Wavelength calibration

Wavelength calibration

Wavelength calibration of MOIRCS spectroscopy data can be done using the OH night emission lines. However, due to the crowding of the night lines, the calibration of R500 data by the OH lines may sometimes does not result in good fit. Keeping the fitting order to low (2 to 4), with the careful selection of some isolated lines is the key for good fit.

Still, sometimes the use of the OH lines would meet difficulty in fitting, especially at the blue end of the zJ500 data or beyond 2.2um of the HK500 spectra. So, it would be safer to take the Calibration Arc Lamp (ThAr) during the dome flat data acquisition.

The important tips for the use of the Arc Lamp for the science data is to take care of the shift of the slit mask. As the slit holding mechanism is not perfect, there is always a possibility that the position of the slit mask during the wavelength calibration and the science data acquisition is different. To correct this, we will always take the Arc Lamp data with the slit mask image. You can check the alignment of the masks using the mask image data. If the shift is observed, you can shift the Arc Lamp data to the science data by the measured offset before the calibration.

Though such treatment for Arc Lamp can eliminate the first-order of the shift, the science data itself could have a slight drift of the spectra on the detector with time, due to the flexure of the instrument. If the observation data consists of the multiple observation periods, the spectra could show some offset among each observing dataset. Applying the Arc Lamp data without enough consideration for such cases would result in a large wavelength calibration error.

Night Sky Spectra for R500 Grisms

The figures below show the example of the night sky emission lines taken by zJ500 and HK500 grisms with 0.3"-width slit. These figures are from the data taken at 30 Aug, 2006 (after refurbishment for stray-light suppression: see information section).

    Figure 1: The OH Night-Sky Spectra for zJ500 (upper) and HK500 (lower), respectively. The bottom horizontal axis indicates the wavelengths [A] and the upper horizontal axis indicates the relative distances from the straight-through wavelenghths (slit position) in pixels. Longslit with 0."3 widths were used (channel 1). Note that the sky count level will change propotional to the slit width used. Some fraction of the sky counts may come from the scattered stray-light.

Th-Ar Comparison Lamp Spectra by R500 Grism

The Figures below show the example of the ThAr comparison lamp spectra taken by zJ500 and HK500 grisms with 0.7"-width slit.

Figure 2: ThAr Comparison Lamp Spectra for zJ500 (left) and HK500 (right), respectively. A 0."7-width slit was used (channel 2). Postscript figures (zJ500/HK500)

The list of ThAr lines used for wavelength calibration of the above images can be found here (zJ500/HK500). The 1-D fits file of the above data can be downloaded from here (zJ500/HK500).

Night Sky Spectra for R1300 Grism

The following figure shows the example of the night sky emission lines taken by R1300 grism with 0.8'' longslit. Note that the spectra shown here is from the data before the install of the stray-light suppression mechanism.


    Figure 3: Night-sky spectra for R1300 grism for H (upper) and K (lower) bands. The bottom horizontal axis indicates wavelengths [A], and the vetical axis show the count level per seconds (per pixel) under 0.8" slit. The J-band atlas will be available in the future (TBD). Note that the low-level stray light component (~1 ADU/sec level) is still seen at the wavelength longer than 2.45um.

OLD MOIRCS Wavelength Calibration Page

The old MOIRCS Calibration page before the 2016 upgrade can be found here.

Please note that all data on these pages are subject to change as the evaluation of the performance of MOIRCS progresses. Updated by Ichi Tanaka (2016-07-30).

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