Wavelength Calibration Lines in Near-IR Bands
Heeyoung Oha,b, Soojong Pakb, Chan Parka, In-Soo Yuka, Moo-Young Chuna, Sungho Leea,c,
Haingja Seob, Tae-Soo Pyod, Won-Kee Parke, Daniel T. Jaffec
a
Korea Astronomy & Space Science institute, Korea
School of Space Research, Kyung Hee University, Korea
c
Department of Astronomy, University of Texas at Austin, USA
d
Subaru Telescope, National Astronomical Observatory of Japan, Japan
e
Center for the Exploration of the Origin of the Universe, Seoul National University, Korea
b
ABSTRACT
KASI is now developing IGRINS (Immersion GRaing Infrared Spectrograph) in collaboration with University of
Texas at Austin. Now we are designing the calibration system for IGRINS based on that of BOES (Bohyunsan
Optical Echell Spectrograph) and Subaru Nasmyth IR calibration unit. We selected four main sources for the
wavelength calibration, i.e., a Th-Ar hallow cathode lamp, OH airglow emission lines, telluric absorption lines in the
standard star spectrum, and an ammonia gas cell as the source for the more precise calibration. We also selected the
tungsten halogen lamp as a blackbody source for the flat imaging. We will make a relay optics design for the
calibration system and integrate to the IGRINS mechanical design.
Keywords: Instrumentaion: spectrographs — method: calibration — near-infrared
1. INTRODUTION
IGRINS (Immersion GRating Infrared Spectrograph) is a high resolution infrared spectrometer which uses the
immersion grating as a echelle grating. It is the forerunner instrument for GMTNIRS (GMT Near Infrared
Spectrograph) which is proposed as a GMT (Giant Magellan Telescope) 1st generation instrument. It will be attached
on the McDonald 2.7m Harlan J. Smith telescope and probably on 4-8m class telescope. In a single exposure,
IGRINS can observe the whole H- and K-band spectra simultaneously. IGRINS will have resolution of 40,000 with
0.68" entrance slit width at a 4m telescope.
In IR spectroscopy, wavelength calibration is important in the process of data analysis. To identify the wavelength
of the spectral line and to correct the distortion of the spectrum, the appropriate calibration methods are required. In
this paper, we will introduce our calibration method we selected and will show the preliminary design of IGRINS
calibration system. The spectral coverage of IGRINS is H-band (1.49μm -1.80μm) with 23 orders and K-band (1.96
μm -2.46 μm) with 20 orders. We studied several previous developments (e.g., CRIRES, BOES) to select
appropriate methods for our calibration system.
2. CALIBRATION SOURCES
Figure 1 shows the conceptual structure of four sources for the IGRINS wavelength calibration. Th-Ar hallow
cathode lamp and ammonia gas cell will be placed in a calibration box of IGRINS, OH airglow emission lines and
telluric absorption lines will be generated by Earth’s atmosphere.
2.1. Telluric absorption line
The molecules in the Earth’s atmosphere (e.g., water vapor, carbon dioxide) absorb the light from the astronomical
objects. In the process of data reduction, telluric correction function is made by dividing a model spectrum of a star
by an observation of astronomical photometric standard stars. And also the telluric features in standard star spectrum
itself can be used as line references for the wavelength calibration. In our application, we will mainly use the telluric
absorptions at wavelength longer than 2.2μm, which are caused by water vapor and CO2. We can use CO2 lines more
usefully because the lines of CO2 are not changed with place and time, while the lines of H2O vary with them.
Figure 1. Conceptual drawing of the calibration sources
Figure 2. (left)Telluric absorption features in H- and K- bands wavelength, (right) Energy levels involved in the
infrared rotation-vibration bands of the OH molecule (Rousselot et al. 2000).
2.2. Telluric OH emission lines
Emission lines produced by the OH radicals dominate near-infrared spectra. They appear between 0.61 and 2.62
μm, and correspond to transitions with Δv= 2 to 5(Figure 2). The OH radicals are created in an atmospheric layer of
6 – 10km thickness at an altitude of about 87km. Removal of OH emission lines from astronomical spectra is an
essential part of the spectral data processing since they are the dominant sources of noise. Nevertheless, a detailed
atlas of OH lines with accurate wavelength and reasonable relative intensities allows direct wavelength calibration
on the sky spectra (Rousselot et al. 2000). We will use the OH line atlas data of Rousselot et al. (2000) that are
available at the URL http://www.eso.org/instrument/isaac.
2.3. Ammonia gas cell
NIR Radial velocity measurement is powerful tool to search for planet and sub-stellar objects. But there is no
technique currently available for the radial velocity precision. Ammonia gas is a well established wavelength
standard for the NIR (e.g., Urban et al. 1989). A glass cell filled with ammonia gas to calibrate the spectrograph
would be useful like the “iodine cell” at visible band (Bean et al. 2009).
Figure 3. Example model components and fit for the radial velocity measurements. The Model components are
given in top three panels. Top: the spectrum of the ammonia cell, middle: synthetic telluric absorption spectrum,
bottom : the template stellar spectrum. A comparison of a fit to the data is the bottom two panels. Top: the observed
spectrum (points) and the best-fit model (line), bottom: the residual from the fit (points). (Bean et al. 2009)
2.4. Th-Ar Lamp
Th-Ar hollow cathode lamp is well known as a calibration reference source at visible and near infrared bands. The
lamp has thorium filament and is filled with argon gas. Kerber et al. (2008) has established more than 2400 lines that
are suitable for wavelength standards in the range of 900-4500 nm and the line list is used for the wavelength
calibration of CRIRES (Cryogenic High-Resolution IR Echelle Spectrometer). BOES (Bohyunsan Optical Echell
Spectrograph) also uses Th-Ar lamp as a calibration source at optical band. We will use the established line data of
Kerber et al. (2008) for our H- and K- band application. We will use Th-Ar lamp with the integrating sphere to form
the uniform source.
3. NUMBER OF REFERENCE LINES
H- and K-bands spectra of IGRINS have 23 and 20 orders respectively and we need to have more than 3 lines in
each order. From the selected line data, we plotted histogram of Th-Ar and OH line at each order. Th-Ar and OH
lines will cover the whole H-band and the most of K-band orders. There is not enough reference lines at the ends of
K-band, thus we will use telluric absorption lines for the wavelength calibration.
Figure 4. Number of reference lines at H- and K- bands
4.
FUTURE WORK
We will finish the relay optics design for the calibration system using 2-3 lenses and integrate them to the IGRINS
mechanical design. We need to list the suitable telluric absorption lines to complete our reference sources, and we
plan to study more about the ammonia gas cell for the purpose of the radial velocity observation.
5. REFERENCES
Rousselot, P., Lidman, C., Moreels, G., & Monnet, G. 2000, A&A, 354, 1134
Kerber, F. 2008, ApJS, 178, 374
Bean, Jacob L. et al. 2009, arXiv:0911.3148v1