3rd Order Rainbow Hunting

Alec Jones (Bolton Halo Triangle), an avid and skilled sky observer, maybe hit the jackpot on 8th September '13 with his two images of the long hunted and exceeding rare 3rd order rainbow.

"The conditions were absolutely perfect; super bright primary and secondary, dense black clouds around the sun and pouring with rain. I had the umbrella in one hand and camera in the other. Both camera and myself soaked.. ..If this isn't a tertiary then I'm giving up atmospheric optics for tiddlywinks or some such!"

All images ©Alec Jones, shown with permission
Another image taken 10s later.

12bit TIFF from RAW image given slight noise reduction followed by small levels shift and then colour saturation.

The 3rd order (surrounded by a fainter and even broader 4th order) is sunwards. It lies in the glow from rays passing straight through raindrops and the general intense glare of the sun.
The third order rainbow (three reflections inside raindrops) is almost the same size as the familiar primary bow.   But it is around the sun rather than opposite.  It is broader.   It is fainter.    It's light is swamped by the sun’s glare and by light passing through raindrops without reflection – the zero order glow.

Fiendishly difficult conditions.

Historically there are only one or two inconclusive accounts of visual sighting but no photographs.   The first image was at last obtained in May 2011 by Michael Grossman, experienced observer and skilled rainbow maker, who received world-wide publicity.

Dark cloud near the sun - essential as background.
No enhancement needed to reveal primary and secondary bows
Real or artefact?

Strong image processing can produce artefacts. Cameras pointed near the sun give deceiving internal lens reflections and CCD effects. Is it real?

Two hand held images show the arc. Always take several hand-held pictures of a strange phenomenon. Artefacts like lens reflections will shift and change but a real object will not.

Several different image processing routes give the same arc.

Colours are the right way around.

Geometry on the arc gives the sun position as just off the lower right hand edge. The arc is near circular except for a slight curvature difference at its indistinct top. The sun position combined with the lens focal length in the image EXIF data yields an arc radius of 43 and on the other image 41. Theoretical radius is 42.5. Arc width is hard to measure but is perhaps only 3.5 compared to a theory value of ~4.4.

To be sure, we need exact reproduction of the original camera position for one of the images, exposure of a star field and subsequent calibration of the lens followed by computation of the precise sun position and then more geometry.




Unprocessed image

Atmospheric
Optics
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Same 12bit TIFF this time processed by subtraction of blue and red channels followed by shift in levels. This accentuates coloured objects.

This version might also hint at a 4th order bow.
         

The sun is just out of frame at lower right.

RAW image converted to 12bit TIFF. Processed by slight noise reduction, levels shifted and strong colour saturation followed by further slight cosmetic noise reduction. No unsharp masking.

The brown circle is probably a raindrop on the lens. Artefacts crowd in irregular bands from lower right.