Safe Observation of an Occultation

Safe Observation of an Occultation 2016-10-07T17:28:06+00:00

HOW TO SAFELY OBSERVE ASTEROID OCCULTATION

 

First, assess the prediction for the occultation found at: www.asteroidoccultation.com and understand the circumstances of the occultation. Examine the map that illustrates the overall path and expected path error. The DETAILS file has the star position, path width, error in the path, latitude/longitude of the shadow and times of central occultation as it moves across the country, elevation and azimuth of the star, and elevation of the sun. Star charts are posted showing different views (wide angle, 15 degrees, 5 degrees, 2 degrees, and 30 arc minutes) of the star to be occulted (the “target star”). It is not necessary to see the asteroid, which can be many magnitudes fainter than the target star. But if you do, you will see it gradually approach the star in the hour prior to central occultation.

SAFETY PRECAUTIONS (A to Z)

All participants in occultation expeditions should recognize and understand the following before venturing to an occultation site.
Occultation expeditions are strictly voluntary activities the data from which could result in contributing to important science research programs for asteroid research.  You are responsible for your own actions and for making decisions that may or may not affect your safety and the safety of others.  Safety is the number one priority in occultation observing with mission success being number two.  If you cannot proceed with the expedition without putting yourself or others in an unsafe position, you should not proceed.  An example of this might be going on an expedition the day after being released from hospital.
Most observations cannot be made at home. Travel is often required/mandated by the location of the occultation path and you will need to transport yourself by some method or through the efforts of others.  Since transport is most often by surface motor vehicle you must observe proper speed limits and consider the timeline in getting to/from a site.  If it is not possible to safely make it from your point of origin to the observation site, then you should not proceed. Good judgement is inherent in this process.
Your site assignment may be made by you or someone else. If by someone else, note the offset from the centerline prediction and consider more than one site on a different highway/roadway in case the primary chosen site cannot be reached.
You can check out possible sites by using the respective Google Map associated with it by consulting http://www.poyntsource.com/New/Global.htm.  You can use the occultation-specific map to zoom in on possible sites, though sometimes this information is incorrect depending upon how long ago the imagery was updated.
If you are planning either to set up a remote site or to stay at a fixed location for any period of time you should consider the following points. These are provided to you to help keep you safe and to maximize the chance of mission success:
a. tell others where you will be going and estimate the time of your return
a1. make a checklist of all the equipment you plan to take with you and use it before you leave for your site! If you are able, have a backup plan to observe in case your primary plan fails. That is, consider the various failure modes that would cause you to have to abort after you have left home. For example, you leave your eyepieces, connector, chord, finder scope, star charts, chargers, extra batteries, laptop (if required), smart phone, cell phone, cash for road tolls, etc.
b. take a cell phone or other form of communication device being sure it is in proper working order; give other observers or family members your contact number.
b1. some sites may have to be operated in areas where there is no internet/cell service reception Do not depend on this for your information!
c. take appropriate hard copy (and/or electronic) maps or information that will properly guide you to/from your destination including meeting point or site(s). You want to minize the chances of getting lost.  If you have an internet connection you can use this to consult the interactive Google Map to find a new site or check local weather radar / satellite data to attempt to negotiate bad weather.
d. recognize that once you reach a site you may have to may even have to replan in real time for one reason or another (weather, traffic, etc.). Allow enough time to abort the preselected site and move to another location.
d1. to avoid running out of time, always set your watch/clock to a recognized time source (if your watch/clock is not already synchronized automatically). It is very easy for a watch to drift in time after having been set months or weeks earlier. Shortwave time signal station WWV on 5, 10, 15, and 20MHz in the US is a good source as is CHU at 3.333, 7.850, 1and 4.67MHz in Canada.
e. if possible, select or check out any site(s) in daylight. That way you can spot potential hazards such as fallen trees, construction, proximity to bars or other lit up structures, uneven ground, fire ants, swampy surface, areas where rain can accumulate, unsafe distance from a roadway or railroad, cliffs;  keep away from properties with barking dogs. Be aware of the potential for poisonous snakes or other wildlife threats in the site area.
e1. animals can be active in remote areas at night. If you leave an unattended station, ensure that you have set it up in such a way to prevent animals from knocking it over or to place a barrier around it.  Carry extra equipment in case one or more unattended stations are damaged, destroyed, stolen.
