Use of computer generated holograms for alignment of complex null correctors

The Giant Magellan Telescope (GMT) uses seven 8.4-m diameter segments to create a giant primary mirror, 25 meters across with focal ratio f /0.7. The off-axis segments will be difficult to measure accurately, as they have 14.5 mm departure from the nearest fitting sphere! The test configuration adopted uses a large 3.75-m powered mirror to fold the light path and provide most of the aspheric correction, with a smaller mirror and computer generated hologram (CGH) providing the additional correction. These optics will be aligned to a vibration-insensitive interferometer using a combination of optical references created by the CGH and metrology with a laser tracker. Some key challenges for this system are presented here including, the system alignment, the large fold mirror, and the mechanical structure. Analysis of the optical test shows that it will meet GMT specifications, including the difficult requirement that the separate segments have matching radius of curvature. Additional corroborative testing will be performed to assure that the mirror segments are correctly figured.

show abstract

“…8 (See Figure 5.) The 10-µm positional tolerance for the CGH and the mirror were achieved using these references.…”

Section: Heritage For Measuring Large Aspheric Mirrorsmentioning

confidence: 95%

Design and analysis for interferometric measurements of the GMT primary mirror segments

et al. 2006

View full text Add to dashboard Cite

show abstract

“…Through the interferogram, we can find out whether there is tilt, defocus and decenter. If the CGH is in its nominal position, we will get a null fringe [7] . Figure 5 shows the design layout of alignment CGH.…”

Section: Alignment Cghmentioning

confidence: 99%

Design and location deviation of the computer generated holograms used for aspheric surface testing

2013

View full text Add to dashboard Cite

Computer generated holograms (CGHs) are widely used in aspheric surface testing and metrology. The primary role of the CGHs is to generate any desired wavefronts to realize phase compensation. In order to eliminate alignment errors of CGH and the tested aspheric, this paper designs a new layout of CGH that has the advantages of high alignment accuracy and simple experimental operation. And further the influence of the location deviation of CGH and the tested aspheric mirror to the entire system is discussed; it shows that the error introduced by the CGH and the tested aspheric mirror tilting around the x-axis or y-axis as well as the tested aspheric mirror moving along the x-axis or y-axis direction is large.Then the CGH design principle and design process are reviewed. Finally, a CGH is designed for measurement of an aspheric mirror (diameter=226mm, F-number =2.2). Interferometric test results of an aspheric mirror show that the whole test system obtains the demanded high accuracy.

show abstract

“…8,9 The approach is not applicable to freeform surfaces and off-axis sections of aspheres due to the missing symmetry in the illuminated surface. Burge et al 10 and Zehnder et al 11 propose projected reference marks for the alignment of the surface under test. Additional diffraction structures are placed at different positions within the illuminated aperture on the substrate or are multiplexed with the diffractive null.…”

Section: Alignment Of the Surface Under Testmentioning

confidence: 99%

Freeform mirror fabrication and metrology using a high performance test CGH and advanced alignment features

et al. 2013

View full text Add to dashboard Cite

The metrology of mirrors with an off-axis aspheric or freeform shape can be based on optical testing using a Computer Generated Hologram as wavefront matching element in an interferometric setup. Since the setup can be understood as optical system consisting of multiple elements with six degrees of freedom each, the accuracy strongly depends on the alignment of the surface under test with respect to the transmission element of the interferometer and the micro optics of the CGH. A novel alignment approach for the relative positioning of the mirror and CGH in six degrees of freedom is reported. In the presented work, a proper alignment is achieved by illuminating alignment elements outside the Clear Aperture (CA) of the optical surface with the help of auxiliary holograms next to the test CGH on the substrate. The peripheral holograms on the CGH substrate are used to generate additional phase maps in the interferogram, that indicate positioning errors. Since the reference spheres represent the coordinate system of the mirror and are measured in the same precision as the optical surface, the registration and shape has to be appropriate to embody the mirrors coordinate system. The alignment elements on the mirror body are diamond machined using freeform turning or micro milling processes in the same machine setup used for the mirror manufacturing. The differences between the turning and milling of alignment lenses is discussed. The novel approach is applied to correct the shape error of a freeform mirror using ultra precision machining. The absolute measurement of the quality of freeform mirror shapes including tilt and optical power is possible using the presented alignment concept. For a better understanding, different metrology methods for aspheres and freeforms are reviewed. To verify the novel method of alignment and the measurement results, the freeform surface is also characterized using ultra high accuracy 2½D profilometry. The results of the different techniques for the absolute measurement of freeforms are compared

show abstract

Use of computer generated holograms for alignment of complex null correctors

Cited by 16 publications

References 9 publications

Design and analysis for interferometric measurements of the GMT primary mirror segments

Design and analysis for interferometric measurements of the GMT primary mirror segments

Design and location deviation of the computer generated holograms used for aspheric surface testing

Freeform mirror fabrication and metrology using a high performance test CGH and advanced alignment features

Contact Info

Product

Resources

About