Introduction to Observing Occultations:
Occultation observing is a fun venture that almost anyone with minimal equipment and astronomy background can do, and one which can lead one toward a potential career in astronomy. This tutorial is intended to describe the basics of occultations and the techniques for observing them. It may help the reader decide whether or not to explore interest in one aspect of occultation observing or another.
The International Occultation and Timing Association (IOTA) is a volunteer organization which predicts, gathers, analyzes and publishes observations of occultations. It has many resources available to help you get started in this fascinating adventure. The references at the end of this document will give you more information about IOTA and observing occultations.
If you aren’t already familiar with the basic attributes of lunar and asteroid occultations, read What is an occultation?
The basic goal of occultation observations is simple: measure the time of the occultation events. We use the term “event” to refer to the time when a star is covered or uncovered by the moon or asteroid. In a total lunar occultation, we measure the time of a single disappearance or reappearance event. In a grazing lunar occultation the observer may record multiple events. In most asteroid occultations the observer will record two events, a disappearance and a reappearance. Clearly accurate time is an important part of occultation observations. For most lunar and asteroid occultations the goal is an accuracy of 0.1 seconds or better. And it is important to note that these times must be accurate, 0.1 sec or better, with respect to UT (Universal Time). The following paragraphs will describe several observing techniques which IOTA observers have utilized. Both equipment and techniques for asteroid occultation observing have evolved over the years. For some observers, developing new techniques has been one of the more exciting aspect of occultation work.
Knowledge and Equipment
Most importantly, all changes in the star’s light must be accurately recorded and timed. Most experienced IOTA observers observe with specialized sensitive video cameras. The observer records the video from the camera during the expected time of the occultation and the observer can review the tape to note the times later. The best method of establishing time on the video is with a GPS based time inserter which inserts the time on each frame of the video. In lieu of GPS time inserter for a time base, observers can record the NIST radio station WWV on the audio track of the video recorder. A video camera and GPS time inserter are currently the best tools for observing occutlations. Passing clouds or poor seeing conditions can confuse a visual observer and the video provides a more accurate record of the star’s signal during the event. But visual observations are still useful and can be a good starting point for first time observers. When observing visually, the observer can record their voice while observing the event and the observer will verbally mark D (disappearance) and R (reappearance) events vocally. For visual observations, WWV is usually the best source of time (recorded along with the observers voice). Time services based on NNTP (internet) protocols have appeared on computers and phones in recent years. Some of these solutions may be accurate enough for occultation work. But it is best to check with IOTA to verify before using these time services.
When observing lunar occultations finding the star is usually a relatively easy task. However, locating the target star for an asteroid occultation can be challenging. You will need first a good, independent ability to find stars in the sky, a telescope with or without a tracking platform, and basic internet access (from your base of operations, usually at home). One cannot completely depend on automatic GO-TO telescopes to do the work of finding a target star which is scheduled to be eclipsed by a passing asteroid. It is mandatory to learn how to use star charts and in particular, to know how to “star hop” to the target star which could be anywhere from 6th to 13th magnitude depending upon the level of difficulty of the occultation target star. This area of occultation work provides the greatest opportunity for real discovery – unknown double stars, determination of the shapes and sizes of minor planets, and possible detection of a new natural asteroid satellite!
To observe a total lunar occultation visually you will need:
- recorder that can record your voice, either cassette or digital voice recording device (e.g. cell phone voice recorder)
- shortwave radio that can pick up time signals at either 5, 10 or 15 Mhz (North America)
- predicted time of the occultation event at your location.
You will begin by observing a star undergoing a lunar occultation, then recording it and consulting the IOTA Manual (chapter 3 and other online IOTA resources) to reduce and report
The next phase will be to use your new found abilities to observe (and perhaps record using the same equipment) an asteroidal occultation. Your prediction resources will be found via IOTA’s main website. You can peruse the events listed to determine which events might be observed from near your location. Or you can install the free program OccultWatcher to automatically search for events (see IOTA’s software page). With these resources you can determine the geographic areas of best viewing, key information on each occultation, and star charts. The IOTA Manual (Appendix F) explains how to submit your observations to IOTA.
In many cases an observation of a predicted asteroid occultation will result in no occultation observed (a “miss” or negative observation). Negative observations may be due to inaccurate data on the star position and/or asteroid orbit. Negative data are always useful as these “misses” define the limits of the asteroid’s shadow and therefore determine the extent of the asteroid’s actual profile. Negative observations are important results and observers should report both negative and positive (event) observations.