e2. sometimes the best remote sites are located along roads with property controlled by the military. In these cases you may expect a visit from military police. Be prepared with a written handout or plan for a potentially extended explanation and delay.  Never set up inside military zoned property. Even outside the fence line this could be an issue but is less likely to cause you to have to tear down a previously set up site. If leaving equipment, place a reusable sign next to the gear so that anyone stopping by will be convinced to not disturb it.
f. if you are only able to select a site at night be sure to bring a bright enough portable light to examine the surface for the applicable concerns above before the site is selected and use the same for insuring you have collected all of your gear before you leave.  Be sure you check out the flashlight and battery state before you leave the house.  In humid environments watch for areas of fog as you are driving to your site. This may indicate better spots to set up than your selected site.
f1. in humid areas, as time progresses, watch the surface of your optics and/or car windows for evidence of dewing.  You should plan to carry a 12VDC portable hair dryer to keep your optics clean, have a viable dew shield or electric dew heater for your scope.  This means you should not set up your optics too early to help preclude formation of dew.
f2. having a small flashlight that can be strung around your neck is very helpful, rather than one you have to pick up and put down each time you use it.
f3. if dewing begins to occur it may help to cover the optics with a plastic bag. If optics such as binoculars become dewed, consider using your internal car heater to blast the surfaces. That will keep  the optics clear for a while.
g. keep your site hidden as best as you can from oncoming headlights
h. avoid setting up on private property unless you have obtained prior permission from the owner. If you accidentally set up on private property it might be a good idea to have a handout which you prepared explaining what you are doing there so you do not have to waste valuable time.
i. consider to notify local law enforcement that you will be in the area.
j. be sure your vehicle is properly maintained, checking tire pressure, gas tank and other vehicle parameters before travel. Be certain you have a spare that is fully inflated and information on AAA or a towing company in case you are stranded.
k.take necessary provisions and dress appropriately for the temperature if planning to spend many hours on this expedition
l. take a watch (Android or IPhone) or other device calibrated to WWV or other time signal
m. if wind is expected consider to set up behind a structure, to bring sandbags, and to use your vehicle as a wind break.
n.always park your vehicle well off the roadway so that it does not pose a hazard or attract unwanted attention checking out the forward surface or hazards such as deep sand, holes, inclines, ditches, etc.
o. maintain a reasonable level of quiet at the site also to avoid attracting any attention
p. never consume alcoholic beverages before or during the trip
q. avoid taking any medication which will make you drowsy before or during the trip
r. do not take anyone with you who may use illegal drugs
s. do not take children with you who need supervision or who may cause distractions during the observation; try to take ONE companion.
t. if you are driving your own vehicle with or without other persons, you are responsible for your own safety and that of passengers. Exercise safe driving habits.
u. take identification and proof of insurance with you; take only enough cash and perhaps one credit card that might be needed on the trip.  If you will encounter toll roads be sure you have proper method of payment.
v. determine more than one route (if possible) that will get you to your destination to protect against road closures. If living in a highly populated area be sure to consider traffic jam issues and time of day/night when it might be best to avoid them.
w. leave the observation site as you found it. Remove all trash.
x. if your site becomes ‘unsafe’ for whatever reason after you have selected it, have a back up site in mind and move to it as quickly as you can based on the timeline of events.  This means if you “feel” a threat from any source, act on it and get out of there. Point your vehicle in a way that does not cause you to waste time. You should be able to drive straight out of a site.
x1. never park on a road that has no shoulder or set up a site on any kind of a road that is in use.
x2. if someone has assigned you a site that seemed to be good initially, feel free to move your site (preferably not more than a mile or so) if in your judgment the site is clearly not suitable.
y. if you are on the road and find yourself unable to reach your site in the proper time, consider to make a real time decision to set up at the nearest possible location even if outside the predicted path. Better to have a miss observation than not to observe at all. Besides, occultation prediction ground tracks are never 100% accurate including the error bars.
y1. if you live in a metropolitan area avoid leaving for the site such that going through town will result in your getting trapped or delayed in rush hour traffic.
y2. if traveling near the Mexico border you may encounter an immigration check point. If you have deployed more than one site along this road, ensure you have enough time to make it through the checkpoint assuming you have to travel by it more than once for set up and activation.
z. if it is legal to carry a firearm or other form of self-defense weapon, consider doing so.

