Combination sun-moon filter

A combination sun-moon filter employs a first filter that is mounted at the front of the telescope and a second that is mounted in the telescope near the eyepiece. Each filter transmits approximately one percent of the light that strikes it, and both filters together reduce the amount of light leaving the viewing opening of the telescope to a value low enough for comfortable solar viewing. To convert to lunar viewing, it is only necessary to remove the front filter, and the remaining filter reduces light intensity enough for comfortable viewing of a bright moon.

BACKGROUND OF THE INVENTION 
The present invention relates to optical filters for telescopes, 
specifically those used for viewing the sun and the moon. 
It is common knowledge that direct viewing of the midday sun with the naked 
eye can be damaging to the eye, and this is particularly true when the sun 
is viewed through a telescope. Telescopic solar viewing therefore requires 
the use of filters to block out all but a very small fraction of the light 
reaching the telescope from the sun. Viewing of a bright moon through a 
telescope can also be uncomfortable, and moon filters are therefore often 
employed. 
From the point of view of the accuracy of the telescope system, it is best 
that the filter be positioned near to the eyepiece, or at least after the 
objective, so that the image will have been formed before any errors 
resulting from the filter are introduced. As a result, filters are 
sometimes positioned just inside the eyepiece. However, solar filters are 
frequently positioned at the front of the telescope so that the interior 
of the telescope will be shielded from the bulk of the solar radiation. 
This avoids the internal heating that would otherwise result, but the 
heating is avoided at the expense of accuracy because any distortion 
caused by the filter occurs before the focusing and image forming of the 
telescope. 
The object of the present invention is to permit easy conversion from sun 
viewing to moon viewing and back again. A further object is to accomplish 
this in an assembly that fulfills the purpose of avoiding solar heating of 
the telescope interior while minimizing the effect of any distortions 
introduced by the filter. 
SUMMARY OF THE INVENTION 
The foregoing and related objects are achieved in an improvement in a 
telescope having a front opening, a viewing opening, and an objective for 
focusing light rays traveling from the front opening to the viewing 
opening. The improvement includes a first optical filter removably mounted 
on the telescope substantially at the front opening. The first filter 
passes only a portion of the light rays that strike it. A second optical 
filter is removably mounted on the telescope so that light rays traveling 
from the objective to the viewing opening pass through it. The second 
filter also passes only a portion of the light rays that strike it, and 
the first and second filters together reduce the light intensity to a 
first predetermined percentage suited to reducing the intensity of the 
sun's rays to a comfortable level. The second filter reduces the light 
intensity to a second predetermined percentage suited to reducing the 
intensity of the moon's rays to a comfortable level when the first filter 
is removed from the telescope. Conversion from sun viewing to moon viewing 
is thereby possible by removal of the first optical filter. 
The first and second optical filters can conveniently include first and 
second sheets, respectively, of a synthetic organic material, each sheet 
having a metallic coating covering its surface, and the metallic coating 
can include a layer of aluminum. In the preferred embodiment, the sheets 
of synthetic organic material consist essentially of polyethylene 
terephthalate. It is recommended that the sheets of synthetic organic 
material be less than about 0.05 mm thick. 
Typical first and second optical filters include first and second support 
means, respectively, on which the sheets are mounted, the support means 
including mounting means for removably mounting the support means to the 
telescope. 
When the telescope includes a mirror mount mounted in the telescope 
centrally of the front opening, the first support means typically includes 
a generally circular peripheral portion, a hub portion disposed centrally 
of the peripheral portion, and strut portions extending from the hub 
portion to the peripheral portion to join it thereto. The sheet is 
attached at its edge to the peripheral portion of the support, and the hub 
is removably mounted on the mirror mount at the front opening of the 
telescope. A permanent magnet may be mounted on the mirror mount, and the 
hub portion of the first support means may include magnetically 
susceptible material magnetically attracted to the permanent magnet to 
hold the hub in place on the mirror mount. 
The telescope will include a tubular portion positioned for passage 
therethrough of light rays traveling from the objective to the viewing 
opening. The second filter support can include a generally cylindrical 
member received in the tubular portion of the telescope. 
The first predetermined percentage should be less than about 0.1%, and the 
second predetermined percentage should be between about 0.1% and about 
10%. The first optical filter should reduce the light intensity to a third 
predetermined percentage between about 0.1% and about 10%. The second and 
third predetermined percentages can conveniently be made to be essentially 
equal. 
