NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Looking at the Sun through a telescope
From: George Huxtable
Date: 2006 Aug 8, 17:39 -0500
Fred Hebard asked-
| Regarding the optics, wouldn't the diameter of the objective be the
| main parameter involved in potential damage, since one is
| concentrating the energy from that cross section into a smaller
| one? Isn't magnification secondary here?
No, it's not so simple as that. To get the energy into the retina, it
has to get through the pupil, the small hole in the iris diaphragm. At
its biggest, in very low light conditions, that can be up to 8mm
across. In bright daylight conditions, it automatically shrinks to
about 2mm diameter.
There's an unavoidable fact-of-life about telescopes; that a pencil of
parallel light from a distant body, occupying the diameter of the
objective of a telesope, is compressed into a narrower pencil when it
emerges from the eyepiece, and that compression is by exactly the same
factor as is the magnification of the image.
So, taking a x3 scope and an objective with a 25mm aperture, the
emerging pencil that meets the eye is 8.3 mm across. It's already
bigger than the entry pupil of the eye, even in night conditions. So
if you changed that telescope for another, with the same magnification
but a larger aperture, say 50mm., it would put no more energy into the
retina. Instead, the extra light would be "wasted", by meeting the
iris rather than passing through the hole. Instead, if you wanted to
collect more light for better night vision, with that 50mm objective,
it would be necessary to double the magnification to x6. Then, the
light pencil would be 8.3 mm across once again, and it would nearly
all enter a dark-adapted eye. That's necessary, whether or not the
extra magnification was really needed for its own sake. Indeed, on a
small vessel, higher magnification is often only an embarrasssment,
because of the motion.|
In general, what you find is that sextant telescopes have sufficient
aperture that nearly all the light they collect can go into a
dark-adapted iris, so that it's always about 8mm x magnification.
Which means that in daytime conditions, a far smaller aperture would
give exactly the same performance. And another implication is that the
most light that can be put into an eye is determined by the
magnification, not by the aperture.
George.
contact George Huxtable at george@huxtable.u-net.com
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
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To post to this group, send email to NavList@fer3.com
To , send email to NavList-@fer3.com
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From: George Huxtable
Date: 2006 Aug 8, 17:39 -0500
Fred Hebard asked-
| Regarding the optics, wouldn't the diameter of the objective be the
| main parameter involved in potential damage, since one is
| concentrating the energy from that cross section into a smaller
| one? Isn't magnification secondary here?
No, it's not so simple as that. To get the energy into the retina, it
has to get through the pupil, the small hole in the iris diaphragm. At
its biggest, in very low light conditions, that can be up to 8mm
across. In bright daylight conditions, it automatically shrinks to
about 2mm diameter.
There's an unavoidable fact-of-life about telescopes; that a pencil of
parallel light from a distant body, occupying the diameter of the
objective of a telesope, is compressed into a narrower pencil when it
emerges from the eyepiece, and that compression is by exactly the same
factor as is the magnification of the image.
So, taking a x3 scope and an objective with a 25mm aperture, the
emerging pencil that meets the eye is 8.3 mm across. It's already
bigger than the entry pupil of the eye, even in night conditions. So
if you changed that telescope for another, with the same magnification
but a larger aperture, say 50mm., it would put no more energy into the
retina. Instead, the extra light would be "wasted", by meeting the
iris rather than passing through the hole. Instead, if you wanted to
collect more light for better night vision, with that 50mm objective,
it would be necessary to double the magnification to x6. Then, the
light pencil would be 8.3 mm across once again, and it would nearly
all enter a dark-adapted eye. That's necessary, whether or not the
extra magnification was really needed for its own sake. Indeed, on a
small vessel, higher magnification is often only an embarrasssment,
because of the motion.|
In general, what you find is that sextant telescopes have sufficient
aperture that nearly all the light they collect can go into a
dark-adapted iris, so that it's always about 8mm x magnification.
Which means that in daytime conditions, a far smaller aperture would
give exactly the same performance. And another implication is that the
most light that can be put into an eye is determined by the
magnification, not by the aperture.
George.
contact George Huxtable at george@huxtable.u-net.com
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
--~--~---------~--~----~------------~-------~--~----~
To post to this group, send email to NavList@fer3.com
To , send email to NavList-@fer3.com
-~----------~----~----~----~------~----~------~--~---