A) YOUR EYE AND A TELESCOPE

For stars that can be seen in binoculars (i.e. brighter than 7th magnitude), you can use a pair of binoculars only if they are mounted on a sturdy tripod. I recommend 10×50 but any good set that will allow you to easily view the star should work. But, the observer must be seated during the occultation in a comfortable position since he/she must watch the target star to be occulted without interruption for 4 minutes: 2 minutes before and after the predicted time of occultation. If the elevation is higher than 40 degrees it is recommended to use a telescope on a clock driven mount or else serious discomfort can result to the neck unless the binoculars have right angle prisms or the observer uses a chair that is inclined.For stars to +12 magnitude, I recommend to use a Celestron 8 or Meade 8 telescope on a motorized mount. A right angle finder is a must. Also, the finder and scope must be boresighted well before the evnet. This is incredibly important. In addition, the finder scope must be able to be focused! The tracking must work well and any eyepiece used should be wide enough to allow you to track the star with comfort and see at least 3 or 4 comparison stars at the same time. Minimum observer skills needed:

  • ability to navigate the night sky and to use star charts to locate the star
  • ability to point binoculars or polar align telescope
  • ability to locate a safe, dark site from which to observe
  • ability to be mobile (optional) which increases your odds of getting away from clouds and lights

Minimum equipment needed:

  • portable cassette tape recorder with cassette rewound and fresh batteries installed
  • shortwave time signal receiver capable of picking up the main frequencies of radio station WWV = 5.0, 10.0, 15.0 MHz (or equivalent time signal frequency in your area) for observers in North America
  • star charts showing the star to be occulted (available from http://asteroidoccultation.com/ after the occultation prediction has been posted to the web
  • topographic map or GPS receiver or locate yourself at a physical address from which the latitude/longitude can be obtained by entering the address into http://www.mapsonus.com; an accurate position is important for analysis purposes
  • extra batteries for radio and recorder
  • digital watch to keep track of time; watch should be set to WWV
  • flashlight
  • if using a clock driven telescope be sure it can be powered in the field from 12v and that you have packed all necessary accessories including multiple eyepieces, attachements, chords, light shield, etc.

Go for it!:

  • Notify occultation organizer where you plan to be located (unless you have already agreed to occupy a site assigned by the organizer) and what equipment you are using.
  • Locate the star at a few days to a week prior to the occultation so you can familiarize yourself with the difficulty/ease in locating the star. If you don’t do this you are risking failure right off the bat! It is vital to become familiar with the field of view before the night of the occultation.
  • Check out all of your equipment to be sure it is working properly!!
  • Scout a location at night in the days before the occultation if that is possible. Never set up on someone’s property without proper notification. If you are forced to find a site in an unfamiliar area, obtain permission to use private property.
  • On the day of the occultation, watch the weather and consider where to go in to ensure best chances of observation if clouds will threaten.
  • On the night of the occultation:a. notify local police that you will be in the area and bring a companion with you and cell phoneb. be set up 2 hours before the occultation c. at least 10 minutes before predicted time of central occultation: run a test to be sure your voice and time signals record at the same volume level d. 2 minutes before: start radio and begin nonstop observing of the target star; look for one or more disappearances or magnitude drops in the target star and call out ‘out’ when it occurs, and ‘back’ when it reappears or regains its normal brilliance. Estimate your reaction time for each event as it happens. e. 2 minutes after: stop observation
  • Next morning: notify the occultation organizer of your results whether positive or negative. Provide your site coordinates and times obtained as well as any comments on timing errors.

