BASIC TIPS ON TIMING OCCULTATIONS & GRAZES


This is written to help those who are just starting to time occultations. First, especially North Americans should read my "Lunar Occultation Highlights" article in the January issue of SKY AND TELESCOPE magazine. This outlines the most important occultation events that are predicted to occur during the year, and gives sources where more detailed information can be found. Other valuable SKY AND TELESCOPE articles discuss timing occultations (September, 1990 issue, p. 288) and the value of occultation observations (November, 1988, p. 480). Also in the January (or sometimes February) issue of SKY AND TELESCOPE, I discuss planetary (mainly asteroidal) occultations that will occur during the year.

Accurate time signals are needed for timing occultations. The time provided by the telephone company in most North American cities is not accurate enough; you should not use it. In the U.S.A., accurate time signals, from the U. S. Naval Observatory master clock, can be obtained by calling 900-410-8463; the call should be placed via AT&T to ensure use of land lines, which will give an accuracy of a few hundredths of a second, more than sufficient for visual timings. If the call is not made via AT&T, or if the National Bureau of Standards' WWV 303-area-code number are used, the call might (or might not, you would not know) be routed through a geosynchronous satellite, causing a quarter-second delay, which is unacceptable, even for visual timings. In the Washington, DC area, the USNO master clock can be reached with a local call to 202-762-1401, but those outside of the DC toll-free area should not use that number, because it might go through a satellite. Some other nations, such as the U.K., also have accurate time signals avialable by telephone.

Often more convenient than telephone time, especially for field use such as grazing occultations, are short-wave radio time signals, such as WWV and WWVH at 5, 10, and 15 megahertz. Radio Shack used to sell a convenient receiver, the "Weatheradio-Timekube", for these frequencies for about $40, but they are no longer available. Some grazing occultation expedition leaders have extra Timekubes that they may be able to loan. A more expensive alternative (kit, $59; wired board, $99) that requires some assembly is the Hamtronics' WWV Receiver, described on pages 48-49 of the January 1996 issue of SKY AND TELESCOPE. It gives a much more reliable signal than similarly-priced general-purpose short-wave receivers. If you buy a general-purpose short-wave radio, try to get one that at least covers 5 and 10 megahertz, the best nighttime frequencies. Also, one with digital tuning is highly recommended; many observations have been lost while observers tried and failed to find a weak time signal with a dial (analog) tuner. Reception with Timekubes and general-purpose receivers can be improved by connecting a copper wire, at least 50 feet long, to the receiver's antenna, and stringing it out approximately perpendicular to the direction of the transmitter (WWV is in Ft. Collins, Colorado, and WWVH is on Kaui, Hawaii). The wire should be suspended above the ground and attached to a tree or a wooden (or other non-electrically-conducting) post. Time signals are also broadcast at 5, 10, and 15 megahertz from Japan, China, and Russia, so WWV receivers are also useful in much of the rest of the world.

Unfortunately, short-wave time signals are no longer available in much of Europe, but accurate time can be obtained there from the German long-wave DCF time signal transmitted at 77 kilohertz. Conrad Electronic made a DCF 77 receiver, Best.-Nr. 19 89 35; it gives a digital LCD display of the time, and the DCF signal can also be fed into a speaker or tape recorder for an audible signal. The unit is no longer sold, but about 200 European observers bought it via group purchases while it was available.

Most observers find stopwatches most convenient for making timings. Unfortunately, most stopwatches now are digital. Stopwatches with dial displays (such as those on most mechanical stopwatches) are needed for some of the timing methods discussed in Van Flandern's Precision Timing of Occultations (P.T.O.). Methods that don't use a stopwatch include the Eye-and-Ear method and tape recording method, discussed in P.T.O. If a tape recorder is used, total occultation times should be marked with a mechanical clicker (such as a toy cricket) or buzzer (such as for a door bell) rather than with voiced calls. Voiced calls are sufficient for grazing occultations, where half-second timings are acceptable.

