Digital Camera Patent AbstractA terrestrial telescope with digital camera has an imaging optical
system having a group of objective lenses and an imaging element
disposed at a position at which an optical image is formed by the
group of objective lenses. An observation optical system is provided
for observing the optical image transmitted by the group of objective
lenses. An optical element is inserted into the optical axis so
as to be retractable from the optical axis during imaging. The optical
element has a transmission surface having a single-eccentric toroidal
configuration with different curvatures in vertical and horizontal
planes of the optical element for correcting the position on the
imaging element at which the optical image is formed so that the
position on the imaging element at which the optical image is formed
remains substantially unchanged regardless of whether the optical
element is inserted into or retracted from the optical axis of the
group of objective lenses.
Digital Camera Patent ClaimsWhat is claimed is:
1. A terrestrial telescope with a digital camera comprising: a
group of objective lenses having an optical axis; an image pickup
device disposed along the optical axis of the group of objective
lenses at a position at which an image is formed by the group of
objective lenses; a quick-return half-mirror inserted into the optical
axis and at a rear side of the group of objective lenses so as to
be retractable from the optical axis during imaging; and an observation
optical system for observing an optical image reflected by the quick-return
half-mirror; wherein the quick-return half-mirror has a transmission
surface having a single-eccentric toroidal configuration with different
curvatures in vertical and horizontal planes of the quick-return
half-mirror for correcting the position on the image pickup device
at which the image is formed so that the position on the image pickup
device at which the image is formed remains substantially unchanged
regardless of whether the quick-return half-mirror is inserted into
or retracted from the optical axis of the group of objective lenses.
2. A terrestrial telescope with a digital camera according to claim
1; wherein the single-eccentric toroidal configuration of the transmission
surface of the quick-return half-mirror corrects image-formation
positional deviations in the horizontal and vertical planes and
astigmatism arising from the image-formation positional deviations
in the horizontal and vertical planes.
3. A terrestrial telescope with a digital camera according to claim
1; further comprising means for decentering the transmission surface
of the quick-return half-mirror with respect to the optical axis
to correct a shift in the optical axis in a direction intersecting
the optical axis.
4. A terrestrial telescope with a digital camera comprising: an
imaging optical system having an optical path and including a group
of objective lenses and an imaging element disposed along the optical
path at a position at which an image of a subject is formed by the
group of objective lenses; an optical element disposed on the optical
path of the imaging optical system between the group of objective
lenses and the imaging element so as to be retractable from the
optical path of the imaging optical system, the optical element
having a transmission surface having a single-eccentric toroidal
configuration with different curvatures in vertical and horizontal
planes of the optical element for correcting the position on the
imaging element at which the image of the subject is toned so that
the position on the imaging element at which the image of the subject
is formed remains substantially unchanged regardless of whether
the optical element is disposed on or retracted from the optical
path of the imaging optical system; and an observation optical system
for observing an optical image of the subject via the optical element
disposed on the optical path of the imaging optical system.
5. A terrestrial telescope with a digital camera according to claim
4; wherein the single-eccentric toroidal configuration of the transmission
surface of the optical element corrects image-formation positional
deviations in the horizontal and vertical planes and astigmatism
arising from the image-formation positional deviations in the horizontal
and vertical planes.
6. A terrestrial telescope with a digital camera according to claim
5; wherein the optical element comprises a quick-return half mirror.
7. A terrestrial telescope with a digital camera according to claim
4; further comprising means for decentering the transmission surface
of the optical element with respect to the optical path to correct
a shift in the optical path in a direction intersecting the optical
path.
8. A terrestrial telescope with a digital camera according to claim
7; wherein the optical element comprises a quick-return half-mirror.
9. A terrestrial telescope with a digital camera according to claim
4; wherein the optical element comprises a quick-return half-mirror. Digital Camera Patent DescriptionBACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a terrestrial telescope with a
digital camera that uses a quick-return half-mirror to split the
optical path so that the optical path goes to the image pickup device
and the observation optical system.
2. Description of the Prior Art
Terrestrial telescopes having a magnification factor ranging from
about 20 to 60 are used extensively for observing wild birds and
other fauna. Terrestrial telescopes include those based on a Galilean
telescope configuration comprising a positive (convex) lens and
a negative (concave) lens that functions as an erecting system,
and those based on a Keplerian telescope configuration comprising
just a positive (convex) lens, to which are added prisms or other
such elements to constitute an erecting system. Telescopes thus
configured enable a user to observe an erect image.