B) VIDEO AND A TELESCOPE (STARS TO +12 MAGNITUDE)

Here is my vision of a “standard configuration”, but there are any number of telescopes that can be used. You need enough aperture to do the job correctly. I recommend either a Celestron 8 or Meade 8 telescope. It must track so you don’t have to push it by hand. The telescope should be mounted using a wedge and tripod (not shown) and have the capability to be powered from a battery, preferably a cigarette lighter plug that lets you get automobile power or portable 12VDC long life battery that can be recharged as required. This view shows a C8, the right angle finder and a right angle eyepiece for optical timing of an occultation. A finder scope is mandatory and it must be boresighted with the telescope BEFORE the night of the occultation. “Boresighted” means when you look through the finder and center it on an object, it appears also in the telescope eyepiece. The finder should be able to be focused clearly and locked in place with small screws as in the figure below. You can easily do that during the day time. A right angle finder is preferred to make high elevation occultation target stars easier to locate. The limiting magnitude under clear skies of this setup with a PC164 camera is around +12 magnitude.

For dew prevention, I recommend a dew cap and Dew Zapper available from Orion Telscopes. The Dew Zapper is powered from a cigarette lighter plug. Alternatively you can use a portable hair dryer but this requires a lot of power and may not be as convenient as a Dew Zapper. The Dew Zapper wraps around the metal tube just below the corrector plate while the dew cap sits on the edge of the telescope and is held in place by Velcro.

If you are using both of the above and powering them via the cigarette lighter of an automobile, you can use a y-plug available from Radio Shack (shown below). A car battery can run the C8 clock drive for hours without impact to the car battery and a drive corrector makes that possible (shown in lower right portion of next photo).

You must polar align the C8. If using a tripod/wedge, it is also useful to have a door stop or fulcrum so you can tilt the tripod leg facing north to get the north star in the center of your finder without having to keep adjusting the wedge to the right latitude.

Wide angle binoculars are useful in locating the field before you proceed further. Use star charts provided through www.asteroidoccultation.com. There are 5 levels: wide angle, 15 degree, 5 degree, 2 degree and 30 arc minutes. Fixating on the wrong star is always a possibility. Confirm for sure you are looking at the correct star by verifying the presence of other key star groups to the north/south/east and west of what you think is the correct target star using the 2 degree chart. Have several eyepices available when locating the star field. If you plan to visually time it rather than using video, also have a right angle viewer. Be sure to use good quality eyepieces! I suggest a combination of 32mm, 24mm and 12mm. (not shown) Visual timing must be recorded on tape, preferably a portable battery powered one with a microcassette tape. (not shown) To get your precise geographical position, a GPS receiver is recommended. It will typically have an accuracy of 3m. Record your latitude and longitude in degrees, minutes, seconds and tenths of seconds. Altitude is normally not necessary. Almost any model is acceptable.

Other equipment can be very helpful: a compass, inclinometer and a set of Allen wrenches. All can be found at hardware stores. The compass can be used in daylight to help point the mount to true north, while the inclinometer can be used to tilt your scope to the correct latitude. Allen wrenches are vital for many things such as loosening the wedge, securing the wedge to the tripod, etc.

The most economical and sensitive TV camera is a black and white model PC164 available from www.supercircuits.com. in Austin, Texas. This camera can reach 12th magnitude under good sky conditions. Moonlight, twilight, sky conditions can all impact the ability for faint star detection. To attach it to the C8 or M8, use a T to C adapter available from many telescope dealers.

