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"slit" = the row number of the center of the slit, as seen in a direct (no grism) image (also for DS9).
Red_Lambda = 628.20 nm + 0.1414 nm/row * (row - slit)
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This biases things a bit away from the optical axis center (at row
2032), but puts more of the optical spectra onto the detector.
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pick fairly bright alignment stars in the optical (not the NIR, as I mistakenly
did the first time!).
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