Wavelength Calibration of Near-Infrared Spectra

Transcription

1 Publications of the Astronomical Society of the Pacific, 113: , 2001 May The Astronomical Society of the Pacific. All rights reserved. Printed in U.S.A. Wavelength Calibration of Near-Infrared Spectra Kenneth H. Hinkle, Richard R. Joyce, Abigail Hedden, 1 and Lloyd Wallace National Optical Astronomy Observatory, 2 P.O. Box 26732, Tucson, AZ ; hinkle@noao.edu, joyce@noao.edu, heddens@carleton.edu, wallace@noao.edu and Rolf Engleman, Jr. University of New Mexico, Department of Chemistry, Albuquerque, NM 87131; engleman@unm.edu Received 2000 December 21; accepted 2001 January 12 ABSTRACT. An atlas of a thorium-argon hollow cathode lamp in selected intervals of the mm region is presented. Accurate wavelengths of the 500 lines recorded are given in a table. This material is intended for wavelength calibration of near-infrared spectra and is especially critical for high-resolution work. 1. INTRODUCTION High resolution astronomical spectroscopy in the 1 5 mm infrared for objects other than the Sun was first undertaken in the late 1960s. At that time the Fourier Transform Spectrograph (FTS) was applied to astronomical problems by Connes and others (Connes & Connes 1966; Connes, Connes, & Maillard 1967; Beer, Norton, & Seaman 1971). Several FTSs at major telescopes in the 1970s and 1980s set the foundation for much of the research now being carried out using infrared spectroscopy (e.g., Hinkle, Wallace, & Livingston 1995). Fourier spectroscopy, based on rapidly sampling single element infrared detectors, is a multiplex technique ideally suited for high signal levels and relatively noisy detectors where the photon noise exceeds the detector noise. With the advent of infrared arrays it became possible to build cryogenic spectrographs employing a grating as the dispersive element and an infrared array as the detector. With low-noise infrared arrays, cryogenic spectrographs can be orders of magnitude more sensitive than an FTS (Ridgway & Hinkle 1993). The decade of the 1990s saw a number of cryogenic grating spectrographs developed for high-resolution infrared spectroscopy in the 1 5 mm region (e.g., Mountain et al. 1990; Tokunaga et al. 1990; Hinkle et al. 1998; McLean et al. 2000). A feature of spectra acquired with an FTS is that the frequency scale is automatically calibrated during the observation. A laser is used as a reference for the moving carriage. The interferogram is sampled at evenly spaced intervals determined from the laser interferogram. In transforming this evenly spaced 1 Based on research conducted at NOAO as part of the Research Experiences for Undergraduates program. 2 Operated by the Association of Universities for Research in Astronomy, Inc., under cooperative agreement with the National Science Foundation. interferogram to produce a spectrum, a frequency scale in evenly spaced inverse distance units (wavenumbers) results. Thus, the frequency calibration of spectra obtained with an FTS is limited by noise rather than instrumental characteristics. For an FTS operated in vacuum FTS spectral frequencies 3 require only the addition of a small (typically 1 kms 1 ) correction for collimation differences between the reference and signal beams. Unlike FTS data, spectra taken with a grating spectrograph must be wavelength calibrated. This includes the assignment of the wavelength interval as well as correction for the dispersion relation. To some extent, infrared absorption or emission lines contributed by the Earth s atmosphere can be used as the wavelength standards required to carry out these corrections. However, telluric lines frequently are not a usable standard. It is of course necessary to observe the sky to see these lines and under bad weather conditions or when the dome cannot be opened for other reasons (e.g., daytime) it is desirable to have a convenient wavelength calibration source at the spectrograph. Furthermore, there are extensive regions of the infrared without telluric lines, and use of a stellar reference (e.g., Arcturus) requires telescope time. In astronomical optical spectroscopy the standard wavelength reference is a thorium (Th) hollow cathode lamp. Th was selected as a wavelength standard because the lines lack hyperfine structure and the spectrum is quite rich. On the basis of these criteria, a competing or perhaps slightly superior choice of element for an infrared wavelength standard would be uranium. However, Th hollow cathode calibration sources are universally available and discussion of a different source for a marginal improvement would not be advisable. 3 This paper follows the standard convention for the infrared that wavenumbers are in vacuum and wavelengths are in air at STP. 548

