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PS reprint
Daniel A. Klinglesmith III, George Sonneborn, Ronald S. Polidan
Code 681, NASA/GSFC, Greenbelt, MD, 20771
Randall W. Thompson, Lyla L. Taylor, Michael T. Carini
Computer Sciences Corp., Code 684.9, NASA/GSFC,
Greenbelt, MD 20771
Patricia J. Lawton
Hughes STX, Code 631, NASA/GSFC, Greenbelt MD, 20771
Michael E. Van Steenberg
Code 631, NASA/GSFC, Greenbelt MD, 20771
The third Orbiting Astronomical Observatory ( Copernicus ) satellite
(1972--1981) was the first major orbiting UV spectroscopic mission,
which covered the wavelength range from 910 to 3200Å. Copernicus observed
primarily bright, early-type stars (
6.5) at high resolution,
producing many important results, particularly for the
interstellar medium (Spitzer & Jenkins 1975). Copernicus spectra represent
the vast majority of our knowledge in the important bandpass between
the Lyman limit (911Å) and 
Å. The only additional data
in this wavelength region have been obtained recently for a small
number of objects by the Shuttle-based HUT, ORFEUS, and IMAPS
payloads.
Copernicus was launched in August 1972 carrying a Princeton University
80cm Cassegrain telescope and Paschen-Runge scanning spectrometer with
six data channels. The instrument had
two wavelength regions (U channel: 910--1450Å; V channel:
2000--3000Å), each with two detectors providing high (U1, V1;
) and moderate (U2, V2;
) spectral resolution and high
photometric accuracy. Co-added Copernicus spectra having S/N>100 were
common. The remaining two channels (U3 and V3) were at fixed
wavelengths to monitor slit throughput. The U channels provided the
majority of and the best quality data. The V channels (2000--3000Å)
were used for selected bright stars. These data were accompanied by
high dark counts in the V1 and V2 detectors, believed due to effects
of cosmic high energy particles, even outside the South Atlantic
Anomaly (SAA). Extraction of V1 and V2 data with high photometric
accuracy (S/N>10) is difficult for all but the brightest stars, but
lower accuracy spectra were obtained for many objects, much of which
has never been published. A typical observation consisted of multiple
scans of one or more spectral regions for a particular star that are
then combined to obtain a final spectrum. High S/N spectra were
achieved by making multiple scans of
individual spectral features. However, the individual scans are potentially an
important source of data on spectral variability (e.g., stellar winds,
binary system mass loss).
A total of 687,718 spectral scans of 553 targets were observed between 1972 and 1981. The 254 MB of data were archived by Princeton at NSSDC on 6250-bpi 9-track tapes in a format unique to the mission. The Copernicus Archive Project began in 1993, supported by the NASA Astrophysics Data Program, to create an electronic archive of ultraviolet spectra from the Copernicus mission and make them available through the World Wide Web (WWW).
We have recovered the Copernicus spectral scans from Princeton's 9-track tapes at NSSDC and reformatted them as disk files using the Flexible Image Transport System (FITS) format. Each file contains the raw scans for a given target, with each spectral scan stored as a row in a Binary Table extension. Because of the varying length of the individual spectral scans, count rate and heliocentric wavelength vectors are stored using the proposed variable length array facility as described in the current NOST FITS draft standard. The largest file contains more than 23,000 rows. The only change made to the original spectral data was to convert the wavelengths from the star's rest frame to a heliocentric rest frame by correcting for the star's radial velocity.
We have also recreated the Princeton processing pipeline which corrects for radiation background and produces ``stacked'', or co-added, spectra from individual scans. This process is essential to produce research-quality spectra. Additional FITS files will be generated in which contemporaneous scans are combined and stored using standard single-row binary table extensions.
As mentioned above, the U channel data is the most scientifically important component of the archive and our efforts are concentrated initially on the high resolution spectra. As part of the archive we are creating stacked scans for the U1 archive. Our intent is to provide a quick-look version of the stacked data on-line so that astronomers can evaluate the usefulness of specific Copernicus data for a particular purpose. Since the decision about which scans to stack together, what corrections to apply (e.g., scattered light, small wavelength shifts), and how to fine-tune the extraction will depend on the research program, users of Copernicus spectra are strongly encouraged to stack their own spectra in order to obtain the best data quality. We have developed such a capability and it is being tested at GSFC. Since the WWW is not very efficient for interactive graphics, we will provide software to support remote X-window sessions to run the Copernicus stack programs. Further information will be posted on the Copernicus archive WWW site.
The nature of the Copernicus spectral scans precludes the use of a fixed length Binary Table. Since there are several parameters that describe the circumstances of individual spectral scans (date, time, wavelength range, photomultiplier, orbital position, etc.), the number of points in an individual scan varies from only a few to a maximum of 350 points per scan. This results in a set of tables (time, wavelength, counts, background, and status) that are of a variable length for each scan. A proposed FITS Binary Table Extension (Cotton, Tody, & Pence 1995) provides the variable length features that are needed to present the Copernicus scans as usable format for modern computer workstations.
All the scans for each object are placed in a single FITS file. The FITS Binary Table Extension provides for two sets of headers and two sets of data. The primary header contains the information about the object: names, location, magnitudes, spectral type and spatial velocities. The secondary header describes the data set formats. The first data table is of fixed length. It contains one 40-column row of information for each scan. Five of these columns point to the locations within the second data table, called ``the heap''. These are the pointers to tables of wavelength, observation time, counts, background and status values for each scan.
The Copernicus spectral data archive currently contains the FITS
files of the raw spectral scans, basic mission and instrument
documentation, and the Copernicus spectral atlases published by J. B.
Rogerson and W. L. Upson. The Copernicus archive WWW
site provides access to this information and data files for
the six spectral atlases: 
Sco (Rogerson & Upson 1977), 
Her (Upson & Rogerson 1980), 
Ori (Rogerson & Upson 1982),
Peg (Rogerson 1985), 
CMa (Rogerson 1987), and
Lyr (Rogerson 1989).
The raw data files contain the complete set of Copernicus scans for a particular stellar object. The naming convention used for these files is Cnnn.PEP. The 3 digit number in the file name represents the original Copernicus target number. A list of targets is available via WWW and ftp. Table 1 is a list of the objects with the largest number of U1 scans. The primary header contains general data describing the target (e.g., object name, coordinates, magnitude, etc.). All other data is stored in a binary table extension with non-scaler data stored using the FITS variable length array facility (Cotton et al. 1995).
Each row of the binary table contains the data for a single scan, with scans stored in chronological order. Scans from all 6 channels (U1, U2, U3, V1, V2, V3) are included. The largest file ( C002.PEP) contains roughly 23,000 scans (13 Mbytes). However, most of the remaining files are much smaller.
The Copernicus raw data files are available from our anonymous ftp site. Users should first go to the WWW site to find which objects and files to obtain. The atlas data files are also available via ftp in the directory pub/copernicus/ atlas. The capability to examine stacked scans will be available later this year. The schedule for this and other additions to the archive will be updated on the WWW site.
Table 1: The 40 Copernicus targets with the most U1 scans
Rogerson, J. B. 1985, ApJS, 57, 751
Rogerson, J. B. 1987, ApJS, 63, 369
Rogerson, J. B. 1989, ApJS, 71, 1011
Rogerson, J. B., & Upson, W. L. 1977, ApJS, 35, 37
Rogerson, J. B., & Upson, W. L. 1982, ApJS, 49, 353
Spitzer, L., & Jenkins, E. B. 1975, ARA&A, 13, 133
Upson, W. L., & Rogerson, J. B. 1980, ApJS, 42, 175