Occultations - Brian Loader

Software for Occultations

The software used for occultations divides into 3 or 4 main groups. Predictions of events, observing, ie the software for video capture, analysis of the resulting video record and lastly recording and reporting results. Much of the software used in association with observing occultations has been developed by two or three observers. That is by 'amateurs'.

The first requirement in observing occultations is to have predictions of when events will occur. In the case of lunar occultations predictions were originally provided by some of the professional observatories, including in the past the Greenwich Observatory and the Washington based United States Naval Observatory. These have largely been replaced by OCCULT, with predictions generated by the user for their observing position.

OCCULT by Dave Herald

OCCULT is the mainstay for occultation predictions and for a lot of other occultation features. Predictions of occultations by both the moon and asteroids can be generated by the user.

For lunar occultations the listing shows times and other details of events visible from the observer's site. These can be for the current date or for a range of dates. The programme takes into account the aperture of the observer's telescope and the effect of the phase of the moon on what it is possible to observe.

Details of the position of the star at the time of occultation round the moon are also shown. Essential information for observing reappearances!

Local predictions for grazing lunar occultations can also be produced. As well as times and info about the star and the position of contact on the moon's limb, coordinates of the graze path and the profile of the moon's limb for any selected point on the path are available.

For occultations by asteroids the user is able to generate predictions for over 5000 asteroids. The latest GAIA star positions May be used. The predictions will show details of the asteroid and the star it is expected to occult with a path within a specified range of the observer. Included are the date and local time of any event, together with the change of brightness of the combined objects when an occultation occurs. A map showing the predicted path of the occultation across the Earth can also be produced. There are also facilities to assist the observer locate the target star – not quite as easy as for lunar occultations!

See Occult Watcher below for other ways of obtaining predictions of stars by asteroids.

More details of some of the features of Occult are included later.

The first version of Occult appeared ca 1984. This was before the advent of Windows so was written for DOS systems. Dave has been maintaining and developing the suite of programmes ever since. I gather he has written, in this time, over one million lines of code.

Occult Watcher by Hristo Pavlov

Occult Watcher (OW) displays details of coming asteroidal occultations with a path near to an intending observer. The predictions used are sourced from a number of 'feeds' containing predictions. The user can select the feeds they want used. The basic feed is a set of Occultation predictions initially produced, on an annual basis, by Edwin Goffin of Belgium. They are usually updated close to the date of the event using the latest positional information for the asteroids. The updates are produced by Steve Preston, the current president of the International Occultation Timing Association (IOTA).

As well as local details of the predicted event, OW shows the probability of an event being seen by the observer. Using OW, an observer can 'announce' their intention to observe an event and see who else also plans to observe it. Users of OW will get computer reminders of their intended events two hours before the predicted time.

As if that was not enough, if an observer finds some likely events for themselves using Dave Herald's Occult which they think are worth observing, then by placing a file of details, as produced by Occult, into one of OW's folders on the observer's computer, they will appear on OW itself as a 'personal' prediction that others can sign in for.

As you can tell, Occult Watcher is an essential programme for anyone intending to observer occultations by asteroids. In addition one of the feeds will give information about coming lunar occultations of double stars.

See also the use of OW in reporting observations, below.

2. Software used for observing

VIRTUAL DUB by Avery Lee and OCCUREC by Hristo Pavlov

These programs provide the software for capturing and saving videos to a computer.

VIRTUAL DUB is free software, available on line, designed for recording and editing videos in general and saving them in .avi format. It is useful for recording Lunar occultations, the .avi format produced can then be used by LiMovie (below) to analyse the results.

OCCUREC is more specialised, specifically designed for use in occultation observing, although it can also be used for observing minima of eclipsing binary stars. It functions in association with cameras capable of integration so is very useful for occultations of faint stars by asteroids. It can be used for lunar occultations as well.

A benefit of OccuRec is that it produces smaller files than VirtualDub when integration is used. It does this by detecting the level of integration being used and saving only one copy of each integrated frame not multiple copies as output by the camera. The output uses a format developed by Hristo, and needs specialised software to analyse, see Tangra below.

3. Analysing the Results

These programmes enable the analysis the video records of occultations. They do this by measuring the brightness of the star image on the video, frame by frame. The resulting data can then be graphed to give a light curve. In both, clicking on a plotted point results in the display of the corresponding video frame, including the GPS derived time inserted on the frame.

LIMOVIE by Kazuhisa Myashita

LIMOVIE was developed by Kazuhisa Myashita of Japan mainly for analysing the results of lunar occultations. It is particularly useful in cases where a close double star is occulted. The presence of a double star is indicated by a step in the light curve. In many cases the presence of a double would not be visible to the unaided eye. By measuring the light levels, Limovie can also make an estimate of the relative magnitudes of the two stars.

Figures showing LiMovie light curves of lunar occultations:

Figure 1: Occultation of a single star.

Figure 2: Occultation of a double star.

Figure 3: Occultation of a new close double star.

TANGRA by Hristo Pavlov

TANGRA is Hristo's equivalent to Limovie, but it is particularly aimed at occultations by asteroids. In many of these events the star disappears for only a very few seconds, having the light curve will confirm an event actually occurred. In other cases when the star is fainter than the asteroid the light change at an occultation will be small and not visible to the unaided eye. Again the light curve May make the event obvious.

Figures showing Tangra light curves of occultations by minor planets.

Figure 4: a deep occultation with star brighter than asteroid.

Figure 5: a shallow occultation, asteroid brighter than star.

Another feature of Tangra is its interaction with Dave Herald's Occult. If an event occurs, the output of Tangra can be saved and passed to a section of Dave's Occult. This detects the event using the change in brightness of the target and produces a report on the times of the event. When saved a file is produced which Occult Watcher can pick up and use to pre-fill a report form of the event. The report can then be sent to the appropriate regional co-ordinator for inclusion in published results. In the case of New Zealand and Australia this person is Steve Kerr, the current director of the RASNZ Occultation Section.

Reporting Observations

Which leads to the last phase in observing occultations, reporting what is seen. Again what happens is different for lunar and asteroidal occultations.

For lunar occultations, OCCULT provides an editor where the time and other details of an observed occultation can be entered. It also provides a visual check that the time data look to be correct eg that there is no error in the star number and that the time is close to prediction. Results are forwarded to a regional coordinator on a regular basis and hence to Dave who adds them to an archive. Observations are archived at Astronomical Data Centres.

The editor also provides facilities for producing a file of light curve information derived from a csv file, generated by LiMovie. The light curve data is also archived at VizieR.

Observed times of Occultations by asteroids are usually entered on a spread sheet report form. The observer can download a copy of the form and save it on their own computer. Much of the data required will remain the same for all observations made by the observer at their normal site, so only needs to be entered once.

Occult Watcher makes reporting even easier. After the event, OW allows the observer to send a brief report giving the result, or even that they were unable to observe due to eg cloud. This report is sent to all others who have announced their intention to observe that event. After sending the brief report, OW then gives the observer the opportunity to complete a full report form. Most of the details needed are filled in by OW. Only the actual details of times and conditions for the observation are needed.

Obtaining the software and other information

Almost all the software mentioned in this article can be obtained through the RASNZ Occultation Section website: www.occultations.org.nz/software/software.htm or at the IOTA website: www.occultations.org/observing/software.

Hristo Pavlov's software may also be downloaded from his website: www.hristopavlov.net.

A guide to observing occultations using video by Jacquie Milner can be found at www.occultations.org.nz/videotime/manual.htm.

The RASNZ Occultation Section website www.occultations.org.nz has more information on occultation observing, as does the IOTA website www.occultations.org