GRAZE OBSERVER'S FIELD GUIDE
Being part of a Grazing Occultation observation team is interesting
and rewarding. This simple field guide was put together by members of the
Occultation Group of the Lunar and Planetary Section of the Astronomical
Society of Victoria. Your comment and suggestions are most welcome.
If you are in a hurry: go ahead and follow these simple steps:
Observe one star continuously over a 20 minute period, starting about 10 minutes before the predicted time. See the star first approach the Moon, then being occulted one or more times and finally move away. Over the full period record all observations and voice comments on a cassette tape recorder together with the accurate time signal from a nearby shortwave radio.
Preparation Check your equipment out
before leaving home:
Records Make a complete recording of all you see, use, and do;
Position Describe your position accurately on tape and on paper;
Afterwards After passing your results on to the coordinator, keep the original tape and a copy of your notes for some time.
WHAT TO EXPECT
* To find the star and see how the
Moon will move past it look at the "Moon Star Diagram", if we made one
for this event. The explanations and an example are on the next page.
On the later pages we have put together some of the things we learnt
from experience the hard way over the years. You probably don't need many
of these, because if all goes well, observing a graze is very straightforward
(weather permitting) but it never hurts to come prepared! Also
later in this guide is a sample observer's report.
USING THE MOON-STAR DIAGRAM: EXPLANATIONS AND EXAMPLE
A new Moon-Star diagram needs to be prepared for each graze, to show the different circumstances. It helps observers to find the star and to know in which direction it will appear to "skim" over the Moon in the telescope. Normally we provide two diagrams for each graze: one for Newtonians and another one for refractors and Schmidt-Cassegrains with star diagonals. Each pair of diagrams shows the relative positions of the Moon and the star, from 30 min before the graze until the actual graze.
Choose the diagram for your type of telescope. Then rotate the paper with the diagram until it looks just like the view in your eyepiece.
(A) The diagram "View in a Newtonian telescope" (or in a refractor or Celestron type scope used without the star diagonal) is for telescopes without any mirrors or with 2 mirrors only (Newtonian) that give a correct image. What you see could be upside down or rotated, but not flipped over.
(B) The other diagram "View in a telescope with star diagonal" is for scopes in which the light is reflected 1 or 3 times, so that you see a "mirror" image that is left right reversed, flipped over. The same is true for a right angle finder - that is just a small refractor with a star diagonal. In each diagram we show the N, S, E, W celestial (star chart) compass directions.
In the Moon-Star diagram you will
1. The dark part of the Moon shaded and labelled "DARK".
2. The star is fixed, and the Moon moves towards east across the sky. If your telescope has a motor drive: centre the star in the eyepiece and watch the Moon come closer. (of course, all the time, both appear to move westward with the whole sky due to the earth's rotation).
3. The star is shown as a * in its position relative to the Moon 30 minutes before the predicted graze time. The line of dashes/dots shows how the Moon's edge will approach the star over those last 30 minutes and roughly where on the Moon's limb the graze will occur.
Here is an old (made up) Example:
Graze of 30 February 1962, at 9.40 pm. as seen in a Newtonian telescope. The Moon was 42% sunlit and it was a southern limit dark limb graze.
* Voice on tape: important, keep talking, you cannot
have too much on tape. Tape is never wasted, bring plenty and use
it. If you see something: put it on tape. If you see nothing or no change:
say so on tape. If you think you just made a mistake: confirm that on tape,
to avoid confusion later. If you think that a particular disappearance
or re-appearance was not instantaneous, but gradual, say so immediately.
Even if you are not
sure, say that you are not sure and compare later with the results from other observers. Towards the end of the graze, when the star is already moving away from the Moon: keep observing for a few more minutes and comment on tape on the sky conditions: stability of star image (seeing) and transparency. Also say whether the outline of the dark limb of the Moon was (faintly) visible or not.
* When the Moon and the star come closer, the star will be more difficult to see. Before the actual graze starts, check the focus - it may have shifted a bit while the telescope cooled down. Precise focus really helps the star stand out as a sharp point in the glare from the nearby Moon.
* During the "central" few minutes, the period full of action, keep your eye glued to the eyepiece and keep observing and talking. It is extremely useful to continually keep confirming what you see, like "visible visible visible .....", "gone gone gone ...". If then an unexpected event occurs, this can be timed by noticing the first hesitation in the voice, which is often a fraction of a second before you are able to clearly say what you see.
