Direct Comb-Line Measurement: A New Calibration Concept for Astronomical Spectrographs

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For decades, astronomers have used hollow-cathode lamps as calibration sources for the spectrographs in optical telescopes, but their limitations mean that the community is now turning to laser frequency combs (LFCs), which provide a sequence of ultra-narrow, drift-free, regularly spaced optical frequencies on a selectable multi-GHz grid.Supported by STFC, HWU is leading the development and deployment of LFCs on several fronts, with current funding from PPRP (ST/S001328/1) and consortium (ST/V000403/1) grants.This capital equipment request directly supports our consortium grant, ST/V000403/1, in which a key goal is to develop LFC-based concepts to extract the wavelength solution of an astronomical spectrograph i.e. its "pixel-to-wavelength" mapping.In ST/V000403/1 we have already developed a comb-line-resolving spectrograph to pilot absolute calibration ideas. Unlike a sealed commercial instrument, we can access the focal plane of this device and place a mask there to use it as a high-resolution spectral shaper, before the light is then directed to an astronomical spectrograph. We propose to achieve this using a high-resolution digital micromirror device (DMD) containing 4 million tiny (7.6 micron) digitally-addressable mirrors.This concept offers some exciting possibilities for conditioning the light sent to the astronomical spectrograph. The most radical of these is to isolate a SINGLE COMB MODE and measure its wavelength to high precision with a commercial wavemeter before injecting it into the astronomical spectrograph. Repeating this process for each order of the spectrograph gives a direct means of obtaining the wavelength solution of each order, unlike current methods which provide just one fiducial wavelength marker for the whole spectrum.Preliminary calculations and an experimental sensitivity check show that this idea should be entirely feasible. The concept removes the uncertainty of which exact comb mode is observed on each order of the spectrograph, a problem normally addressed only by resorting to secondary measurements with a hollow-cathode lamp, or using a complex single-frequency laser locked to the LFC.Further opportunities exist to use the shaper to provide full-spectrum dynamic-range control by selectively attenuating brighter parts of the comb, and to implement switchable comb-mode spacings by selectively removing every nth comb line.Specifically, two items of equipment are requested to construct a prototype spectral shaper (forming a single equipment item): 1. DMD and driver board (Vialux DLP SuperSpeed V-Module) @ £12,202 2. Precision wavemeter (High Finesse WS7-60) @ £26,130Both items have a value below the regulated procurement threshold of £50,000, so no tender process is required. Orders will be placed under a non-competitive action, because of the unique performance capabilities of each item.Lead times for both items are a maximum of 16 weeks, so both can be ordered, delivered and receipted within the lifetime of the capital equipment grant.Including VAT and a 20% contribution from Heriot-Watt University, the final cost to STFC is £36,799, making this a relatively low cost capital equipment request that could offer considerable value to the astronomy community by piloting a new digital calibration approach for astronomical spectrographs.