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Spectroscopic Accuracy

Spectroscopic Accuracy

The spectroscopic accuracy has been evaluated using the white dwarf orbiting HD 8049 discovered by Zurlo et al (2013), believed to have a temperature of 18800+/-2000 K. Observations were performed during the second commissioning run on December 12, 2013. The recipes, calibration files, and reduced cubes used to create the plots in this section are available as part of the first light data release. For this data, zero-point offsets to the wavelength solutions were determined using an Argon arclamp image taken immediately after the exposures. The data were flux calibrated using the spectra extracted from the satellite spots using the 2mass magnitudes combined with a Pickles model of a K2V star. The error bars on the data points in the plots below are determined by normalizing the spectra of the individual satellite spots to a common integrated intensity, then looking at the standard deviation in each wavelength channel. This gives an approximation of the uncertainty of the extracted spectrum but is most certainly an underestimate. Furthermore, the effects of systematic error are not properly included. The effects of systematic error are evident in the extracted spectra of the white dwarf. The following plots compare the extracted spectra in the H, K1 and K2 bands to a 18800 K black body.

H band spectra

The H-band spectrum was extracted out a single 60s image. Overall, the spectral slope is well reproduced, however, systematic error can be seen in both the long and short wavelength extremes (~0.06um) of the band. The large deviation in the first couple data points is probably due to a small error in the wavelength calibration. Because the throughput of the filter is so low in this region, a large correction factor is applied when performing the flux calibration, therefore any small error is greatly amplified. The single blue data point indicates the absolute flux level measured using H-band photometry by Zurlo et al (2013). The blue error bar (upper right in the above figure) indicates the standard deviations in the integrated photometric flux of each satellite spot. It is believed, but not confirmed, that the ratio between mean satellite intensity and the intensity of the central star is constant. However, if this is not the case then this error bar indicates the uncertainty on the flux normalization. Another possible scenario leading to our increased flux level may be a result of an incorrect calibration between the mean peak intensity of the occulted star and the satellite spots. Research into this is on-going.

K1 band spectra

The K1 spectrum of HD 8049b (above), whose wavelength calibration was performed using the telluric absorption lines at ~2.01 and ~2.06 um also shows evidence of systematic error at shorter wavelengths. However, previously obtained spectra of this object show evidence that it is not a perfect blackbody, but at the moment we do not possess the measured spectrum and cannot do a direct comparison. The extreme deviations are a result of spectral crosstalk at the extreme ends of the bands, therefore this region of the spectrum should be disregarded (at wavelengths shorter than ~1.93um). At this time, the resulting systematic error between 1.93-2.02 remains unclear (assuming it is indeed systematic error). One possible scenario is correction to compensate for flux that is not included in the 3-pixel box extraction algorithm. Because no flat-field image was taken at the position of the science target, no correction could be applied. More thorough extraction algorithms are currently under development that will make such a correction unnecessary. If the observer requires a precise replication of the broad spectral shape, it is recommended to take a flat-field exposure before and/or after the science target. Although no previous K-band photometry exists for HD 8049b, the fact that the flux appears higher in K1 (at ~1.90 um) then H (at 1.8 um) suggests that the current ratio between the satellite and occulted star flux is incorrect by a factor of ~2.

K2 band spectra

The K2 data had particularly low signal-to-noise in the satellite spot images below 2.15 um and above 2.32 um therefore these regions of the spectrum are subject to large error. Overall, the spectrum does match the slope of the blackbody to within uncertainty. However, we do emphasize that because no systematics are apparent in this plot, they most certainly exist and may vary as a function of field position. Furthermore, the percent error for the K2 data are larger than the other bands and may be larger than the level of systematics.