Ultra-smooth finishing of aspheric surfaces using CAST technology

Abstract

Abstract:Growing applications for astronomical groundbased adaptive systems and air-born telescope systems demand complex optical surface designs combined with ultra-smooth finishing. The use of more sophisticated and accurate optics, especially aspheric ones, allows for shorter optical trains with smaller sizes and a reduced number of components. This in turn reduces fabrication and alignment time and costs. These aspheric components include the following: steep surfaces with large aspheric departures; more complex surface feature designs like stand-alone off-axis-parabola (OAP) and free form optics that combine surface complexity with a requirement for ultra-high smoothness, as well as special optic materials such as lightweight silicon carbide (SiC) for air-born systems. Various fabrication technologies for finishing ultra-smooth aspheric surfaces are progressing to meet these growing and demanding challenges, especially Magnetorheological Finishing (MRF) and ion-milling. These methods have demonstrated some good success as well as a certain level of limitations. Amongst them, computer-controlled asphere surface-finishing technology (CAST), developed by Precision Asphere Inc. (PAI), plays an important role in a cost effective manufacturing environment and has successfully delivered numerous products for the applications mentioned above. One of the most recent successes is the Gemini Planet Imager (GPI), the world's most powerful planet-hunting instrument, with critical aspheric components (seven OAPs and free form optics) made using CAST technology. GPI showed off its first images in a press release on January 7, 2014 . This paper reviews features of today's technologies in handling the ultra-smooth aspheric optics, especially the capabilities of CAST on these challenging products. As examples, three groups of aspheres deployed in astronomical optics systems, both polished and finished using CAST, will be discussed in detail.

Numerical modeling of the proposed WFIRST-AFTA coronagraphs and their predicted performances

“…Other powered optics are given RMS wavefront errors of 2.5 nm and flats are 1.5 nm. These are similar levels as the Gemini Planet Imager optics 22 . The DM facesheets and the unconventional PIAACMC optics have 5 nm RMS wavefront errors.…”

“…The HLC results are shown in Figs. 22-24 and Table 4. The unaberrated HLC provides a mean contrast of 2 × 10 -10 from r = 3 -4 λc/D and over the full field.…”

Far- and Lyman-ultraviolet imaging demonstrator: a rocket-borne pathfinder instrument for high efficiency ultraviolet band selection imaging

“…The telescope optics for F110M and F140M were fabricated by a division of Coherent [formerly Precision Asphere (Note that II-VI acquired Precision Asphere and Coherent)] with polishing capabilities, such as low microroughness and mid-spatial frequency control and metrology, demonstrated on UV optics. 7 Two sets of telescope optics were acquired with the current funding, and the remaining two sets will be purchased pending future proposal selection. Filters for the FLUID instrument are all reflective in nature and consist of multilayer coatings applied to the telescope optics optimized for high reflectivity of in-band light and low reflectivity of out-of-band light.…”

“…The detector-area limited field-of-view has a 20′ diameter. The telescope optics for F110M and F140M were fabricated by a division of Coherent [formerly Precision Asphere (Note that II-VI acquired Precision Asphere and Coherent)] with polishing capabilities, such as low microroughness and mid-spatial frequency control and metrology, demonstrated on UV optics 7 . Two sets of telescope optics were acquired with the current funding, and the remaining two sets will be purchased pending future proposal selection.…”

FLUID: a rocket-borne pathfinder instrument for high efficiency UV band selection imaging

“…The Figure 1 telescope optics are fabricated by a division of Coherent (formerly Precision Asphere) * with polishing capabilities demonstrated on UV optics. 6 Two sets of telescope optics were acquired with the current funding and the remaining two sets will be purchased pending future proposal selection. The FLUID flight filters are defined by the primary and secondary telescope mirrors, each coated with a multilayer prescription specific to their designated bandpass (Section 3).…”