The grism is a versatile dispersion element for an astronomical instrument ranging from ultraviolet to infrared. Major benefit of using a grism in a space application, instead of a reflection grating, is the size reduction of optical system because collimator and following optical elements could locate near by the grism. The surface relief (SR) grism is consisted a transmission grating and a prism, vertex angle of which is adjusted to redirect the diffracted beam straight along the direct vision direction at a specific order and wavelength. The volume phase holographic (VPH) grism consists a thick VPH grating sandwiched between two prisms, as specific order and wavelength is aligned the direct vision direction. The VPH grating inheres ideal diffraction efficiency on a higher dispersion application. On the other hand, the SR grating could achieve high diffraction efficiency on a lower dispersion application. Five grisms among eleven for the Faint Object Camera And Spectrograph (FOCAS) of the 8.2m Subaru Telescope with the resolving power from 250 to 3,000 are SR grisms fabricated by a replication method. Six additional grisms of FOCAS with the resolving power from 3,000 to 7,000 are VPH grisms. We propose “Quasi-Bragg grism” for a high dispersion spectroscopy with wide wavelength range.
The germanium immersion grating for instance could reduce 1/64 as the total volume of a spectrograph with a conventional reflection grating since refractive index of germanium is over 4.0 from 1.6 to 20 μm. The prototype immersion gratings for the mid-InfraRed High dispersion Spectrograph (IRHS) are successfully fabricated by a nano-precision machine and grinding cup of cast iron with electrolytic dressing method.
In ultra-precision oblique axis grinding process for machining micro aspherical mould, form error of aspherical surface is caused by the inconsistence elastic deformation of grinding system, which derived from differences velocity from inside to out. In this case, whole PV value can meet requirements, however, pits are produced in central after error compensation, which is unworkable. In this paper, mechanism of machining error caused by grinding system rigidity is analyzed, and experiments are carried out. Form error compensation grinding are carried out in the central local area, based on traditional error compensation method, which can effectively eliminate the pits of surface center. In this method, cemented carbide YG8 which diameter is about 6mm is ground. The results showed that the form accuracy under PV 200 nm and under PV 50 nm within the scope of 1 mm, and the surface roughness under Ra2nm.
This paper presents a ultra-precision manufacturing process of large stamping dies with confocal paraboloidal and
hyperboloidal surfaces of Wolter mirror for an X-ray telescope. In order to improve the alignment of the two substrates
and reduce the degeneration in imaging quality, two compound reflectors were formed from one thin substrate utilizing
synthetic ELID grinding and arc-enveloped grinding method. Because the thin foil substrate was obtained by press
forming, large stamping dies of foil substrate must be machined with high Fig. error and low roughness. In this ELID
arc-enveloped grinding system, cast iron fiber bonded (CIFB) diamond wheels were 3-dimentional controlled to scan the
workpiece and generate required surfaces. Through truing grinding wheel and ELID grinding for upper concave and
lower convex stamping dies, attainable form accuracy, surface roughness were investigated.
An ultra-precision synergistic finishing process integrated MRF and ELID grinding was proposed for shorten total
finishing time and improve finishing quality. Sets of ultra-precision experiments were carried out to grind and finish
some optical glass BK materials. ELID grinding, as pre-finishing, was employed to obtain high efficiency and high
surface quality; and then, MRF, as the final finishing, was employed to improve further surface roughness and form
accuracy. BK13 plane glass was processed by integrated ELID grinding using #4000 wheel and MRF. After ELID
grinding, λ/5 P-V was obtained, and then, MRF 30 minutes, the form accuracy was improved to λ/28 P-V. Likewise,
curvature BK7 glass lens was also used to test. After ELID grinding using #4000 wheel, form accuracy λ/5 P-V and
surface roughness 261 nm rms were obtained; and then, MRF 17 minutes (2 cycles), the form accuracy and the surface
micro-roughness were improved to λ/18 P-V and 0.56 nm rms. By applying the ultra-precision synergistic finishing
process of ELID-grinding and MRF, the glass materials could be finished to sub-nanometer surface micro roughness and
~20 nm figure accuracy in a short time.