For reporting occultations, you need to determine your accurate geographical longitude, latitude, and height above sealevel. These can be measured from a large-scale topographic map of your area, preferably with a scale in the range of 1:50,000 to 1:24,000. Check your telephone book for map, engineering supply, or sporting goods stores in your area that sell these maps. College, University, and sometimes municipal libraries have map collections that you might be able to use to measure positions. The maps can also be ordered by mail, usually for less than the price charged by local dealers; index maps and order forms can be obtained from your national mapping agency. In the U.S.A., the address is: Map Distribution Office, U. S. Geological Survey (USGS); Box 25286, Building 810; Denver Federal Center; Denver, CO 80225; telephone 1-800-USA-MAPS. USGS service can be slow; in general, you are better off trying to find a local map dealer. But if there is no nearby map store, or an expedition is planned outside the area of their coverage, Richard Wilds suggests using Powers Elevation Co. in the Denver area for a quick response (they can get any USA map over the counter). Their address is P.O. Box 440889; Aurora, CO 80044; phone 1-800-824-2550. They accept credit card orders. Richard recommends ordering several maps at once to offset their $5.00 service fee per order. Richard finds they are very friendly and will send maps by overnight delivery. In Canada, contact the Canada Map Office; 615 Booth St.; Ottawa, ON, K1A 0E9.

For planning grazing occultations, smaller-scale large-area topographic maps are most useful; I prefer the 1:250,000-scale maps that show all rural roads and cover an area of 1 deg. of latitude by 2 deg. of longitude. These are also sold by dealers and the national mapping agencies. Some good maps are also available commercially, although they are not detailed enough for determining longitude and latitude to the necessary 1" (100 feet or 30 meters on the Earth's surface) for reporting lunar occultation timings. One of these is a series of atlases covering several of the states of the U.S.A. at 1:150,000-scale sold by the Delorme Publishing Co., Freeport, Maine, telephone 207-865-4171. After using these maps to select locations from which the graze might be observed, if time permits, it is helpful to also get the detailed large-scale maps of those areas. Then, you can find sites whose positions can be accurately determined and avoid newly-constructed buildings and roads that are not shown. In any case, you will need the detailed maps for determining accurate coordinates of those stations from which the graze is successfully observed. But take the large-area maps with you for selecting possible alternate sites, since bad weather or other last- minute problems can chase you away from your prime sites. Another source of large-area maps are aeronautical charts available at many airports; these are less detailed than the 1:250,000-scale topographic maps and not as useful for grazes, but they are useful for those who want to travel out of their immediate area with portable equipment for asteroidal occultations. Note that most civilian GPS receivers are NOT accurate enough for determining geographical positions for lunar (especially grazing) occultation timings. Simultaneous differential GPS measurements, from a know base station to a field station using two differential-capable receivers, are needed to obtain the 30-meter accuracy required for lunar occultation work. The situation will change if and when the degradation caused by Selective Availability (S/A) is turned off. Although a decision was recently made to phase out S/A, it will be a few years under the current plan before that will be accomplished, and in the meantime, some who want to keep S/A on could prevail. A detailed article about GPS use for occultations, published a year ago in IOTA's Occultation Newsletter, is available upon request.

Even if you don't join the International Occultation Timing Association, you might want to write to them at 2760 SW Jewell Ave.; Topeka, KS 66611-1614, enclosing $1.00 and asking for a copy of their most recent list of members. Perhaps one lives near you who could help you get started, or you could find others for organizing grazing occultation expeditions or attempts to observe asteroidal occultations that might occur in your region. Information about upcoming events can be found by telephoning the IOTA occultation line at 301-474-4945 in Greenbelt, MD. For e-mail notification of updated asteroidal occultations, send a message with your location and brief telescope description to the e-mail address below.

Good luck with your observations! David W. Dunham 1996 April 12
email: dunham@erols.com


TOTAL OCCULTATIONS:

If you do not have the means of generating your own total occultation predictions (e.g. David Herald's OCCULT program), or there is no one in your area to run predictions for you, send your requests to:

Walt "Rob" Robinson, 515 W. Kump, Bonner Springs KS, 66012, USA
phone: 1+913-4221280; Email: webmaster@lunar-occultations.com

Please include your longitude, latitude and elevation above sea level; size and focal length of the telescope; and your experience with timing occultations. I can return your predictions either by email (.txt or .zip file) or by US Post Office (be sure to include your address).

