General Observing Notes for the 06 October 2022 Stellar Occultation by Triton

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Basic Information:

star magnitudes from APASS, 2MASS, Gaia EDR3 catalog

Detailed information on the occultation prediction is available here.

Finder Charts for the Star

Finder charts to confirm the star position are located here.

Goals

The scientific goals of this event are (i) to accurately determine the current half-light diameter of Triton, (ii) generally characterize Triton's atmosphere, probing for spikes, knees, or other light-curve features that can shed light on deeper atmospheric phenomona, and (iii) establish the position of Triton with respect to the star and Triton's ephemeris for aiding future occultations.

Observing Notes

Event Basics

As the 11.5 magnitude occultation star is expected to be occulted by a 13.5 magnitude Triton, the main observational result in the joint star/Triton light dimming by approximately 90% and then brightening again. The maximum expected duration for this event is likely to be approximately 2 minutes 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 ~23 km/sec shadow travel rate, the exact event time depends on your site location. It will occur around 14:40 UT in China.

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 Triton shadow moves across the earth at approximately 23 km/sec, every one second uncertainty in the disappearance or reappearance time corresponds to approximately a 23 km uncertainty in the measured radius of the Triton.

Thus for small aperture telescopes (~14-in), we hope to measure the star's brightness at least once every 1-2 seconds, if possible. We suggest that an observing efficiency (exposure time / cycle time) of >= 50% be employed. Thus, if the detector read time is 5 seconds, the shortest exposure time considered should be 5 seconds, unless saturation forces shorter exposure times.

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, 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 Triton'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

We recommend continuous observations from at least 20 minutes before through 20 minutes after the expected Triton occultation midtime. Depending upon your location, camera, disk size, and conditions, you may be able to increase this to 30 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.

Astrometry

Telescopes with the capability of imaging the the star and Triton 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 Triton 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 Carlos Zuluaga (czuluaga@mit.edu) 2022-01-21 23:41

Please direct all inquiries to the MIT Planetary Astronomy Lab (planetary-astronomy@mit.edu)