Chiron Fiber-mode Data Reductions
Disclaimer: this is applicable ONLY to Chiron data taken in fiber mode.

The standard Chiron data reductions are optimized for the standard Chiron observing program - radial velocity studies of bright stars searching for exoplanets. Radial velocity stability is crucial. For these purposes the reduced data products more than suffice.

Chiron is fiber-fed: there is no sky subtraction capability (except after the fact). The standard reduction does not seem to subtract the instrumental background, which is not a problem for short integrations in bright stars. The flat field is flattened prior to division into the data, retaining the blaze function.

Our needs are different. Novae fade rapidly, and the spectra are often dominated by broad emission lines. We need to be able to subtract the instrumental background (our exposures run up to 30 minutes). We need to be able to flatten the spectra, because when the width of the line is comparable to the free spectral range it can be difficult to determine where the continuum is.

We also discovered that the algorithm employed to reject cosmic rays has the unfortunate side effect of rejecting strong emission lines. This is unacceptable. So I wrote a new reduction scheme in IDL. The salient features include:

Full details and documentation are available here.

IDL procedures and master calibration files are available in these tar files.

Figures from the full writeup follow.

Order 10 of achi150728.1127.fits. The target is the nova N Sgr 2015b. The emission line is Fe II λ4921. The narrow dropouts are an artifact of the default data reduction process.

The normalized ratio of the boxcar-extracted to the Gaussian-extracted flat for order 108. The green trace is the scaled width of the Gaussian used to extract the flat. They track well, with the larger ratio (less flux in the Gaussian extraction) when the fit is broader. This occurs when the trace is centered between two pixels. I suspect this is attributable to the non-Gaussianity of the PSF. In any event, the effect seems to divide out.

The Na D lines in N Oph 2015 on 150728. There are few narrow lines in novae - these are interstellar. The achi spectrum is in aqua. The wavelength solutions are in good agreement.

The ratio of the boxcar-extracted to the Gaussian-extracted spectra, after flat division, for order 108 of chi131014.1143 (μ Col). The RMS scatter of the ratio is 0.6%. The median is 0.999; no normalization has been applied. The structure visible in the flat ratio is greatly reduced. The SNR of the spectrum, from counting statistics alone, peaks about 900, and exceeds 500 between pixels 250 and 850. Within this region the RMS is about 0.4%, comparable to the 0.3% expected from counting statistics alone.

A comparison of the boxcar extraction (black), Gaussian extraction (dark blue) and the Yale extraction (aqua) for the H-alpha of Nova Sco 2015 (image chi150728.1124). The Yale extraction is scaled so the medians match. Note that the central peak of the H-alpha line is missing in the Yale extractions. It has apparently been flagged as a cosmic ray or some other type of defect. The top of the line is not saturated.

A comparison of the boxcar extraction (black) with the Yale extraction (aqua) for the sodium D line region of Nova Sco 2015 (image chi150729.1124). The Yale extraction is scaled so the medians match. Note that the line strengths are much weaker in the Yale reduction, while the S/N is higher. This suggests that the background has not been subtracted in the Yale reductions. The emission line is He I λ5876. Multiple velocity components are visible in the Na D1 and D2 lines. The low velocity components are galactic foreground; the blue-shifted absorption lines may be ejecta from the nova.

This is the same data as shown in the figure above, except that the data (black) are not flattened, and the background has not been subtracted. This provides a reasonably good match to the Yale reductions (magenta).

Order 67, showing strong emission from the λ8498A component of the Ca II IR triplet. The line is not evident in the Yale reductions. The apparent strengths of the λ8446A O I line are also very different.

Order 66, showing strong emission from the λ8542A component of the Ca II IR triplet. As in the previous figure, the line is not evident in the Yale reductions.

The bright [O I] λ6300A line in N Sgr 2015b. The two extractions agree well when the background is negligible. Differences in the slopes are attributable to the flat-fielding.

The H-delta λ4100A line in N Sgr 2015b. The other line is probably Fe II λ4128A (multiplet 27). This is as blue as Chiron goes. All 1028 points are plotted; nothing is masked. There is not much sensitivity at the ends of the order.

A particularly boring part of the spectrum of the O9.5V star μ Col, a spectrophotometric standard. Note that division by the actual flat extraction yields a very flat spectrum.

The Hα region of V745 Sco on 150603. This is the sum of two 1800 second integrations. The R mag at the time of observation was about 16.2. The dotted red vertical lines mark the spliced regions. No background has been subtracted; the data are unsmoothed. The default trims have been expanded by 10 pixels. The high points are due to low counts at the edge of the detector rather than to cosmic rays. The narrow emission is Hα from one of the two stars in the system; the source of the broad emission near λ6585A is not currently known. No sky has been subtracted; much of the continuum and the λ6563A absorption may be from the night sky.