* For the graze report we send to I.L.O.C. we need to for
(a) When you started monitoring the star,
(b) All event timings; each disappearance, reappearance, blink or flash is an "event".
(c) Any interruptions: always say on tape when and why you stopped temporarily; this could be due to cloud, or the need to sneeze, or insect in your eye and note when it resumed,
(d) The time observing finally stopped.
The graze coordinators will help you to get this information from your tape and put it on the standard report form to send to I.L.O.C.
* Start the tape well before any events are expected, say 20 minutes before the predicted "central graze time". For a 45 minute tape (1/2 of a C 90) this would give: about 15 minutes recording time before the events, then 5 minutes for highlights, and then plenty of tape (more than 20 min) to continue monitoring and recording additional information (sky stability and transparency, time check, temperature, position, scope details, special weather problems, etc) after the event, ie. after you see clearly that the star is separating from the Moon's limb.
* Before the graze, with all equipment set up and going, make sure that the tape is moving and not accidentally put in upside down. Use your torch to see that the spools are turning and that there is plenty of tape left!
* After the graze, describe on tape and make a sketch showing your exact position with respect to the centre of the road, nearby road junction, power pole, fence, gate, Telecom/Telstra marker, road sign etc. Look for anything useful: numbers on power poles, names on gates, advertising signs etc. To get the distances, pace (step) them out, or use a long tape measure. The accuracy required by I.L.O.C. is about 10-30 metres in all directions. In the field we like to aim at an accuracy of about 3 metres so that we do not add any uncertainty to the errors already present in 1:25,000 maps.
* Dust: Set up on the upwind side of the road, in case cars do come past.
* Trees: We want to observe the Moon for less than an hour, during which it only moves 15*. If you are setting up while it is still overcast, a magnetic compass can be useful, eg for aligning an equatorial mount. Note that at present in Victoria: magnetic North is about 11* East of true North.
* Power lines (Low or High Voltage): can be a problem in two ways: blocking the view or causing severe static on the short wave radio. If you need to use a site under lines for other reasons (wind break, clear view etc), then try your radio first (before setting up the scope) and walk around a bit to see if you can find a "quiet" spot. Another source of radio interference can be the inverter power supply to the drive motor of your scope; find that out beforehand at home.
* Visitors: We often find that a passing car stops and asks if we "are OK", because the driver thinks our car has broken down in the middle of the night and offers to help. If you tell them "yes, we are OK, no worries, thank you", most will go away. If the visitor does not leave, he/she is probably a local resident who shows an interest in what we are doing. In that case: explain what a graze is, what is being measured, that this is an activity of the Astronomical Society of Victoria, that we send our results overseas to professional astronomers at the U.S. Naval Observatory, the Royal Astronomical Society of New Zealand, the International Lunar Occultation Centre in Japan, etc. then the visitor will understand that you need to concentrate on observing for the actual graze period. After the graze, let the visitors (and their kids) have a look at the Moon through your telescope. They will be impressed - we might even gain some new society members!
* Concerned Residents: In remote areas, residents can be worried about "what those strangers are doing out there" and we have to appreciate their concern. We make an effort not to set up close to homes or farms, but some dogs have very keen ears and can notice us from a very long distance. If a local resident comes to investigate, first give him/her the full explanation as in "Visitors" above. In Victoria in the past, we have never had any serious problem and always found that a simple explanation and offer to look through the telescope will overcome any misunderstanding. However, if in the future we might come across a situation where residents are still worried or unsympathetic and they ask us to leave, then we politely leave, without complaints, problems or argument. We must respect their privacy, because it is their farm and livelihood or their home environment and we are only temporary visitors. Even if it would spoil an observation, we accept their request to leave, because maintaining good public relations is more important in the long term.
IN TIME OF CRISIS - DON'T WORRY BUT KEEP OBSERVING
If things start to go wrong, always (i) concentrate on observing, because the observation is the prime aim of the graze expedition and (ii) keep the tape recorder going right to the end of the tape because it is crucial for getting the full sequence of events. Even without a radio, a tape recorder running at constant speed can give good relative timing information. Then, finally, if even the tape recorder stops just before or halfway through a graze: keep observing, try to remember what you see and immediately after the event write it all down.
Some examples of mishaps that are important, but should not be allowed to interrupt the observation, are:
* If you have a motor drive, and it stops, don't try to fix it, simply move the scope by hand and keep observing - many of our graze observers do not have a motor drive (or use Dobsonians) and still get good results.
