As originally conceived, the NVO concept emphasized all-sky imaging surveys at selected wavelengths. In this document we describe a different acquisition mode in which only a few square arcseconds of sky are imaged, but many thousands of channels of high-resolution spectra in the visible and infrared are recorded, say 110,000 points from 350 to 900 nm. The mass of data that will be collected in this mode will also be substantial. Almost every large telescope now has an echelle spectrograph, cross disperser, and CCD detector, with resolution of 30,000 or better. Most spectra will be recorded by small PI-led research groups studying specific distant objects. As such, the astronomy value of the data appears to be less transferable to other investigators and modes of inquiry.
A hidden science resource exists in the form of the background or sky spectra recorded by the pixels imaging the empty neighborhood of the astronomer's target object. The principal contributors to this background are sunlight scattered by interplanetary and galactic dust, optical emissions by galactic gas, spatially unresolved starlight, and emissions from the terrestrial atmosphere. The astronomer needs to resolve the spectral complexity of this sky background in order to subtract it from the spectrum of the target object. For an atmospheric scientist, and especially for an aeronomy specialist studying the dayside photochemistry and nightside recombination, excited state generation, and the resulting optical emissions, such spectra would represent a huge leap in observational capability. For example, at the end of a night's data collection, one would be able to retrieve hourly mesospheric temperatures from OH rotational distributions, mesopause temperatures from O2 rotational distributions, sodium nightglow intensities, and the decaying intensities of ionospheric atomic nitrogen and oxygen. With the same information available on a given day from a network of a dozen observatories scattered worldwide, one would have access to a data set that in many ways is superior to the global coverage provided by a satellite, precisely because the platforms are stationary.
We propose to generalize and expand the pioneering work of Don Osterbrock (described below) by collecting these background noise spectra as an aspect of normal operations at multiple large-telescope observatories world wide. The data would be organized in the form of two-column (wavelength, intensity) plain text spectrum files (with both axes calibrated) along with associated header files containing the observing conditions. Access would be provided through the world-wide-web. A simple example is the solar spectrum available from http://mesola.obspm.fr/form_spectre.html More sophisticated users with access to and experience with astronomical analysis software could download data as IRAF/FITS files. Application-specific user-interface software would be developed to allow inexperienced users to ask "science" questions without having to understand the details of telescope observations.
The PIs have decades of experience using their expertise in laboratory spectroscopy and chemical kinetics to interpret the optical emissions of terrestrial and planetary atmospheres. The unique resource of the Keck/HIRES spectra has enabled them to make a number of advances that have important implications both for understanding atmospheric processes and for dealing with near-earth night sky emissions that interfere with astronomical observations. Some of these are recounted here. See also the references cited in the Web page http://www-mpl.sri.com/projects/pyu02424.html The first precise spectroscopic characterization has been made of 25 vibrational levels in the lowest three electronic states of O2. The first determination of the distribution of population in the vibrational levels of O2(b) shows surprising structure that will have significant implications for atmospheric chemistry. Numerous atomic emissions are assigned, some observed for the first time in the atmosphere. The superb spectral resolution and calibration of the HIRES spectra has resulted in new determinations of emission wavelengths for some supposedly well-known transitions in the potassium and nitrogen atoms. Surprising is observation of 36 strong lines from atomic neon, along with a few lines assigned to atomic argon, and two to atomic xenon. The neon emissions are significantly stronger than those of atomic mercury. Identification of the source of the atmospherically unexpected atomic emissions is under investigation.
The PIs are currently examining the individual unsummed spectra, typically integrated over an hour or less, correlating the intensities of individual emission features during the night from sunset to sunrise, and following the viewing direction with respect to atmospheric, solar-system, galactic, and extra-galactic sources. As a result we have learned about astronomical IRAF/FITS/makee data formats, how to understand what the astronomer has done, to extract the observing conditions, what is needed to perform wavelength and intensity calibration, and the requirements and problems we will encounter in making astronomy data accessible and useful to other atmospheric scientists.
Based on our understanding of the relationships between the atmospheric chemistries of Earth, Venus, and Mars, the PIs have initiated on-purpose telescope observations of the night airglows of Venus and Mars. The first high-resolution visible spectrum of the night sky of Venus was taken at Keck in November 1999 by Tom Bida (then at Keck, now at Lowell Observatory), producing a big surprise. The O2 emissions reported by the Venera 9/10 orbiters were confirmed, but the strongest emission is the atomic oxygen green line at 558 nm, which was not present in the Venera spectra, but should have been conspicuous.
As a result we are meeting and developing new collaborations with additional astronomers (including Wal Sargent and Tom Barlow of Caltech, Jeff Kuhn of U. Hawaii, Dave Crisp of JPL, and Nancy Chanover of NMSU), learning more about astronomical observations at various observatories, and developing a better understanding of the details of extraterrestrial background sources such as moon light, Venus day-side scattered light, zodiacal light, and galactic line emission. Wal Sargent has agreed to find us contacts at Japanese and European/VLT observatories.
The data to be included are taken in well-defined observing campaigns with narrowly focused objectives. Nevertheless, one of the key results of our past and ongoing analyses of Keck/HIRES night sky spectra is that they are automatically "surveys." We discover things even though we were not looking for them. We can see something interesting, then jump to another wavelength, or a different time on the same night, or a different season or year, to ask follow-up questions. This fits well with the overall NVO concept. Someone else takes the data. Anyone can analyze it, making discoveries that were unanticipated by the original observer.
Better results are likely when the data sets are relatively homogeneous and limited in scope and when data archiving and subsequent use are planned in advance. Such would be the case for the original NVO target of "sky image" surveys and for the "sky spectra" surveys addressed by the NVAO proposed here. The sky spectra present a somewhat more difficult case in that the data to be archived are merely incidental to the primary objectives of the original observer. This means that contiguous spectral coverage is not guaranteed and that photometric intensity calibration requires additional analysis. Furthermore, the region of the atmosphere observed varies during the night as the telescope tracks the astronomer's target object or as a new target is selected. In addition, the sky spectrum has multiple contributions from atmospheric, solar system, galactic, extragalactic sources, probably only one of which is of interest to any individual user.
We plan to design the user interface based on the way we have been analyzing our initial sets of Keck/HIRES data. For example, selecting a wavelength range should result in presentation of a menu of available spectra and scripting tools for (1) choosing a sequence of spectra versus observing conditions (telescope location, local time, month/season, azimuth, elevation, ecliptic coordinates, declination, and right ascension), (2) co-adding, (3) graphical display, (4) downloading, (5) peak fitting, and (6) spectral simulation. Similarly, selecting a single or small number of known or user-defined peaks or bands should lead to (1) graphs of intensities or ratios versus observing conditions, (2) correlations of intensities, (3) statistical variations, with (4) graphical display and (5) downloading.
Even more important to the success of the proposed concept is the comfort level of observing astronomers whose data would be abstracted. Donating data should require no additional effort beyond their normal analysis. There must be no pressure to change the way they do their observations. Nothing about their own research objectives and results should be revealed in advance of publication. Having seen what we have already found in their data, a small but growing number of astronomers have indicated their interest in contributing. We plan to solicit the broadest possible participation by publications in society newsletters and journals, by frequent presentations/workshops at society meetings, and by asking senior astronomers to join our planned Scientific Advisory Group.
PHASE 1 (9 months)
PHASE 2 (15 months)
PHASE 3 (12 months, then ongoing)
White Paper MP 00-051
30 November 2000
Submitted to NASA Office of Space Science
david.huestis@sri.com
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