Original occultation path plot from David Herald, 3/01. Go here for the latest, updated plot!

Remarks:
 -------

 - Detecting a 15 mubar atmosphere (like Triton) is pretty obvious. Note that a
detection is possible even with b= 1000 km, i.e. more than 200 km OUTSIDE the 
geometrical radius of the satellite (790 km). Note that a grazing occultation
(b= 790 km) is pretty obvious. 

 Hence the importance to observe even if one is outside the predicted
 geometrical path. 

 - A 1 mubar atmosphere is pretty obvious too. Note that grazing occultations
(b= 700-750 km) detect more easily an atmosphere that central ones. This is 
because the drop of signal due to the atmosphere last longer and has a shallower
slope for grazing events than for central ones.

 Again grazing observers are welcome!

 - A 0.1 mubar atmosphere is probably the limit of what we can do. Once again 
grazing occultations are more sensitive to an atmosphere than central 
events. Note that the drop due to the atmosphere just before the drop due to
the physical limb is about 7%, not much!

 - I am thinking about another method to detect a faint atmosphere. Refraction 
"shrinks" a little bit the physical shadow of Titania. For p= 0.1 mubar, this
shrinking amounts to about 4 km radially. Thus, even if we cannot see the
faint 7% atmospheric drop, we can at least measure the radius of the
shadow, using the timings at various stations. 

 The shadow moves at about 20 km s-1, thus we need an sub-second accuracy for
the absolute timing at each station, on order for this method to work. Also, we
need a good sampling for the light curve, since the star is going to smooth the
signal over ~ 0.5 sec anyway.

 Best regards

 Bruno Sicardy








From: Bruno.Sicardy@obspm.fr Thu Jul 26 10:23:06 2001


Dear Titania observers, 

I found in a double star list 
(see http://www.mapug.com/AstroDesigns/MAPUG/DbleStar/DbleStar1.htm)
a system which nicely simulate (IN THE VISIBLE) Uranus and Titania
during 
the occultation: 

SAO 10937 (HIP 207 and 208), 
RA: 00h 02m 36.087 sec
DE: +66d 05' 56.28"		(J2000)

with separation 15.2 arsec, 
V= 5.9, type G8 III, 
V= 7.2, type A2 V, 
for each star respectively. 

It could be useful for tests of saturation, scattered light, scale,
etc... 
and easy because it is in Cassiopea. 

Note however that the fainter star is rather blue, while the occulted
star is
read (type K0 III) so that the test will be misleading outside the V
band. 

Cheers,

Bruno




From Bruno....

 Some remarks about the observations in Venezuela:

 ->The most important is to get a lightcurve of the event with
 sufficient time resolution. Titania is going 20 km/sec with respect to
 the star, projected onto the the plane of the sky. Thus we need 1sec
 resolution or better in order to resolve an atmosphere, which is
 likely to have a scale height of no more than 30-40 km.

 ->Fast CCD images or video, or any fast imager with reasonable
 linearity are ok. A fast aperture photometer could be tricky becaus
 Uranus will be close and bright (V= 5.7 at 16 arcsec from the star, V=
 7.1). Scattered light from the planet would contaminate the aperture,
 and make the interpretation of the lightcurve difficult after the
 event. The best would be at have some IOTA cameras in Venezuela, or
 some good video camera whose signal could be digitized afterward.

 ->It is important to record the all event, not just the ingress and
 egress. If the atmosphere is thick enough, the occultation will be
 rather smooth and the star may not disappear completely even when it
 is the geometrically behind Titania. 

 ->It is important to note that there is still an uncertainty which 
 corresponds to about one radius or more of Titania. In other words, the 
 gray stripe on the diagrams (track of the shadow on the Earth) may still 
 shift north or south by half its width or even more. So, it would be wise 
 to observe even if you are outside the nominal track.

 ->I do not think that spectra will be very useful, since the atmos.
 will be so tenuous that absorption lines will be impossible to detect.

 ->We do NOT know if Titania has an atmosphere. This is a prospective
 experiment. The problem is the star scintillation in the Earth atmos,
 NOT the photon noise, since the star is so bright. Thus, a big
 telescope like the 1-m is good not because it collects more photons,
 but because it reduces  the scintillation wrt a 20cm telescope. Under
 good conditions, we can detect down to 10^{-7} bars at the surface of
 the satellite.