The PC164 is a tiny, lightweight, and sensitive plug and play device and must be powered by 12V. The black chord connects to 12VDC, the yellow BNC connector is the video OUT chord. You can also obtain a 12V battery and charger system from  www.supercircuits.com. Next you need a VCR, also preferably portable battery powered. A camcorder is ideal, but choose the camcorder carefully to ensure it will take an RCA plug for VIDEO IN recording. SONY makes a wide variety of camcorders and many of them have the ability to record from an external camera. Below you can see how the RCA jack connects from the PC164 into the Sony camcorder.

To record your voice and time signals you can get a lapel microphone (upper right part of photo below) also from www.supercircuits.com. Then you can power both PC64 and microphone using a Y-chord from the same 12V battery. This chord is also available from www.supercircuits.com. The camcorder should use HI-8 or 8mm tape though at some point we should transition to a more long lived medium such as DVD. Be sure that the camcorder has a pullout display screen or you are somehow able to route the video into an external monitor of some sort. Most camcorder view screens of 2.5 inches in diameter are sufficient to get a decent focus and verify you are on the correct star. The photo below shows the video system and microphone where both the PC164 and mike are powered jointly by it, while the camcorder has its own internal battery. Follow the chords. The battery chord goes to the base chord of the y-cable. The one part of the y-cable leads to the lapel mike, the other leads to the video camera. But the video camera comes with a BNC connector(female). You need another chord with a BNC connector (male) on one end and an RCA jack on the other. You may have to buy a chord with RCA jacks on both ends, then a separate male BNC connector that will simply slip on to one RCA jack to make this all work. The power chord of the video camera connects to the other part of the y-cable while the RCA jack takes the video signal from the video camera into the camcorder.

Time signals must be recorded through the camcorder audio channel. Use a short wave radio battery powered that can receive 5.0, 10.0 and 15.0 MHz. You need three frequencies to account for changing atmospheric conditions. Get a radio that is digital and not one that you are required to tune using a hand-turned knob. You can typically find short wave radios at Radio Shack and some boat supply dealers.

A rat’s nest of wires can result from this configuration; so you must ensure that most of the parts are contained in a briefcase and set on a table about the same height as wedge. Use twist-ties to keep the wires from getting tangled in the dark. You should endeavor to sit during the observation and be able to have ready access to the slow motion controls on the telescope in order to make minor adjustments to keep the star centered in the field of view. Try to keep at least 2 other stars in the field in order to detect seeing or transparency impacts as well as a reference in case clouds drift through. Rotate the camera on the T to C adapter so that the resultant field of view in the camcorder display screen matches the star chart. Keep the star focused at all times. Focus can change unpredictably. Avoid having to make tracking adjustments during the minute preceding and following predicted central time of occultation. This can be assisted by ensuring the mount is aligned to the pole properly and the motor is tracking well. Since the PC164 camera is lightweight, there is no need for counterweighting the telescope.

C) VIDEO AND A TELEPHOTO LENS (STARS TO +7 MAGNITUDE)

I use a camera lens instead of a telescope and my preference is for Nikon but any removable camera lens will work. For the example here, all that is needed is a) a tripod b) the PC164 video camera c) a Nikon telephone lens (here is shown a 75-150mm f/3.5 zoom lens). A zoom is good because you can change the field size based on the brightness of the star. d) a c-mount adapter for Nikon lenses I don’t want to repeat the instructions for getting power to the camera and the video signal into the camcorder, so see the above section for that data. You will need the camcorder, lapel microphone, RCA to BNC chord, y-cable, 12V gel cel battery. The lens to choose is based on your sky and the brightness of the target star. It pays to try two or three lenses to make the optimal decision. In the first photo the components are laid out of the floor before assembly.

Here they are tripod mounted. The PC164 has 1/4-inch 20 threads/inch adapter which fits the standard tripod. You may want to use regular or lock washers if needed to get a tighter connection.

