn, 2006 Jan 25 Visual At 25cm aperture, the star was hard to see adjacent the planet. It could be detected in the Cassini gap and possibly seen traversing the ring system, but visibility was marginal at best, and required fairly good seeing. Video The star was easily seen against the planet and rings when filtered to the I-band. All observers found the star easier to detect near the pole. Faint detection of the star in the Cassini gap with 20cm aperture and stacked images. Conclusions Visual observation of an occultation by a mag 7.9 star by Saturn is at best marginal in a 20 to 25cm telescope. Under good conditions the star will be visible in the Cassini gap. Video observations are much enhanced by using a deep red filter - but very large telescope aperture is required to compensate for the lower light intensity. Stars brighter than 7.0 should be reasonably visible at 20cm aperture. stars brighter than 6.0 should be visible through moderate telescopes - although presumably a large magnification will be required (and hence manually-driven telescopes may have difficulties). *************************************************************************** Observability of future occultations by Saturn The following is a table of future occultations by Saturn, up to the year 2040. Two events that occur at small solar elongation have been excluded. Date U.T. Star Star y m d h m mag No. Region of visibility EASILY VISIBLE 2032 Apr 07 6 18.4 5.8 HIP 23883 central Pacific, Hawaii, Alaska, western USA The star is 105 Tauri = SAO 76972 = ZC 766 = X06555 = BD +21 766. The star may be a close double, according to a visual total occultation observed by R. Melley, Birmingham, England, on 1974 Nov. 3 (ON I, #11, p. 120), with equal components perhaps 0.3" apart in PA 270 deg. I think this duplicity is doubtful since a separation that large should have easily been resolved by Hipparcos, which, as far as I know, found the star to be single. MARGINALLY VISIBLE none VISIBLE ONLY WITH HEAVY FILTERING, TOGETHER WITH LARGE APERTURE 2023 Apr 15 6 21.4 8.7 TYC 5807-01344-1 west Africa, Brazil 2030 Apr 06 22 24.9 8.3 TYC 1233-00635-1 Newfoundland, Brazil 2032 Apr 15 3 43.9 8.4 HIP 24129 north America 2035 Jan 10 4 17.3 8.8 TYC 1386-00517-1 Americas, Europe, Africa It should be noted that over this period there will be many occultations of fainter stars. While these will not be observable against Saturn, occultations of those stars by one or more moons of Saturn may be observable. ************************************************************************** Summary of reported observations - Occultation of HIP 42705 by Saturn, 2006 Jan 25 Telescope: 8" SCT Equipment: Visual, at 133x and 206x; also video Location : Busto Arsizio (Va) - IT Observers: Simone Bolzoni Report : Before the event, I saw quite well the star around 18 UT. The planet was quite low to the horizon, and a video taken at that time don't show neither the star nor Titan. Then, I quickly had supper and returned a few minutes before the disappearance through the A ring. From this moment (about 18.35 UT) I never succeeded to see the star, even if with various combinations of eyepiece, filters, and so on. Another video don't show the star too, notwithstanding it was better than the first. For my location, reappearance of BY Cnc from the south pole of Saturn was around 20.49 UT: I had the first impression to see the star at 20.51, but very uncertainly till around 20.54 UT, when the impression became certainty. Then, I quickly mounted the Philips Toucam Pro II to the S-C with a long T-adapter, in a way to increase focal length without to reach the Barlow values, and introducing a red W 25A filter. With this combination I took the image in attach at 21.06 UT, where the star is quite visible near the planet limb. The seeing was fair bad: I mean it was not possible to do more with my instruments... --------------------------------------------------------------------------- Telescope: 105cm Reflector Equipment: I-filter (Schott RG780), Watec 902H black-and-white camera plus a DV camcorder - with time insertion. Location : Pic du Midi, FR Observers: J.Lecacheux, E.Frappa, F.Colas, P.Laques Report : The most spectacular phases, as observed on the control monitor, were an abrupt drop at the A-ring ingress, a spectacular full reappearance through the Encke division, good star visibility along all the A-ring crossing, another abrupt drop at the Cassini division egress (alias the B-ring ingress), then finally