by Raffaello Lena (a) and Giancarlo Favero (b)

(a) Geologic Lunar Research Group; American Lunar Society, Coordinator of the Lunar Impact Project (e-mail gibbidomine@libero.it)

(b) Osservatorio "Guido Ruggieri" Padova (e-mail faverogian@libero.it)

Introduction

Two recent papers [1][2] showed that a flash recorded during a video camera monitoring of the waxing Moon’s dark side is not recognizable as the trace of a real lunar impact on the basis of purely photometric and statistical considerations. The papers suggested that the reality of the record could be ascertained by the criterion alone, consolidated by W. Haas [3], i.e. the confirmation by a remote observer obtaining an independent and contemporaneous identical record.

In paper [2] one of us (R.L.) suggested to defocus the image recorded by the video camera, or to place a dispersing element in front of the CCD, as methods for a single observer to validate the impact origin of his/her recorded flash. Here we describe these techniques, particularly the second one, much more sophisticated and expensive, which could give interesting information about the nature of the lunar impact.

Discussion

The image of a star, and presumably that of a lunar flash, recorded by a CCD placed at the focus of a telescope is all but distinguishable from the image of a spurious flash caused by: i) cosmic ray, ii) local radioactivity, iii) CCD thermal noise (the video cameras are not cooled), iv) other CCD noise [1][2]. This conclusion was reached by comparing star images obtained with a telescope-CCD-recorder with the flashes recorded during the so called "dark tests", i.e., videotape records obtained by capping the telescope. Both signals appeared statistically significant, but during the dark tests only spurious flashes were recorded.

Things change if the CCD camera is slightly defocused, e.g., some tenths of a millimeter. The star image, and presumably the one of a lunar flash, will be consequently defocused and will become a doughnut several pixels wide if the telescope is a reflector with a secondary mirror, or a disk of comparable diameter if a refractor is employed. The defocusing will have no consequences on the images of spurious flashes, which will remain limited to a few pixels, thus allowing them to be distinguished from lunar flashes.

Defocusing will spread the flash’s (or the star’s) light on a larger CCD surface, and it will lower the content of each pixel involved in the image so that it will raise the CCD threshold limit. We find that enlarging the doughnut image of a star up to 10 pixels of a Cookbook 245 camera fitted to a 305 mm diameter Newton having 2000 mm of focal length require an exposure time 17 times greater than a focused image. On this basis we can foresee about a 3 magnitude waste on the limiting magnitude reached by the instrument, which will record only the brightest flashes. In the case of small meteors most of the flashes will probably be lost, but in the case of large meteors – e.g. the Leonids - most of the flashes will probably remain detectable (Fig 1).

FIG 1 (stella sfuocata is defocus star, Rumore is noise and Raggio cosmico is cosmic ray)

If the optics depicted in fig. 2 are placed between the telescope objective and the CCD, a spectrum of the flash light will be recorded. The negative, plane concave, lens acts as a Barlow lens and produces a parallel beam when its focus is coincident with the original focus of the telescope. The parallel beam traverses a dispersing element, a prism or a grating (the zigzag line), and then it is refocused by a positive, plane convex, lens. If the two lenses have the same focal lengths and are built of the same glass, they behave as a plane parallel plate, which will not introduce any noticeable aberration on the beam of an objective having F/D > 5. All the optics are available from science dealers such as Edmund Sc. Co..

FIG 2

The image of a lunar flash focused on a CCD fitted at the focus of the spectroscope described in fig. 1 will appear as a spectrum some pixels long, and will be the longer the higher the dispersing power of the prism or of the reticule employed is. This spectrum may give interesting information regarding the composition and the temperature of the light source, i.e. the material vaporized by the impact.

Dispersing the flash light with a spectroscope will probably cause a larger loss in magnitude compared with the previously described defocusing, so that it is suggested only with large instruments and large meteors.

References

[1] Eric Douglass, Francesco Badalotti, Giacomo Venturin, Raffaello Lena, and Guido Santacana, 2001, JALPO, vol. 43, n. 2, p. 24-29

[2] Raffaello Lena, 2001, Selenology, vol. 20, n. 3, p.11-13

[3] Thomas R. Williams, 2001, JALPO, vol. 42, n. 4, p. 177-185