Ideally, you should have two of everything. That is usually not possible, so as a backup plan you should go ‘visual’ if a critical component in the video recording chain causes an unrecoverable failure. At a minimum I recommend two 12V batteries, two microcassette tapes (for tape recorder), two video cassettes.

TIMELINE OF AN OCCULTATION

The following is detailed information that you can use to train yourself and others in organizing for an asteroid occultation. O-7 days (where “O” means occultation): email or phone alert issued/IOTA web site is updated. This update might also come even closer to the date of the occultation. O-7 days to O-1 day: locate the target star. Practicing ahead of time is invaluable and can save the day. PRACTICE! If you are training others, finding the star before occultation night will be like an investment in success. There are many times when clouds are near the star on occultation night and locating it can consume lots of time. The star might be also be situated in an area of the sky devoid of other reference stars or it could be in a very rich area with confusing star patterns. Other factors such as proximity to the moon or twilight makes early detection very important.   O-1 day: determine where you plan to observe. If from your house, notify the local coordinator so we will know that that part of the occultation path is covered. Send either GPS coordinates or an address. If you plan to be mobile, also notify the coordinator so you can be assigned a site area.   O-2 hours: you should be set up at your chosen site by this time. Polar align your mount and be sure your clock drive is working. If using a Schmidt-Cassegrain watch out for dew formation. Use a ‘dew zapper’ or portable hair dryer to mitigate this possibility. Be sure you have the following: at least 2 eyepieces (wide field), bug spray, right angle finder, red flashlight, binoculars, copies of the star chart, tape recorder with fresh batteries and tape rewound to the beginning, short wave radio source of time signals. This latter item is the key to precision timing.   O-30 minutes: find the target star; observe from a comfortable position either sitting or standing that will not cramp your neck. Be sure your hands are free as possible.   O-20 minutes: test your voice and the radio and recorder to be sure the time signals and your voice evenly record. Play back to verify. If one is drowned out, reposition and repeat the test until you get it right. Be prepared to comment on passage of clouds, distractions, seeing changes, stability of the target star, etc. during your 4 minute observation window. Be quick about it and be aware to quickly call out when the star disappears and reappears. Use either “D” and “R” or “out” and “back”.   O-2 minutes: begin continuous observing. Start radio and recorder (or video). If the radio fades in and out, another person at your site could assist in helping get the signals back.   O+2 minutes: end observation. Turn off radio and recorder (or video).   O+1 day: within 24 hours report positive or negative observation to your coordinator. Provide your name, exact location of site, telescope info and duration of any event(s) seen.   O+3 days: fill out the online IOTA reporting form and submit to the occultation organizer or IOTA.

PITFALLS OF POOR PLANNING

The following is a list of possible excuses as to why observers failed to see an occultation when otherwise, they should have been successful. They are based on my personal experiences:

  • did not find the star prior to the night of the occultation
  • misidenitifed the target star
  • used someone elses telescope with which they were unfamiliar
  • did not use a telescope with a clock drive
  • had a battery failure associated with portable short wave radio, tape recorder, or video system
  • Schmidt-Cassegrain telescope dewed over because it was left out too long and too far ahead of occultation without being covered
  • eyepiece being used either had too high or too low magnification
  • no finder on telescope or the finder was useless in helping to locate key stars
  • finder and telescope were not collimated
  • a key screw or wrench was not present when assembling the telescope
  • polar alignment not successful because Pole not visible and no compass was available.
  • tape recorder ran out of tape
  • radio drifted off frequency and no time signals were recorded
  • lights from police car or other approaching cars blinded observer at key time
  • observer was held at bay by armed person when trespassing without permission on private property in the middle of the night
  • vibrations from passing trucks shook instrument
  • seeing conditions were so poor that image of target was not stable enough to ascertain an occultation
  • temperature so cold that observer could not concentrate for long period due to inadequate clothing
  • jacket held over observer and eyepiece to keep out nearby lights caused fogging of eyepiece due to temperature/humidity combination
  • dark adaption lost due to not using a red flash flight
  • telescope tripod shifted due to being set up on unstable ground
  • observer lost concentration due to being set upon by hordes of mosquitoes and not having repellant
  • star fainter than predicted and observer confused
  • observer unsure of occultation because depth of drop was less than one magnitude
  • tape recording failed to record observer’s voice because time signals were too loud and no test was done before hand to ascertain equal level of recording between voice and radio
  • occultation not seen because observer was talking to people at the site and not paying attention; lost track of time
  • telescope aperture inadquate to see a star that dim
  • too large a scope used and target star could not be properly confirmed
  • “GO TO” telescope computer would not work and observer had no experience using the telescope to manually star hop to the target star. Excessive dependence on technology!
  • image intensifier battery failed and no spare was available
  • occultation occurred at too low an altitude and it was not possible to find the star in enough time
  • occultation occurred in too bright sky and this was not anticipated before the observer attempted the event

