Thanks very much for all the kind comments.
I didn't record the exposure, but I 'd guess at the least the camera would give: 1/1000 s at f/22, on Kodachrome 64. When you say 'a bigger version' do you meant a higher resolution file or a print?
This is, I'm told, a 22 degree halo with an 'oval circumscribed halo'.
It was taken in late October. The air temperature was well below freezing, but the strong sun helped when it shone. Once you are acclimatised, the air at 18,500 ft is OK and life is normal. It took a while to get there, so it wasn't as if I went from sea level to 18,500 ft in a day. I'd been there for about a month by the time the picture was taken. A friend had been killed on Shivling shortly before, so it had more than the usual feeling of dangerous beauty about it.
Thanks,
Helen