NavList:

   

Reply

I still disagree with Mr. Huxtable except in his correction of my 
miscalculation error of foveal area. Area is indeed 0.07 mm. not 0.28 mm. and 
I am sure he will appreciate the effect of this if anything is to potentially 
increase the I/R energy density at the fovea by a further four times, making 
my simple calculation an underestimate by this amount. 

He still seems reluctant, however,  to accept light energy can be focussed.

If the flux density of light cannot be focussed from an objective lens and 
hence increased by optical means onto a small area, then all of those people 
in the Lawrence Livermore facility concentrating huge aperture lasers onto a 
pinpoint of about one mm. for fusion research are going to be badly 
disappointed.

Although he now agrees that all of the light energy into an objective 
(neglecting inefficiency of the optics) will emerge through the exit pupil of 
the optical device, he seems determined not to let that light into the eye 
from a telescope.  I do not think he is being realistic about this or is 
familiar with the optics of the eye.

One or two more points to try to shed light on the matter (sorry about pun):-

The exit pupil of an astronomical telescope is the image size of the objective 
lens itself which forms the aperture stop.

Take my simple cheap binoculars. Magnification ten; objective aperture 50 mm;  
focal length of objective 195 mm, focal length of eyepiece 19.5 mm. The 
aperture of the exit pupil(x) is therefore:-

x/19.5 = 25/214.5

which  = 2.2 mm.  real, and at 19.5 mm. from the eyepiece.
 
.....and hence is quite small enough to have the entrance pupil of the eye at 
this point and allow all the light pencil into the eye for a full field of 
view.  This is normal for astronomical telescopes.

The Galilean eyepiece however, produces a virtual exit pupil so there can be 
no field stop, no sharp boundary and the whole field cannot be seen at once, 
but the limiting factor is eventually the objective diameter. 
Magnification affects the field of view and illuminance at the image as normal 
with inverse functions with both.
--------------

There is a simpler analysis for consideration with the eye however, for 
magnification and image of the sun.

It is a curious co-incidence of nature that the diameter of the Sun, and the 
diameter of the Moon and the diameter of the fovea all subtend the same angle 
of about half a degree.
This means that the image of the Sun, or Moon, covers almost exactly the fovea in the naked eye.

If a telescope has unity magnification (1.0)  then the energy entering the 
objective is equal to that emerging frmo the eyepice; and as just indicated, 
all of which can enter the eye.  The energy enering the eye is now increased 
by the greater aperture of the objective compared to the eye pupil and is in 
proportion to the different aperture areas.  The energy of that part of the 
image which is the Sun is fully covering the fovea - and increased by the 
larger aperture of the telescope compared to the eye pupil.

Now consider the case if the magnification of the telescope used is say, two. 
Then the angle of emerging rays at the telescope is doubled, and image size 
on the fovea is doubled - but that is still half of the Sun's object size 
occupying the foveal area - and as discussed,  the flux density of light from 
the full aperture of the objective is placed there. It might be half of the 
flux energy from the telescope if it had a magnification of one - but is 
still enormous when the difference in apertures between eye pupil size and 
objective lens aperture size are considered.

Same argument if you increase the magnification of the telescope to four: you 
then have a quarter of the Sun's disc on the fovea, but still a massive flux 
increase in proportion to the objective aperture and eye pupil areas .. and 
quite enough to burn a hole in the retina.

Enough of that. I have made my point. It is reckless to use telescopes looking at the sun - anytime.

===============
I shall make another posting shortly and that involves the other thread with 
star distances and accuracy, but that will have to be later when I can find 
time to write about it and scan some documents which should give further 
interest/enlightenment.

Douglas Denny.
Chichester.  England.



--~--~---------~--~----~------------~-------~--~----~
Navigation List archive: www.fer3.com/arc
To post, email NavList@fer3.com
To , email NavList-@fer3.com
-~----------~----~----~----~------~----~------~--~---

   

Reply