Friday, February 25, 2011

Gemini GMOS tilted slits

Posting this here for future reference, as the Gemini documentation on the issue of tilted slits is ambiguous.

Spectroscopic slits for GMOS are designed on an x-y coord system, with the dispersion direction being along the x axis. The slits are described by their tilt angle "slittilt" (in degrees, with slittilt=0 for untilted slits oriented perpendicular to the dispersion direction) and by "slitsize_x" (the width of the slit in the x direction; the resolution of the spectrum varies inversely with slitsize_x) and "slitsize_y" (the height of the slit in the y direction). Untilted slits are rectangles, but tilted slits are parallelograms, with two sides of width slitsize_x in the x direction and two tilted sides of height slitsize_y in the y direction.

* The software "gmmps" displays tilted slits as parallelograms but with the WRONG WIDTH in the x-direction (it uses slitsize_x/cos(slittilt) instead of slitsize_x).
* The document "" draws tilted slits as rectangles; that is incorrect.

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