* If the radio fades, when the graze is just about to start, don't try to retune, but keep observing. By playing the tape back later on the same tape recorder and by comparing your results with those of neighbouring stations, an acceptable time scale can usually be worked out. Then after the end of the graze: keep the tape running and then adjust the radio, or read your watch on a full minute and say something sharp on the tape (plus explanation afterwards, of course). Then, only when you are 100% sure you have useful time marks on the tape, you can stop the recorder. Later at the meeting point compare your watch with a radio time signal provided by one of the other radios. Even better: if you know before the graze that your radio has died, but the tape recorder is still OK, then: read your watch onto tape a few times both before and after the graze.
* If, just before the graze, you think another eyepiece would do a better job but you can still see the star reasonably well: do not necessarily rush off to swap it, but keep observing - better a less accurate result than no result at all, because you could miss one or two events during the time it takes to change eyepieces.
Of course, all the above things can be fixed if you still have plenty of time left before the predicted time, but if you only have a few minutes or if you are already halfway the event, it is more important to keep observing.
To understand this advice, consider these two kinds of accuracy: (i) Your position determines the accuracy of the observation in latitude. Even if you set up in a hurry, finding the exact position of the telescope can be done later, by pacing out to the fence, power pole etc. Even if your timing has been disrupted, it is important to keep recording on tape because we can learn a lot from your results by comparing with the next stations to the North and South of you. (ii) Your timing determines the longitude of the features observed. It is excellent if you can produce results that are reliable to ±0.5 sec or even better. However, timing to one or a few seconds is still very useful and even rougher timing can be helpful, again when seen in comparison with nearby stations.
CASSETTE TAPE RECORDERS
* Length of tape: standard C 90 tapes (45 minutes per side) are good and give you plenty of spare time. Regular C 60 tapes are OK, but a bit on the short side. The long C 120 tapes are very thin and can give spooling problems; they can get tangled in the dark when it is hard to notice this.
* Microphone: Most simple cassette tape recorders and radio-cassette combinations (stereo or mono) have very good built-in microphones and are OK for graze work. Many recorders also have a socket for an external (hand-held) microphone, which can be used, but has no real advantage over the built-in microphone, except in the following case:
Some radio-cassette units, have a "dubbing" or "tape over" facility, but this only works when you tape from radio and use their external microphone socket at the same time. This means that you can get the radio plus the external microphone input on tape. If such a radio-cassette unit has a good short wave radio band then it can be used to both receive the time signal and record the voice as well. Very handy, as you have to carry only one piece of equipment.
* Signal meter or LED: Some recorders have a signal indicator, a small red LED, flashing to show the incoming signal. While not necessary, this can be handy in the dark, if each VNG seconds time marker shows up as a small flash of red. At least you then know that it got as far as the microphone and the amplifier. Still check with the torch that the spools turn. Some tape recorders have a "VU meter" that measures the signal, but need to be viewed with a torch. Some others have an "audio monitor" socket where with an earphone you can hear what is being recorded.
In Australia and New Zealand, the best signal is typically VNG, transmitted from eastern Sydney. Most simple domestic portable radios with some kind of short wave band have no problem receiving VNG on 5.000 MHz (or 8.638 MHz) at night. A self-adhesive label on the radio tuning scale with a pen mark will assist finding the right spot on the scale. But if you have a short wave receiver, especially one with a digital frequency read-out, that is very useful and will make it easier to tune.
If at any time you cannot find VNG, it is possible to use WWVH from Hawaii (with female voice) or WWV from Colorado (male voice) on 5, 10 or 15 MHz, but they are much weaker in Australasia and often mixed with tones and signals from other sources (Japan, China). If you have used WWVH or WWV, ask for help in reading out your tape, because the minute marker can be confusing.
In practice, travelling to a graze and making the arrangements on-the-spot always takes much more time than you think. We always aim to get to the sites one hour before the graze, but life is never perfect. Despite good planning efforts, try to be prepared for the situation where you could get to the actual observing site only 20 or 30 minutes before the first predicted events. This leaves very little time for setting up equipment. Make sure that you know your gear and all preparation has been done beforehand at home, including:
* For beginners only: in your garden do a complete dry run, setting up all gear and actually recording something on tape. See if the tape makes sense, e.g. check that the sound level of your voice and the level of the time signal are roughly the same so that both can be heard clearly. Remember how you set it up, so that you can repeat it in the field.
