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13 June 2013

First Attempt of Recording Lunar Halo

Ah.. the beauty of halos. I remember asking myself when I was much younger what was that big bright circle thing in the night sky which always appears to encircle the glorious full moon. Well, now although the natural mystery has been largely solved and understood (as the mechanism for this phenomenon is similar to the formation of rainbows), it is still interesting to whip out my camera and try if I can pack this occasional nature's wonder into a retrievable memory.

So I used my Nikon D5000, took 3 isolated photographs of the event which later I found two shows clearer profile of the spectral dispersion. The clearest photo was taken on May 25 at ISO400, 18mm, f4 and 2 second exposure time showing clear spectral dispersion around the lunar disc. Under this exposure, Scorpio's eye, Antares, appears as a bright dot at approximately 8 o'clock direction of the picture on top. The picture was taken as JPEG (as I originally intend to do a stop-motion video, not aesthetic photography) is then level adjusted, increased in overall saturation for spectral dispersion visibility and increase selective channel of colour to make the blue sky appear more natural.

Now, it is important that the large halo around the moon is formed by refraction and reflection of moonlight upon tiny ice crystals or supercooled water aerosols in the high-skies (by floating cirrocumulus in our case), in some cases when the dispersing particulates are in the right size and geometry, a large spectral dispersed (of rainbow colours) halo can be seen which in our case, doesn't shows perhaps due to incomplete refraction or probably just simply not resolved under my camera setting.

The second photo shows an interesting spectral dispersion in a better spatial profile which its formation is puzzles me. Its vicinity so close to the moon suggest a phenomena of optical diffraction? Anyway, the image was cropped from a 18mm (wide field) photograph, level adjusted to enhance contrast, increased overall saturation to bring up visibility of the spectral dispersion and lastly, selectively adjusted the yellow channel to approximate natural colour of night sky.

Other than these two featured images (the third was bad and omitted) I have taken a series of photographs using a function called "interval shooting" with my camera. Using ISO400, 18mm, f4 and 1.6 second exposure time, the interval cycle was 10 seconds and I have took 72 photographs of the moon without tracking. The images were not post-edited and they were copied into two groups where one was directly stitched into a stop-motion film and another after cropping out the rainbow-ed ring around the moon from the rest of the photo, are then made into stop-motion video using the same method.

So from the short video above, we see a time-resolved evolution of the halo. Before any analysis, it is obvious that video like this shows how precious each photo-worthy moment are to astrophotographers as a good photo (usually with long exposure time) has to be taken before volatile atmospheric conditions interfere with the photography. So our works are really up to the mercy of the skies.

It is simple to see that the spectral profile immediately around the moon (shape of the "rainbow") depends heavily on the homogeneity of the dispersing elements. i.e. ice crystals and water droplets. To have a "perfect rainbow circle" around the moon, therefore, is to desire an even distribution of a thin layer of almost uniform size of dispersing particulates in our atmosphere.

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