 ->To make tests, you can use a double star which simulates nicely
 Uranus + the star at the moment of the occultation:

 SAO 10937 (HIP 207 and 208 in Cassiopea, RA: 00h 02m 36.087 sec DE:
 +66d 05' 56.28"             (J2000)

 with separation 15.2 arsec, V= 5.9, type G8 III, V= 7.2, type A2 V, 
 for each star respectively.

 It could be useful for tests of saturation, scattered light, scale,
 etc... Note however that the occulted star is red, while the fainter
 star in the binary above is blue, so tests could be misleading outside
 the V band.

 Cheers

 Bruno Sicardy 
 Observatoire de Paris 
 92195 Meudon
 Cedex 
 Principal France

<+><+><+><+><+><+><+><+><+><+><+><+><+><+><+><+><+><+><+><+><+><+><+>

Dear Titanians,

 I received last week astrometric results from Bordeaux, giving some
 O-C's for the Uranian system. Michel Rapaport obtained 11 positions for
 Uranus, 7 for Titania and 7 for Oberon, wrt Hipparcos stars, with the 
 meridian instrument. The average O-C obtained from Titania and Oberon
 positions is:

 O-C alpha*cos(delta)= -0.065"
 O-C           delta = -0.057"

 which yields the track shown in the attached ps file. 

 This new prediction is north of, but otherwise essentially the same as the
 one previously derived from Bill Owen's observations at Table Mountain,
 and from which I derived the tracks posted on Rik Hill's site on July 20
 (see http://www.lpl.arizona.edu/~rhill/planocc/titania/titania.html)  

 I did not include the O-C of Uranus in the present track, since the planet
 is 3.8" across, and thus with a more poorly defined photocenter than
 the satellite. In any case the O-C of Uranus would not change much the
 present track. 

 There is still a problem of error bars: the Bordeaux observations have
 typical dispersions of < ~ 50 mas, i.e. about one Titania radius. Thus,
 the track shown in the attached ps file can still be off by +/- one
 Titania radius, i.e. one Titania diameter full width.

 In other words, all the observations (Ron Stone, Bill Owen, Michel
 Rapaport) are pretty consistent, but none of them really pin down the
 error bars below +/- one Titania radius. Averaging all these results is
 tricky because some systematic errors could linger in each of these
 observations.

 What would be needed now is having Titania and the star on the same plate
 and make direct differential astrometry. The two bodies are about 1 deg
 apart now. 

 Can anybody do that ????????????

 Cheers

 Bruno







From: Bruno SICARDY 
Subject: Titania astrometry update



Here is a brief account of the latest astrometric measurements of the
Uranian system:

 - Bordeaux results (Michel Rapaport): 
 O-C alpha * cos(delta) = -0.075 arcsec, O-C delta=  -0.030 arcsec. 

 Then the occultation is grazing in southern Portugal, Madeira, and the
 shadow cuts in half Ecuador. 
 Venezuela and Aruba are in full shadow.

 - NOFS: O-C= -0.119, -0.079  arcsec in alpha and delta, resp.
 Than Portugal, Madeira, Venezuela, aruba and Ecuador and in full shadow.

 - Table Mountain:  (Bill Owen) O-C= -0.153, -0.034 arcsec. Portugal,
 Madeira and Ecuador are outside the shadow. Venezuela and Aruba are
 in. This later results is at contrast with all the other previous
 measurements, essentially because of a relatively high offset in alpha
 (-0.175 arcsec instead of more typical values of -0.070 arcsec).
 There is no explanation for this divergent result, according to Bill
 Owen. 

 Thus the shadow is still wandering around with no definite
 convergence... Madeira, Southern Portugal  and Southern Ecuador are
 dangerously close to the southern limit of the shadow. But remember that
 grazing occns are best for detecting an atmosphere.

 Note that all the astrometric masurements indicate that the occn by
 Titania will occur SOONER than predicted by the nominal JPL Titania
 ephemeris. It should occur around 1:54-1:55 UT sept. 8 in Europe, and
 around 2:00-2:01 UT in S. America. Be prepared! (and correct the diagram
 in Sky and Telescope, Sept. issue, p. 95