= Anamorphic magnification of a grating
{title2=$|d\beta/d\alpha|=\cos\alpha/\cos\beta$}
At fixed <wavelength>, the <grating equation> gives $d\beta/d\alpha=-\cos\alpha/\cos\beta$. Thus an incident angular width is magnified by $\cos\alpha/\cos\beta$ in the dispersion direction. Including the ratio of camera and <collimator> <focal lengths> gives the projected slit width. The https://www.eso.org/sci/libraries/historicaldocuments/Operating_Manuals/Operating_Manual_No.9_A1b.pdf[ESO B&C operating manual, Appendix A] explicitly includes this anamorphic factor.
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