Introduction
Impact Plots for
2004
Suggested Methodology
Making
Scientifically Useful Videotapes
For success, it is vital that at least two people observe, with at least one recording with a video camera or some other recording device. While an observation of such an event from a single person is potentially useful, a simultaneous, independent observation from a second person, preferably at some geographical distance from the first, is essential for valid confirmation. We coordinate observations of two classes of lunar meteors: those derived from annual streams with terrestrial ZHR's greater than 10, and those from the background sporadic meteors, in the form of "earthshine observations" or “monthly campaign” observations.
We recommend observing the moon when it has an elevation of greater than 20 degrees above the local horizon, but one can go to 15 or even 10 in the event of exceptionally clear skies or exceptionally high meteor activity. Try to avoid observations in strong twilight unless videotaping with an IR filter and/or the above conditions apply. Also, if videotaping, try to get a set of stars of known magnitude near the elevation range of the moon, to allow comparison for magnitude determination of any possible impact observations. If the latitude of the observer, the UT of observation, and the RA and Dec. of the star observed is known, the Coordinator can obtain airmass information to better correct for atmospheric interference. This procedure, however, is only possible for at least a half-dozen stars at various altitudes, and a range of magnitudes should be covered in order to provide an ample list of comparison stars for lunar meteor studies. More detail on this is presented in the section, "Making Scientifically Useful Video Tapes".
The following list, suggestions of which most are courtesy of Richard Hill of ALPO and the Lunar and Planetary Laboratory, is a starting point for the coordination of observations and are listed below.
The above is merely a preliminary list, subject to adjustment. A standardized observing form is provided to aid observers in the reporting of their observations. In the meantime, it is important for each person to carefully check his/her videotape of previous impact watches (if he/she has not done so already) to see if any additional impact events can be confirmed. If we can confirm impact events, this would go a long way in opening up the new field in observational astronomy discussed above. Even if no confirmed events arise from this study, we are able to develop and refine our program to increase the probability of observing an event during a future shower, and increase the quality of the observations themselves.
Care must be taken to ensure that video tapes of Lunar Meteors are of the highest quality to enable useful science to be done from the tape. Primarily, though, a video recording of an impact event provides a permanent record of the event as well as confirmation of another's observation, and through digitizing and image enhancement, coupled with accurate grid placement, can provide a position of an impact with an accuracy of at least a few kilometers (compared to tens of kilometers for good visual estimates). It is useful to select a CCD camera with low light sensitivity--a few examples include the PC23C from Supercircuits, the Watec 902HS CCD, and the AstroVid video camera--plus a good frame-grabbing device, such as Snappy. The frame grabber will enable frames with event candidates to be saved as images to a PC. A focal reducer is also useful for increasing the field of view, while preserving low-light sensitivity. It is vital that a time element of some sort, whether an accompanying audio recording of the WWV signal, or an accurate to-the-second time display on the video (again calibrated to WWV), be made with the observations. A video event with an inaccurate, or no time signal, is of little value for attempting to confirm lunar meteor events.
A telescope that is clock driven is important for producing high-quality videos. One could, if one is careful, use a small, hand-drivcn scope, but that puts things in constant motion, blurring out (to invisibility in the faintest cases) lunar events. Some slight motion, such as vibration, may be useful for distinguishing events from spurious signals, but the problem of signal diffusion (the blur) once again comes into play. Start the video with a peek at the sunlit portion of the Moon to get your bearings straight, then slowly move the bright part out of the field of view (FOV), so as to only be observing the earth-lit part of the lunar disk. Keep the bright crescent out of the FOV for the duration of your observing session, then at the end, observe the crescent again. Once the session is ended, it is suggested that many of the standard stars listed here be observed, both the blue ones (A- and B- spectral types) and the red ones (the G- and K- spectral types), at the same elevation as the Moon and points higher, to provide the "marks on your ruler" to be able to measure the brightness of any impact flash. At least five pairs should be video recorded, at different elevations and different magnitudes, to provide useful references. They should be recorded every night lunar impact observations are made, since transparency conditions can change from night to night (e.g. one night is hazy but cloudless, another night is crystal clear).
To be able to measure brightness accurately it is very important that the gain of your video camera be the same throughout an evening's observing session. Cameras tend to have a gain control/auto iris that varies the amount of light which gets through to allow a comfortable light level to be recorded. This is fine in most Earthbound sessions, but not for astronomical observations. It is important that the gain be the same throughout the session (with the possible exception of viewing the lunar crescent, since it is so much brighter than any stars or impacts observed) to enable some accurate measure of the intensity of the impact flash. When you observe your standards, at least a time should be recorded, and I can use that, along with your latitude and longitude, and the star's RA and Dec. to find the exact elevation of the star, which will indicate the amount of air the star light has to pass through to get to your camera (known to the professional astronomers as "air mass").
It is almost certain that one will get an impact candidate on one's video, perhaps many will appear during a session. The candidate may appear on or off the moon in the tape as well. What I am referring to are spurious signals that appear on videotapes with regularity, and these include cosmic ray hits, signals from static, tape dropouts, etc., and they are difficult at first glance to tell these signals from any real meteor activity on the Moon. The first "line of defense" in this case is a second, corroborating video of the event, made at the exact time, which will indicate whether the event is real or not. Cosmic ray signals tend to come in a variety of shapes and sizes, and sometimes can be picked out by the trained eye. Streaks, multiple dots, blips, elongated flashes are all likely cosmic ray hits or other forms of non-impact signal. Cosmic rays tend to be sharp in appearance, whereas stars and starlike phenomena tend to have some fuzz to them due to the blurring of the atmosphere. Slightly defocusing the scope will make any astronomical object blurred, but the cosmic ray hits will remain sharp--this may be a useful method, except any faint flash may be blurred out of existence. Again, the best way to find lunar impacts is to have simultaneous observers, two or more, sufficiently remotely placed from one another, with similar instrumentation, to be able to make observations that can be independently confirmed.
Any further questions, comments, or suggestions concerning the observations of lunar meteors, video or visual, are very much welcome and can be sent to the coordinator.