2 IR WAVELENGTH CALIBRATION 549 Fig. 1. FTS spectrum of a Th-Ne-Ar hollow cathode in the 1 5 mm region is shown compressed onto one panel. Note the thermal continuum, absorptions by molecules between the lamp and the spectrograph, and the density of the emission lines. This paper reviews our efforts to develop a Th reference list to extend this wavelength reference into the near-infrared. A survey of the literature revealed that there were few existing line lists suitable for a high-resolution wavelength reference. It would be desirable to have at least two and preferably four or more lines in each spectral interval on the detector. At a resolution of 10 5, a 1024 element detector covers a bandwidth of only about 0.5%. Sensitivity to weak hollow cathode lines is critical since infrared spectra can be sparse. In order to increase the sensitivity we have purposefully limited our search for weak lines to the nonthermal infrared, i.e., wavelengths short of 2.5 mm. Our initial effort at producing a line list was from an FTS observation of a hollow cathode lamp. While this had large wavelength coverage, frequent reference to the resulting atlas has shown it to be only marginally useful. We provide a link to that data below. Later we obtained an extensive FTS Th line list. With this line list as a reference, we have used an infrared grating spectrograph to produce an atlas. This atlas should permit wavelength calibration from Th hollow cathode spectra observed at a large variety of infrared spectrographs. 2. FTS ATLAS A Th-Ne-Ar hollow cathode lamp was observed with the National Solar Observatory (NSO) McMath-Pierce 1 m laboratory Fourier Transform Spectrometer. For this observation a commercial lamp was operated at a current of 14 ma, the manufacturer specified value. The resulting FTS spectrum provides a map of nearly the entire spectrum available with an InSb detector. The short-wavelength limit was set at 1.13 mm by a silicon filter. The long-wavelength limit of 5 mm was set by the long-wavelength limit of the detector. Figure 1 shows the complete spectrum at full resolution but compressed to a single page. A number of the features of the experiment are nicely shown in this figure. There is prominent thermal continuum increasing to long wavelengths. Against this continuum intervening absorption is conspicuous. This absorption arises in the air between the lamp and the FTS input window and is largely attributable to CO 2 at 2350 cm 1 and 3700 cm 1 and H 2 O near 3755 cm 1 and 5300 cm 1. The continuum appears broadened due to fringing in the lamp window. There is a general increase in emission-line strength and density

3 550 HINKLE ET AL. Fig. 2. Section of the FTS spectrum of the hollow cathode. Line identifications are marked. The intensities of the lines that are off-scale are in square brackets. from low frequency out to about 8000 cm 1. Note that no correction has been attempted for the instrumental response. At the observed resolution of the FTS spectrum the hollow cathode lines were unresolved. To eliminate fringing from the instrumental profile, the spectrum was apodized to a resolution of cm 1 (or j/dj ranging from 50,000 at 2200 cm 1 to 200,000 at 8800 cm 1 ). The continuum level was then determined and subtracted. A line list was then derived from the line center frequencies. Identifications were made from the compilation of Outred (1978) with updates for Ar i by Palmeri &Bièmont (1995) and Ne i by Chang et al. (1994). There is little work on atomic spectra below 4 mm, and in this wavelength region many lines are unidentified. A short interval of the spectral atlas is shown in Figure 2. Lines with known identifications have been labeled. For lines that are off-scale, the line intensity on the scale of the plots is given in square brackets. The intensity scale of these plots is, as noted above, not corrected for instrumental transmission. The strongest line observed in the 1 5 mm region is Ar i cm 1. On the arbitrary intensity scale shown on the plots this line has a strength of 44. The strong lines in this spectrum are contributed largely by Ar and Ne lines. The Th lines present are weak. In addition, seven lines of Al i, four of Ca i, and three of K i, all presumably from contaminants in the cathode, were clearly present in the spectrum Th-Ar ATLAS The FTS Th-Ne-Ar hollow cathode atlas has been referenced for wavelength calibration with various infrared grating spectrographs. However, the density of lines in the FTS atlas was often inadequate to provide a dispersion solution, even when lines were utilized that were barely visible in the noise of this atlas. The only adequate solution was to use the superior sensitivity of a grating spectrograph itself to generate an atlas by observing a Th-Ar hollow cathode lamp Observations Observations were undertaken with Phoenix, a highresolution ( R 80,000), long-slit, cryogenic spectrograph which has been used for astronomical spectroscopy at Kitt Peak since 1998 (Hinkle et al. 1998, 2000). Phoenix spectra are confined to a single echelle order selected by a narrowband 4 The entire spectrum is available on the NOAO Web site,

