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Geoffrey,

Since we are airing out eye stories I will confess that I have had
blind spots come and go over the years (not on the fovea). I often
wondered if they were caused by 30 plus years of sextant use but
dismissed the sextant as the culprit because of the off center areas
affected. I did not get an eye exam but probably should have just for
the peace of mind. Full vision always returned sometimes quickly
sometimes not so quickly. There are so many illuminated things that we
look at during the day besides the sun. Blood pressure, neurological
glitches, fatigue, alcohol, etc must all contribute to visual
anomalies also. Now that I'm over fifty I can now blame being old as a
cause ;-)

Greg

On Jun 29, 11:39�pm, Geoffrey Kolbe 
wrote:
> >Ken Gebhart wrote:
>
> >Now that this thread is coming to a close, can the following
> >questions be answered?
>
> >1. �Does normal sextant use, even by a neophyte, pose any significant
> >risk of eye injury?
>
> Peter Fogg wrote
>
>
>
> >I also think that whether its "significant" as Ken Gebhart has
> >defined this term will vary with individual perception as to the
> >risk, with or without scope.
> Geoffrey Kolbe writes:
>
> I think it is "significant" that nobody has chirped up and said
> (admitted) that they have had eye damage through sextant use, or the
> use of any telescope. As the people on this list probably spend more
> time staring at the sun than any other group of people, (outside
> India), I think we should expect that if there was any "significant"
> danger of looking at the sun through small power telescopes using
> suitable filters, somebody reading this thread would have reported
> damage or anecdotal evidence of damage in somebody else. Nobody has.
>
> Having said that, it is now a year since I last used a sextant to do
> a sun sight. A year ago, I discovered a small scotoma (blind spot) in
> my left eye, the eye I normally used with my A-12 Link aircraft
> sextant. This sextant is one I "won" on ebay and resuscitated.
> Normally, the A-12 sextant has one low density filter and one high
> density filter, but the filters on this example were both quite low
> density. I used it for a year or so, in particular during one of my
> Sahara trips, but eventually I figured the image of the sun (no
> magnification) was uncomfortably bright and I replaced one of the
> filters with a high density glass Neutral Density Filter. It was
> shortly after that when I noticed my little scotoma, smack in the
> middle of my fovea.
>
> One would think that a blind spot in the middle of the fovea would be
> easy to recognise. When looking at a white surface, you would expect
> to see a small black spot where the blind spot was. But the brain is
> quite clever and it knows that these cells in the eye are not working
> correctly and so it 'fills in' the blind spot with what the rest of
> the eye is seeing and what it assumes the defective cells would see
> if they were working correctly. So, if I look at a white surface, it
> is uniformly white. If I look at a red surface, it is uniformly red.
> If I look at a black surface, it is uniformly black. If I look at
> printed text, however, where there are very rapid changes of contrast
> through the visual field, there is a small spot about ten MOA across
> where I see nothing except a blurred, slightly scintillating smudge.
> I only noticed it while trying to read with the left eye when
> drinking a cup of coffee, which obscured the right eye.
>
> "Oh dear!" I thought. Either this is the onset of macular
> degeneration (from which my mother went blind) or I have fried my
> fovea. Off I went to the optometrist and he had a good stare. Nothing
> to be seen. Macular fine, no burnt out area. Another optometrist had
> a good stare and did various field tests. Nothing came up. So, I
> reported to the top ophthalmologist in this area of Scotland at the
> eye department of the regional hospital. He had a good stare and took
> some high resolution photographs of the back of the eye. The 'burnt
> out' spot would be about 70 microns (about ten cells) across and the
> resolution of the digital images was certainly good enough that if
> there had been something to be seen, he would have seen it. Nothing
> there. He said that if I had fried the fovea, the pigmentation layer
> would be burnt and it would be easy to spot. But the pigmentation
> layer across my macular was smooth and uniform. So, he sent me up to
> the top man in Scotland and he used Optical Coherent Tomography (the
> only machine in Scotland) which can penetrate several millimetres and
> build up cross-sectional image slices through the back of the eye.
> Fascinating piece of kit. But the pictures of the retina, layers of
> blood vessels, pigment layer, and the ganglion layer behind that
> which pre-processes the retinal image before passing it onto the
> brain, were perfectly normal. I had definitely not fried my fovea.
>
> So, I asked the good doctor, what is causing the scotoma? "No idea."
> he said. I am glad to report, however, that it is gradually improving.
>
> I relate this little tale just to point up that if anyone out there
> did damage their eye through staring too long at the sun, the damage
> may be quite subtle and not as easy to recognize as one might think.
> The symptoms of my little blind spot are exactly what would be
> expected if I actually had gently fried my fovea.
>
> As for me, I still have lingering doubts. Since the good doctor was
> unable to come up with any other reasonable explanation for my blind
> spot (or any explanation at all for that matter), I am reluctant to
> pick up the sextant again.
>
> Geoffrey Kolbe
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