Spectro-selenography

Spectro-selenography consist in assembling a lunar image from a line of its spectrum. As in spectro-heliography, the Moon is forced to pass in front of the spectrograph's slit while an AVI or an image sequence is recorded. Due to the  Moon's luminosity being much inferior than Sun's luminosity, some of the procedures are necessarily different. First, a higher aperture telescope is used, in this case a 9.25" Schmidt-Cassegrain. The spectral resolution of the spectrograph is also reduced in order to gain light. In our case we use a diffraction grating of 300 lines per millimetre. If a webcam is used with a 5.6µ pixel, a dispersion of  1Å/pixel is obtained. Although with this setup some image detail is achieved, due to the lack of light the signal-to-noise ratio (SNR) is very low, not allowing the visualisation of eventual different detail of images with different wavelengths. The dynamics of an 8 bit image doesn't allow big manipulations, either. The figure at right show a reduced mosaic of the Moon assembled from a set of images captured by a webcam. This image, the first that I captured with this process, has a bandwidth of 1Å in the Hα zone. Many images were obtained with this process, but the method was abandoned due to the reasons above exposed.

We opted to use a CCD, in this case a KAF3200. The main obstacle to this choice is the read time for this CCD: the Moon doesn't stop between  CCD readings.  The solution found was to reduce the speed of the Moon's pass over the slit, by taking advantage of the difference between sidereal and lunar speeds. Moon travels its diameter in one hour relative to the sidereal movement, or 0,5" /s. Maintaining the mount at sidereal rate and letting the Moon pass over the slit at its lunar rate, is enough to achieve results. Related to this, if a 3x3 binning is used, the reading time of the CCD is reduced, in the case of a KAF3200 USB from 8s to 1s. With this configuration a lower spectral resolution is attained, of 3,4Å/pixel, but on the other hand, an excellent SNR and 16 bit dynamics is obtained. Another advantage of this CCD is its size, allowing the coverage of all the visible spectrum only in two grating positions. The figure bellow represents the visible  spectrum of the Moon covered by the CCD in two positions of the diffraction grid, as well as some lunar images obtained with this method Interesting to note that the spectrum obtained is the solar spectrum reflected by the Moon.

 

 

 

The aim of spectro-selenography is the acquisition of lunar narrowband images. By inspection of those images try to obtain information about the formation and geologic evolution of our natural satellite. For a method to get information of the obtained data, follow this link.

 

 

 

All this pioneer work at amateur level has been done in collaboration with Alberto Fernando and Filipe Alves.

WebCam

 

<Back>    <Home>