Off-axis Parabolic Mirrors
The need for powerful reflective optics operating over a wide range of wavebands has driven the development of off-axis parabolas that have higher imaging performance than ever before. Everything about off-axis parabolas has changed in the past five years – tolerances for quality and performance, manufacturing methods, testing methods, thin film coatings, and alignment techniques. OAS has redesigned off-axis parabolas to fit today’s application needs.
To achieve the highest quality requires comprehensive consideration of mechanical design, optical quality, and mounting and alignment.
On the mechanical design side, we factor in reference surfaces that allow us to calibrate off-axis angles, focal lengths, and the positioning of the OAP to its parent axis. Tolerance drawings are provided free of charge to all customers. On the optical quality side, we consider not only surface form errors, but also wavefront quality, gradient errors, mid-spatial frequency tolerances, and directly relate wavefront quality to the quality of focus you can expect to achieve. Finally, the best OAP is useless if it is deformed in its mount and its alignment cannot be optimized. Therefore, we provide mounting solutions with the alignment position resolution you need to optimize the performance of the mirror.
Specifications:
- Sizes: 50mm-800mm
- Materials: Fused Silica, Zerodur / Clearceram, Single Crystal Silicon and Silicon Carbide
- Off-Axis Angles: 0-90°
- Focal Ratio (F/#): F/1 - F/20
- Design Services
- Custom Optomechanical Assemblies / Mounting Brackets
- Quality Grade Options for Off-Axis Paraboloids (OAPs): We offer a variety of quality grade options, depending on the desired application. These quality grades can vary significantly in price so that customers can choose the most economical option that best suits their application and budget. When specifying the optical quality of aspheric optics, traditional approaches do not typically address mid-spatial error tolerance, which is critical to predicting imaging quality.
| Grade | OAS Definition | Diffraction Limit |
|---|---|---|
| Standard | For spatial scale length > 10 mm, RWE < 63 RMS | not applicable |
| Precision | For spatial scale lengths > 1 mm, RWE < 32 RMS | = 1.3-1.5 times the diffraction limit |
| Controlled PSD | ||
| High Performance | RWE < 12.6 RMS for spatial scale length > 1 mm | = 1.2-1.3 times the diffraction limit |
| Relative intensity of diffraction features in the image plane relative to the central focal peak < 10-6 | ||
| Controlled PSD |
Surface and Reflected Wavefront Specifications:
The most common tolerances used to specify the quality of aspheric mirrors, such as off-axis paraboloids, are surface accuracy and reflected wavefront error. Surface error is the deviation of a surface from its perfect shape. Wavefront error is the deviation of the resulting reflected or transmitted wavefront from its perfect shape.
At first glance, the decision whether to specify an optic based on wavefront quality or surface shape may seem purely a matter of personal preference. One may be tempted to think that they are simply related to each other by a scaling factor. However, this is incorrect. It is possible to convert between the two, but not with a simple scaling factor like for planar or spherical optics. It turns out that this decision can be very important when considering how precision optics are measured. Perhaps some will be surprised to learn that the interferometers used to measure aspheric surfaces in a typical test configuration do not measure surface form error at all – only wavefront error, and there is no single scaling factor that can extract the surface error of an off-axis parabola. The consequence of incorrect scaling is that people who specify optical quality based on surface accuracy may not be getting the quality they think they are.
Off-Axis Parabolic Coatings:
Metallic Coatings: Metals offer the most effective means of enhancing the reflectivity of optical mirrors. Metallic coatings are easy to manufacture, offer broadband performance, and are less sensitive to angle of incidence, group velocity dispersion (GVD), and polarization than dielectric coatings. Due to high absorptivity, metals, in general, typically have lower laser damage thresholds than dielectric materials. Since metallic coatings are relatively soft, a protective dielectric coating is often applied to enhance the metal’s durability, cleanability, and resistance to environmental damage. With proper care, protected metallic coatings can be cleaned with lens tissue and standard solvents.
Multilayer Dielectric Coatings (MLD): MLD coatings offer lower absorption reflectance than metallic coatings, typically resulting in higher resistance to laser damage. MLD coatings can be tailored to very specific performance requirements for reflectors, partial reflectors, leaky mirrors, beamsplitters, polarization control coatings, and antireflection coatings.
Size:
The definition of off-axis parabolic geometry is not universally applicable. The following figure and subsequent definition are the specifications we adopt.
| Parent axis: | The optical axis perpendicular to the center of the parent parabola |
| Segment Axis: | The segment axis is parallel to the parent axis and corresponds to the centerline of the reflected (or incident) collimated beam. |
| *Parent focal length (fp) | The distance from the vertex to the focus along the parent axis. This specification is fundamental to all parabolas - even off-axis ones. |
| Focal length (fs) | The distance from the intersection of the off-axis parabola surface and the segment axis to the focus |
| *Off-Axis Angle (OAA) | Angle between segment axis and segment focal length |
| *Off-Axis Distance (OAD) | The distance between the parent axis and the segment axis. Some manufacturers define this as the distance from the parent axis to the inner edge of the segment - we do not define it this way because measuring this quantity is less reliable and prone to large errors. The center of the parabola is a more reliable reference point. |
| Sag (Z) | The displacement of the surface at the radial distance from the vertex to the parent axis |
| Degree of roundness (d) | It is often helpful to determine the difference in out-of-roundness between a parabola and a best-fit sphere. This quantity is called "out-of-roundness" and determines the degree of asphericity. This is usually related to the difficulty of manufacturing. |
| Shape error | Also called "figure error" or "surface irregularity." This is the deviation from the perfect expected surface profile. |
| Medium Spatial Frequency (MSF) Error | Periodic surface ripples with spatial scales between 1 mm and 10 mm (some definitions extend this to 33 mm). |
| Roughness | Roughness Surface error on a scale between 0.0025 mm and 0.080 mm. |
Ordering Information:
Send us your drawings or specifications and we will start preparing a quote for you.
The table below lists what we need to fully specify your off-axis parabola (if you are unsure about the specifications, that’s ok, we can work it out with you). You can find information about definitions and other technical data under the “Specifications” and “Dimensions” tabs.
- The size of the incident or reflected beam:
- Focal length (can be segment focal length or parent focal length)
- Off-axis angle or off-axis distance
- Preferred materials
- Coating requirements
- Installation Requirements
- Required quality (reflected wavefront error and appearance quality)
