Triton Occultation - 2017 October 5

Artwork by Liem Bahneman
Prediction   Occultation Star Information   Station Lists   Finder Charts   Observing Tips

Observing Notes

Event Basics
As the 12.5th magnitude occultation star is expected to be occulted by a 13.5th magnitude Triton, the main observational result is the joint star/Triton light dimming to approximately 40% and then brightening again. The maximum expected duration for this event is likely to be almost 3 minutes at the center of the shadow, and shorter as one approaches the limbs.

Event Time
The time given in the prediction page is the time the shadow center should pass across the Earth's center. Each site will be either several minutes beofre or after this time as the exact event time depends on the ~17 km/sec shadow travel rate and your site location. It will occur around 23:48 UT in Europe and 23:54 UT in the eastern US. Individual site predictions will be provided once observing lists are finalized.

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 16.8 km/sec, every one second uncertainty in the disappearance or reappearance time corresponds to approximately a 16.8 km uncertainty in the measured radius/position of the Triton’s atmosphere.

For small aperture telescopes (~14-in), we hope to measure the star's brightness at least once every 2-4 sec, if possible. We suggest that an observing efficiency (exposure time / cycle time) of >= 50% be employed. Thus, if your detector read time is 5 seconds, the shortest exposure time considered should be 5 seconds, unless saturation forces shorter exposure times. Obviously detectors with significantly shorter read times are preferred.

Larger aperture telescopes can record the star at a faster cadence if supported by the camera, as exposure time sets the limit on our vertical resolution in Triton’s atmosphere, 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 finder charts.).

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. 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 (e.g. NTP), 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 3 minutes around the midtime, the predicted midtime is uncertain, perhaps as large as several minutes if the star turns out to be an unresolved double.  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.

For this event, we are simply looking for the largest signal to noise possible, to enable the shortest exposure times. Thus, for most stations no filter is required, and minimum pre-camera optics are suggested, to gain the maximum light from the star.  We’re considering various moon (30deg away from event) and Neptune blocking options, and will update for particular stations.

Camera Calibrations
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/Williams 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 photometric 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 (and fixed telescopes who haven't checked lately!) are asked to record their GPS location both before and after the event to be sure the location was stabilized during the event time.