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Off-Axis Parabolic (OAP) Mirrors: Precision Solutions for Advanced Optical Systems

An off-axis parabolic mirror is an optical device that converts plane waves to spherical waves and vice versa. This makes it handy for both collimating light from a point source and focusing collimated light on a specific spot. It does so with remarkable precision and is independent of wavelength. Shanghai Optics’ high-quality off-axis parabolic mirrors can be utilized for broadband UV, visible, and IR applications.

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Understanding Parabolic Mirrors

A parabolic mirror is a reflector with the shape of a circular paraboloid. A conventional parabolic mirror, which is usually composed of a highly reflective material like aluminum, will be centered on the paraboloid's vertex. This is the point at which the curvature is highest and the axis divides the paraboloid symmetrically.

Parabolic mirrors are good at collimating light and focusing it to a specific point, even though the receiver frequently casts a shadow on the paraboloid’s vertex. This prevents a considerable portion of the mirror from reflecting light, resulting in severe signal loss.

Off-axis parabolic mirrors are designed to solve this problem. These mirrors are sections of a parent paraboloid obtained from a position other than the central axis. An off-axis parabolic mirror can have a high or small angle, depending on how far it is off the axis.

However, since these mirrors focus light to offset points outside the collection region, there is no problem with receiver shadowing. This makes off-axis parabolic reflectors the optimal optic for long-range and precision instrumentation. The mirrors’ free access to the system’s focus point allows for more compact system designs.

Off-set angle: This represents the angle between the optical axis and the direction of incoming or outgoing light in the off-axis parabolic mirror

Parent focal length: This is the focal length of an on-axis parabolic mirror with the same shape as the off-axis mirror

Effective focal length: This represents the practical focal length of the off-axis parabolic mirror, taking into account both the offset angle and any deviation from the ideal configuration.

The following variables are also highly important in determining the effectiveness of OAP Mirror:

Protected Coating and Reflective Coatings

The mirrors can be coated with a protective aluminum layer to increase durability and reflectivity over a range of wavelengths. The mirrors can be applied with a variety of reflective coatings that are suited to specific spectral regions and application requirements.

Coating Options Included

  • Ion-plated protected silver coating (for NIR, up to 2 µm)
  • Protected gold coating (for 750 nm to far infrared applications)
  • Improved aluminum coating with 99% reflectivity (for visible spectrum)
  • 99% dielectric coating for laser use (visible spectrum)
  • UV aluminum/MgF2 coating for laser use (near-infrared region)

Chromatic Aberration

OAP mirrors reduce chromatic aberration by focusing all wavelengths at a single point, improving image clarity and precision.

Diamond Turned Optical Elements

The OAP mirrors are diamond turned, which provides great accuracy and surface quality in demanding optical applications.

Mounting the Off-Axis Parabolic Mirror

Each mirror features a flat back with three threaded mounting holes for sturdy installation. Typically, an adapter plate is installed between the OAP’s rear surface and a kinematic mirror mount. Off-axis mirrors are not rotationally symmetric, hence rotation around the optical axis must be limited. Precision machined mounts are suggested for accurate alignment of the six degrees of freedom.

Discover Off-Axis Parabolic Mirrors at Shanghai Optics

Video credit: Shanghai Optics Inc.

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Applications of Off-Axis Parabolic Mirrors

OAP metal mirrors are utilized in spectrometers, interferometers, astronomical optical instruments, spectrum analyzers, as well as beam expanders and collimators. They can be used in series to form a relay system since several mirrors allow users to transition between the focal plane and pupil plane of the system without sacrificing image quality.

Limitations of an Off-Axis Parabolic Mirror

Off-axis parabolic mirrors should only be utilized with infinite conjugates. They can generate a completely collimated beam from a spherical wave or perform diffraction-limited imaging when focusing a collimated beam, but when utilized at finite conjugates, the image quality is poor.

While employing OAPs, keep in mind that orientation is vital. When a collimated beam is incident on the off-axis mirror from an off-axis focal angle, it does not generate a diffraction limited image. If a spherical wave enters on axis, it will not form a collimated beam.

How are Off-Axis Parabolic Mirrors Produced?

Manufacturers have numerous options for producing off-axis parabolic mirrors. One way is to use a rotating furnace to produce mirrors from a molten base material. A simpler approach is to cut and shape off-axis paraboloids from metal blanks. Shanghai Optics uses the latter technology to make high-quality standard and custom OAP mirrors quickly.

To provide the best results, Shanghai Optics hand-polish the parabolic mirrors from large aluminum blanks with an accuracy of 1/20 lambda RMS. The regular mirrors range in diameter from 2.5 to 10 inches, with curvatures of 15, 30, 45, and 60 degrees off-axis.

