optical illusion or atmospheric effect?

[Yousuf Khan]

Tonight, I was looking up at the sky around midnight. I noticed that
almost all of the stars looked doubled. The brighter the star, the more
noticeable the effect. Just looking at them naked eye, not through
telescopes or binoculars or anything. The funny thing was that when I
looked at them true naked eye (i.e. without my glasses), the doubling
appeared horizontal, but when looking at them through my glasses, the
doubling appeared vertical. Is there a known atmospheric effect that can
do this? I would estimate the separation between the doubled stars was
about 2-5 arcseconds, either horizontally or vertically.

Yousuf Khan

[Sam Wormley]

This effect is in your eyes--aberrations are exaggerated at larger
apertures expected in the dark. I'm assuming you have corrections
for astigmatism in you glasses prescription.

[Yousuf Khan]

Well that's possible, and I do have astigmatism, but today was the only
day I've ever seen this.

Yousuf Khan

[Martin Brown]

I suspect if you look at your glasses prescription you will find it has
a fair amount of correction for cylindrical astigmatism. The formula
only works correctly at the right distance and light levels.

When your eye is dark adapted and the pupil wide open abberations are
much worse than normal (also why they do eye tests in a darkened room).

With your glasses on your long distance vision is over corrected at
infinity and without them it is under corrected. The result is you see
each star as two spots or a line : or ..

ITYM 3-5 arc minutes separation too.

--
Regards,
Martin Brown

[Mike Dworetsky]

I'll agree with the other responses that this is astigmatism in your eye,
and add that spectacle corrections are not always perfect, which is why you
still saw some residual astigmatism in the perpendicular direction. I once
got prescription lenses that actually made vision in one eye worse, and was
told that, "Well, the angle tolerance allowed in the workshop is 20 degrees
+/-, good enough for most people." I told the optician to send it back and
get a new one made, which was spot on, and gave complete correction, at
their expense.

--
Mike Dworetsky

(Remove pants sp*mbl*ck to reply)

[Mike Dworetsky]

I'll just add, I took off the faulty spectacles and held them up while I
rotated them, and at one point with about 25-30 degrees tilt, the vision
correction was excellent, which is how I knew the angle ground into the lens
was off.

[William Hamblen]

On 5/25/2012 3:49 AM, Mike Dworetsky wrote:

> I'll agree with the other responses that this is astigmatism in your
> eye, and add that spectacle corrections are not always perfect, which is
> why you still saw some residual astigmatism in the perpendicular
> direction. I once got prescription lenses that actually made vision in
> one eye worse, and was told that, "Well, the angle tolerance allowed in
> the workshop is 20 degrees +/-, good enough for most people." I told the
> optician to send it back and get a new one made, which was spot on, and
> gave complete correction, at their expense.

That optical shop was lying to you. The angle tolerance in cylinder is
supposed to be only TWO degrees, not twenty, depending on the amount of
cylinder in the prescription.

Bud

[Yousuf Khan]

On 25/05/2012 3:06 AM, Martin Brown wrote:
> I suspect if you look at your glasses prescription you will find it has
> a fair amount of correction for cylindrical astigmatism. The formula
> only works correctly at the right distance and light levels.
>
> When your eye is dark adapted and the pupil wide open abberations are
> much worse than normal (also why they do eye tests in a darkened room).

Which would make some sense, as we just got back from watching a movie
(in a darkened theatre, obviously). On other nights, I don't have any
problems, since I'm coming out from lighter environments.

> With your glasses on your long distance vision is over corrected at
> infinity and without them it is under corrected. The result is you see
> each star as two spots or a line : or ..
>
> ITYM 3-5 arc minutes separation too.

Interesting, describes almost exactly what I saw.

I wonder if it's worth getting a new prescription done, based on this
situation? Or will getting a prescription corrected on watching stars
make the glasses useless for regular use?

Yousuf Khan

[Mike Dworetsky]

Thanks. It was the optician's assistant who said this to me, not the lens
maker, who is located a long way from London.

Well, you may be right. Though judging 2 degrees from looking at letters
and spots, and listening to the patient saying, "Maybe this one is a bit
better than the last one," and adjusting the angles on test lenses during
an exam, seems to be expecting a lot from the optician. OTOH if you say the
tolerance allowed for the lens maker is 2 degrees variation from the
prescription, this makes more sense.

I'll check it out. Since changing opticians a decade ago (after moving to a
new address) I haven't had any further problems with these prescriptions.

[William Hamblen]

The tolerance, of course, is in the lens fabrication, which is why the
statement about plus or minus twenty degrees _in the workshop_ was bunk.
Clinical error is another thing altogether, but a careful eye doctor
and a willing patient should be able to get the angle closer than plus
or minus 20 degrees. Your correction isn't ever 100% stable, either.
You need to get it checked periodically. I myself have irregular
astigmatism in my right eye that makes it impossible to reach an
end-point correction in the eye doctor's chair. We go until we get
tired of trying. Roughly 20/25 (6/7.5) is about the best acuity I've
ever had in that eye. The left eye, although it takes a stronger
prescription in both sphere and cylinder, will be 20/20 (6/6) with the
right prescription.

Bud

[Martin Brown]

Probably not a good idea. I have just reached the stage where I can tell
my uncorrected night vision is no longer tack sharp at infinity. It
isn't a problem but it does affect the limiting magnitude.

Correcting for the worst case maximum aperture at one extreme of
distance isn't likely to be helpful in ordinary every day use.

--
Regards,
Martin Brown

[Mike Dworetsky]

Certainly true, I have a check once a year.

> have irregular astigmatism in my right eye that makes it impossible
> to reach an end-point correction in the eye doctor's chair. We go
> until we get tired of trying. Roughly 20/25 (6/7.5) is about the
> best acuity I've ever had in that eye. The left eye, although it
> takes a stronger prescription in both sphere and cylinder, will be
> 20/20 (6/6) with the right prescription.

Mine does not vary so much, but from time to time a new prescription is
necessary. Which I hate, because the blended lenses are really expensive...

[Paul Schlyter]

One can get two pairs of glasses, one for everyday use and another
for astronomical use.

[Martin Brown]

On 25/05/2012 21:03, Paul Schlyter wrote:
> On Fri, 25 May 2012 17:53:33 +0100, Martin Brown
> <|||newspam|||@nezumi.demon.co.uk> wrote:
>> On 25/05/2012 15:18, Yousuf Khan wrote:

>> > I wonder if it's worth getting a new prescription done, based on
> this
>> > situation? Or will getting a prescription corrected on watching
> stars
>> > make the glasses useless for regular use?
>
>> Probably not a good idea. I have just reached the stage where I can
> tell
>> my uncorrected night vision is no longer tack sharp at infinity. It
>> isn't a problem but it does affect the limiting magnitude.
>
>> Correcting for the worst case maximum aperture at one extreme of
>> distance isn't likely to be helpful in ordinary every day use.
>
>

> One can get two pairs of glasses, one for everyday use and another for
> astronomical use.

I am planning on doing just that if it gets any worse. For now I can
still see enough of the constellations ~mag 4.5 to do public stargazing
talks. I have a tame optician friend in the village but it is finding
time to do an eye test when dark adapted and he has brought his kit
home. I estimate my far point is now about 30m instead of infinity, and
my near point has now receded well beyond the length of my arms.

In daylight +0.25 diopter is enough to bring infinity tack sharp again
(and also in the classic opticians test where infinity is taken to be
about 30' by convention). But dark adapted with a fully dilated pupil I
estimate my correction should be about -0.25 - no amount of positive
correction seems to help (easily tested with reading glasses).

--
Regards,
Martin Brown