The Calibration System for IGRINS, a High Resolution Near-IR Spectrograph

We present development of the calibration system for IGRINS (the Immersion GRating INfrared Spectrograph). IGRINS is a high resolution infrared spectrograph which is developed by a collaboration of the University of Texas at Austin, Korea Astronomy and Space Science Institute, and Kyung Hee University. IGRINS is the forerunner for GMTNIRS (the Giant Magellan Telescope Near Infrared Spectrograph) which has been selected for study of GMT first-light instruments. IGRINS uses a silicon immersion grating as an echelle grating and it has a resolving power of 40,000 in H(1.49μm -1.80μm) and K(1.96 μm -2.46 μm) bandpass. To establish the calibration concept of our high resolution instrument, we reviewed a few existing similar spectrographs, e.g., BOES, CRIRES, and IRCS. There are two main purposes for the calibration. Those are: 1) to get a distortion solution and a wavelength solution using line reference sources; 2) to correct pixel-to-pixel sensitivity variations by flat frames. For the IGRINS wavelength calibration, we can use hallow cathode lamps to get the reference lines. The telluric OH emission lines and the telluric absorption lines can be other reference sources. We investigated the advantages and disadvantages of each calibration sources. For flat-fielding, we will use a tungsten halogen lamp with an integrating sphere to get blackbody radiations. For selecting an appropriate line lamp, we count the number of available lines per each order based on the acceptable dynamic range of the line intensity. For our Hand Kband application, we confirmed that the uranium lamp is the most preferable. We designed calibration optics with Code V and conducted its performance analysis using LightTools software package. We designed lens barrels and a conceptual calibration box containing the integrating sphere, the line lamp, folding mirrors and moving parts. We organized the moving positions of calibration hardware considering the observing scenarios.