Monitoring / recording asteroid occultations of stars is highly valued among planetary observers / scientists. It provides precise position and size measurements of small objects with two time measurements: the start and end time of the event.

In the last few years, three asteroid moons have been confirmed/discovered in video recordings, while several more await further analysis. Much remins to be discovered by this simple measurement technique.

Occultation of a brighter star by a fainter asteroid will cause the star to vanish for a short while, and can be recorded.

Video Recordings,
We record the image of the star (and sometimes the asteroid) with either the WAT-910HX integrating video camera, or a CMOS astro camera.
The astro camera is typically monochrome, but colour sensors are becoming more common in use because they are used by planetary imagers.

There are two types of CMOS camera read-out modes: those with a Global Shutter (IMX174) which read out the whole frame at once, and secondly the Rolling Shutter type. Most newer CMOS cameras (since about year 2020) have the second type. Either can be used

Timing
Adding the UT time stamp to each frame of the recording is the most important aspect of the observation. This can be done in a number of ways and how this can be achieved reliably (and least expensively) can be found on asteroid occultation forums and groups.io New observers exploring occultation timing can use the PC clock (W10/11) synchronised by NTP (Network Time Protocol).

This synchronosation will set the PC time to an accuracy of +/- 10ms, but the clock time will subsiquently wander up to 1sec per day depending on the computer and applications.

CMOS GPS cameras
These have a GPS receiver built into the camera ( for accurate timing ), and are the most straight forward is use. Analogue video needs a video time inserter and video digitiser

1) QHY174m GPS (China) useable with SharpCap software (12bit images)
Based on the IMX174, became available in 2017.
Is a Global shutter camera with cooling, 5.9x5.9um pixels, in 1200 x 1900 array (7.1 x 11.2mm) 2x2 bin possible.

Although the QHY174gps is becoming "old tech", this camera has no substitute at present, has an exposure range from 1ms to 10min+, and cooled so is a jack-off-all-trades.

2) DVTI+Cam (Swiss) as IMX430 (12bit). Start-up company in 2018, now selling cameras.
New products are being developed. IMX430 is 1240x1624, 4.5x4.5u pixel camera (5.6 x 7.3mm) - Dedicated in-house software with many features. Passive cooling. Limited exposure to 5 seconds at present. Global shutter. A popular camera for observers upgrading from video.

3) ASTRID (USA/Canada). Sony IMX296 1440x1080 px 3.4x3.4um (10bit)
Integral GPS with WIFI connection to phone or other device. Special record format .RAVF. Based on RasPi.
Liked by our US colleagues for pre-point observation mode. Useful for push-too coordinates (alt-az).

VIDEO 8bit Analogue cameras

4) WAT-910HX integrating from 0.04 to 2.56 sec.
Used with a GPS video time inserter such as IOTA OpenVTI or GPSBOXSPITE 3, and the output digitised into laptop. This is considered as the "gold standard" for time accuracy. Camera were made between 2010 and 2020. No longer made however. A few are available as new (from old stock), or used.

5) RunCam Night Eagle 3 analogue (8bit) (USA)
This Astro edition of the RunCam has 3 integration settings, 0.04, 0.08 and 0.16. It is small and runs on 12V. There is a kit that includes power supply, 0.5x focal reducer and the video converter to digital. The camera has the same sensitivity as the WAT 910HX but lacks the longer integration settings: 0.32 0.64s etc.

Timing for non-GPS CMOS cameras.

6) The PC clock time (W10/11) can be controlled by GPS through the USB port. This is achieved with RasPi kits, and more recently with the TIMEBOX-II from Shelyak Instruments. This means that many more cameras can be time calibrated and used for occultations.

7) The ZWO ASI432 camera with 9x9u pixels is used successfully with longer focal length instruments. Has very good sensitivity and low noise characteristics with global shutter. [ Global shutter cameras read out the whole frame for timing purposes.] This camera can be used with a TIMEBOX-II or other well disciplined time signal. For example a C11 at F6.3 with ASI432, has an image scale of 1.1 arcsec/pix, and a field of view of 0.5 x 0.3 degrees.

PREDICTIONS

Event UT:
The predicted time of an asteroid occultation is reasonably well known. (to within a few seconds in many cases now). It's advisable to begin observing/recording about one or two minutes before the event, and continue for up to 2 - 4 min duration. The asteroids with well defined orbits can be predicted accurately to within a second. Smaller or more distant objects (e.g. TNOs) with less well defined orbits will have UT errors of up to 1 minute, maybe more. Check the prediction for the event UT. To detect unknown satellites of asteroids, observation will need to commence may be 5 min earlier than the time for the parent asteroid./or Occult, and record for up to 10min.

Use Occult Watcher Desktop (OWD) and Occult Watcher Cloud (OWC). OWC needs a login. Use the same one as OWD if you already have this installed and set up to share positions.

Reporting Negative and Positive Asteroid Occultations
All observations are gratefully received. Results will be forwarded to the central body for reducing and archiving. Please join the UKoccultations.group.io for a advice on recording technique and reporting observations to the European Occultation Results Database - SODIS

Join: https://ukoccultations.groups.io

Clear Skies and good observing,

Tim

Author: Tim Haymes
Assistant Director (Occultations)
Asteroids and Remote Planets Section
- and Lunar Occultation Coordinator
British Astronomical Association.

Oxfordshire, UK
2025 July