Astigmatism is left uncorrected in traditional Czerny-Turner spectrometers with spherical mirrors, which leads to low throughput of modern instruments applying line-array detectors. By gradually varying the sagittal curvature, freeform mirrors are introduced to suppress astigmatism for a wide wavelength range simultaneously. So as not to reduce spectrum resolution, further calculations of the light path are performed for extra coma compensation. A design example is presented with optimized parameters. The ray-tracing result has revealed a reduction in sagittal spot size from several hundred micrometers to around 10 microm in the wavelength range from 200 nm to 800 nm.

Design of freeform mirrors in Czerny-Turner spectrometers to suppress astigmatism

The Cryogenic Near Infrared Spectropolarimeter for the Daniel K Inouye Solar Telescope is designed to measure polarized light from 0.5 to 5 μm. It uses an almost all reflective design for high throughput and an R2 echelle grating to achieve the required resolution of up to R = 100,000. The optics cooled to cryogenic temperatures reduce the thermal background allowing for IR observations of the faint solar corona. Both the spectrograph and its context imager use H2RG detector arrays with a newly designed controller to allow synchronized exposures at frame rates up to 10 Hz. All hardware has been built and tested and the key components met their design goals. 1) The cryogenic system uses mechanical closed cycle coolers which introduce vibrations. Our design uses a two stage approach with a floating mounting disk and flexible cold links to reduce these. The vibration amplitudes on all critical stages were measured and are smaller than 1μm. 2) The grating stage of the spectrograph uses a double stack of harmonic drives and an optical encoder to provide sub-arcsecond resolution and a measured repeatability of better than 0.5 arcsec.

Cryogenic near infrared spectropolarimeter for the Daniel K. Inouye Solar Telescope

We present the design and practical implementation of a visible/near-infrared imaging spectrometer. The design of the spectrometer is based on an Offner relay. The approach presented allows an effective design for the Offner type imaging spectrometer to achieve a large flat field and to have minimum astigmatism, which is the dominant residual aberration over the full interested wavelength range. The system works at the wavelength range from 400 to 1000 nm with a high spectral resolution of 0.6 nm/pixel, a relatively low f-number of f/2.3, and a large flat field of 12 mm. The performance measurement of the achieved prototype shows that only a low smile distortion (<0.1 pixel) and a small spectral keystone distortion (<0.07 pixel) are present. The instrument has the advantages of a compact configuration and a light weight. We present some actual images at different working wavelengths.

Analytical design and implementation of an imaging spectrometer

The Cryogenic Near Infrared Spectropolarimeter for the Daniel K. Inouye Solar Telescope

Tandem gratings of double-dispersion mount make it possible to design an imaging spectrometer for the weak light observation with high spatial resolution, high spectral resolution, and high optical transmission efficiency. The traditional tandem Wadsworth mounting is originally designed to match the coaxial telescope and large-scale imaging spectrometer. When it is used to connect the off-axis telescope such as off-axis parabolic mirror, it presents lower imaging quality than to connect the coaxial telescope. It may also introduce interference among the detector and the optical elements as it is applied to the short focal length and small-scale spectrometer in a close volume by satellite. An advanced tandem Wadsworth mounting has been investigated to deal with the situation. The Wadsworth astigmatism-corrected mounting condition for which is expressed as the distance between the second concave grating and the imaging plane is calculated. Then the optimum arrangement for the first plane grating and the second concave grating, which make the anterior Wadsworth condition fulfilling each wavelength, is analyzed by the geometric and first order differential calculation. These two arrangements comprise the advanced Wadsworth mounting condition. The spectral resolution has also been calculated by these conditions. An example designed by the optimum theory proves that the advanced tandem Wadsworth mounting performs excellently in spectral broadband.

Advanced astigmatism-corrected tandem Wadsworth mounting for small-scale spectral broadband imaging spectrometer

A high-resolution, flat-field, plane-grating,
f/10 spectrometer based on the novel design proposed by Gil and Simon [Appl. Opt. 22, 152 (1983)] is demonstrated. The spectrometer design employs off-axis parabolic collimation and camera mirrors in a configuration that eliminates spherical aberrations and minimizes astigmatism, coma, and field curvature in the image plane.
In accordance with theoretical analysis, the performance of this spectrometer achieves a high spatial resolution over the large detection area, which is shown to be limited only by the quality of its optics and their proper alignment within the spatial resolution of a
13 μm×13 μm pixelated CCD detector. With a
1500 lines/mm grating in first order, the measured spectral resolving power of
λ/Δλ=2.5(+/-0.5)×104 allows the clear resolution of the violet
Ar(I) doublet at 419.07 and
419.10 nm.

Ved P. Nayyar

Papers

High-resolution, flat-field, plane-grating, f/10 spectrograph with off-axis parabolic mirrors.