the atmospheric slow emersion at the dim southern limb of Saturn. The star then displayed during 3 minutes, as I have predicted, many spectacular flares or "spikes", whose physical origin was in the Saturn mesosphere, high above the polar clouds. I had a look at the eyepiece of a 20-cm telescope, at magnification x180, only two minutes after the emersion had ended. The star was seen easily against the polar limb, almost like on our control monitor, besides with colour view ! So I am sure than the polar emersion was observable even with a 20-cm telescope, either visually, or with any sensitive camera. I think from the spikes duration we recorded at the 1-meter (slower than expected) that a 8 images/s cadence would have been OK. Some bad news... Looking at our mini DV video recording, we had the unpleasant surprise to see that the camcorder had recorded only a fraction of the signal amplitude provided by the Watec 902H. Although the image was perfect on the *control monitor* during the event, the image of Saturn on the recording is strongly (but not totally) saturated. Fortunately, the target star and the south pole of the planet are OK and we'll be able to get accurate light curve of the star emersion with the very spectacular atmospheric spikes. But we are not able to provide a nice unsaturated video of the event as we planned, and the crossing of the rings by the star is lost. We never paid attention to this video problem before, because it not occurs for asteroidal occultations due to the faintness of the target (or is it a malfunction?). What we don't understand is that the spec video output of the Watec 902H is 1 Vp-p/75 ohms and the AV terminal video input of the camcorder used (Canon MV600i) is 1 Vp-p/75 ohms too. Any idea? We are very disappointed not to be able to show you perfectly the splendid crossing we have seen live on the screen. The only consolation is that the emersion recording has the better S/N ratio possible for our setup, with a bright unsaturated star and a relatively dark south pole. --------------------------------------------------------------------------- Telescope: 20cm SCT Equipment: Visual [15-mm (133x) and 11-mm (182x) Plossl oculars] Location : Norway Observers: Bjorn H. Granslo Report : The star was first seen shortly after the start of the observing session at 17:20 UT and it was quite easily seen until 17:45 UT. After a break the star was again seen with certainty for some minutes until 18:30 UT. It was then about a challenging object as Dione (Saturn IV), but easier to see than Tethys (Saturn III). The latter moon was only seen with difficulty. I was not able to see the star when it should be situated within the Cassini gap around 19:00 UT. The star was first definitely seen again at 21:00 UT, or nearly ten minutes after the end of the occulatation. Five minutes later it was clearly separated from Saturn's disk and its visibility was then comparable to Dione. When the observing ended at 21:40 UT the star was again a rather easy object. It then appeared slightly brighter than Rhea (Saturn V), but still considerably fainter than Titan (Saturn VI). These observations were made in a clear sky, but under rather unfavourable seeing conditions. --------------------------------------------------------------------------- Telescope: 25cm SCT [10" LX200] Equipment: Visual Location : High Wycombe, GB Observers: Chris Rowland Report : After writing it off because of the cloud it cleared in High Wycombe UK. So I had a go visually with a 10" LX200 I used a 9.7 or 12.4mm eyepiece and no filters. I missed the start and the star crossing the Cassini division but think I could see it through the rings sometimes. There seemed to be something in the right place. It passed behind the planet at about the right time. The egress was difficult to see, the first indication was at 20:55 UTC when it looked like a discontinuity near Saturn's pole, and by 20:58 the star was obviously separate from the planet. There's no precision to this, I couldn't say when the star appeared to closer than a minute and for the disappearance it is several minutes. There's obviously no scientific merit to these 'observations', just personal interest - and it is the first time I've seen an occultation :-) --------------------------------------------------------------------------- Telescope: 40cm Cassegrain, 11" SCT, 10.5cm Maksutov [ETX105] Equipment: Visual Location : Naples, IT Observers: Massimo Corbisiero and others Report : >From Naples