4 IR WAVELENGTH CALIBRATION 551 blocking filter. The wavelength coverage of this atlas was limited by the available blocking filters. The wavelength coverage of a single integration is limited by the 1024 pixels available in the dispersion direction. For each of the 11 available order separating filters, spectra of the Th-Ar hollow cathode lamp were observed at grating settings calculated to yield at least 50% overlap with adjacent observations. This large overlap allowed the unambiguous combination of sections after the observations. Exposure times of both 10 and 300 s were used. The shorter exposures were necessary to obtain intensities for strong lines which were saturated in the 300 s observations. A continuum lamp was also observed at each grating setting as a flat-field calibration. The blocking filters have typical widths of 100 cm 1, but as noted above the wavelength coverage of a single integration is limited by the 1024 pixels in the dispersion direction. Typically 10 grating settings were used to cover a filter bandpass. Because of the volume of data, it was necessary to automate the observing process, and all the hollow cathode exposures were taken before the flat-field exposures. The grating mechanism does set quite accurately, but some small offset is unavoidable. This can result in a small amount of fringing in the final spectrum but proved negligible in this experiment. The data were reduced with IRAF using standard software packages. A dark frame of the same exposure time was subtracted from each Th-Ar hollow cathode lamp and flat image. The dark-subtracted images were then divided by the exposure time to normalize the data set. The Th-Ar hollow cathode lamp images were divided by the flat-field images for that grating setting to remove variations in pixel sensitivity as well as overall system response. The central 106 columns, which are illuminated by the slit length, were then combined, using a median algorithm to reject bad or noisy pixels, to obtain the one-dimensional spectrum. The thermal continuum from the lamp was fitted to a high-order polynomial and then removed. Note that this step occasionally results in weak absorption adjacent to very strong emission lines. This absorption does not exist in the data and is an artifact. The normalized 300 s data were then plotted as an atlas. For strong lines, the 10 s data were spliced into the atlas to avoid saturated line cores. The array does have some memory, and even though short exposures were used to clear previous images from the array, 300 s exposures of strong lines produce artifact lines in the spectrum. These have been identified on the spectrum. The continuum lamp used for flat field calibrations was located at a greater distance from the instrument window than the Th-Ar hollow cathode calibration lamp. Even over this short (50 cm) distance, absorptions by H 2 O lines can be found in the flat-field spectra. These result in weak emission lines in the flattened Th-Ar hollow cathode spectra. The water lines are of course not actually observed in emission. These features are below 4205 cm 1 and are noted on the spectra. The intensities of the reduced spectra have not been corrected for the temperature of the lamp, absorption by the lamp window, air in the path, etc. Comparison of the similarly uncalibrated FTS intensities ( 2) shows a scale factor (PHX # scale factor p FTS) of about 1.2 at 2.3 mm to2.5at1.3mm. Different lamps were observed for these two data sets Thorium-Argon Line Identifications Preliminary identifications of the features in the Phoenix spectra were made on the basis of the spectral summary by Outred (1978). In the process of working with the data we became aware of an analysis of an infrared Th-Ar hollow cathode spectrum obtained with the NSO FTS by Engleman. The Th-Ar hollow cathode observed was a water-cooled device with a replaceable cathode. With this device, high cathode currents not possible with a commercial cathode can be used. As a result the spectrum has a rich assortment of Th lines, but the overall spectrum is very different in appearance from the spectrum of commercial hollow cathode lamps typically used to wavelength calibrate spectrographs. Further discussion of this spectrum can be found in R. Engleman (2001, in preparation). The analysis of this spectrum, along with Palmer & Engleman s (1983) analysis of a Th-Ne hollow cathode spectrum in the visible and near-infrared, is the basis of Th identifications which we used to identify the lines in our spectra. We have also found six weak lines of Th ii reported by Giacchetti et al. (1974). An analysis of Ar i by W. Whaling (2000, unpublished) and Ar ii and Ar iii by Whaling et al. (1995) was the basis for the identification of Ar lines in the spectrum. Four weak Ar lines are also identified in the 9232 cm 1 bandpass using Minnhagen (1963, 1973). The hollow cathode that was used for the Phoenix observations did not contain Ne lines owing to difficulties in obtaining hollow cathode tubes containing a Ne-Ar gas mix. We began the identification process with the approximate frequency centers for the exposures and approximate dispersions estimated from the difference in frequency centers of adjacent exposures which had features in common. This, the FTS atlas ( 2), and the laboratory material suggested possible identifications. We then fitted a third-order polynomial (determined to be necessary and sufficient) to the laboratory frequencies of these lines versus pixel number. This process allowed the detection of wrong identifications and the extension of the identifications to weaker features and adjacent exposures. Th and Ar are the source of all of the identified lines, aside from H 2 O, and probably the source of many of the unidentified lines as well. The Al i, Cai, and K i lines present in the FTS spectrum were not observed the the Phoenix spectra. The hollow cathodes used were, as noted above, different. Spectral lines from impurities are certainly present, but in the regions observed we could not identify spectral lines from any contaminant Atlas Plots and Tables Frequency scales were established from the polynomial fits of the various exposures. The ends were trimmed off the best

