Optical design of the NASA-NSF extreme precision Doppler spectrograph concept "WISDOM"

Abstract

The WISDOM instrument concept was developed at MIT as part of a NASA-NSF funded study to equip the 3.5 m WIYN telescope with an extremely precise radial velocity spectrometer. The spectrograph employs an asymmetric white pupil optical design, where the instrument is split into two nearly identical "Short" (380 to 750 nm) and "Long" (750 to 1300 nm) wavelength channels. The echelle grating and beam sizes are R3.75/125 mm and R6/80 mm in the short and long channels respectively. Together with the pupil slicer, and octagonal to rectangular fibre coupling, this permits resolving powers over R = 120k with a 1.2 diameter fibre on the sky. A factor of two reduction in the focal length between the main collimator OAP and the transfer collimator ensures a very compact instrument, with a small white pupil footprint, thereby enabling small cross-dispersing and camera elements. A dichroic is used near the white pupil to split each of the long and short channels into two, so that the final spectrograph has 4 channels; namely "Blue", "Green", "Red" and "NIR". Each of these channels has an anamorphic VPH grism for cross-dispersion, and a fully dioptric all-spherical camera objective. The spectral footprints cover 4k×4k and 6k×6k CCDs with 15 µm pixels in the short "Blue" and "Green" wavelength channels, respectively. A 4k×4k CCD with 15 µm pixels is used in the long "Red" channel, with a HgCdTe 1.7 µm cutoff 4k×4k detector with 10um pixels is to be used in the long "NIR" channel. The white pupil relay includes a Mangin mirror very close to the intermediate focus to correct the white pupil relay Petzval curvature before it is swept into a cylinder by the cross-dispersers. This design decision allows each of the dioptric cameras to be fully optimised and tested independently of the rest of the spectrograph. The baseline design for the cameras also ensures that the highest possible (diffraction limited) image quality is achieved across all wavelengths, while also ensuring insensitivity of spot centroid locations to variations in the pupil illumination. This insensitivity is proven to remain even in the presence of reasonable manufacturing and alignment tolerances. Fully ray-traced simulations of the spectral formats are used to demonstrate the optical performance, as well as to provide prefirst-light data that can be used to optimise the data reduction pipeline.

WISDOM: the WIYN spectrograph for Doppler monitoring: a NASA-NSF concept for an extreme precision radial velocity instrument in support of TESS

“…While this present paper did not discuss the optics in detail, we refer to the accompanying WISDOM optical design paper 12 , as we feel the issue is important enough to be listed here in this section.…”

“…The 4 cameras were also designed with a constraint to follow the same (or at least very similar) optical formula, so that the opto-mechanical design elements could be recycled and thus allow savings by non-reoccurring engineering costs. The WISDOM camera designs 12 do not follow the recent trend 21,22 of employing very few elements (4-5) that exploit the fact that echelle spectrograph cameras do not have to be corrected either for lateral or axial color (chromatic focal shift). Latter now is often compensated by the tilt of the detector.…”

“…The WISDOM optical design 12 cleverly utilizes a symmetrical and balanced arrangement of optical elements, which results in a very simple optical bench. In some ways the ability to do so was also the result of pupil slicing and the re-investment of the achieved budget savings into multiplication of the spectrograph and cameras.…”

“…We have used the EchMod software package 17 to simulate RV errors arising from extreme cases of non-uniform illumination of the spectrograph pupil (i.e. far-field instabilities), for details see [12]. These sources constitute the bottom level of the non-calibratable instrument error branch of the RV error budget, indicated in orange as allocations in Figure 15.…”

Fiber link design for the NASA-NSF extreme precision Doppler spectrograph concept "WISDOM"

“…If there is one area, though, in which ways of improvements are not yet fully explored that is very cautious optical design and careful illumination control of the spectrograph. Our papers on WISDOM 3 and its optical design 4 elaborate on the former, while this paper is mainly concentrating on the latter, by describing a fiber optic link that is designed to provide high throughput and a very uniform, stable spectrograph illumination.…”