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The Pluto stellar occultation of 23/24 August 2009

What will happen?

On 23 August 2009 near ~ 11:30 UT (i.e. the EVENING of 23 August 2009 in Australia and New Zealand), Pluto will occult a star.
Star coordinates (J2000):

ra= 18h 02m 33.5038 sec
de= -17d 52' 16.346"
(measurement from Rio de Janeiro group, Assafin et al., as of July 2009)

The corresponding apparent coordinates of the star are:
ra= 18h 03m 09.1 sec
de= -17d 52' 17"

Click on images for enhancement

The shadow on maps moves from right to left. Dots on centerlines are plotted every mn. The larger dot on centerline is for geocentric closest approach, namely 11:32:05 UT (NB. August 23 UT, i.e. **evening** of August 23 in Australia and New Zealand). Solid lines is where the star reaches its half-light level, due to differential refraction by Pluto's atmosphere. Dotted lines are where the stellar drop is only 1% drop, and mark the limits where the occultation can practically be detected. Red star symbols are potential observing sites. NB. error in **timing** can be up to 3-5 mn. Error in shadow path can be up to 700 km in N-S direction
NB'. the event is pretty slow, ~-9.9 km/sec, so that a central event may last for a little more than 5 mn.

Click on images for enhancement
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- the event is slow (about 9.9 km/sec), so the occultation can last for up to 5 mn, if central. Even data taken at cycle times of 1 to 5 sec or more can be useful. Try to minimize the dead time between frames, compared to the exposure time of each frame. For instance, there is not much advantage having an exposure time of 0.5 sec if the read/write time of the image is 6 sec, as most of the photons will be lost. It is better than to have an exposure time of 6 sec, to get at least 50% of the photons.

- be very careful in getting a GOOD ABSOLUTE TIMING of each of your image. This is of great importance when reconstructing the geometry of the event using various stations.

- if the occultation is central, an increase of light (the so-called "CENTRAL FLASH") may be visible at mid-occultation for several seconds. Such flash is most useful to constrain the amount of dust possibly present in the deepest Pluto atmospheric layers. If observed from various sites, it is also very useful to measure the speed of winds in Pluto's lower atmosphere.

- the star is VERY RED. The contrast of occultation will be POOR in visible. Even in R, the star is 10 times fainter than Pluto see table below. The I band may be more favourable, but the star is not measured in that band in available catalogs. So, TESTS SHOULD BE PERFORMED BEFOREHAND to optimize SNR according to your instrument and receptor. Remember that filters can help increase the contrast, but they reduce the photon flux. So, compromises should be attained, that WILL DEPEND ON TELESCOPE SIZE.

- IT IS EXTREMELY IMPORTANT, for further analysis, to be able to take away Pluto's flux from the light curve, which will consist in the SUM of the star and Pluto fluxes. For instance, if the occultation is grazing, then the star will not disappear entirely behind Pluto. Thus, the real depth of occultation can be determined only if Pluto's contribution is properly subtracted to the flux. The real depth of occultation is then important to constrain both Pluto's atmospheric pressure and the reconstruction of the occultation geometry.

This means carefully calibrating Pluto's flux vs. the stellar flux before or after the event, when they are well separated. The problem is that the star being VERY RED, and Pluto more neutral, the ratio star/Pluto WILL CHANGE WITH AIRMASS. So, such calibration may result useless. Also, measuring Pluton on a different night for comparison is not advised, as the planet brightness varies by up to +/- 15% during its 6.4 rotational period.

- A solution is then to take another star in the FoV (or **better**, several other stars) as reference(s), and measure their relative fluxes, together with the occulted star, on other night(s) AT THE SAME AIRMASS as for the occultation, with the SAME FILTER, DETECTOR, TELESCOPE, etc..., thus allowing us to retrieve Pluto's contribution to the occultation light curve.

Note also that using MORE THAN ONE reference star has the advantage to be able to measure those reference stars with respect to each other, thus giving the opportunity to check the overall consistency of the calibration. For instance, if the ratio of two (or more) reference stars is not the same during the occultation and during the calibraton night(s), then it means that conditions varied and chromatic effects are not the same anymore, or that the stars are variables, etc...

Be careful also that some detectors (e.g. video, or Watec cameras when the "gamma" is no set to unity) are NOT LINEAR, so that calibration is meaningless in those case, unless non-linearity is accounted for. Again, do not forget to set "GAMMA=1" (or "NO GAMMA") on some cameras.

- Note that the calibration NEEDS NOT BE DONE WITH THE SAME INTEGRATION TIME as for the occultation. Taking longer integration time will improve the SNR of the calibration and reduce the amount of data. This assumes, however, that the receptor is linear, i.e. that a longer integration time will respect the various flux ratios measured in the FOV.

- Finally, do not forget to take DARKS and FLATS.

The star is very red, with V~17.7, R~16.4 and K~11.7. It is not documented in I. (see Vizier service)

Note that Pluto has V~14.4 and R~13.9

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