Frequently Asked Questions

Where do I get Copernicus archival data?

• At the end of the mission, Princeton provided the National Space Science Data Center (NSSDC) with a compact set of data on 9-track tapes as the final archive of the Copernicus science mission. The data were formatted in such a manner that special programs were needed to extract the information, bit by bit, from the archive. Under a grant from NASA's Astrophysics Data Program, the Laboratory for Astronomy and Solar Physics and the IUE Data Analysis Center have converted the tape files into FITS format disk files and created an on-line archive of Copernicus ultraviolet spectra.

  There is one FITS file of raw scans for each of the 551 objects observed by Copernicus. These files can be retrieved from the raw target list, the Copernicus search page, and via the anonymous FTP at archive.stsci.edu in the /pub/copernicus/raw subdirectory. The FITS files containing the raw spectral scans are constructed using a Binary table extension and the proposed "variable length array facility" as described in the NOST FITS Draft Standard. Each row of the binary table contains the data from one spectral scan.

  In addition to the raw data files, coadded contemporaneous scans from the U1 high resolution channel (900-1560 Å) and U2 (900-1650 Å) channel are available from the Copernicus Coadded Scan search page, and via the anonymous ftp at archive.stsci.edu in the U1 coadded scan directory and the U2 coadded scan directory The files are stored as standard Binary table FITS files using fixed length vector fields. The files are intended primarily for quick-look data analysis.

How do I download the Copernicus Spectral Atlas Files?

• The spectral atlas files are available online from the Copernicus spectral atlas page. References to the original published papaers are also given. The files can also be retrieved via anonymous ftp at archive.stsci.edu in the spectral atlas directory. All the spectral atlas files use FITS binary table extensions with scalar fields.

What are the wavelength ranges of the Copernicus detectors?

• The table below shows the spectral coverage of each detector. Note however that the individual scans generally covered a much smaller wavelength range.

  Name	Exit Slit* (Å)	Wavelength (Å)	Step Size* (Å)
  1 - U1	0.05	710-1500**	0.025
  2 - V1	0.10	1640-3185	0.05
  3 - U2	0.20	750-1645	0.20
  4 - V2	0.40	1480-3275	0.40
  5 - V3	?	3430 (fixed)	-
  6 - U3	?	1320 (fixed)	-

  *Note: Exit slit and step size vary slightly with wavelength.
  **For brighter stars, the 1500-1560 angstrom range can be scanned with U1 in the first order.

Did the instrument sensitivity significantly degrade with time?

• The principal malfunction in the Copernicus instrumentation was the rapid decline in the far-UV sensitivity. The decision to terminate the spacecraft was based partly on the loss of far-UV sensitivity. The following figure shows the relative sensitivity of the high-resolution far-UV phototube U1 from orbits 100 to 44000. A rapid (approximately 50 percent) decline at shorter wavelengths was experienced during the first year followed by smaller declines in subsequent years. The low-resolution far-UV phototube U2 exhibited a behavior similar to that of U1. The near-UV phototubes, V1 and V2, did not exhibit the rapid decline seen in the far-UV phototube. For further information, see the Final Operations Report.

• The raw data sets are probably the most difficult files to read since they use the proposed variable length array facility for storing vectors in a binary table extension. In any event, <examples> of how to read all the Copernicus data sets using a variety of FITS readers are available. Many users will want to work with individual scans. Examples of how to work with them are on the <third example> page just cited.

• The IDL program STACK allows U1 or U2 Copernicus scans to be coadded. This software was used to create the "quick-look" coadded scan files and is available to users from either the Copernicus Spectral Scan page (which currently requires you to know the name of a coadded scan file), or from the browse files accessible after searching the coadded scan database. In principle, STACK can coadd any Copernicus scans, however only scans taken close in time and using the same detector can produce meaningful results. For this reason, use of STACK is restricted to addiing or subtracting the existing sets of coadded scans.

How do I know what wavelength regions were observed for a particular object?

• The min and max wavelength for each scan is stored as scalar fields in the raw data sets (note wavelengths could be scanned in either direction, so some vectors are in order of decreasing wavelength). However, to help users determine wavelength coverage for U1 and U2 scans, a spectral coverage table was created. This can be a useful tool since most U1 scans only cover a couple of Angstroms, and U2 scans normally covered less than 50 Angstroms. The rows of the spectral coverage table represent the observed targets, while the columns list various wavelength regions. Each entry in the table shows the number of scans obtained in that particular spectral region and is also a link to a gif-format plot file showing the spectral range of each scan within that spectral region.