5 552 HINKLE ET AL. Fig. 3. Atlas of a Th-Ar hollow cathode in intervals over the mm region. The wavelength coverage is limited to the intervals shown by the available blocking filters. The intensity axis is a combination of logarithmic and linear as described in the text. of the exposures to provide 0.5 cm 1 overlap and then overplotted and labeled with identifications to provide the figures. In the K-band region the bandpasses of the blocking filters overlap, so the spectrum used is from regions of good filter transmission. However, we also present spectra taken with spectrally isolated filters. For these filters we have kept the spectrum well past the region of good filter transmission in order to increase the wavelength coverage. This can be seen in the decrease of the signal-to-noise ratio. Figure 3 is the Th-Ar hollow cathode atlas that resulted from the Phoenix observations. The intensity scale on the figures is a combination of linear and logarithmic. The upper two-thirds of the intensity scale is logarithmic, and the lowest one-third is linear. The range in intensity of the emission lines covers more than a factor of While this plotting technique allows comparison of the strengths of all the lines, it produces an odd distortion of the profiles of the very strong lines. A line list for the observed regions is given in Table 1. This gives laboratory frequencies in vacuum and corresponding wavelengths in air from the sources described above as well as our own measured frequencies. The laboratory frequencies are given to three decimal places, while measurements taken from our spectra are limited to two decimal places. The intensities are peak intensities on the same scale as the plots. We thank the NOAO director, Dr. Sidney Wolff, for supporting the development of the Phoenix spectrograph. Dr. Ward Whaling kindly proved unpublished lists of Ar frequencies. We are indebted to Mr. Charles Corson of the WIYN observatory for the loan of the hollow cathode lamp use for this research when our lamp was broken. Dr. Mike Dulick (NSO), Mr. Dave Jaksha (NSO), and Dr. Peter Bernath (University of Waterloo) undertook the original observation of the hollow cathode with the NSO FTS.