As well as being able to use such telescopes to observe natural
flora and fauna, users want to be able to record the images they
are seeing. The prior art includes a configuration comprising a
terrestrial telescope to which an imaging mechanism is added to
constitute a terrestrial telescope having a digital camera.
Except for the structure of the observation optical system, the
structure of the main optical system of the terrestrial telescope
with the digital camera is similar to that of a single lens reflex
camera. That is, a quick-return (hereinafter "QR") mirror
is inserted into the optical path of the main optical system (which
functions as the observation optical system) to guide the image
to the observation optical system.
The QR mirror can be a total-reflection type mirror. However, a
QR half-mirror is used in the case of a digital imaging system.
A spatial image formed by light reflected by the QR half-mirror
and passed through an erecting optical system can be viewed by a
user via the ocular lens of the observation optical system. In addition,
light transmitted by the QR half-mirror falls incident on the image
pickup device. By retracting the QR half-mirror from the main optical
axis during imaging, the total amount of the light that has been
split can be made to fall incident on the image pickup device. The
advantage of this configuration is that during the observation period,
autofocus control and exposure control can be carried out based
on image signals acquired by the image pickup device via the half-mirror.
However, if the QR half-mirror is a parallel plane glass plate,
there is a problem of deviation along the optical axis between the
position at which an image is formed by light from the optical system
transmitted by the QR half-mirror and the position at which the
image is formed when the QR half-mirror is retracted from the optical
axis. Moreover, shifting of the optical axis also results in a discrepancy
between the images obtained by the image pickup device during observation
and during imaging, producing differences between the image during
autofocus processing and the image that is actually taken. This
causes the imaging to be carried out with a focus position and field
of view that are different from those the user intended to use.
Astigmatism of the image transmitted by the half-mirror disposed
at an angle of 45 degrees further arises from the difference in
the length of the optical paths in the vertical and horizontal planes.
This degrades the sharpness of the image, which has an adverse affect
on the autofocus processing.
In JP2002-340426 A1, the present applicant proposed a configuration
to correct the axial deviation in image position by inserting a
parallel plane glass plate perpendicular to the optical axis, and
in JP2003-12953 A1 proposed a configuration in which a parallel
plane glass plate is inserted into the optical axis to correct the
shift in the image position.
However, these prior-art configurations increase the number of
optical components and require a mechanism for retractably inserting
the correctional parallel plane glass plate in addition to the QR
half-mirror, increasing the complexity of the mechanism and the
cost. Also, since the parallel plane glass plate is inserted during
the imaging, the amount of light incident on the image pickup device
is decreased, resulting in a photographic image that is dark compared
to the original optical performance.
An object of the present invention is therefore to provide a terrestrial
telescope with a digital camera that uses a simple configuration
that enables the image-formation errors caused by the QR half-mirror
to be corrected without the insertion of correctional optical components.
SUMMARY OF THE INVENTION
To attain the above object, the present invention provides a terrestrial
telescope with a digital camera that comprises an objective lens
group, a quick-return half-mirror disposed at a rear side of the
objective lens group that can be retractably inserted into an optical
axis of the objective lens group, an image pickup device disposed
at a rear side of the quick-return half-mirror, and an observation
optical system for observing an image reflected by the quick-return
half-mirror, wherein a transmission surface of the quick-return
half-mirror is formed as a correctional surface that corrects a
position on the image pickup device at which an image is formed
such that the position remains substantially unchanged regardless
of whether the quick-return half-mirror is inserted or retracted.
Further features of the invention, its nature and various advantages
will be more apparent from the accompanying drawings and following
detailed description of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a view illustrating the terrestrial telescope with a
digital camera according to the present invention, showing the optical
system of the telescope in vertical section together with the control
system;
FIG. 2 is a view illustrating the optical system of the telescope
of FIG. 1 in horizontal section;
FIG. 3 is a view illustrating a parallel plane glass QR half-mirror
in vertical section in a prior art arrangement;
FIG. 4 is a view illustrating a parallel plane glass QR half-mirror
in horizontal section in a prior art arrangement;
FIG. 5 is a view illustrating astigmatism produced in the case
of a parallel plane glass QR half-mirror arrangement according to
the prior art;
FIG. 6 is a view illustrating the correction of deviation of an
image-formation position in the QR half-mirror of the telescope
of FIG. 1; and
FIG. 7 is a view illustrating the correction of imaging optical
axis shift in the QR half-mirror of the telescope of FIG. 1.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The embodiments of the invention will now be described with reference
to the drawings.
