2006 January 19
[PLANOCCULT] Preparing the occultation by Saturn (polar atmosphere)
From J. Lecacheux


The special problem is to detect the occulted star (V= 7.96, I= 7.66)
at the saturnian polar limb.

On January 25 the mean V-brightness of the saturnian globe will be
+6.68/square arc second according to the "Horizons" ephemeris.
Taking the mean colour index as V-I= +1.15 for the saturnian globe,
we can predict for the mean I-brightness +7.83/square arc second.


I measured on the planet Saturn the following brightness in the
I-band at Pic-du-Midi on the last 2005 November 07.
(Note: I used a 0".10 pixel and the seeing was good.)

My results in arbitrary units were the following :

I-brightness of the brightest region of Saturn
(south-equatorial zone, at the central meridian) :  1.42

Mean I-brightness calculated upon the whole
southern hemisphere of Saturn
(2D integration excluding any ring contribution) :  1  (arbitrary unit)

Mean I-brightness at 1" from the southern limb
of Saturn, on the dusky polar cap, at the central
meridian                                         :  0.54

Mean I-brightness in the square arc second
just centred on the saturnian south pole
(at 0".47 from the limb)                         :  0.21

Mean I-brightness between the south pole and the
southern limb (inside the 0".47 interval)        :  0.09

So the I= 7.66 occulted star will be in close contact with a small
region of Saturn dimmer by a factor 1/0.09 ~ 11 than the mean
planet, i.e. of brightness I ~ +10.4/ square arc second.


I have made some simulations by adding a synthetic star to my real
images of Saturn, and by making gaussian convolutions to adjust
the effective resolution.

I found that the star visibility during the incoming emersion will
depend critically from the instantaneous seeing.

In case of good seing (~ 0".5), the observers using a C12 at f/10
(for example) and any sensitive camera with small pixels (between
6 and 9 microns) should have no problem to separate the star from
Saturn as soon as it will become brighter than I= 11, i.e. nearly
comparable to the satellites Dione or Thethys.
In case of medium seeing (~ 1"), the contrast star/planet should
be lowered by a factor 2 or 2.5, and so the observable part of the
emersion will begin near I= 10. This still corresponds to the more
interesting part of the phenomenon.
In case of bad seeing (~ 2"), the practical limit will near I= 8.5,
so that only a poor observation, near the terminal phase, would
result.


Please remember some usual characteristics of the occultations by
giant planets :
1 - The scintillation caused by the planetary atmosphere is a huge
   phenomenon. Abrupt flashes or "spikes" can exceed one magnitude
   in a short fraction of one second.
2 - The thin atmospheric layers responsible from this scintillation
   seem possess an horizontal coherence larger than 2000-3000 km
   and seem keep temporal stability, at least at minutes scale.
   So all the european observers should detect essentially THE SAME
   spikes, although delayed appreciably for locations more and more
   north : for example in England they should occur 3 minutes later
   than in Italy, like the nominal emersion time itself.

Note that the vertical motion of the star relative to the Saturnian
horizon will be 8.9 km/s. As the star size will represent 0.33 km at
the Saturn distance, the spike duration should not exceed 1/27th of
a second, or one full video image (2 frames).
The Fresnel diffraction scale will be 0.7 km in the I-band, 0.6 km
in the V-band, possibly making the spike pattern more complex.

If we assume that the atmospheric scale height is ~ 50 km at 750 km
above the Saturnian polar clouds, at the level where the refractive
mid-occultation actually should occur, one can estimate that the
fluctuating increase of the star light (at the emersion of course)
could take 1.5 minute.
So begin any observation at least 3 minutes before the nominal
egress time.


We can imagine that Watec cameras (or similar) would be ideal
receivers to record the occultation spikes. However I suspect that
the window of the Sony "ExView" CCD is made of a KG3-style glass,
i.e. an efficient infrared attenuator... If my hypothesis is true,
we might be forced to observe with a red filter closer to R than I,
so almost losing all the contrast gain brought by the saturnian
methane.

Tests on the sky (Saturn and target star) several nights before the
occultation are strongly recommanded.