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15 562 HINKLE ET AL. REFERENCES Beer, R., Norton, R. H.,& Seaman, C. H. 1971, Rev. Sci. Instrum., 42, 1393 Chang, E. S., Schoenfeld, W. E., Bièmont, E., Quinet, P., & Palmeri, P. 1994, Phys. Scr., 49, 26 Connes, J., & Connes, P. 1966, J. Opt. Soc. Am., 56, 896 Connes, J., Connes, P., & Maillard, J. P. 1967, J. Phys. Suppl., 28, C2 Giacchetti, A., Blaise, J., Corliss, C. H., & Zalubas, R. 1974, J. Res. NBS, 78A, 247 Hinkle, K. H., Cuberly, R. W., Gaughan, N. A., Heynssens, J. B., Joyce, R. R., Ridgway, S. T., Schmitt, P., & Simmons, J. E. 1998, Proc. SPIE, 3354, 810 Hinkle, K. H., Joyce, R. R., Sharp, N., & Valenti, J. A. 2000, Proc. SPIE, 4008, 720 Hinkle, K. H., Wallace, L., & Livingston, W. 1995, Infrared Atlas of the Arcturus Spectrum, mm (San Francisco: ASP) Minnhagen, L. 1963, Ark. Fys., 25, , J. Opt. Soc. Am., 63, 1185 McLean, I. S., Graham, J. R., Becklin, E. E., Figer, D. F., Larkin, J. E., Levenson, N. A., & Teplitz, H. I. 2000, Proc. SPIE, 4008, 1048 Mountain, C. M., Robertson, E. J., Lee, T. J., & Wade, R. 1990, Proc. SPIE, 1235, 25 Outred, M. 1978, J. Phys. Chem. Ref. Data, 7, 1 Palmer, B. A., & Engleman, R., Jr. 1983, Atlas of the Thorium Spectrum (Los Alamos Rep. LA-9615) Palmeri, P., & Bièmont, E. 1995, Phys. Scr., 51, 76 Ridgway, S. T., & Hinkle, K. H. 1993, in High Resolution Spectroscopy with the VLT, ed. M.-H. Ulrich (ESO Conf. Workshop Proc. 40; Garching: ESO), 213 Tokunaga, A. T., Toomey, D. W., Carr, J., Hall, D. N. B., & Epps, H. W. 1990, Proc. SPIE, 1235, 131 Whaling, W., Anderson, W. H. C., Carle, M. T., Brault, J. W., & Zarem, H. A. 1995, J. Quant. Spectrosc. Radiat. Trans., 53, 1

16 IR WAVELENGTH CALIBRATION 563 Wavenumber (cm 1 ) TABLE 1 Thorium-Argon Hollow Cathode Line List Wavelength (Å) Intensity Note K Band Ar i Th i Th i Ar i Ar i Atmospheric H 2 O Ar i Atmospheric H 2 O Ar i Ar i Atmospheric H 2 O Artifact Th i Ar i Atmospheric H 2 O Ar ii Th i Th i Ar ii Th i Unidentified Th ii Unidentified Th i Unidentified Ar i Th ii Ar ii Th i Th i Ar ii Ar i Th i Th i Ar i Unidentified Th ii Ar i Ar i Th i Ar i Th ii Th i Ar ii Ar i Th i Th i Th i Th i Th i Th i Ar ii Unidentified Ar ii Th i Ar i/th i Th i Ar i Th i Wavenumber (cm 1 ) TABLE 1 (Continued) Wavelength (Å) Intensity Note Th i Th i Ar ii Ar ii Th i Th i Ar i Ar ii Ar ii Th ii Th i Th ii Ar ii Th i Th i Ar ii Th i Ar ii Ar i Th i Th ii Unidentified Ar i Th i Th ii Ar i Ar Ar i Ar i Ar i Th i Ar ii Ar i Th i Ar ii Ar i Artifact Ar ii Artifact/Th i Unidentified Th i Unidentified Th i Th i Ar ii Th i Th i Th ii Ar ii Th i Th i Th i Th i/th ii Ar ii Ar ii Ar i Th i Th i Th i Ar i

