An Occultation Timing Primer

Updated: 2002 October 29; limited update 2007 Oct. 8

This was requested in a message by Steve Gemeny; below is my response, updated Oct. 10 and 29.  Another good beginners article 
was written by Guy Nason and placed on the Web site of the Toronto Centre of the Royal Astronomical Society of Canada.
Since this was written, an important video timing device has become available; it is the Kiwi OSD (On Screen Display) GPS 
video time inserter, designed by Geoff Hitchcock, "Kiwi Geoff", in New Zealand and now manufactured by PFD Systems in Bethesda, MD.  
Details about it are on the PFD Systems Web site here.
Also, now available, giving much more detailed information about observing occultations of all kinds, for those who want more 
information than the summary information given here, is IOTA's online observing manual, "Chasing the Shadow:  The IOTA Occultation 
Observer's Manual" available for free download here. 
You asked about a primer for occultations, so I'll attempt to give one below.  Besides the main IOTA site, , there is some more basic information in the observing/occultations section of Sky and 
Telescope's Web site,
When you get into Sky and Telescope's site, in the upper left, click on "observing".  From that menu, click on "celestial objects", 
and in the list of those, click on "occultations".  The Sky and Telescope site is set up so that you have to go through this procedure; 
their subpages are not directly accessible.
Joe Sedlak commented that my write-up below is too advanced, especially for scouts and middle/high-school students, and provided 
additional background information to help explain asteroidal occultations, and timing them, here.
Basically, make timings with whatever resources you have.  First, for time signals, it's most convenient to use a shortwave receiver 
that can pick up WWV time signals at 5.0 and 10.0 megahertz.  For those who want WWV, Radio Shack sells a digital short-wave receiver 
for between $70 and $100, depending on sales; a digital-tuning radio is much preferred to an analog one since it's so much easier to 
quickly find the station.  Those who don't have short-wave receivers for WWV or CHU might get a WWVB-controlled alarm clock, such as 
item 63-964 available for $30 from Radio Shack.  You can ensure accurate time by briefly removing the batteries an hour or so before 
the occultation; that will force the clock to reset using WWVB.  You should set the alarm to go off 2 or 3 minutes before the occultation; 
start your tape recorder (or camcorder used for audio recording before that to record the alarm for a few seconds, then your voiced calls 
of the occultation disappearance(s) and reappearance(s), and then set the alarm to go off again 1 or 2 minutes in the future, and keep the 
recording going to record it as well.  The audio alarm is accurate to 0.01 sec. or so, but the LCD display has a delay of about 0.1 second 
at room temperature, and much greater as the temperature approaches or drops below freezing.  So use the alarm for timing, not the display, 
which, however, is useful for keeping track of the time to the nearest second so that you know when to observe (this makes these clocks 
useful even if you have a short-wave receiver).  Similar radios are available using the German DCF77 transmissions from Germany.  
Another source for accurate time would be just a car radio, if in a given area, a coordinator will record a selected strong local AM or 
FM standard broadcast radio station along with WWV time signals.  This master tape can then be used to determine the times of syllables 
spoken in the selected standard broadcast that those with only a car (or transistor) radio record along with their event calls.  In the 
Washington, DC area, the strong AM station WTOP at 1500 kilohertz can be 
A few observers use stopwatches for timing, just starting the watch at the occultation and stopping it to a time signal.  But for an 
asteroid occultation, where there are two events to time, you need either two stopwatches, or one with a split arm, or digital method 
for two timings.  And the star might be a close double (as we know to be the case for ZC 593 that will be occulted by 431 Nephele on 
Nov. 2/3), perhaps producing 4 events instead of 2.  So I do not recommend stopwatches for timing events in real time; they can be used 
to determine times from a recording of a visual observation. 
It is better to make at least an audio recording during the occultation, so that either WWV or WTOP can be recorded along with the 
observer's calls of the occultation events.Tape recorders are relatively inexpensive, selling for about$25 at numerous stores.  These days, 
many tape recorders use voice activation to save tape, only recording when there is sound; that, of course, makes them useless for timing.  
So if someone buys a tape recorder for timing, be sure to get one that either doesn't have voice activation, or that can turn off that feature.  
After the occultation, the recording can be played back several times with a stopwatch (even the stopwatch function available on most digital 
wrist watches) to time from a WWV minute tone, or a distinct syllable in a word in the WTOP broadcast, to the event call.  Also, when you 
record the observation, you can give some estimate of your reaction time, or at least say something like "I think I was about a full second 
slow in calling out the event".
These days, many families are more likely to have camcorders than tape recorders.  A camcorder records audio as well as video, so they can 
be used like tape recorders to record a visual observation.  Those with large telescopes,about 12 inches or larger, could even video record 
a 9th-mag. occultation by pointing the camcorder into a low-power eyepiece [for a star as bright as ZC 593 or pi Arietis that will be 
occultation by Nephele or Lindemannia (respectively) 
during November, even a 2-inch or 3-inch telescope should work].  If you do that, practice beforehand, first on the Moon, then on bright 
stars.  It's best to use manual focus rather than automatic focus; you can maintain the proper focus better if it is fixed.  Video recordings 
eliminate the reaction time uncertainties of visual observations and so are preferred for timing occultations, when they can be made to work.  
But visual observations are better than no observations, since the shape of an asteroid, especially near the path edges, is best determined by 
observations from many locations. 
The best observations are made with a small video camera attached to the telescope, with its output, and audio from a microphone, recorded 
with either a VCR, or a camcorder that accepts input lines for recording in VCR mode (inexpensive camcorders don't have that capability).
IOTA recommends the PC-164C camera, available for about $130 from (to place orders quickly, call 1-800-335-9777).  
That camera can see about as well as you can with an eyepiece with most any telescope, and in some cases even better; it is very sensitive, 
and fora 9th-mag. star, it should be able to record it with a 3-inch telescope or larger.  For this, you also need a C-mount to eyepiece 
adaptor so the camera, which only weights a few ounces and is about 2 inches long and 1.3 inches wide, can be inserted into a standard 
1.25" eyepiece holder; Adirondack Video sells such an adaptor for about $35.  The PC-164C has a BNC male for the video, so you need to get 
a female BNC to female RCA adaptor from Radio Shack to connect to standard video (RCA) wires.  You also need a video microphone; tape recorder 
microphones are too weak (mismatched impedance) to work with VCR's.  Supercircuits (above) sells a small video microphone called the PA3 for 
about $13; if you order one, you should also ask for a TAB adaptor, about $2 more, touse it with the standard small 9-volt batteries.  Also 
recommended for those with SCT's is a focal reducing lens to obtain a wider field of view with the small camera chip; best is the f/3.3 
Meade focal reducer sold by FocusCamerain New York for about $150 (and available from some other sources of Meade products); also useful and 
less expensive is an f/6.4 focal reducing lens available from Orion and other sources.  For more on video, see the main IOTA Website, especially 
the item near the top by Richard Nugent, and the items right after it that give some additional information.  I will help answer questions 
from those who start to get into this.
Most important is being able to locate the star. Print the detailed finder chart from the main IOTA Website and practice locating the star 
well before theoccultation, preferably some night before the event. If your telescope doesn't have a good go-to capability, then it should 
have a good finder scope, 50mm or larger aperture, so that it can see 7.5 to 8th-mag. stars in a 4 deg. or larger field of view.