The following description teaches a method of changing from solar to lunar 
telescopic viewing that includes the steps of providing a telescope and 
first and second filters as previously described, removing the first 
filter, and viewing the moon through the telescope. 
There is also taught a method of changing from lunar to solar telescopic 
viewing that includes the step of providing a telescope as previously 
described and a first optical filter removably mounted on the telescope so 
that light rays traveling from the objective to the viewing opening pass 
through it, the filter reducing the light intensity to a first 
predetermined percentage suited to reducing the intensity of the moon's 
rays to a comfortable level. The method further includes mounting a second 
optical filter on the telescope substantially at the front opening, the 
second filter and the first-mentioned filter together reduce the light 
intensity to a second predetermined percentage suited to reducing the 
intensity of the sun's rays to a comfortable level, and viewing the sun 
through the telescope. 
A combination sun-moon filter kit also achieves the objects of the 
invention when used with a telescope having a front opening, a viewing 
opening, and an objective for focusing light rays traveling from the front 
opening to the viewing opening. The kit includes a first optical filter 
that has a first filter sheet passing only a portion of the light rays 
that strike it, the first optical filter further including first support 
means on which the first sheet is mounted. The support means includes 
means for removably mounting the first support means substantially at the 
front opening of the telescope. The kit also includes a second optical 
filter including a second filter sheet passing only a portion of the light 
rays that strike it. The second optical filter further includes second 
support means on which the second filter sheet is mounted. The second 
support means include means for removably mounting it on the telescope so 
that light rays traveling from the objective to the viewing opening pass 
through the second filter sheet. The first and second filters together 
reduce the light intensity to a first predetermined percentage suited to 
reducing the intensity of the sun's ray to a comfortable level. The second 
filter alone reduces the light intensity to a second predetermined 
percentage suited to reducing the intensity of the moon's rays to a 
comfortable level. Mounting of the second optical filter by the second 
mounting means on the telescope so that light rays traveling from the 
objective to the viewing opening pass through the filter sheet thereby 
permits comfortable viewing of the moon through the telescope, and 
subsequent mounting of the first optical filter substantially at the front 
end of the telescope by the first mounting means without removing the 
second optical filter thereby permits comfortable viewing of the sun 
through the telescope.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 illustrates an exemplary telescope 10 of the type in which the 
filters of the present invention can be employed. The external 
configuration of FIG. 1 suggests that telescope 10 is a refractor, but 
those skilled in the art will appreciate as the description progresses 
that the principles of the present invention can be employed in any type 
of telescope. 
Telescope 10 is of conventional design, having a telescope tube 18 mounted 
on suitable mounting apparatus 20. At the front end of the telescope is a 
front opening 22 through which light is received that travels down tube 18 
and ultimately passes out of the telescope at the viewing opening 12. 
Between the front opening 22 and the viewing opening 12, the light is 
suitably focused in such a way as to magnify the image seen by the viewer. 
A prism assembly 16 is provided so that the viewer may look in a direction 
perpendicular to the axis of telescope tube 18, but those skilled in the 
art will recognize that the use of a prism 16 is not required. It is only 
necessary that a telescope tube 18 be provided and that suitable optics 
interior to the telescope properly focus the image for viewing. In a 
refractor telescope, the optics would include at least an objective near 
the front of the telescope for focusing the light entering front opening 
22. In refractors the objective is a lens, but the objective in a 
reflector would typically be a spherical mirror positioned near the end 
opposite the front opening. Whether the objective is a lens or mirror, its 
function is the same; all light entering the front opening 22 at a given 
angle is focused to a single point, and light entering at a different 
angle is focused to a different point. The light is the telescope 
continues past the focal plane defined by the points of focus and would 
typically be received by further lenses or mirrors. However, in the 
simplest case of a telescope used for photography, the telescope optics 
could consist of nothing more than the objective. Whatever the 
configuration of the telescope may be, the arrangement of the present 
invention employs a first filter at the front end and a second filter 
positioned in the optical path somewhere after the objective. 
FIGS. 2, 3, and 4 illustrate one embodiment of the first filter of the 
present invention. The operative feature of the filter is a filter sheet 
42 of some type of synthetic organic material, preferably a polyester such 
as polyethylene terephthalate. The sheet should be quite thin; in the 
preferred embodiment, the thickness is approximately one-hundredth of a 
millimeter, although sheets as thick as five-hundredths of a millimeter or 
more could be employed. However, thicker sheets add more refraction 
distance and more possibility for distortion, so sheets in the suggested 
range are preferred. 