17 564 HINKLE ET AL. Wavenumber (cm 1 ) TABLE 1 (Continued) Wavelength (Å) Intensity Note Ar ii Ar i Th i Th i Ar i Ar Th i Unidentified Unidentified Th i Th i Ar ii Th i Th i Ar i Th iii Ar ii Artifact Ar ii Th i Ar i Ar i blend Ar ii Unidentified Ar ii Th i Th i Ar ii Th i Ar i Ar i Th i Ar i Th i Th i Ar ii Th i Ar ii Th i Ar i Th ii Th i Th i Ar i Th i Ar ii Ar i Th i Th iii Ar ii Unidentified Ar i Ar ii Ar i Ar ii Th i Th ii Th i Ar ii Th i Wavenumber (cm 1 ) TABLE 1 (Continued) Wavelength (Å) Intensity Note Ar i Th i Unidentified Th i Th i Th i Ar ii Ar ii Ar ii Ar ii Ar ii Ar ii Ar i Th i Th i Th i Th i Th i Ar ii Ar ii Th i Ar i Th i 6073 Filter Unidentified Th i Th i Th i Th i Th i Th i Th i Unidentified Unidentified Th iii Ar ii Ar i Ar i Unidentified Th i Th i Th i Unidentified Unidentified Artifact Artifact Ar i Th i Th i blend Artifact Ar ii Th ii Th iii Th ii Ar ii Artifact Th i Ar i Th ii Th ii

18 IR WAVELENGTH CALIBRATION 565 Wavenumber (cm 1 ) TABLE 1 (Continued) Wavelength (Å) Intensity Note Ar i Th i Ar ii Th i Th i Artifact Unidentified Unidentified Unidentified Th i Unidentified Artifact Th ii Unidentified Th i Th i Ar ii Artifact Unidentified Th i Th i Th i Unidentified Ar ii Th i Th i blend Unidentified Th i Ar i Th i Unidentified Ar ii Th i Th i Ar ii Ar i Th i 6420 Filter Unidentified Th i Ar i Ar i Unidentified Ar i Th i Ar i Th i Unidentified Ar i Unidentified Th i Th i Th i Unidentified Unidentified Unidentified Th i Th i Th i Wavenumber (cm 1 ) TABLE 1 (Continued) Wavelength (Å) Intensity Note Unidentified Ar ii Th i Th ii Th ii Th i Th ii Unidentified Th i Th ii Th i Th i Th i Th ii Th i Th i Unidentified Th i Th i Th i Th i Th i Unidentified Th i Th i Unidentified Th ii Unidentified Ar i Unidentified Th i Th i Th i Th i Th i Th i Th i Ar ii Unidentified Th i Th i blend Th i Ar i Th i Ar i Th i Th i Th i Ar ii Ar i 7799 Filter Th i Th i Ar i Th ii Th i Th i Th i Th i

19 566 HINKLE ET AL. Wavenumber (cm 1 ) TABLE 1 (Continued) Wavelength (Å) Intensity Note Ar i Th i Th i Th i Unidentified Th i Th i Ar ii Th i Unidentified Th i Th i Th i Unidentified Ar i Th i Th i Th i Th i Ar i Ar i Ar i Ar i Th i Ar i Ar i Artifact Ar ii Th i Th i Ar i Th i Ar i Ar i Th i Unidentified Ar i Unidentified Th i Ar i Ar i Th i Ar ii Ar i Ar ii Artifact Th i Th iii Artifact Artifact Ar ii Th i Th i Ar i 9232 Filter Ar ii Th i Th i Wavenumber (cm 1 ) TABLE 1 (Continued) Wavelength (Å) Intensity Note Th i Ar ii Ar i Th i Ar i Th i Ar i Th i Th i Ar i Unidentified Th i Ar ii Ar ii Th i Th i Th i Th ii Ar i Th i Unidentified Th i Ar i Th i Unidentified Ar ii Unidentified Th blend Th ii Th i Ar i Ar ii Ar i Unidentified Ar i Th ii Ar ii Ar ii Th i Ar ii Th i Th i Ar i Th i Th i Th i Th i Unidentified Unidentified Ar i Th ii Th i Th i Ar ii Th i Th i Ar i Ar i