General Observing Notes for the 10 February 2011 Stellar Occultation by Varuna

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Basic Information:

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

2011 February 10 12:38:31 ± 00:01:21 UT

Occultation Star Position (J2000):

RA (hh mm ss.ss): 07 35 38.856

Dec (dd mm ss.s): 26 16 06.49

Occutlation star UCAC2 magnitude 15.5
KBO approximate visual magnitude 20.2

Detailed information on the occultation prediction is available here.

Finder Charts for the Star

Finder charts to confirm the star position are located here. Star charts designed for manual telescopes are here

Goals

The scientific goals of this event are (i) to accurately determine the diameter of Varuna, (ii) to probe for any tenuous atmosphere of Varuna, and (iii) to detect possible nearby satellites of Varuna.

Observing Notes

Event Basics

As the 15th magnitude occultation star is going to be occulted by a 20th magnitde KBO, the main observational result should be that the occultation star vanishes from the field and then reappears. The maximum expected duration for this event is likely to be just over 50 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 22 km/sec shadow travel rate, the exact event time depends on your site location. It will occur centered around 12:39 UT in East Asia, around 12:37 UT in Alaska, and around 12:35 UT in Hawaii .

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. As the KBO shadow moves across the earth at approximately 22 km/sec, every one second uncertainty in the disappearance or reappearance time corresponds to approximately a 22 km uncertainty in the measured radius of the KBO.

Thus for small aperture telescopes (~14-in), we hope to measure the star's brightness about twice a second.

Larger aperture telescopes can record the star at a faster cadance 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, as we are more concerned with timing than with the star's actual brightness. 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 the KBO. 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.)

Observing Duration

While the event is expected to last no more than a minute around the midtime, 1) the prediction is somewhat 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.

Thus, we recommend continuous observations from at least 10 minutes before through 10 minutes after the expected occultation midtime. Depending upon your location, camera, and conditions, you may be able to increase this to 20 minutes before and after.

Filters

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.

Calibrations

Camera

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 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.

Location

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.

Astrometric

Large aperture telescopes, with the capability of imaging the KBO itself (~20 mag), rather than just imaging the star, 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 the KBO are separated enough to do accurate measurements of their separate positions. The provided finder charts indicate how much the KBO moves in a day.

If your dynamic range does not allow observations of the 20th mag KBO without saturating the 15th mag occultation star, alternating sets of frames with exposure times suitable to each are requested. Such frames should always be centered on the occultation star and allow the KBO to move across the field. This way a consistent astrometric reference network can be used for all reductions.


Last updated by Michael J Person (mjperson@mit.edu) 2011-01-11 3:00

Please direct all inquiries to Jim Elliot (jle@mit.edu)