Please pay attention to the above pitfalls and you will likely never have to worry about failure again.

STRATEGY FOR ORGANIZING A GROUP OF OBSERVERS

The best way for amateur astronomers to map an asteroid topography is to enlist the aid of a group of likeminded individuals who are willing to engage in an organized effort to time the event from equally spaced locations perpendicular to the path of the asteroid shadow. Since the path is known to within a certain level of accuracy (e.g. plus or minus 1.0 path widths or less), the idea is to first choose an event with the least error in minor axis. Assuming there are just 3 observers the best strategy for this case is have an observer at the center, one 1/2 path width north of center, and the third 1/2 path width south. I first described this strategy in articles in the early 1980’s. If you have 10 observers, for example, you might distribute them at equal distances north and south of the center so that each observer has no chance of duplicating another’s observation. This strategy breaks down when groups in differernt US states try to observe. In many cases, observers might have fixed locations that make it impossible to move; other observers have restrictions on the distance they can travel based on personal and other commitments. There are geographic considerations. If on the big island of Hawaii and we consider the following diagram, you can see that island topography, the ocean, altitude and cloud buildups dictate careful planning. The path is shown crossing the island. In this case the width is predicted to be 180 miles; setting up sites on this island allows a limited coverage over the southern portion of the asteroid.

In the diagram the lines are 10 miles apart relative to the predicted center line. But the elevation of the target star is just 30 degrees above the western horizon. Due to mountains, setting up sites along the east coast is inadvisable. Assuming there are just 7 observers, sites at 10 mile intervals can be assigned. For additional observers, the organizer can make assignments at perhaps 7 to 9 mile intervals, for example. The value of setting up observers in this fashion is that they can be complemented with observers on other islands where overlap is not possible due to the angle of the path as projected onto the earth (see the inset of the other islands in the upper right part of the above diagram).

MORE DETAIL ON HOW TO OBSERVE AN ASTEROID OCCULTATION

The ground based effort, primarily by amateur astronomers, continues to make strides in improving our knowledge of the asteroid population. Equipment used has varied from small telescopes and tape recorders (employed to record disappearance and reappearance information) to video cameras and image intensifiers. This is an excellent application of scientific field work that either an individual or an astronomical society can undertake. Since most stars occulted by asteroids are between +9 and +12 magnitude, it is necessary to develop the necessary skill to locate relatively dim stars, or else to have a telescope that is capable of navigating to a set of coordinates using computer input. Still, you need to be able to interpret the orientation of a star chart, a finder eyepiece, and a telescope eyepiece effectively due to inversion of optics which will show the star field in a different aspect from that shown on the star chart. It is sometimes easy to locate the wrong star, so double checking the target star by “star hopping” to it from more than one key star is a good aid in assuring you have not misidentified the target.

