The Critical Importance of Data Reduction Calibrations In the Interpretability of S-type Asteroid Spectra Michael J. Gaffey Space Studies Department University of North Dakota

Only briefly -- Extinction Corrections – –Accurate extinction corrections are essential in the NIR Spectra – –Corrections should depend on objective criteria Avoid using adjustable parameters unless there is are independent objective criteria for determining the adjustment factor. “It looks right” is not an adequate criterion.

Preface This presentation uses examples of asteroid spectra in the recent literature.

Preface This presentation uses examples of asteroid spectra in the recent literature. Some of these are used as examples of data with significant problems which effect their interpretability.

Preface This presentation uses examples of asteroid spectra in the recent literature. Some of these are used as examples of data with significant actual or potential problems which effect their interpretability. No citations or references are given to these particular spectra since the intent is not to attack or insult any individuals. – –In most cases, the asteroid identifications have also been omitted.

Preface This presentation uses examples of asteroid spectra in the recent literature. Some of these are used as examples of data with significant actual or potential problems which effect their interpretability. No citations or references are given to these particular spectra since the intent is not to attack or insult any individuals. – –In most cases, the asteroid identifications have also been omitted. The intent is to show the nature of the problems and to discuss the methods to ameliorate these problems.

Unsmoothed, unedited average spectrum. This should be the current expectation for NIR asteroid spectra. Current State-of-the-Art H2OH2O H2OH2O Hardersen et al. (2004) 1459 MagnyaV mag = 15.7

Spectra with Serious Problems in the 1.4 & 1.9  m Telluric Features V mags = Are these problems due to fainter objects or shorter integrations?

Similar 1.4 & 1.9  m problems for a bright asteroid target (Vesta)

Can these problems be ameliorated by smoothing? Smoothing will only work if the “spikiness” is due random noise in the spectra.

0.05 airmass difference Exposure = 60 sec V Mag = 12.4 What is the nature of these variations? Simple Asteroid / Standard Star Ratio

Effects of Channel Shifts Much of the “noise” is actually an interference pattern due to small offsets in the location of the dispersed spectrum onto the detector array. +1 Channel Offset -1 Channel Offset Raw Flux Data

The Pattern Results from the Fine Structure in the Water Vapor Features At this resolution, slight channel offsets produce an interference pattern in the resulting ratios of raw spectra. At much lower resolution there is no problem. At much higher resolution there is no problem.

Correction for extinction using a SPECPR Starpack Extinction coefficients were calculated from standard star observations without correction for channel offsets

Extinction correction with channel offsets Most of the “noise” in the 1.4 & 1.9  m regions was not random, but due to uncorrected channel offsets

Pattern for 1 channel Offset on SpeX Smoothing does not remove this pattern 37 Pt Smooth ~0.15  m Interval 13 Pt Smooth ~0.05  m Interval 25 Pt Smooth ~0.10  m Interval 51 Pt Smooth - Edited ~0.15  m Interval

Effects of Uncorrected 0.5 Channel Offset -0.5 Chan Chan. No Shift A mall offset produces major deviations in Band II

Implications for Analysis Irrespective of the analysis technique Curve matching Gaussian fitting Parameter extraction Interpretations would differ for these spectra.

Effect on Band II Parameters SpectrumBand II AreaBII Center Properly Corrected 3.16 units 1.93  m Uncorrected +0.5 Channel Shift 3.48 units 10% high 1.95  m Uncorrected Channel Shift 3.89 units 23% high 1.82  m Double Band?

Implications The structure of the 1.4 & 1.9  m “noise” is not random. It’s an interference pattern due to slight channel offsets in the spectra. – –Offsets due to instrument flexure – –Offsets due to position of the object in the slit or aperture Fractional channel shifts produce significant spectral effects. Smoothing will not ameliorate this problem!

Effects of Smoothing Smoothing the uncorrected data introduces significant artifacts in the spectral region of the water vapor absorptions. This problem is particularly significant for S- type asteroids which have relatively weak 2  m features. The S-type asteroid used in this example has a relatively strong 2  m feature. For more weakly featured S-asteroids, the effect on the 2  m feature will be more pronounced.

Correction Process The pattern identifies presence of an offset. Channel offset determined for each set of observations relative to some reference set. Offsets are derived by using the steep edge of the 1.4  m atmospheric water vapor feature. Offsets should be established to ~ pixels. Pixel offset corrections are applied to the raw standard star spectra prior to calculation of the extinction coefficients (or their use in ratios). Pixel offset corrections are applied to the raw object spectra prior to extinction corrections.

Conclusions All medium resolution NIR asteroid spectral data should be corrected for channel offsets as the initial reduction step. – –Preferably the offsets should be determined from the data itself. Extinction should use “objective” criteria. – –Standard extinction coefficients adjusted until the spectrum “looks right” should be avoided. Smoothing of spectra should be avoided unless the previous steps have been accomplished.

The Upside -- Asteroid spectra obtained with medium resolution NIR spectrographs must be routinely checked and corrected for channel offsets. The parameters needed to make the offset correction are derivable from the raw data itself. Routines exist to make the corrections as a regular step in data reduction.

In consequence -- It should no longer be acceptable to publish asteroid spectra which exhibit this correctable problem. As reviewers and editors we should assist our colleagues in identifying this problem and ask that they resubmit their manuscripts with the appropriate correction. This will be good for both our colleagues and for asteroid science by providing the best attainable spectra of our subject bodies.