The side of sheet 42 shown in FIG. 3 is coated with a thin layer of 
aluminum. The layer is deposited to a thickness such that it will reflect 
all but about one percent of the light striking the filter. Of course, 
filters employing the teachings of the present invention may well reflect 
different amounts; the determining factor, as will be seen below, is 
whether the first and second filters in combination reduce the amount of 
light sufficiently for comfortable viewing of the sun, and filters 
individually transmitting on the order of one percent of the light achieve 
this result. Aluminum coatings with sheet resistivities in the range of 
one-half to five ohms per square have been employed for such purposes. 
Sheet 42 is mounted, as seen in FIG. 4, on a support including an annular 
portion 44 concentric with a hub portion 48 and attached to it by means of 
strut portions 46 extending from the hub portion to the annular portion. 
As FIG. 3 shows, hub portion 48 has a knob 50 extending from the front 
side and adapted for convenient grasping by the user. The other side of 
the hub includes a relatively wide disc-shaped portion 40 made of a 
magnetically susceptible material and a boss 38 extending from its center. 
A ring magnet 34 is provided with an opening in which boss 38 can be 
received. 
FIG. 4 shows the first filter in an arrangement adapted for use on 
reflector telescopes having a mirror mount 62 disposed centrally of the 
front opening. Mirror mount 62 is disposed in the center of a glass piece 
60, which may be merely a flat glass plate in some types of telescopes. In 
others, member 60 could be a corrector plate. In any event, disc magnet 34 
is appropriately attached by cement or some other means to mirror mount 
62. Disc magnet 62 is intended to remain in position relatively 
permanently; it remains even when the first filter is removed. 
For normal solar viewing, central boss 38 is received centrally of disc 
magnet 34, which attracts the magnetically susceptible material in the hub 
of the filter support. Under ordinary conditions, this is sufficient to 
hold the first filter in place. By grasping knob 50, removal of the first 
filter is a relatively simple matter. Under windy conditions, it is 
advisable to provide some assistance, by any appropriate means, to hold 
the filter in place, the most reliable means being to attach it to the 
telescope about its periphery. 
An alternate for mounting magnet 34 is illustrated in FIG. 5, which 
portrays a secondary mirror 64 mounted at the front portion of another 
type of reflector. It is common in this type of telescope to have a 
threaded rod 66 extending forwardly from the front of the telescope. On 
such telescopes a T-nut 68 could be attached to ring magnet 34 so that the 
combination can be threaded on to the front of the telescope. The first 
filter would then be attached as before. 
FIG. 6 is a perspective view of the second filter of the present invention. 
The first and second filters are used together for solar viewing, and 
their combined effect is required to reduce the intensity of solar light 
rays to a comfortable level. In the preferred embodiment, the second 
filter includes a filter sheet 32 made of the same material as that out of 
which sheet 42 is made. Of course, it is not necessary that the materials 
or the fraction of light that they transmit be the same, but this is a 
convenient arrangement. Like the first filter, the second filter transmits 
one percent of the light that strikes it, so the first and second filters 
together only permit 0.01% of the light from the sun to pass out the 
viewing opening. (Solar filters that permit somewhat more or less than 
0.01% of the light to pass through can also be effective, but it is 
recommended that no more than about a tenth of a percent of the light be 
transmitted through the viewing opening.) 
Sheet 32 is mounted on a generally cylindrical support 26 that has a lip 30 
formed about the end to which sheet 32 is attached. The generally 
cylindrical portion 26 has protruding tabs 28 formed in it for a purpose 
illustrated in FIG. 7. In FIG. 7, second filter 24 is mounted in a 
horizontal tubular portion of the telescope extending horizontally from 
prism assembly 16. Tabs 28 are free at their ends near lips 30, and they 
are forced inwardly as the second filter is inserted into tubular portion 
56. The resultant outward force applied by the tabs effect a snug fit of 
filter 24 in tubular portion 56. When second filter 24 has been inserted 
to the desired distance, lip portion 30 of second filter 24 abuts the edge 
of tubular portion 56. 
As is well known in the art, prism 16 bends light traveling generally 
horizontally in FIG. 7 so that it adopts a generally vertical path. 