GRAZING OCCULTATIONS:

Below are the Super Standard Station assignments for 1999. Please contact the person who computes the predictions for your area:

               SUPER STANDARD STATIONS
                                  
The coordinates of the centers of all super standard
station regions for which 1998 graze data were computed,
or might be computed in the future, are listed below.  
The Grazereg graze coverage selects all grazes usually 
within 650 miles of these coordinates. 

REGION LNG. LAT. COMPUTOR  AREA

A     75.0W +41.5 Timerson ne USA, Ont., s. Que.
                  Bader    CT, DE, NJ, PA, and RI
                  Dunham   DC, MD, NC, and VA
                   [so Timerson should get stations in
                    IN, KY, MA, ME, MI, NH, NY, OH, VT, 
                    WV, and Ontario (ON) & Quebec (PQ)]
B    115.0W +37.0 Dunham   SW USA; NW Mexico
C     95.0W +43.5 Senne    Midwest USA; Manitoba
D     84.5W +30.0 Kazmierczak  SE USA - all but FL
                  Campbell Florida (FL)
E    101.0W +32.5 Hutchinson  South central USA
F    115.0W +47.5 Hutchinson  NW USA; SW Canada
H    162.0W +18.0 Senne    Hawaii
I*     1.5W +40.0 Gomez C. Iberia
J     48.0W -26.0 Lourencon Southern Brazil
K    115.0E +26.0 Yue      SE China; Taiwan; Hong Kong
L     64.0W -33.0 Lourencon Chile; Argentina; Uruguay
M     31.0E -16.0 Fraser   Malawi; Zimbabwe
N    173.0E -41.0 Kruijshoop New Zealand
O    134.0E +37.0 Soma     Japan except Hokkaido
P     66.1W +18.4 Senne    Puerto Rico; Hispaniola
Q    122.5E +12.0 Morgan   Phillipines
R     40.5E +53.5 Riedel   w.Russia,Belorus,e.Ukraine
S     25.5E -29.5 Fraser   South Africa
T    147.0E -22.5 Kruijshoop Queensland
EUR   Europe "box" Bode     western and central Europe
                  Elliott  United Kingdom
                  Bulder   Benelux -NADIR
                  Gomez C. Iberia (I-region)
V    146.0E -36.6 Kruijshoop SE Australia
W     64.8W +32.3 Dunham   Bermuda
XB    61.0W +49.5 Dunham   Canadian Atlantic Provinces
XC    67.0W -18.0 Lourencon Peru and Bolivia
XD    70.0W  +6.0 Lourencon Colombia and Venezuela
XE    23.0E +63.8 Bode     Scandinavia; Leningrad
XF    42.0W  -5.0 Lourencon northeastern Brazil
XG    55.0W  +8.0 Dunham   Guyana, Trinidad, Barbados
XO    77.0E +28.0 Riedel   northern India; Nepal
                            Shankar might help
XQ   145.0E +47.0 Soma     Hokkaido and Sakhalin
XR    99.2W +19.5 Hutchinson  Mexico
XT    77.0E +13.0 Riedel   Sri Lanka, southern India
                            Shankar might help
X1   120.0E -29.0 Kruijshoop southwestern Australia
X5    17.5E -20.5 Fraser   Namibia; Botswana; Angola
Z     22.3E +45.1 Manek    w. Ukraine; central Europe
       (Czech Rep., Hungary, Slovenia, and countries in
        Europe east and south of them; Buromsky may be
        able to help for Ukraine)

* indicates a region for which data have not yet been 
computed for 1998 and are needed from E. Riedel.    
In some cases, more than one computor calculates
predictions for the region. In these cases, the computor
covering the largest area is listed first, and the whole
region is described for him. However, he is responsible
for the region minus the areas covered by the other
computors for the region. Predictions for regions M, P, 
Q, and XC have not been computed since they contain no 
current IOTA members who have reported coordinates to 
us, but they could become active during 1999. 