In the embodiments described below, the transmission surface of
the quick-return half-mirror, as an optical element, for transmitting
light to the image pickup device during observation is constructed
as a correctional surface that corrects an image-formation position
so that it is substantially the same when the quick-return half-mirror
is inserted and when it is retracted. For this, the correctional
surface of the quick-return half-mirror is formed with a given curvature.
FIG. 1 is a side view of the optical system and control system
of the terrestrial telescope with a digital camera of this invention,
and FIG. 2 is a top view of the configuration shown in FIG. 1. FIG.
1 shows a vertical cross-section that contains the optical axis
of the optical system, and FIG. 2 shows a horizontal cross-section
that contains the optical axis of the optical system.
In FIGS. 1 and 2, a light beam transmitted by a group of objective
lenses comprised of a fixed lens group 1 and a movable focusing
lens group 2 falls incident on a quick-return half-mirror (abbreviated
to "QR half-mirror" hereinbelow) 3 that normally intersects
the main optical axis (the optical axis of the objective lens group)
at an angle of 45 degrees. The movable focusing lens group 2 can
be moved along the main optical axis by an AF (auto-focus) motor
4.
Unlike in the prior art in which the half-mirror is a parallel
plane glass plate, the QR half-mirror 3 has the shape shown in the
FIGS. 1 and 2. Before describing details of the shape of the half-mirror
3, the basic configuration of the optical system will be described.
An imaging optical system disposed along the main optical axis
includes the group of objective lenses and an imaging element. The
light transmitted by the QR half-mirror 3 falls incident on an image
pickup device 5 (such as a CCD or CMOS) as the imaging element located
on the focal plane. The light reflected by the QR half-mirror 3
falls incident on the observation optical system and, via an erecting
optical system composed of a combination of a penta roof prism (not
shown) or a reflecting mirror 6 and a relay lens 7, forms a spatial
image on a reticle 8 located at a position conjugate to the position
of the focal plane. Via an ocular lens 9, the user can view the
image as an erect image. The reflectance of the QR half-mirror 3
is arbitrary. However, a reflectance is selected to be 80% to 90%
to facilitate observation so that most of the light goes to the
observation optical system.
The QR half-mirror 3 is located at the position at which the image
is viewed via the ocular lens 9. The QR half-mirror 3 is rotatably
supported so that it can pivot about axis 3a. When a shutter release
button (not shown) or the like is operated to take an image, a quick-return
mechanism (not shown) swings the QR half-mirror 3 up in the direction
indicated by the arrow, retracting the QR half-mirror 3 from the
main optical system. With the QR half-mirror 3 thus retracted, all
the light from the objective optical system impinges on the image
pickup device 5.
The image pickup device 5 is driven by a CCD driver 13, via which
the image output of the image pickup device 5 is input to a control
circuit 14 composed of a microprocessor, memory and other such components.
Image data received from the image pickup device 5 during the imaging
is stored on a memory card or other such recording medium (not shown)
by the control circuit 14. In this embodiment, during observation
the light from the object is also able to enter the image pickup
device 5 via the QR half-mirror 3. Therefore, based on the image
information thus obtained from the image pickup device 5, it can
be processed for showing on a monitor (not shown), processed for
autofocusing by using the AF motor 4 to control the movable focusing
lens group 2, for exposure calculations (controlling the amount
of exposure when the shutter release button is half-pressed), and
for carrying out other such processing.
The configuration of the QR half-mirror 3 will now be explained.
The objective lens side of the QR half-mirror 3 is a plane constituting
a semi-transmitting surface, and the image pickup device side has
a three-dimensional (aspherically curved) shape as shown in FIGS.
1 and 2. The QR half-mirror 3 is given this shape so that, when
it is in the inserted position, it corrects deviation in the image-formation
positions along the optical axis and perpendicular to the optical
axis (image shift).