* Check your telescope is complete and bring a number of eyepieces, especially your favourite, best, cleanest (good contrast) eyepiece that will give somewhere between 80 and 120 times magnification. We have found that fairly high magnification makes it easier to separate the star image from the bright parts of the Moon's surface. On the other hand, when working without a motor drive, a lower power eyepiece with a large field of view of would allow for tracking errors and gives a larger observation area for reappearances.
* If you have a motor drive for the telescope, then bring it and use it if there is enough time to set up. However, also learn how to survive without it, if we don't have time to plug it in
* If you are using an inverter (DC to AC converter) to drive the motor of your telescope from the car battery, check whether it causes buzzing interference in the radio. You may need to place the inverter a few feet away from the short wave radio.
* Have a blank tape already in the tape recorder, ready at the starting point.
* Check all batteries etc at home. For your graze radio and tape recorder, Alkaline batteries are preferred, because they have the greatest capacity and die slowly (rechargeable nickel-cadmium cells die suddenly).
* Bring spares of everything: cassette tapes, batteries, eyepieces etc.
TORCH (U.S. "FLASHLIGHT")
Carry a solid, bright torch, which can have normal clear glass. Unlike on deep sky or photography trips, there is no need for dim red torches at a graze, because we always observe near the Moon.
* Dust can be a problem: we often set up along edges of secondary roads, to avoid traffic, and these can be unsealed. Therefore, when driving the last few miles to the site, have your car heater fan set to "recirculating". Otherwise fine dust will enter the car without being noticed in the dark.
* Petrol can be hard to get: start off with a full tank of petrol and, if you think you need to fill up later, do so as early as possible. In many small towns service stations close at 8 or 10 pm.
The day after the graze, listen to your tape and make a full written record of all your observations and comments. Then add any other points you still remember, but did not have on tape. From the tape, notes and memory, complete a sketch showing your exact position with respect to the centre of the road, a road junction, nearby power pole, fence line across a field, gate, bridge, creek, road sign etc. Anything the coordinator can use to find your site on a 1:25,000 topographic map.
If you have never before analysed a graze tape recording, please ask for advice on finding the exact timings and do it together with a more experienced observer.
The completed observers report should go to the "graze expedition leader" who will look after finding your coordinates from the map and entering your results in the official I.L.O.C. forms. An example of the report is at the end of this guide.
After a graze it would be nice if you could keep the cassette tape and your original rough notes for a few months, until we have heard back from I.L.O.C. and know that our report has been accepted and that all important information has been read from it. This is because you might want to check your timings again, or the graze organiser might ask you to check a particular period on your tape because another observer nearby had problems (you could help by e.g. timing the sound of a passing car or a cow in the distance - to use as a time marker for the other person).
We report graze timings to:
(a) The Royal Astronomical Society of New Zealand Occultation
Section in Wellington which keeps track of them and, where necessary, corrects
our reporting procedure,
(b) The International Occultation Timing Association (I.O.T.A.) in the U.S. where they are listed in the I.O.T.A. Occultation Newsletter and used to study the measured "shift", the difference between the predicted and observed profiles.
(c) The International Lunar Occultation Centre (I.L.O.C.) in Tokyo, the official organisation that collects all (amateur and professional) lunar occultation observations worldwide. After a few months, I.L.O.C. sends back to us a copy of their computer analysis, showing how it entered our results in their computer database.
We find that I.L.O.C. is a very careful and accurate organisation and
always sends us the copy of their computer print-out for us to check as
well, to see if they made any mistakes typing in the results. It is also
interesting to know that I.L.O.C. processes our graze timings as a long
series of lunar occultations; they enter each event (disappearance, reappearance,
blink, flash, miss) separately in their records and then send the overall
information on the lunar limb to Washington to be added to the United States
Naval Observatory database. I.L.O.C. itself also has a small group of professional
graze observers, who travel to about 4 grazes per year using 3-4 stations
equipped with photometers or video equipment.
TO BLEEP OR NOT TO BLEEP?
Most observers worldwide mark the events on tape with sharp short words,
like "in", "out", "gone" etc. A few people use small metal hand-held clickers
(toys that make a click-clack sound) and "bleepers", where you hold a small
box with a button that "bleeps" when pressed. These make a sound so that
you can both hear the bleep and record it on tape at the same time. Some
fully electronic bleepers are "silent" and put the marker direct on tape.
Whether such a clicker or bleeper is helpful or not is a matter of personal
choice and you will have to decide:
* On the one hand: if you have good eye brain hand co-ordination, then pressing the bleeper will give a good sharp marker on tape that can be timed accurately. Of course, when using the bleeper, other comments, explanations, etc. must still be spoken into the tape recorder as usual.