The International Occultation Timing Association (IOTA) linkweb site contains star maps and also updated information in the days prior to the occultation that will assist observers in knowing possibly where the predicted path has shifted. Serious observers should check this site several times a week prior to the occultation event. But, what if you look for an occultation and end up seeing nothing? There is a high value in obtaining a ‘negative observation’. It will actually be of benefit in confirming where the occultation did NOT take place. This can be especially useful in the case where the path is not known with any degree of certainty. Even if the path is well understood and you are predicted to be in the path and do not see any occultation, this negative data is extremely vital to the assessment or confirmation of other observations of either the primary occultation or a secondary event (meaning possible evidence of a satellite).

A typical timeline to observe an occultation might start out like this. Assume an event is predicted to occur at 9:04pm CST (03:04UT). You should plan to be at your observation site 2 hours before. This is necessary in order to maintain flexibility in case of changing weather patterns. Your location should already have been known by IOTA prior to the observation in order to preclude duplicate observations by people located too close together geographically. If not, be sure you know the latitude, longitude from GPS or from a good topographic map. If in extreme situations, time is running out and you are trying to outrun a cloud system, set up anywhere and worry about locating your exact site later on.

In humid areas, users of Schmidt-Cassegrain telescopes should expect fogging of the corrector and use a ‘dew zapper’ or hair dryer or dew shield. Success is enhanced if you have located the star at the same time of night at least a day or two before the event. Be sure you have assembled all of your equipment in their exact configuration at least a day before the occultation. Leaving one critical piece of gear behind, such as a tape, microphone, adapter, eyepiece, or power chord can be very devastating. Using a telescope with a clock drive will allow you to keep your eyes focused on the occultation instead of having to manually track the star. If observing from a fixed site, be sure the star will not be masked by houses, trees or other obstructions at occultation time.

On the night of the example occultation above you should locate the target star to be occulted no later than 30 minutes before the event. Begin observing nonstop from 9:02pm to 9:06pm; that is the predicted time +/- 2 minutes. Why such a long interval? To account for possible time errors in the prediction and also to search for satellite companions of the asteroid as well as to establish atmospheric seeing effects. For visual observers maintaining such a vigil can be quite a challenge to avoid eye fatigue. Hence using a video system conquers this problem since the video can record without error all during this period. It can also be replayed for analysis. The clear disadvantage to human observations is that there is no second chance and the observation cannot be visually replayed. There are also such things as human reaction times, possible seeing effects which may or may not be related to the occultation process. All of these are overcome by using video. At completion of the occultation, packing up is usually done within 30 minutes after the event.   Successful audio or video tapes should then be copied and sent to IOTA for analysis. The observer should always maintain the original unless requested otherwise. Again, I want to stress that if you DO NOT observe the occultation but you have watched during the key time, your observation is VERY important. It is critical to know where the boundaries of observation are located in order to establish upper limits on the prediction error and also the size of the asteroid. All observations made whether positve or negative should always be reported using the form on the IOTA website. Immediately after the event, send an email or notify your local coordinator of what you saw or did not see.

Mobile observers should exercise extreme care to drive safely to/from the site. Sometimes it may take extra time to locate a site which will eat into the setup time. Remember it is better to be safe than to take unnecessary risks with your own safety or those with you. Choose a site that is out of the way and free of lights and obstructions. Be sure it is not on private property without the owner’s permission. Notify the local police if that is feasible. Why? Many an observer has had a police car pull up moments before the occultation and this thwarted what might have otherwise been a successful event! Also it will possibly mitigate any phone calls from passers by reporting unseemly behavior along the roadside. If you pick and site and then have bad feelings about it, listen to yourself and find another safer location. Check out the site for presence of hazards such as barking dogs, fire ants, muddy ground, high tension lines, holes, other animal presence, barbed wire, and being illuminated by oncoming headlights. If it is windy, look for a wind break or set up your scope behind your vehicle. Always try to take another person with you for security purposes. Never carry a lot of money and bring a cell phone if possible. Carry a copy of the asteroid alert email, identification, and anything else that might prove to police that you are out there doing astronomical research.