Planning a Lunar Graze Observation
4) The date and approximate
graze track locations are published the BAA Handbook which summarises
observability information for each event. In some situations, as with
a bright star or thin crescent phase, a bright limb event maybe predicted.
The majority of events in the BAA Hand Book are at dark limb. The Cusp
Angle (CA) is an indication of how far the star is from the illuminated
cusp. For dark limb events, the larger the CA, the better from the observers'
point of view.
6) It should be noted that the graze limit is a green line on Google Earth and this corresponds with the white line on the Limb Profile in Fig-3. If the white line doesn't intersect any peak in the limb profile, the observer should find a location that does, and hopefully maximise the number of contacts.
7) There is a light blue histogram on the limb profile (software generated) which gives an indication of the number of D-R contact that might be seen if the observer was in that region of the limb cross-section. Using the limb profile, observer(s) can align themselves at places within the track to observe events of interest, and time them.
8) Observing positions are usually selected to:
8.1) maximise the number of D-R timings, and to compare Observation
to Calculation. The O-C value called "residual" is the result
of interest to the amateur. This is returned to the observers when the
report is submitted and analysed. [ Observers using Occult-4 will be
able to produce preliminary O-C values ]
9) The vertical axis on the limb profile is in km. and this can be
projected in the minds-eye onto GE using a suitable map scaling. Planning
a location is then a question of finding an area that can accommodate
several mobile observers with telescopes at strategic positions based
on the number of contacts expected. The team leader for an expedition
should seek permission for use of non-public locations. Its best to
do this face-t- face or by a telephone call. This writer has found that
email generally doesn't always produce a reply.
Observing and Recording Grazes
10) Graze occultations are considered by IOTA to be more valuable scientifically than Total Lunar Occultations. Observers should aim to visually record the time of D-R contacts to an accuracy of 0.3 sec or better. Visual observations are typically recorded with dictaphones or other devices, while a continuous UT time signal is also recorded. I can use a camcorder and set the internal clock accurately to UT by sync'ing on a Telephone time pip, or I can point the camera at a radio controlled clock and record that. There are many possibilities. I also have a home made MSF (60KHz) receiver with a pizo sounder.
Note: Radio clocks with liquid crystal displays are effected by ambient temperature and the displayed time is subject to a delay, Eg at 5C, 200ms slow, -5C 400ms This makes the LCD clocks unsuitable for observing in cold conditions.
11) Mobile phones have clock or stop-watch functions that will record multiple lap times - practice and care is needed - perhaps with a second phone recording audio. I have found the action of "pressing" a soft-key non-intuative, as there is no tactile feedback.
12) Another possibility to time events by audio is using the mobile app.:"TimeThe Sat" from http://www.satflare.com/ This has an audio clock setting providing a continuous speaking clock 1 sec tone sync'ed to UT. The settings give a choice of NTP time servers. I tested this app. (once) and found the pips in step with the BT land-line (123) speaking clock with no appreciable correction. This App can be recorded along with audio observations. Its original purpose is irrelevant.
13) Accurate wrist watches: An "atomic watch" or "radio watch" can be be used as a relaible secondary standard. I have a Cassio "wave ceptor" that uses a build in radio received (60 or 77 KHz) which is sychronised at midnight or on demand. When the alarm function is set the alarm pulses are coincident with the BT land line speaking clock. This could be applied to timeing occultations or creating time markrs for audio recordings.
14) CCD observers can record the event with planetary web cam. Use a frame rate of 10 to 25 or 30 fps and a Barlow lens to magnify the image. A sidereal telescope drive is recommended. To get times for the events, record the frame times by a reliable method. If this requires a computer internal clock, ensure that the clock is synchronised with UT using an internet time protocol. Dimension 4 software (free windows app by Thinking Man ) will do this. EZ planetary software and FireCapture have the ability to add a time stamp to the web-cam frames.
Be prepared to start recording/observing some minutes before the time of central graze. The duration of the graze can be estimated from the limb profile.
15) Observers with low light analogue video cameras can invest in a GPS time text overlay unit (e.g. GPSBOXSPRITE or the IOTA-VTI). This puts a time stamp on every video frame. This is the most reliable method, and the method of choice. The WAT-910HX will record at field level (50 fps)
There is more information on my pages describing the timing of asteroid occultations.
See here for further information: http://www.lunar-occultations.com/iota/iotandx.htm
"Watts: a 1950's photographic survey conducted by C.B. Watts (U.S. Naval Observatory) in which he measured the heights of lunar features along the edge of the Moon as recorded in thousands of photographs taken at different phases and librations The Watts profile was the best available in the last century"
Watts is no longer used. In fact I have graze data from the 1980s reduced with Watts giving an O-C of 0.53" arc. but when analysed with LRO data the O-C is 0.05". So the observation was pretty good with tape-recorder and clock and careful use of the Personal Equation (reaction time).
18) The BIG advantage with high frame rate video is that fade events and double star features will be time resolved and recorded.