![]() The bottom horizontal axis indicates the wavelengths and the upper horizontal axis indicates the relative distances from the straight-through wavelenghths (slit position) in pixels. These figures are from the data taken at 30 Aug, 2006 (after refurbishment for stray-light suppression: see information section).įigure 1: The OH Night-Sky Spectra for zJ500 (upper) and HK500 (lower), respectively. The figures below show the example of the night sky emission lines taken by zJ500 and HK500 grisms with 0.3"-width slit. Applying the Arc Lamp data without enough consideration for such cases would result in a large wavelength calibration error. If the observation data consists of the multiple observation periods, the spectra could show some offset among each observing dataset. 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 shift is observed, you can shift the Arc Lamp data to the science data by the measured offset before the calibration. You can check the alignment of the masks using the mask image data. ![]() To correct this, we will always take the Arc Lamp data with the slit mask image. 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. 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. So, it would be safer to take the Calibration Arc Lamp (ThAr) during the dome flat data acquisition. 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. Keeping the fitting order to low (2 to 4), with the careful selection of some isolated lines is the key for good fit. 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. Wavelength calibration of MOIRCS spectroscopy data can be done using the OH night emission lines. Wavelength calibration Wavelength calibration
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