Proceedings Article | 18 July 2014
KEYWORDS: Spectroscopy, Spectral resolution, Sensors, Collimation, Astronomy, Diffraction gratings, Telescopes, Imaging systems, Reflectivity, Spectrometer engineering
Tornado Spectral Systems (TSS) has developed the High Throughput Virtual Slit (HTVSTM), robust all-reflective
pupil slicing technology capable of replacing the slit in research-, commercial- and MIL-SPEC-grade spectrometer
systems. In the simplest configuration, the HTVS allows optical designers to remove the lossy slit from pointsource
spectrometers and widen the input slit of long-slit spectrometers, greatly increasing throughput without
loss of spectral resolution or cross-dispersion information.
The HTVS works by transferring etendue between image plane axes but operating in the pupil domain rather
than at a focal plane. While useful for other technologies, this is especially relevant for spectroscopic applications
by performing the same spectral narrowing as a slit without throwing away light on the slit aperture. HTVS can
be implemented in all-reflective designs and only requires a small number of reflections for significant spectral
resolution enhancement–HTVS systems can be efficiently implemented in most wavelength regions. The etendueshifting
operation also provides smooth scaling with input spot/image size without requiring reconfiguration
for different targets (such as different seeing disk diameters or different fiber core sizes). Like most slicing
technologies, HTVS provides throughput increases of several times without resolution loss over equivalent slitbased
designs.
HTVS technology enables robust slit replacement in point-source spectrometer systems. By virtue of pupilspace
operation this technology has several advantages over comparable image-space slicer technology, including
the ability to adapt gracefully and linearly to changing source size and better vertical packing of the flux
distribution. Additionally, this technology can be implemented with large slicing factors in both fast and slow
beams and can easily scale from large, room-sized spectrometers through to small, telescope-mounted devices.
Finally, this same technology is directly applicable to multi-fiber spectrometers to achieve similar enhancement.
HTVS also provides the ability to anamorphically “stretch” the slit image in long-slit spectrometers, allowing
the instrument designer to optimize the plate scale in the dispersion axis and cross-dispersion axes independently
without sacrificing spatial information. This allows users to widen the input slit, with the associated gain of
throughput and loss of spatial selectivity, while maintaining the spectral resolution of the spectrometer system.
This “stretching” places increased requirements on detector focal plane height, as with image slicing techniques,
but provides additional degrees of freedom to instrument designers to build the best possible spectrometer
systems.
We discuss the details of this technology for an astronomical context, covering the applicability from small
telescope mounted spectrometers through long-slit imagers and radial-velocity engines. This powerful tool provides
additional degrees of freedom when designing a spectrometer, enabling instrument designers to further
optimize systems for the required scientific goals.
