Launched in August 1972, almost 6 years before IUE, the
third Orbiting Astronomical Observatory (otherwise known as Copernicus)
represents one of the oldest archives of astronomical UV spectra
and is the oldest mission within MAST. Before ending
operations in 1981, multiple observations were obtained of 551 targets
ranging in brightness from -1.5 to 7th magnitude.
Six single-channel detectors were used (U1,U2,U3,V1,V2,V3 with U3 and V3
mainly used for monitoring) covering
the wavelength range from 900 to 3150 Angstroms.
instrumentation/operations for more information.)
The most useful Copernicus data is generally considered to be
the high resolution U1 and medium resolution U2 scans which covered
subsets of the 900-1600 Angstrom range. Individual
Copernicus scans could begin anywhere within the prescribed wavelength
range and proceeded by stepping up or down in wavelength every 14 seconds.
Individual scans generally covered only a few Angstroms
(i.e., they contained 14-350 points, with most containing less than 75 points).
Observations were typically centered around specific interstellar lines.
Occasionally a series of overlapping scans were taken to obtain
a more complete spectrum of the target.
The most observed target, Zeta Oph, has close to 23,000 spectral scans.
A complete target
list sorted by right ascension, and
spectral coverage plots which
give an indication of the U1 & U2 spectral coverage, are available.
Each Copernicus raw data FITS file contains all the scans for one particular
target. The scans are stored chronologically with the earliest scans first.
Usually an observer would take a set of scans, once with each detector,
and then repeat the process at various wavelengths.
As part of a recent ADP program, the U1 and U2 scans were extracted,
the contemporaneous scans were coadded, and the results were
stored in a separate set of FITS files.
The Copernicus spectral atlas files containing data published in the
six spectral atlas papers by Rogerson, et al, represent a third set of
archived data. More information is available on
data products, and
retrieving Copernicus files.
Except for some of the spectral atlas data, Copernicus spectra
are not calibrated or corrected for scattered light.
The raw data FITS files use the
"variable length array facility" which has not yet been adopted as an
official FITS format and may not be readable by all FITS readers.
The coadded scan FITS files use vector fields in a binary table FITS file,
which, although adopted as an
official FITS format,
may not be readable by all FITS readers.
The Copernicus detectors severely
degraded with time,
obtained in the first few years generally have the highest S/N.
This also implies that scans taken far apart in time are
not suitable for coadding.
Because the observations are scanned, each wavelength increment
is observed roughly 14 seconds before and after its neighbor. Certain
instrumental errors can therefore accrue across the spectrum (e.g., passage
into the South Atlantic Anomaly),
Stray light was found to be a problem with some U1 scans, but
observing methods were adopted that effectively "blocked" the stray light.
For this reason U1 scans were classified as either "blocked" or "unblocked"
and the two were not coadded.