To consider what kind of correction should be effected by the QR
half-mirror 3, FIGS. 3 and 4 show the deviation that arises when
the QR half-mirror is a parallel plane type as used in the prior
art. FIG. 3 is a sectional view that cuts through the vertical plane
of a conventional parallel plane glass QR half-mirror 10 disposed
in the optical system, and FIG. 4 is a sectional view that cuts
through the horizontal plane of the QR half-mirror 10.
When the QR half-mirror 10 composed of parallel plane surfaces
as shown in FIG. 4 (and FIG. 3) is inserted, the image-formation
position undergoes a shift to the rear along the optical axis, as
indicated by a solid line, compared to the image-formation position
when the QR half-mirror 10 is not inserted as indicated by a broken
line. Moreover, as the QR half-mirror 10 is inclined with respect
to the optical axis, the image-formation position also undergoes
a shift downward from the optical axis by insertion of the QR half-mirror
10 compared to when the QR half-mirror 10 is retracted, again indicated
in FIG. 3 by the solid line and broken lines, respectively.
Thus, although the image pickup device 5 has to be located in the
position shown by a solid line in FIGS. 3 and 4, the image-formation
position undergoes a shift or deviation to the position shown by
the broken line when the QR half-mirror 10 is retracted. This discrepancy
between the positions shown by the solid and broken lines gives
rise to autofocus control error and image deviation.
In the case of the invention, the image pickup device side of the
QR half-mirror 3 is provided with a curvature, as shown in FIGS.
1 and 2, such that the image of the object is formed at the image
pickup surface of the image pickup device 5 regardless of whether
the QR half-mirror 3 is inserted or not. That is, in shape, the
transmission surface of the QR half-mirror 3 is a single-eccentric
eccentric toroid that is curved with a given curvature in the horizontal
plane and eccentrically curved in the vertical plane with a different
curvature.
An image transmitted by a QR half-mirror inserted at an angle to
the optical axis is intended for use in AF control or the like in
the invention. Therefore, the image-formation deviations as follows
should be taken into consideration: (1) deviation along the optical
axis, (2) astigmatism produced by the insertion of the QR half-mirror
at an angle, and (3) deviation (image shift) in a direction that
intersects (is perpendicular to) the optical axis. (1) and (3) are
as have been described with reference to FIGS. 3 and 4. The astigmatism
of (2) is illustrated in FIG. 5. The explanation will be made according
to FIG. 5 with respect to astigmatism caused when the QR half-mirror
is inserted at an angle to the optical axis.
In FIG. 5, a difference between the lengths of the optical paths
through the half-mirror (indicated by the thick lines) produces
focal positional deviation in the vertical and horizontal planes
when the parallel plane glass half-mirror 10 is inserted at an angle
of 45 degrees to the optical axis. The deviation .delta.1 in the
image-formation position along the optical axis in the vertical
plane caused by the insertion of the QR half-mirror 10 can be expressed
using Equation (1), taking into account the geometrical relationship
of movement of the image-formation position along the optical axis
by the central and peripheral beams. Here, the glass (or other suitable
material) of the QR half-mirror 10 is assumed to have a refractive
index of n, the angle of incidence of the central beam on the QR
half-mirror 10 is assumed to be 45 degrees, and the angle of incidence
of the peripheral beam on the QR half-mirror 10 is assumed to be
.phi..
.delta..times..times..times..PHI..times..times..PHI..times..times..times..-
times..times..times..PHI..times..times..PHI..times..times..times..times..P-
HI..times..times..PHI..PHI..times..PHI. ##EQU00001##
The deviation .delta.2 in the image-formation position along the
optical axis in the horizontal plane can be approximated using Equation
(2) from the thickness d' and the refractive index n of the QR half-mirror
10.
.delta.'.function. ##EQU00002##
The term relating to the angle of incidence .phi.' of the peripheral
beam is negligibly small, and the image deviation .delta.2 depends
mainly on the thickness d' and refractive index n. It is to be noted
that, when the correction amounts .delta.1 and .delta.2 are calculated
to correct the image-formation positions described below, the beam
in the paraxial region is used as a peripheral beam shown in FIG.
