An integral field spectrograph (IFS) is an instrument combining spectrographic and imaging capabilities. IFSs provide spectral information over a 2D field of view. In lenslet-based IFS designs, a lenslet array is placed in the spectrograph entrance plane. All beams generated by the lenslet array are fed through a dispersive element and imaged by a camera, resulting in a spectrum for each individual lenslet.
Traditional lenslet-based IFS designs stagger the spectra, so they are separated and only partially overlapping. The staggering of the spectra is problematic as one column of the detector will be occupied by different wavelengths of neighboring spectra, where the dynamic range could be quite high.
NASA Goddard researchers have developed the Lenslet-Based Integral Field Spectrograph that makes better use of detector pixels by placing adjacent spectra next to each other, rather than staggering them. For situations that require a high dynamic range, each spectrum may occupy five rows to avoid crosstalk between its neighbor. The design rearranges the lenslet foci so that the foci from any group of lenslets is aligned perpendicular to the dispersion direction, similar to a slit spectrometer. This re-arrangement removes the separation requirement between spectra in each group, which greatly increases the detector pixel usage efficiency; for example, if every four lenslets are grouped together, the detector pixel usage efficiency could increase by up to 150%. The increased efficiency can be used to increase the field of view and the spectral bandwidth.
The lenslet array achieves the efficient detector pixel usage of other existing types of IFS, such as an image slicer IFS or a fiber IFS, without compromising compactness and simplicity. It does not need any fore optic accessories to rearrange the slits or fibers as the input of the spectrometer, which is ideal in situations where the mass and volume are constrained. The spectrograph does not require any new fabrication capability — existing dry-etching technology is capable of fabricating a specialized lenslet array with custom curvature on each lenslet.