General Observing Notes for the 27 June 2011 Stellar Occultation by Pluto and Hydra


Basic Information:

Basic Event Information
Pluto Event (Geocentric) Mid-time (yyyy month dd hh:mm:ss)

2011 June 27 14:17:15± 00:00:161 UT

Hydra Event (Geocentric) Mid-time (yyyy month dd hh:mm:ss)

2011 June 27 14:50:23± 00:00:161 UT

Occultation Star Position (J2000):

RA (hh mm 25 29.0124

Dec (dd mm ss.s): –18 48 47.636

Occultation star UCAC2 magnitude 13.6
Pluto approximate visual magnitude 14.0
Hydra approximate visual magnitude 23

Detailed information on the occultation prediction is available here.

Finder Charts for the Star

Finder charts to confirm the star position are located here.


The scientific goals of this event are (i) to accurately determine the current half-light diameter of Pluto, (ii) generally characterize Pluto's atmosphere, probing for spikes, knees, or other light-curve features that can shed light on deeper atmospheric phenomona, (iii) establish the position of Pluto with respect to the star and Pluto's ephemeris for aiding future occultations, (iv) accurately measure Hydra's radius, and (v) probe for any unknown satellites/debris in the Pluto system.

Observing Notes

Event Basics

As the 13.6th magnitude occultation star is expected to be occulted by a 14th magnitude Pluto, the main observational result should be that the light occultation star vanishes from the field and then reappears. Given that Pluto is fainter than the star, this would manifest as the joint star/Pluto light dimming by approximately a 60% and then brightening again. The maximum expected duration for this event is likely to be just over 116 seconds at the center of the shadow, and shorter as one approaches the limbs.

Event Time

The time given in the table above is the time the shadow should pass over the geocenter. Given the ~24 km/sec shadow travel rate, the exact event time depends on your site location. It will occur centered around 14:15 UT in the Hawaiian Islands and around 14:19 UT in Australia / East Asia. The Hydra occultation will happen approximately 30 minutes later. If observers are able to continue observations through these longer times, there is a possibility of recording an occultation by Hydra.

To find a precise time, go to the table on the bottom of the the occultation prediction page, and choose the site nearest your location. The midtime of the event (midway between disappearance and reappearance) is given in the UT Mid-Time column.

Exposure Times and Frequency

Our primary result from this event is the precise timing of when the occultation occurs. Thus, we would wish to record the star with as short a cycle time as is feasible given your equipment. As the Pluto shadow moves across the earth at approximately 24 km/sec, every one second uncertainty in the disappearance or reappearance time corresponds to approximately a 24 km uncertainty in the measured radius of the Pluto.

Thus for small aperture telescopes (~14-in), we hope to measure the star's brightness at least once every 2-4 seconds if possible.

Larger aperture telescopes can record the star at a faster cadence if supported by the camera, but be sure the star image has enough signal that it is not read-noise limited.

If sky conditions are sub-optimal, increase the exposure/cycle times until you can get a clear image of the star. Time variable sky conditions should be easily calibrated out as there are several stars of similar brightness (that will not be occulted) within a couple of arcminutes of the occultation star. (See finders).

Also, the absolute time of the disappearance and reappearance is as critical as the duration, as having a common and consistent time-base allows us to directly combine the data from separate stations resulting in a more precise picture of the overall size and shape of Pluto's atmosphere. Thus GPS time-tagged exposures are preferred whenever possible, but if not, care should be taken to properly calibrate the recording camera's system clock before the event. (Such as through the use of an internet time server, external GPS time source, etc.) Calibration to better than a tenth of a second will give results accurate to a couple of kilometers of shadow travel. For systems that are simply synchronized to a GPS signal when commanded, we recommend performing the synchronization shortly before exposures begin to minimize the drift since the last sync, and checking the apparent calibration just as exposures end to get a good estimate of variability.

Observing Duration

While the event is expected to last no more than two minutes around the midtime, 1) the predicted midtime is uncertain, perhaps as large as couple of minutes, and 2) the goal of scanning for small satellites/debris requires monitoring the star both before and after the event.

Occultations by Nix and Hydra are difficult to observe due to the small size of their shadows, and orbital uncertainties that greatly affect the position of the shadow on the Earth much beyond the error bars noted on the plots. The occultation star crosses the orbital radius of Hydra approximately 30 minutes after the midtime of the Pluto event.


For this event, we are simply looking for the largest signal to noise signal possible, to enable the shortest exposure times. Thus, no filter is required, and minimum pre-camera optics are suggested, to gain the maximum light from the star. Large telescopes where we request filters will receive individual specific requests.



Camera calibrations, such as bias frames and flat/dark frames if needed, should be taken before and after the event depending upon the camera in use. POETS and PICO cameras supplied by MIT need no dark frames if operated at –30 C for these short exposure times, but bias frames are always needed. Please be sure to take bias and dark frames if needed using the same temperature/exposure settings that was used to record the event. Flat field images, taken on the twilight sky are appreciated from all systems as they aid in careful reduction of the frames.

If your camera supports Automatic Dark Subtraction, we recommend turning this off for these observations, as these modes often result in variable overhead times, and we prefer to do dark/bias subtraction manually.


Portable telescopes are asked to record their GPS location both before and after the event to be sure the location was stabilized during the event time.


Telescopes with the capability of imaging the the star and Pluto separately are requested to take astrometric images of the field before and after the event (possibly the night before and after), in which the star and Pluto are separated enough to do accurate measurements of their separate positions. Note that the field is very crowded, so these measurements are only possible with telescopes that can obtain decent signals with only a fraction of an acrsecond per pixel in resolution.

Last updated by Michael J Person ( 2011-06-26 18:00

Please direct all inquiries to the MIT Planetary Astronomy Lab (