* On the other hand: some people feel that the clicker or bleeper is "an extra thing to worry about", that you could get mixed up as to whether to press or to speak and that it is better to concentrate on observing and making sure you react promptly with short sharp clear words, that can also be timed well.
There is no hard need for a clicker or bleeper, but a personal liking and decision by each observer. When you have gained some experience with grazes and you think it might help, please check with others in the group to see what they use.
Good description of a complete graze observation:
1. Robert Reeves, Ron Dawes and Rick Frankenberger:
"Charting Mountains on the Moon",
Astronomy Magazine (USA), Vol 16, No 8, August 1988, p 71 - 74.
Further background material:
2. David W. Dunham and Joan S. Bixby Dunham: "I.O.T.A.
International Occultation Timing Association, Greenbelt, MD, USA, 2nd Edition 1985.
3. "Guide to Lunar Occultation Observations",
International Lunar Occultation Centre, Tokyo, Japan, March 1982.
4. Harold R. Povenmire: "Graze Observer's Handbook",
J.S.B. Enterprises, Indian Harbour Beach, FL, USA, 1979 (Second Edition)
5. P. Clay Sherrod and Thomas L. Koed: "A Complete
Manual of Amateur
Astronomy", Prentice Hall, Englewood Cliffs, NJ, USA, 1981.
(This example is a "made up" typical report. You could have fewer
or more events than listed here. It is best to read out your tape and make
your own report as soon as possible after the graze, describing what actually
happened. If you have never read out a tape before, ask for help from your
local Society or expedition leader. In the report list clearly which observations
are of good quality and also which are less certain. If in doubt, include
as much description as possible.)
Graze of ZC 2876 at Bonington - Evening of 6/12/87 - Observer: John Monteith I was positioned at site No 7, on the north edge of Harkers Road, just West of the junction of Harkers Road and Leek Avenue. The next observer to my North was Mary Andrews. The next observer South of me was James Bloggs. My telescope was set up near the "Give Way" sign, across the road from power pole No 38. It was 12 paces North of the centre of Harkers Road and 38 paces West from the centre of the junction with Leek Avenue. Later at home I checked my pace length as 80 cm.
(a small sketch of the roads and position of the telescope will be helpful here)
Telescope: 6" f/8 Dobsonian; Aperture 15 cm, Focal Length 120 cm, Alt-Az Mount, Manual movement, Used 15 mm Kellner eyepiece giving a magnification of 80x. Timing using VNG radio time on 5 MHz; good steady reception. The VNG seconds markers were very clear and not mixed up with voices from other standard time stations. Recorded on separate cassette tape recorder. Air temperature was 7*C (thermometer was out for 20 minutes on the box of my tape recorder). There was a gusty strong wind that shook the telescope and caused some problems tracking the star; this could also have been part of the cause of the uncertain blink, see below.
Start observing: 10 h 32 m UTC Or 9:32 pm daylight
saving time; star still well clear of dark limb
Disappearance 10 h 48 m 36.9 s ± 0.3 s
Reappearance 10 h 48 m 44.3 s ± 0.5 s
Disappearance 10 h 48 m 54.0 s ± 0.3 s
Flash 10 h 49 m 12.7 s ± 0.5 s
Cloud moves in 10 h 49 m 20 s Star not visible when cloud moved in
Cloud clears 10 h 50 m 12 s Star not visible when cloud cleared
Reappearance 10 h 50 m 28.2 s ± 0.5 s
Blink ?? 10 h 52 m 08.6 s ± 1 s Not certain of this event - possibly poor seeing?
End observing 10 h 58 m 00 s Star now well separated from the moon
All grazing events took place at the dark limb of the moon. The dark limb of the moon was visible throughout the graze (when there was no cloud). Sky transparency, away from the clouds, was quite good. Stability was only fair during setting up, but later quite good during the graze.
Certain of all events, except the blink at 52 m 08.6 s. The disappearances at 48 m 36.9 s and at 48 m 54.0 s were seen immediately and timed accurately. The other events were timed with a bit more uncertainty. The timings in the list are "as read off the tape" and have not been corrected for Personal Equation, which is known to be about 0.3 sec. The possible blink at 52 m 08.6 s was not very clear on tape and it could be a mistake. I still have the tape for later checking.
John Monteith, 29 Smith Street, Kegworth, Antarctica 9566,
077 222 789 (phone evenings only)