Sun halo

[glowingturnip]

So I had a week in Spain, which was unfortunately clouded over for the whole time so I couldn't get any imaging done (although apparently it was gorgeous the day after I left, sparkiling milky way etc etc).

Anyway, we did see this cracking 22 degree sun halo on one of the days, so I thought I'd post it up anyway.  Not strictly astrophotography I suppose, but it does at least have a star in it.

The 22 degree halo is an optical phenomenon caused by the refraction of sunlight through high uniform cirrostratus cloud which is made of tiny ice crystals.

 

The ice crystals are formed in small hexagonal columns and the refraction seen here is from light entering through one side and leaving through another, refracting thorugh 22 degrees (about the width of an outstretched hand held at arms length).

 

Notice the sky inside is darker since there is no refraction back inwards leaving a 'hole in the sky'. There are rainbow rings with red on the inside, the inside edge of the halo is sharper than the outsides, and in this one, notice the top and bottom are sharper than the sides, which I think is due to the semi-random orientation of the ice crystals (horizontally oriented crystals would give 'sun dogs')

This picture also shows the much rarer 46 degree halo towards the bottom and left of the image, formed when light enters through the sides of the hexagonal column ice crystals and leaves through the ends.  Ice crystals tend not to form with flat ends but often have little cone cut-outs, hence the rarity of this halo.

Hope you enjoy, and that I'm not too off topic !

Cheers

[Pete Presland]

Looks very nice and definitely not "off topic" was as bright as the images show?

[glowingturnip]

quite a bit brighter actually, it was mid-morning, and you couldn't look at the halo without blocking out the sun with a hand, but once you did it was easily visible, like a rainbow.

I was shooting stopped down by about 2 stops from auto-exposure, which came to 1/3200 at f22, no editing of the photos has been done (for once).  I was being very careful not to fry my hardware and wetware !