Ordinarily, an eyepiece such as eyepiece 12 (FIG. 1) is provided whose 
diameter is such as to fit within opening 52 of a tubular portion 54 
extending vertically from prism assembly 16. The inner diameter of tubular 
portion 54 is slightly greater than the outer diameter of tubular portion 
56 so that an eyepiece whose outer diameter is the same as that of 
horizontal tubular portion 56 can be either substituted for prism assembly 
16 or fitted into vertical tubular portion 54. It is thus apparent that 
second filter 24 could alternately be positioned on such an eyepiece. This 
is not ordinarily the preferred arrangement if the prism is being used, 
because the arrangement shown in FIG. 7 permits changing of eyepieces 
without removal of the filter. However, if no prism assembly such as prism 
assembly 16 is used, second filter 24 would have to be inserted into the 
eyepiece. 
It should also be pointed out that photographs are sometimes taken through 
a telescope without the use of an eyepiece. This type of photography is 
not illustrated in the drawings, but it can be appreciated by reference to 
FIG. 7 that tubular portion 56 is intended to be inserted into another 
tubular portion of the telescope having an inner diameter equal to that of 
tubular portion 54. The opening to such a portion becomes the viewing 
opening when eyepieces and prisms are removed, and second filter 24 should 
be inserted into that opening if solar photography is to be undertaken 
without eyepieces. Lunar photography can be performed without filters. 
Some colar correction may be desired in filters of this type in order, for 
instance, to restore the sun to its normal color. For ordinary amateur 
astronomical purposes, the color is not particular important, but it will 
be appreciated that most viewers would prefer to see the sun in the color 
with which they are familiar. Accordingly, colored glass may be provided 
in the second filter to restore the sunlight to its customery color. 
Ordinarily, the filter sheets 32 and 42 would be dyed a copper color to 
reduce to some extent the coloring resulting from the presence of the 
aluminum coating. It has been found desirable in such cases to add an 
orange-colored glass disc 58 in the second filter 24. The resulting solar 
image has the expected color. 
When the sun is to be viewed, both filters are mounted on the telescope as 
illustrated in the drawings. Since each filter reduces the light intensity 
by about 99%, the resulting intensity of the sun's rays as they leave 
viewing opening 12 is on the order of one-hundredth of a percent of that 
which would otherwise result. Both filters transmit equal percentages of 
the light that strikes them, but the first filter effects the overwhelming 
majority of the light removal in absolute terms. Accordingly, the amount 
of solar radiation that is admitted into the telescope is only one percent 
of that which would be admitted if the filtering were totally performed at 
the eyepiece end. As a result, heating of the telescope is greatly 
reduced. On the other hand, only half of the total thickness of the metal 
film is present on the first filter, so only half of the distortion 
introduced by the filter system occurs ahead of the objective; the 
remainder of whatever distortion is introduced occurs after the objective, 
where it will have less effect. Accordingly, telescope heating is largely 
avoided during solar viewing without introducing all of the filter 
distortion ahead of the objective. 
When it is desired to switch from solar to lunar viewing, it is not 
necessary to remove a sun filter and then attach a moon filter. All that 
is required is that the front filter be removed by merely grasping knob 50 
and pulling the front filter off permanent magnet 34. The one percent 
transmission afforded by the remaining second filter 24 is suited to 
comfortable viewing of a bright moon. Conversely, when conversion from 
lunar to solar viewing is desired, removal of a lunar filter is not 
necessary; it is only necessary to attach the sun filter in the manner 
illustrated in FIG. 4. 
It will be apparent to those skilled in the art that a variety of 
modifications of the preferred embodiment can be exployed without 
departing from the teachings of the present invention. For example, the 
division of filtering between the first and second filters can easily be 
different from that illustrated in the preferred embodiment. If it is 
thought that the telescope will tolerate more heating than is permitted 
with the first filter of the illustrated embodiment, the metal coating on 
the first filter could be made, say, half as thick as that in the 
illustrated embodiment. The coating on the second filter could then be 
made half again as thick, and the same amount of total filtering would 
result, but the distortion introduced by the filters would be concentrated 
even more after the objective, where its effect is less. Changes in the 
way that the filters are attached will also be obvious to those skilled in 
the art. For example, it may not be desired in certain telescopes to mount 
the front filter centrally. It would then, of course, be a simple matter 
to attach them peripherally. Changes in other aspects of the preferred 
embodiment that would still result in a device following the teachings of 
the present invention will also be apparent to those skilled in the art.