Addresses of Computors (Note that several e-mail 
addresses have changed from a year ago):

Clifford Bader; 1209 Gateway Ln.; West Chester, PA
19380, USA; phone +1-610-6967361; email cjbader@aol.com

Hans-Joachim Bode; Bartold-Knaust-Str. 8; D-30459 Hannover, Germany; 
phone +49-511-424696; E-mail: IOTA@kph.de 

Henk J. J. Bulder; Insteek 44; NL-2771 Boskoop; Netherlands; phone 
+31-1727-11870; E-mail: HJJBulder@compuserve.com 

N. Buromsky; Astronomical Obs., Kiev Univ.; Observator-naya 3; Kiev 
254053, Ukraine. E-mail buromsky@aoku.freenet.kiev.ua 

Tom Campbell; 13418 Thomasville Circle; Temple Terrace, FL 33617;
USA; phone +1-813-9851842; E-mail: thcamp@tampabay.rr.com

Andrew Elliott; White Lodge; Bank Lane; Warton; PRESTON; PR4 1TB; UK;
Phone: +44-(0)1772-632450
Email: ae@f2s.com


David W. Dunham; 7006 Megan Lane; Greenbelt, MD
20770-3012; USA; phone +1-301-4744722.
E-mail: dunham@erols.com

Brian Fraser; 77 Marathon St.; Kensington 2094; Republic
of South Africa; phone +27-11-6152396 or via M. D. Overbeek,
+27-11-4536918.  E-mail fraserb@intekom.co.za

Jose Gomez Castano; C/.Galicia,17-4D; E-28943 Fuenlabrada (Madrid); 
Spain; phone +34-1-6082455; E-mail jgcas@compuserve.com

Wayne Hutchinson; 2123 Elmgate; Houston, TX 77080-6428; USA; phone 
+1-713-8270828; E-mail: mwhutch@net1.net 

Mike Kazmierczak; 624 Edgewater Circle; Conyers, GA
30094; USA; +1-770-7608502; 

Alfred Kruijshoop; Telecom Research Labs.; P.O. Box 249;
Clayton, Victoria 3168; Australia; phone +61-3-2770139. 
E-mail: a.kruijshoop@trl.oz.au

Romualdo Lourencon; Rua Sao Jorge, 125 Centro; 13200-220 Jundiai SP; 
Brazil; phone +55-11-4348854; E-mail: lourencon@uol.com.br

Jan Manek; Stefanik Observatory; Petrin 205; 118 46 Praha 1; Czech 
Republic; E-mail: jmanek@mbox.vol.cz

Arun Shankar; New Delhi,India; email ashankar@idcindia.com

Mitsuru Soma; National Astronomical Observatory; Mitaka-shi; Tokyo 
181, Japan; phone +81-422-343788; E-mail: somamt@cc.nao.ac.jp 

Walter V. Morgan; 3534 Churchill Ct.; Pleasanton, CA 94588-3413; 
USA; phone +1-510-8463199; E-mail: WVM13@aol.com

Eberhard Riedel; Schubertstr. 7; D-80336 M?nchen; Germany; phone 
+49-89-532128; fax +49-89-534552; E-mail ERiedel@compuserve.com 

Joseph Senne; PO Box 643; Rolla, MO 65401; USA; phone
+1-314-3636233.  E-mail: senne@umr.edu

Bradley Timerson; 623 Bell Rd.; Newark, NY 14513-8805; 
phone +1-315-3317110; E-mail: bwtimer@eznet.net

Eugene Trunkovsky; Sternberg Observatory; Moscow Univ.;
Moscow, Russia; E-mail tem@sai.msu.su

Richard Wilds; 3630 S.W. Belle Ave.; Topeka, KS 66614-4542; phone 
913,271-7187; E-mail darkmatter-at-hart@worldnet.att.net 

W. C. Yue; 1409 Hing Chung House; Mei Chung Court, Tai Wei; Shatin, 
N.T.; Hong Kong, China; phone 3-7152075; E-mail 
astronet@hk.super.net


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