in three different sites the star was not visible: good weather conditions but very poor seeing. --------------------------------------------------------------------------- Telescope: 15cm Maksutov Equipment: Visual Location : Ripon, Yorkshire, GB Observers: David Graham Report : Conditions were initially cloudy and Saturn did not appear until 18h 50m UT. The planet was then watched using a 7.5mm Plossl eyepiece (magnification X200) until 19h 30m UT, when it was anticipated that the star would initially be in contact with the outer edge of Ring A and then pass through Cassini’s Division. The seeing was IV on Antoniadi’s scale though Cassini’s Division was held visually. However the star was not visible at any point in this initial observing run. Returning to the telescope at 20h 45m UT (afer rain). A break in the cloud occurred from 21h 15m to 21h 30m UT and the star was visible as a faint speck of light immediately south and following the south polar limb of Saturn. The seeing was still Ant. IV at this point, and with the threat of further rain, it was judged prudent to terminate the observing run shortly after 21h 30m UT. --------------------------------------------------------------------------- Telescope: 44cm Reflector Equipment: Visual Location : Basingstoke, GB Observers: Guy Hurst Report : Cloud prevented seeing the star entering the rings or disappearing behind the planet and Saturn was too low to reach from the observatory at (51 14 27N, 01 07 50W) using the 0.44-m reflector. I did sight the planet and rings through thin clouds using a portable 0.125-m Maksutov telescope initially but conditions were too poor for meaningful observations. However at about 20.20UT the clouds cleared whilst the star was being occulted and the altitude now just (!) allowed observation of the planet with the 0.44-m reflector. At 20.46UT using a 12mm Nagler eyepiece at x120, I saw Titan, Rhea, Dione and TYC 1395 1199 of magnitude 9.4. In addition Tethys, of predicted magnitude 9.9, was just glimpsed about 10 arcseconds from the rings which illustrates the difficulty of seeing apparently bright objects so close to the planet. I first glimpsed the star emerging from the pole of the planet at 21.01UT broadly in a line from the centre of the planet towards Titan. By comparison with Dione and Tethys it gave the appearance from magnitude estimates to be about 10.3. Although catalogues place it nearer magnitude 8 this difference was clearly due to its closer proximity to the planet than the other moons used as comparisons. By 21.30UT it was also held to direct view in a low power 28mm Meade Ortho eyepiece. This type of observation can, I believe, be open to bias. Predictions of times for reappearance can easily lead the observer to suspect a sighting when, in reality, proximity to the planet coupled with seeing which regularly blurred Saturn meant quite a 'late' sighting. --------------------------------------------------------------------------- Telescope: 30cm Newtonian Equipment: Visual Location : cambridge, GB Observers: Chris Hill Report : I was on Saturn at about 1840 UT, but I couldn't see the star. I didn't get back to Saturn until 2114 UT, by then the seeing was better and the star was clearly visible near Saturn's south pole. An earlier email said "the polar emersion was observable even with a 20-cm telescope" - that might work 3500 m up in the Pyrenees but not at 10 m! --------------------------------------------------------------------------- Telescope: Celestron C9 Equipment: Philips Vesta web cam with 2x barlow Location : Bucharest, Romania Observers: Alex Conu & Adrian Sonka Report : 'star can be barely seen inside the Cassini division on 3 images' --------------------------------------------------------------------------- Telescope: 38cm Reflector Equipment: Mintron video camera equipped with a blue filter Location : Frasso Sabino Observatory (near Rome), IT Observers: Claudio Costa (+ members of the ARA Association) Report : The star was visible up to 18:30 UTC, with a gain settings of the Mintron camera which saturated the rings. Introducing a 2x Barlow lens and reducing the gain to prevent saturation resulted in making the star invisible, also due to the very poor seeing which made Saturn’s globe move constantly. So, in spite of the perfect working of the video recording chain I recorded nothing and gave up at 19:15 UTC. Temperature was -5 degrees Celsius. ---------------------------------------------------------------------------