Thursday, 19th Oct 2017

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Jul 06 2007

The Saturn occultation of May 22nd, 2007

004_composite

On May 22nd, 2007, a Saturn occultation was expected, with much more comfortable time and observing conditions than the previous one of March 2nd, 2007, which I had missed because of some stupid problems with my equipment.

Here are the times calculated for my observing location (downtown Genova, Italy); all times are UTC, have been figured out with Occult 3.6.0 and refer to the planet’s disk only (not including the rings):

 

Event Start time End time Duration (s)
Start of occultation (planet’s disk

obscured by the Moon)

19h 32m 00s 19h 32m 42s 42
End of occultation (planet’s disk

coming back out of the Moon)

20h 37m 39s 20h 38m 17s 38

The Moon was waxing around first quarter, so that Saturn would sink into the unilluminated portion of the Moon’s disk, while it would come back out from the bright crescent. When planning on imaging two so different phases, some factors must be taken into account:

  1. In this case, the imaging conditions for the two events are largely different from each other. In particular, the more difficult task is to shoot the final phase due to the sheer difference in brightness between Saturn and the Moon’s surface.
  2. As with any lunar occultation, one has to figure out the exact spot of the Moon’s disk where Saturn will emerge from.

Problem #1 is really a hassle. A high-dynamic-range camera will surely be of great help, and turning up the brightness and gain as needed will allow for a correct exposure of Saturn’s disk, while certainly overexposing the Moon. For the sake of esthetics, one could also take two separate exposures with the correct exposure settings for each target, and finally combine them into a single composite image.

As for problem #1, we resort again to Occult, whose output is shown in the maps below. Saturn is marked by a a yellow circle, which is just by the lunar disk edge to the lower left (close to the Shickard crater) in the initial phase, and to the right at about half the height of the Moon’s disk in the final phase (in the neighborhood of Langrenus crater). Subsequently, a more detailed map was examined, which enabled me to locate the emerging spot very close to Mare Smythii and Kastner crater. These features lie just on the Moon’s edge and by the time of the occultation the so-called libration phenomenon would ensure better-than-average observing conditions.

Start of occultation:

start

End of occultation:

end

In spite of the poor transparency and high clouds, I managed to shoot both phases through a 9.25″ Schmidt-Cassegrain Telescope, fitted with a Lumenera Lu075M camera, both of which rode atop a Losmandy G11 mount, plus a number of filters and barlow lenses.

The start

As already mentioned, the start phase was much easier to image, since Saturn’s disk would simply be covered by the Moon’s unilluminated portion. So, it would just suffice to frame the planet as with any standard imaging session, and watch it while it gradually sank into the “dark side of the Moon” :-).

Start phase composite:

start_composite

Saturn is back out!

This part was tougher to acquire. I couldn’t count on the scope aligment and tracking to be so precise as to keep Saturn right in the field of view for more than an hour: therefore, I had to carefully find the emerging spot with the maximum possible precision by using the maps from Occult and a detailed lunar atlas.

In the end, I was right spot-on, and managed to have the whole planet (including the rings) within the true field of view of my optical system (made up by my 9.25″ SCT + 2.5X Powermate + Lumenera camera). However, having underestimated the difference in brightness between the ringed planet and the Moon, the former ended up largely underexposed in the first part of the movie. Furthermore, by the end of the occultation the seeing had clearly worsened, forcing me to use a red dichroic filter to somewhat “tame” the infamous atmospheric turbulence. Anyway, I managed to achieve the full acquisition of the whole occultation ending, whose movie will be available when I manage to finish the tedious processing phase.

Shortly after the occultation was over, I took two separate AVIs of Saturn and the Moon, which I later processed and combined into a single composite image.

Composite image with both targets correctly exposed

004_composite

Addendum: comparing the precision of astronomical software

To perform a top-quality measurement of the exact occultation times, I would have had to superimpose a very precise (i.e. atomic) digital clock signal over the streaming image by means of a thingy called “time inserter”. Unfortunately I am no occultation freak and don’t own any such advanced piece of equipment, but a decent estimate can be made by carefully adjusting the notebook’s internal clock with a reliable clock signal within a second’s time. Based on this, if the frame rate is known the exact timing for each frame can be found. The following table depicts my measurements, which have an error of +/- 1-2 seconds due to the difficulty inherent with the visual estimation of the exact time of Saturn’s reappearance.

Main Event “Sub-event” Start time End time Duration (s)
START Ring obscuration 19h 31m 32s 19h 32m 25s 53
Disk obscuration 19h 31m 38s 19h 32m 18s 40
END Ring reappearance 20h 36m 58s 20h 38m 24s 86
Disk reappearance 20h 37m 24s 20h 38m 00s 36

It’s worth making a comparison between the times measured and the calculations obtained from Occult. Also, I wanted to throw in the figures calculated with Solex 9.1 by Aldo Vitagliano, whom I thank for his helpful collaboration. The following table depicts the whole comparison. All times (UT) refer to Saturn’s disk only:

 

Event Measured Occult DeltaT (measured-

occult)

Solex 9.1

DeltaT (measured-

Solex)

Start of disk occultation 19h 31m 38s 19h 32m 00s -22 s 19h 31m 41s -3 s
End of disk occultation 19h 32m 18s 19h 32m 42s -24 s 19h 32m 23s -5 s
Start of disk reappearance 20h 37m 24s 20h 37m 39s -15 s 20h 37m 59s -35 s
End of disk reappearance 20h 38m 00s 20h 38m 17s -17 s 20h 38m 38s -38 s

 

Both programs I tested yielded times delayed by some amount with respect to my measurements. In particular, there’s very good accordance with Solex for the start of the occultation, whereas that’s not the case (surprisingly enough) with the occultation’s end. As for Occult, the discrepancies tend to be a little more consistent.

Conclusion

On the whole, I am quite satisfied with the results; the good observing conditions played a key role, as did the high clouds that were kind enough to scram before the event began :-D ; the software “roundup” was also interesting. I can only hope to succeed next time Mother Nature will be so kind to treat us with another such spectacular phenomenon, which is always a feast for my eyes (and for my camera too! :-D ).

This post is also available in: Italian