Off-Axis Parabolic Mirrors (Introduction)

Parabolic mirrors are the most common type of aspherical mirrors used in optical instruments. They are free from spherical aberrations, and thus focus the parallel beam to a point or point source to infinity. In many optical systems it is not necessary to use aperture that is completely rotationally symmetric. On the other hand, in some applications the central part of the mirror obscures the beam path and therefore is a disaster. For such systems, off-axis parabolic mirrors (OAP) offer many advantages versus the traditional paraboloids. There are OAP mirrors applications: ordinary reflecting mirrors and grazing incidence optics/for Synchrotron radiation/bless angle


Definition of the OAP mirror for ordinary reflecting mirrors ( 0°<β <160°)


Parent focal length (PFL) is the focal length of the parent paraboloid Y2=2pX for YX coordinates where p-parameter of parent Parabola, p=2(PFL);

Slant Focal Length (SFL) is the distance between OAP mechanical center and parabola focus F(p/2;0). This value may be calculated from PFL and vice versa. Please note, that SFL is the REAL FOCAL LENGTH of beam for used OAP mirror commonly known as Reflected Effective Focal Length (EFL).

Optical Centerline is the line parallel to parent parabola optical axis and coming through the mechanical center of OAP.

Zonal Radius (ZR) is the distance between parent parabola optical axis and optical centerline of the OAP. Often are used Off Axis Angle -β.

Off-axis Distance (OAD) is the distance from parent parabola optical axis to inner edge of OAP. This value may be calculated from ZR and vice versa.

To fully describe OAP one has to specify 5 parameters:

  1. PFL (or SFL=Reflected EFL);
  2. ZR (or OAD, or β-Off Axis Angle);
  3. CA (clear aperture) or work Ø;
  4. SA (surface accuracy);
  5. SQ (surface quality) s/d.

All these parameters are not independent. For example longer focal length allows better SA and longer ZR makes for lower SA.

Key data for ordinary OAP mirrors we produce: Diameters are to 2000 mm. Typical values are 200-500mm. Typical materials are: Sitall (analogue of Zerodur), Fused Silica, LK-5, also and BK7 glass. Typical surface accuracy is λ /15 at 633nm P-V (λ /70 RMS). Higher precision surfaces may be produced on the request. Typical coating for ordinary reflecting mirror is Al+SiO, Gold or Silver are available on request. Off-axis distance is up to 500 mm and more, typical value is 150-300 mm. Focal lengths are to 25 meters. Typical value is 1-2 meters. Asphericity depths may be up to 1000 microns and more. Typical range is from a couple of microns to 250 microns.



Definition of the Off-axis parabolic mirror for grazing incidence optics (165°<β <180°)



To fully describe grazing OAP one has to specify:                                                                                           1) PFL (or Parabola formula);                 2) SFL (or Reflected EFL);               3) Off Axis Angle β or ZR;                4) SA surface accuracy Lambda/N@632nm; 5) Roughness (surface quality); 6) CA -clear aperture (LxW) or Diameter  for round substrates; 7) Tolerances of: Focal lengths and Dimensions.

Short description and key advantages of our technology

The manufacturing technology was developed to supply mirrors cheaper and in higher volume for use in aero/defence applications. Typically, OAPs are made by polishing and slicing-up large on-axis parent paraboloids. Obviously, this “traditional” method is quite expensive, especially when perhaps only 1-2 mirrors are needed. This older method also places heavy restrictions on the range of available combinations of focal lengths and off-axis distances. The other “traditional” method is diamond turning. Its main disadvantages are limitations for substrate materials (metals), lower surface roughness, and accuracy. Instead of the above-mentioned methods of OAPs production, our OAP facility uses a sophisticated, computer-controlled spot-correction polishing process. It combines advantages of conventional polishing (smooth surface and possibility to use common glasses) and diamond turning (possibility to produce OAP without polishing of full paraboloid). After each polishing run we measure the surface error using large interferometers that precisely simulate current surface error of the OAP mirror. Then the information from the interferometer is transferred to the polisher’s computerized controller, which calculates the optimum location, trajectory and rotation speed of the compact, spot-polishing head. We name this process retouching. 12,000 square-foot facility is used for this production. A team of highly experienced (and patient!) technicians set up and measure a mirror over and over until it is perfect. Each mirror undergoes typically about 10 cycles of interferometric measurements, followed by retouching. Of course for state-of-the-art mirrors many more cycles are required. This unique production technology allows us to offer precise optics at very competitive prices.


Z I L T A Co. unlimited © 2017, Lithuania European Union, Fax: +370 7000 4444, E-mail: zilta[eta]

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