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)
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
- 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.
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.
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.
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.