5 having an angle of incidence that is very close to 45 degrees
(especially in Equation (1)). So in the case of QR half-mirror 10
having a thickness of 1 mm and a refractive index of 1.51633, for
example, the focal deviation .delta.1 along the optical axis would
be about 0.60 mm in the vertical plane and the focal deviation .delta.2
along the optical axis would be about 0.37 mm in the horizontal
plane.
According to the definition, the astigmatism produced by the vertical
and horizontal focal discrepancies is represented as 0.60-0.38=0.22
(mm). That is, the astigmatism is caused by the image-formation
positional deviations (the above (1)) along the optical axis in
the vertical and horizontal planes when the half-mirror is inserted
at an angle to the optical axis. If this astigmatism is left uncorrected,
it has an adverse effect on the image sharpness and on the AF control
and the like.
As described above, the insertion of the QR half-mirror 10 at an
angle of 45 degrees, as shown in the vertical plane of FIG. 5, also
causes the optical axis to be shifted by .DELTA.y (image shifted)
in the perpendicular direction.
In this embodiment, as shown in FIG. 1, the transmission surface
of the QR half-mirror 3 is formed as a correctional surface that
performs image-formation positional correction so that the image-formation
position relative to the image pickup device 5 when the QR half-mirror
3 is inserted is substantially the same as that when the QR half-mirror
3 is retracted. This correctional surface is formed as a special
curved surface that (1') corrects the image-formation positional
deviations along the optical axis that differ in the vertical and
horizontal planes, (2') corrects astigmatism at the same by means
of (1'), and (3') corrects an image shift (shift of the imaging
optical axis) in a direction that intersects (is perpendicular to)
the optical axis by making the curved surface eccentric.
For (1'), a toroidal surface is used having different curvatures
in the vertical and horizontal planes of the QR half-mirror 3. And
for (3'), the principal point of the curved surface is decentered
(translated in parallel) within the vertical plane.
The working principle of the decentered toroidal surface will now
be described.
FIG. 6 depicts a typical optical system of the embodiment, showing
only the positions of the optical components. In FIG. 6, F2 denotes
a QR half-mirror (QR half-mirror 3 of FIG. 1) and F1 denotes the
imaging optical system up to the front of the QR half-mirror F2
(the objective lens group of FIG. 1). The QR half-mirror F2 has
a semi-transmitting (reflecting) surface F2a on the optical system
F1 side and a transmission surface F2b on the opposite side, and
has a thickness d (not fixed). In FIG. 6, these surfaces are shown
only as straight lines perpendicular to the optical axis.
F denotes the image-formation position of the optical system at
which the image pickup device 5 is located. Position F is a position
with the QR half-mirror F2 in the retracted state. f1 denotes the
overall focal length of the optical system (F1 to F). F' is the
position at which the image is formed when a conventional parallel
plane half-mirror is in the inserted state, producing a focal deviation
of .delta.1 to the rear of the image-formation position F, as shown
in FIG. 5 and elsewhere. This deviation .delta.1 is proportional
to the thickness d of the QR half-mirror F2. In this embodiment,
the transmission surface F2b is a toroidal surface with different
curvatures in the vertical and horizontal planes to individually
correct the optical axial deviations .delta.1 and .delta.2 shown
in FIG. 5.
The effects of the curvatures of the transmission surface F2b in
the vertical and horizontal planes will now be explained.
First, the required refractive power of the curved surface in the
vertical plane will be considered. The transmission surface F2b
of the QR half-mirror F2 in the vertical plane is given a curvature.
The image-formation position F'' obtained when the thin lens constituted
by the transmission surface F2b of the QR half-mirror F2 is inserted
is away from the principal point of the thin lens by a distance
S. The refractive power of the thin lens can be expressed as 1/f2,
where f2 is the focal length of the QR half-mirror F2.
If X is the distance from the principal point of the optical system
F1 to the principal point of the thin lens constituted by the transmission
surface F2b, the above distance S will remain the same even if the
QR half-mirror F2 is inserted at an angle of 45 degrees, as shown
in FIG. 1. This is because the curve forming the transmission surface
F2b in the vertical plane is an arc. The distance S can be shown
by the following equation.