They also provide off-axis parabolic mirrors in custom sizes and coatings upon request, allowing users to choose an OAP mirror that meets the particular project’s needs.

Off-Axis Parabolic (OAP) Mirrors: Precision Solutions for Advanced Optical Systems

Image Credit: Shanghai Optics Inc.


Off-Axis Parabolic Mirror Factory Standard

Positioning a point source or spherical wave at the focal point of an OAP mirror allows it to capture collimated light. Similarly, it can focus collimated light on a single location by directing it along the optical axis.

Source: Shanghai Optics Inc.

Specification Dimension  
Material   Aluminum
Diameter tolerance mm +0, -0.2
Focal length tolerance   +/- 1%
Off-axis distance tolerance mm +6, -0
Surface accuracy   1/10 lambda RMS
Surface quality   60-40
Note   Optics design available upon request


Source: Shanghai Optics Inc.

Specification Dimension Part Number
    OAPM-15-10 OAPM-30-20 OAPM-45-10 OAPM-60-20
Type   15° Off-Axis 30° Off-Axis 45° Off-Axis 60° Off-Axis
Diameter mm 25.4 50.80 25.4 50.8
Effective Focal Length EFL mm 387.60 54.45 148.79 135.45
Y Offset mm 3.95 27.20 4.14 4.62
Note   other size and custom coating are available upon request


Off-Axis Parabolic Metal Mirrors

Source: Shanghai Optics Inc.

Part Number Focal Length Diameter, D Thickness, Te Off-axis distance, Off-axis angle
  (m) (Inches) (mm) (mm) OAD(mm) degrees
  0.5 2.5 63.5 15 60 6.9
  0.75 3 76.2 15 80 6.1
  1 2.5 63.5 5 60 3.4
  1 4 101.6 17 110 6.3
  1.25 3 76.2 15 80 3.7
  1.25 5 127 20 100 4.6
  1.5 6 152.4 25 100 3.8
  2 4 101.6 17 110 3.2
  2 5 127 20 100 2.9
  2 8 203.2 33 150 4.3
  2.5 10 254 45 175 4
  3 6 152.4 25 100 1.9
  4 8 203.2 33 150 2.2
  5 10 254 42 175 2


Mounting Off-Axis Parabolic Mirror

Each of the OAP mirrors’ flat backs has three threaded mounting holes. An adapter plate is typically put between the OAP rear surface and a kinematic mirror mount.

Off-axis mirrors are not rotationally symmetric; therefore, rotation must be controlled around the optical axis. A precision machined mount is recommended since the alignment of the other six degrees of freedom is quite sensitive.

Alignment of an OAP

To align the off-axis parabolic mirror to collimate a spherical wave, follow these steps:

  • Ensure that the incoming beam is at the proper height and travels parallel to the optical reference surface
  • Adjust the height of your mount so that the center of the mirror corresponds to the center of the incident light
  • Position the center of the mirror one focal length (reflected focal length) from the origin of the spherical
  • Match the angle of the mirror to the reflection angle specified in the optical system
  • To check for collimation, use a shear plate interferometer. This is a four-step method in which users first check collimation in one plane, make any required adjustments, check collimation in the orthogonal plane, and then return to the first plane and readjust. Each plane will have to be examined at least twice, if not more because the modifications are not decoupled
  • Finally, after making adjustments, return to the first plane and readjust. Since the modifications are not decoupled, each plane must be inspected at least twice, if not more.
  • Finally, confirm that the output beam is parallel to the reference surface

Specifications of Off-Axis Parabolic Mirrors from S.O.

Shanghai Optics produces off-axis parabolic metal mirrors with curvatures of 15°, 30°, 45°, and 60°. The standard mirrors are made from premium aluminum and range in diameter from 2.5 to 10 inches. Shanghai Optics hand-polishes the mirrors to an accuracy of 1/20 lambda RMS.

Users can locate an OAP mirror that precisely matches the project specifications; the company can also supply off-axis parabolic mirrors in custom sizes and coatings upon request.

Among the potential coatings are:

  • Ion-plated protected silver coating (for NIR, up to 2 um)
  • Protected gold coating (for 750 nm to far infrared applications)
  • Enhanced aluminum coating with 99% reflectivity (for visible spectrum)
  • 9% dielectric coating for laser use (for visible spectrum)
  • UV aluminum/MgF2 coating for laser use (near-infrared region)

The optical engineers can assist users in designing and manufacturing a mirror that best suits their application and are ready to provide off-axis parabolic mirror consulting.

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