If you wish to devote more resources to occultations, you can then advance to the next phase which is video recording of an asteroid occultation but this will require an investment of perhaps $300-$500 more or less to
obtain a Supercircuits PC164C-EX-2 video camera, battery, camcorder (or digital video recorder, DVR), PA3 microphone; Canon ZR camcorder (usually from ebay) associated cables and an IOTA VTI GPS time inserter. In lieu of the IOTA VTI GPS Time Inserter, once can use a shortwave radio (approximate cost $50-$75) to receive time signals as stated above. The GPS based video time inserter will identify and record the observing site location’s latitude and longitude. The GPS time (Universal Time, UT) will be overlaid on the video in real time to 1/100 of a second or better. Bright star asteroid occultations usually require that the observer be
mobile and travel. Only occasionally (typically a few times/year) can one observe an asteroid occultation from his/her home – the path widths (20km – 250 km) must cross over where you live. Being mobile will bring more events into range and provide more opportunities.
Multiple Observers / Multiple Observing Stations
For both lunar and asteroid occultations a single observer can produce but one data point, unless he or she is highly experienced in setting up multiple unattended video stations. One observer can achieve success but must never do so at the expense of other observers. Therefore there is a great need for a team of individuals working in coordination with one another or with other individuals/teams either nearby or in other geographic regions in order to maximize the amount of data that can be collected for a particular occultation. The free software program Occult Watcher allows observers to share with other observers their intended locations for observing asteroid occultations. Each observer inputs a location which then pops up as an icon on the path map. This allows one to avoid duplication of sites to optimize coverage and promote teamwork. Advance coordination of observation should be done in order to maximize results but especially to overcome weather obstacles. An experienced observer should be responsible to help train new people in the proper methods of understanding thepredictions and how to accomplish successful data collection. The goal, beyond that of having fun, is to collect information that will be scientifically useful. Discipline and adherence to procedure will help achieve that.
For grazing lunar occultations, a single observer can only sample one small region of the lunar terrain as the star alternately disappears and reappears. By including other observers located at different places and spaced perpendicular to the first one, coverage of successive areas north and south can be made until the farthest observer is actually outside the path and sees no disappearance or reappearance. This will allow a profile of the mountains along the lunar limb to be drawn. A “miss” (no occultation seen) observation might not seem useful but in occultation astronomy that data can be scientifically valuable as it can define the upper limit(s) of certain lunar features. A team is needed for a different reason where asteroid occultations are observed since the predictions are less reliable. A single observer may or may not actually see an asteroid occultation, but a broadly spaced team of observers will help ensure that the asteroid is actually observed to occult the star and can define areas where no occultation is seen. This provides valuable clues as to the prediction quality and can more accurately define the size of the asteroid shadow and size limitations of the asteroid.
Time is the most important element in occultation observing for two reasons. You must have the most accurate time for your observations. Secondly, time is of the essence in planning for a particular occultation. The observer will have to define a ‘timeline’: planning the location of the site (if not observing at home), traveling to it, assuring its security, setting up equipment, dealing with unanticipated problems, locating the target star, and then making the observation. There are many factors that can impact execution of the timeline. In addition, your position in latitude and longitude needs to be known to ±100 meters for an asteroid occultation and ±10 meters for lunar/grazing occultations. Your elevation (altitude) needs to be known to ±5 meters. The GPS Video Time inserter described above displays the ground position and altitude on your recorder. Or one can determine their position/altitude from a low cost GPS commercially available receiver and/or online topographic maps.
Finding a target star is the biggest challenge for asteroid occultations. The widest possible field of view is required especially for faint target stars. To obtain this a focal reducer is extremely helpful. If one is not used the process of finding the target (or relocating it) tends to be quite long and if an observer is having difficulty it could become an impossible task if one does not have sufficient star finding skills or have enough time.
If you record an occultation on video, you can extract data from your video free software programs LiMovie or Tangra (available via IOTA’s software page). These program will analyze video files (typical in an AVI file) for occultation events (both lunar and asteroid occultations). If you have question on operating these programs, you can consult other resources on IOTA’s website or ask for help from other experienced IOTA observers via one of the IOTA online forums. Dr. Richard Nolthenius has a website explaining how to get your videos into a format for LiMovie. The graph below shows a sample light curve before, during, and after an 13 second occultation by the asteroid (372) Palma.
Reporting your observation
After analyzing your data to determine the event times, you can report your observation to the regional coordinator for your observing area. For asteroid occultation observations, the Worldwide Asteroid Occultation Observations page provides guidelines for reporting in different worldwide regions. For reporting lunar occultation observations, refer to the information on the Lunar Occultation Resources page.