.times. ##EQU00003##
In practice, the focal deviation .delta.1 is produced by the thickness
d of the QR half-mirror F2. The distance S, therefore, has to satisfy
the following condition so that the final image-formation position
is the same F as that when the QR half-mirror F2 is retracted. S=f.sub.1-X-.delta..sub.1
(4)
It is to be noted that, in Equation (4), the deviation .delta.1
produced by the thickness d of the QR half-mirror F2 can be calculated
using Equation (1) with the thickness d whose value corresponds
to that in the vicinity of the optical axis.
Therefore, from the Equations (3) and (4) the focal length of the
thin lens constituted by the transmission surface F2b in the vertical
plane can be obtained as follows.
.delta. ##EQU00004##
For example, when f1 =100 mm, X =70 mm and d =1 mm, .delta.1 =0.603511952
mm and f2 will be 1461 mm.
This focal length f2 and the refractive index of the material of
the QR half-mirror F2 can be used to calculate the required curvature
of the transmission surface F2b in the vertical plane. Providing
the transmission surface F2b in the vertical plane with a curvature
that satisfies Equation (5) results in zero focal deviation in the
vertical plane, making it possible to locate the image-formation
position at F when the QR half-mirror F2 is in the inserted state
or in the retracted state.
The required curvature of the transmission surface F2b in the horizontal
plane is calculated in the same way. That is, the horizontal-plane
focal deviation .delta.2 to be corrected is substituted for .delta.1
in Equation (5), which allows the focal length f2' of the curved
surface in the horizontal plane constituted by the transmission
surface F2b to be obtained as follows.
'.delta. ##EQU00005##
For example, when f1 =100 mm, X =70 mm and d =1 mm, .delta.2=0.36770691
mm and f2' will be 2418 mm.
In this way, the curved transmission surface F2b of the QR half-mirror
F2 is formed as a toroid (donut shape) in which the surface has
curvatures having different refractive powers of 1/f2 and 1/f2'
in the vertical and horizontal planes, respectively. Using this
toroidal shape makes it possible to provide the different curvatures
required in the vertical and horizontal planes.
Of the problems described above, (1) image-formation positional
deviations along the optical axis, which differ in the vertical
and horizontal planes, can be corrected, and at the same time, (2)
astigmatism caused by differences in focal deviations in the vertical
and horizontal planes can be eliminated.
In addition, (3) the shift of the imaging optical axis (image shift)
in a direction that intersects (is perpendicular to) the optical
axis can be corrected by making the curved transmission surface
F2b of the QR half-mirror F2 eccentric to translate the center of
the QR half-mirror F2 through which the optical axis elongates.
That is, as shown in FIG. 7, the optical axis deviation .DELTA.y
can be cancelled by vertically offsetting the principal point of
the curved transmission surface F2b by the amount .DELTA.y'. This
shift .DELTA.y of the optical axis is the same as the .DELTA.y of
FIG. 5, and in accordance with Snell's law, can be calculated based
on the curvature and thickness d of the QR half-mirror F2, again
using a value in the vicinity of the optical axis for the thickness
d.
In FIG. 7, the angle of the optical axis that produces the optical
axis shift .DELTA.y is .theta., so that the amount of eccentricity
.DELTA.y' of the curve imparted to the transmission surface F2b
can be obtained as follows.
.DELTA..times..times.'.times..times..times..theta..times..DELTA..times..ti-
mes. ##EQU00006##
For example, when f1=100 mm, X=70 mm and d=1 mm, .DELTA.y is calculated
to be 0.33 mm for f2=1461 mm, and .DELTA.y' is found to be 16.3
mm. Therefore, the vertical image shift can be cancelled out by
vertically offsetting the principal point of the curved transmission
surface F2b by 16.3 mm downwards relative to the optical axis.
As mentioned above, the transmission surface F2b is curved into
a shape that, (1') corrects image-formation positional deviations
along the optical axis in the vertical and horizontal planes, (2')
corrects astigmatism at the same by means of (1'), and (3') corrects
the optical axis shift (image shift) in a direction that intersects
(is perpendicular to) the optical axis. This allows the corrections
of (1) image-formation positional deviation along the optical axis,
(2) astigmatism produced by the insertion of the QR half-mirror
at an angle, and (3) shift of the optical axis (image shift) in
a direction that intersects (is perpendicular to) the optical axis.
