.965 illuminated reticle

Anybody know if these are/were made? I have built a right angle finder scope
that uses .965 eyepieces, but it has no crosshairs in it so I was wondering
if there is such an animal as an illuminated one?

Thanks

Tom Wales

Tom Wales:

Tom Wales wrote:
Anybody know if these are/were made? I have built a right angle finder scope
that uses .965 eyepieces, but it has no crosshairs in it so I was wondering
if there is such an animal as an illuminated one?

Thanks

Tom Wales

To avoid light adaptation of the eye a specific range of illumination
wavelengths must be used.

Dark adaptation, the opposite of light adaptation, occurs in the eye in two
distinct ways.

One, the iris closes down to reduce the aperture and the amount of light
entering the eye. That also increases the depth of field (or depth of focus
of the eye) and creates a sharper image. Sky viewing requires more light so
the focusing is more the task of the telescope optics. Small apertures of
the eye are not too useful, therefore.

Secondly, the aqueous humour of the eye darkens as a consequence of
increased amounts of light entering the eye. That is an automatic chemical
process of the darkening of the fluid in the eye due to the presence of
light. Similarly, some photochromic sunglass lenses are also
self-darkening. In the absence of light the aqueous humour becomes more
clear, and more light is admitted to the retina to create images. Dark
adaptation in the absence of light occurs to about 50% adaptation after
about 15 min. in the dark. Something like 80% dark adaptation occurs after
about 30 min., and 99% to 100% dark adaptation occurs after about 45 to 60
min. Small amounts of light may make it necessary to return to the dark for
15 to 20 minutes. Scientists wanting to see individual photons would use 2
to 3 hours for total dark adaptation, and to reduce EM or thermal /chemical
noise.

An auto headlamp really does a job on one's visual dark adaptation, and
consequently lowers one's ability to see faint light sources. So does light
cast upwards from towns.

To remain in a completely dark adapted state a non-light leaking pair of
goggles is needed that have only a specific type of red filter lens.
Ordinary red transparent plastic or glass filters are not useful for that
purpose. WWII German pilots would wear red filter goggles at all times on
the ground while waiting to fly. Once airborne they could remove the
goggles, and they would be totally dark adapted.

The ideal red filter material is transparent to light but will only pass
red light of long wavelengths. That same material does not pass other
shorter wavelengths. All other spectral color light sources, e.g., violet,
blue, green, yellow, and orange will appear black when viewed through the
filter. Sources of red light will appear red, and only sources of red light
will have any color, that of red. What one sees is only black and red
images. Not too comforting when walking around for there may not be a lot
of information available by seeing with that type of light. Auto tail light
lenses appear to be red in color, however, in reality they pass many other
colors as well as red. They are more of a strongly red tinted white light
lens because the pass so much other light. One might think that dark
adaptation would be good for that application, and that a sharp cut off at
the far red end of the wavelength spectrum would be good. Not so, but for
another reason. All colors except for a specific small range of
yellow-green wavelengths are only visible in a small central area of the
eye, that is, in the fovea, or the central region of color vision. Only the
fovea sees red. That color vision region is small, and it represents only
approximately 3 degrees of the angle of vision. If a sharp cut-off far-red
filter were used for the auto lights, and if the surrounding area was black
or dark, and one looked at the light source at, say, a few degrees off the
center of the light source, the far-red filtered light source could not be
seen at all. The tail lights would be invisible unless one looked directly
at them.

A reticle, in order to be visible should be lighted. Far-red wavelengths
with a sharp cut off would be good to reduce light adaptation. The red
light would only be visible in the central 3 degrees of vision from the
eye. In a wide angle scope there may be regions of vision, say where the
eye is pointed toward the quadrants between the lines, and no reticle would
be visible.

From the standpoint of dark adaptation green or any non-red lines would
not be good for reticles, and far-red color lines would be the best.

Astronomers who use flashlights or other red lights may not get the full
benefits of dark adaptation, and they may need additional dark time to get
fully adapted. The filters are not sufficient.

Ordinary 1-1/2 volt AA, BB, and CC battery powered incandescent lamp
flashlights produce lots of far-red photons. Halogen, Xenon, or LED sources
produce extra light, but most of that light is in the shorter wavelengths
that would be filtered out by the far-red sharp cut off filters. Sources
with such far-red filters would illuminate the reticle.

To evaluate the filter material, either glass or plastic, consult the
maker's light transmission spectrum plotted graph for that material. The
spectral frequencies would be plotted along a horizontal line, and the
amounts of photons at each wavelength would be plotted vertically. That
tells us how much light of each wavelength would be passed or absorbed by
the filter material. It will pass little or no amounts of light in the
wavelengths shorter than the far-red, and it will pass lots of light in the
far-red. The graph would look like this:

_ red
violet ______/
-------

Wavelength: shorter - longer
Quantity of photons: ^ up is more, down is less.
Zero quantity line: ---------------
The amounts of non-red light should be closer to zero than is shown above.

No other wavelengths should be passed for safety's sake, and small amounts
of illumination should do the job.

Ralph Hertle

"Tom Wales" wrote in message
news:gEmBd.8371$1U6.1545@trnddc09...
Anybody know if these are/were made? I have built a right angle finder
scope that uses .965 eyepieces, but it has no crosshairs in it so I was
wondering if there is such an animal as an illuminated one?

Thanks

Tom Wales


I've seen 'em. They're marked ASTRO and made in japan. Seems to me the
focal lengths were shorter than you'd want for a finder though, I think they
were for guiding.

Ed T.