The Littrow form of spectrograph has a long and storied history in astronomical spectroscopy since its presentation in 1862 by Otto von Littrow. Light from an input slit traverses the same optical elements in reaching the dispersing element (prism or grating) and returning to a focused, dispersed image at the focal plane. This 1:1 symmetry helps cancel aberrations in the reimaging optics while presenting the dispersing element with the geometry most favorable to dispersion, efficiency and anamorphic scale change. Historically, Littrow spectrographs have not been pushed to high throughputs (fast f/ratios). However in the short- and mid-wave infrared particularly, high index, low dispersion materials like silicon and germanium can be combined effectively into compact, high throughput (<f/2.5), well corrected 1:1 reimaging systems that economize volume and cooling resources and are well-suited for moderately high resolution spectrographic space missions such as atmospheric sounders. We present some high throughput Littrow spectrograph concepts designed for infrared atmospheric sounding missions and incorporating both plane and immersion gratings.
Previous works have shown the viability of using the Sine Condition Test (SCTest) to verify the alignment of optical systems. The SCTest uses the Abbe sine condition to measure the mapping between the entrance and exit pupils of an optical system. From this pupil mapping, the linearly-field dependent aberrations can be measured and used to verify the alignment. Specifically, the linear astigmatism is used as a metric to determine how well the optical system is aligned. An advantage to using the sine condition to measure the off-axis performance is that the measurement equipment can be placed on-axis. By doing this, the uncertainty of the measurement is reduced, making this test especially useful for verifying systems with large inherent aberrations. In this paper, we expand the design space of the SCTest by exploring the two different source options: a point source with a grating or a flat-panel display. Additionally, we show experimental results of implementing the SCTest using a flat-panel display. Last, we explain how the SCTest can be implemented on more complex systems, such as a three-mirror anastigmat (TMA) and a double Gauss. By exploring the design space, we provide more design options for selecting the SCTest source, increasing the flexibility and utility of the SCTest.
An experimental test for violations of the sine condition is presented. This test is particularly useful for identifying the
state of alignment of an imaging system because it provides a direct measurement of the linear astigmatism (astigmatism
that varies linearly with field) in a system using only on axis measurements. The concept of the test is explained from
the perspective of both geometrical optics, using the sine condition, and wave optics. In addition, the results of an
experimental proof of concept are presented. This experiment shows good agreement between the measured and
Imaging aberrations that have linear dependence on field angle are caused by pupil aberrations that can be described
using the Abbe sine condition. This well-known relationship is frequently used to guide the design of optical imaging
systems. For example, the aberration of coma is eliminated in the design of axisymmetric systems by controlling the
pupil distortion, as defined by a standard implementation of the sine condition. An optical system with misalignments of
surface irregularities will suffer pupil distortions that are quantified using a more generalized form of the sine condition.
Such pupil aberrations create image aberrations that have linear dependence on field angle. While it is possible to infer
the state of alignment by measuring multiple field points, it may be more straightforward to perform a single on-axis
measurement of the sine condition violations. This paper summarizes the generalized sine condition and relationship
between violations of this condition and aberrations with linear field dependence. An application is discussed for
measuring sine condition violations of a 4-mirror system, which allows determination of the off-axis aberrations.
An integrated miniature multi-modal microscope (4M device) for microendoscopy was built and tested. Imaging performance is evaluated and imaging results are presented for both fluorescence and reflectance samples. Images of biological samples show successful imaging of both thin layers of fixed cells prepared on a slide as well as thick samples of excised fixed porcine epithelial tissue, thus demonstrating the potential for in vivo use.