Thus, even when the QR half-mirror is inserted, the image pickup
device 5 can capture an image in the same way as when the QR half-mirror
is retracted, eliminating the problem of the imaging being carried
out with a focus position and field of view different from those
the user intended. The arrangement also eliminates the adverse effects
on the autofocus processing.
Next, the operation of the terrestrial telescope with digital camera
thus configured will be described.
In an observation mode, the shutter release button (not shown)
is half-pressed with the QR half-mirror 3 in the position shown
in FIG. 1 under the control of the control circuit 14. Light from
the object then enters the image pickup device 5 via the QR half-mirror
3 for photoelectric conversion to detect the brightness of the light
and the contrast using a conventional contrast detection method.
Based on the detected brightness of the object, the control circuit
14 sets the speed of the electronic shutter of the image pickup
device 5 and, based on the detected contrast, operates the AF motor
4 to control the autofocusing by moving the movable focusing lens
group 2 along the optical axis. The control circuit 14 drives the
AF motor 4 to move the movable focusing lens group 2 to the focus
position in accordance with changes in the object contrast so that
the contrast of the image of the object formed on the image pickup
device 5 is made maximum. Instead, a configuration can be used in
which autofocusing control is always carried out rather than being
triggered by half-pressing the release button.
Following this, the imaging timing is decided by fully depressing
the release button under the control of the control circuit 14.
The controller 14 then operates a quick-return mechanism (not shown)
to retract the QR half-mirror 3 from the main optical system in
the direction indicated by the arrow. Then, the electronic shutter
is opened for the set time to image the object. The electronic image
data thus obtained can be stored in a storage medium such as a memory
card or the like, and transmitted out over a network or the like.
After imaging has been completed, the control circuit 14 uses the
quick-return mechanism to return the QR half-mirror 3 to the observation
position of FIG. 1.
In carrying out the above observation and imaging operations, the
transmission surface of the QR half-mirror 3 used to split the optical
path in the terrestrial telescope with digital camera according
to this invention has a single-eccentric toroidal configuration
having a given curvature in the horizontal plane and an eccentric
different curvature in the vertical plane, thereby making it possible
to correct the imaging optical axis shift in a direction that intersects
(is perpendicular to) the optical axis as well as image-formation
positional deviations along the optical axis.
Since in accordance with this invention the image-formation position
remains the same regardless of whether the QR half-mirror 3 is inserted
into the optical path during observation or retracted from the optical
path during imaging, it is possible to substantially eliminate autofocusing
control error and image deviation during observation and imaging.
Moreover, this is done using just the QR half-mirror 3, unlike in
the prior art arrangements in which it was necessary to provide
a correctional glass plate and a mechanism for inserting and retracting
the plate, so the present invention has the merit of having a simple
structure. Furthermore, since there is also no loss of light intensity
caused by the use of a correctional glass plate, there is no degradation
in the performance of the optical system, making it possible to
obtain high-quality images.
Thus, this invention makes it possible to correct the image-formation
position of the QR half-mirror using a very straightforward arrangement
that does not require additional correctional optical components.
The invention is therefore highly effective with respect to obtaining
high-quality images because the sharpness of the images is not degraded
by autofocusing control error and image deviation.
To facilitate the above explanation, the QR half-mirror 3 was described
as being inserted into the main optical system at an angle of 45
degrees. However, it is to be understood that the invention is not
limited to this, but that the angles at which the QR half-mirror
3 is disposed relative to the main optical system, as well as other
design conditions, can be suitably modified as required.
As described in the foregoing, the terrestrial telescope with a
digital camera according to this invention comprises an objective
lens group, a quick-return half-mirror disposed at a rear side of
the objective lens group that can be retractably inserted into an
optical axis of the objective lens group, an image pickup device
disposed at a rear side of the quick-return half-mirror, and an
observation optical system for observing an image reflected by the
quick-return half-mirror. In the invention, a transmission surface
of the quick-return half-mirror is formed as a correctional surface
having a toroidal shape that corrects image-formation positional
deviations caused by insertion or retraction of the quick-return
half-mirror. This makes it possible to correct the image-formation
positional deviations caused by the QR half-mirror using a very
simple arrangement that does not require additional correctional
optical components. The invention is therefore highly effective
with respect to obtaining high-quality images because the images
are not degraded by autofocusing control error and image deviation.
|