OLED - Limbos

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Lunar limb in stellar occultations: some caveats

By definition, in the moments close to the occultation, any aperture centred at the star will contain the lunar limb, which will by definition occupy half of the aperture at the time of occultation. The lunar limb can be slightly brighter than the sky background, but for faint stars, which require high gain/exposure times, the limb can become significantly brighter than the sky background, even brighter than the star itself (see Fig. 1).

Figure 1. Emergence of a star from the dark lunar limb (left). The star is so faint that its brightness is comparable to that of the limb. An aperture with a radius of six pixels (right panel) includes a significant region of the limb, resulting in a strong non-uniform signal that decisively contaminates the measurement. In this case limb-suppression techniques are required. Data obtained from an observation carried out by Rosendo Jorba (Agrupación Astronómica de Madrid).

In these cases, standard photometry programs like Tangra or Maxim DL may fail, and considering the non-stellar signal from the field becomes critically important, which requires special analysis (Fig. 2).

In addition, the light curve of the sector where the star is occulted may not be strictly horizontal, due to a limb advancing or receding in our aperture. Note how in the right panel of the figure, the maximum level of the star signal is lower than the level of the lunar limb, making the process of centering an aperture on the Moon (even a small one) very difficult and may be impossible for general analysis software.

In any case, the standard software programs may fail in the process of centering an occulted star, or may lose automatic centering due to a highly bright limb.

Figure 2. Limb suppression technique. A special analysis allows the subtraction of the limb signal (middle panel) in the image of the left panel, without essentially modifying the source signal. Astrometry is performed on the image of the central panel, whose average background is zero. The right panel shows a radial cut of the signal, in which the illumination field of the Moon varies smoothly. The sequence of two images corresponds to the emergence of a faint source: in the lower image, the star has not yet emerged from behind the limb, while the upper one is the first image in which the star is visible, 20 ms later. The case represented by these data is a "difficult" case, which requires special analysis techniques as the process of centering an aperture on the star is greatly hindered by the high level of signal from the lunar limb. Data obtained from an observation carried out by Rosendo Jorba (Agrupación Astronómica de Madrid), analyzed by Enrique Velasco.

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