Data Set Information
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| DATA_SET_NAME |
IUE LWR DATA OF COMETS
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| DATA_SET_ID |
IUE-C-LWR-3-EDR-IUECDB-V1.0
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| NSSDC_DATA_SET_ID |
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| DATA_SET_TERSE_DESCRIPTION |
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| DATA_SET_DESCRIPTION |
Raw Image Data and Label Parameters:Each raw image consists of an array of 8-bit picture elements or'pixels'. Each vidicon scan line consists of 768 pixels or'samples' obtained in minor frame units of 96 pixels; 768 such scan linescompose the entire image. Line 1, sample 1 is at the upper left corner ofthe image; line 768, sample 768 is at the lower right corner of the image.Each raw pixel value lies in the range 0 to 255 (integers only). The unitsof raw pixel values are data numbers (DN), which are proportional (up tothe telemetry system limit of 255) to the integrated charge read out fromthe SEC Vidicon target in the camera scanning process. Since the telemetrysystem saturates at 255, the DN/charge proportionality breaks down at thatlevel. Associated with each raw image is a set of 20 header, or label, records.Each record is 360 8-bit bytes long (a concatenation of five 72-bytelogical records). This set of 20 label records is generated by theOperations Control Center (OCC) software during image acquisition andcontains various identifying parameters and scientific/engineering datapertinent to the image. Raw images must be corrected for the instrumental effects of the SECVidicon camera system before quantitatively meaningful data can beextracted from them. The methods of compensation for the radiometric(photometric) non-linearities and non-uniformities and the geometricdistortion introduced by the vidicon system are described in the NEWSIPSManual, Chapters 5 - 11(Garhart et al., 1997 [GARHARTETAL1997]).In addition, figures 2.1-15 of the same manual illustrate schematically thespectral formats in both dispersion modes, for both apertures and for alloperational cameras. IUE Final Archive Data Products for Comets: The output files for the IUE Final Archive are fundamentally different fromthose produced by IUESIPS, both in content and format. They are based onthe Flexible Image Transport System (FITS) format (NOST 1995) andincorporate the FITS binary table extensions (NOST 1995) and FITS imageextensions (Ponz, Thompson, and Munoz 1994). Although some FITS readingroutines may not yet support these new FITS extensions, it was felt thatthere was no convenient alternative FITS format available for storing IUEdata. Note that only those features included in the basic binary tableproposal (i.e., excluding the conventions described in the appendices ofthe proposal) have been used in the Final Archive file formats. The formatsdescribed below (as originally described in DCG 1995) have been approved bythe IUE Three Agencies as well as the NOST FITS Support Office. --------------------------------------------------------------------------- * IUE Filename Conventions * Resampled Low Image (SILO) * Resampled High Image (SIHI) * Extracted Low-Dispersion Spectra (MXLO) * Extracted High-Dispersion Spectra (MXHI)--------------------------------------------------------------------------- IUE Filename Conventions: The basic FITS keywords define the structure and content of the files.These basic keywords include both the required FITS keywords and, whenappropriate, certain optional reserved FITS keywords. A project-defined keyword that needs to be mentioned is FILENAME. Thiskeyword describes the camera image number and the type of data contained inthe particular FITS header-and-data unit (HDU) and appears in every HDUcontaining data. One purpose of the FILENAME keyword is to provide users with a namingconvention when separating FITS file. FILENAME is useful for verifyingthe contents of the various data sets. The value of the FILENAME keyword is formed by the concatenation of thefollowing codes: * Camera: 3 letter code (LWP, LWR, SWP). * Image number: 5 digits. * File type: 2 letter code as: RI raw image RO original RI (low dispersion only, in the case of partial-read images) VD vector displacements XC binary table extension of the VD file containing the cross correlation coefficients LI linearized image LF nu flag image extension of the LI file SI resampled image WL binary table extension of the high-dispersion SI file containing spectral wavelengths and spatial centroid positions of the orders SF nu flag image extension of the high-dispersion SI file CR cosmic ray image extension of the high-dispersion SI file MX merged extracted image (large, small or both apertures) * Dispersion: 2 letter code (HI, LO). Resampled Image (SILO):The resampled low-dispersion image is an array produced by resampling thephotometrically corrected portion of the LILO/LIHI image using the modifiedShepard algorithm taken from the Numerical Algorithms Group (NAG) softwarepackage. Each pixel is resampled to the position determined by thesummation of the vectors needed for: 1. shift to photometric correction (ITF) raw space, 2. shift from ITF space to geometrically-rectified space, 3. rotation such that orders are horizontal, 4. wavelength linearization, 5. detilting of large-aperture spectra for low-dispersion extended sources only, 6. alignment of the low-dispersion apertures for constant wavelength in the line direction, 7. adjustment so that both LW cameras provide coverage of the same spectral range, 8. adjustment to maintain the spectrum at approximately the same location in the file in the spatial direction (low dispersion only), 9. adjustment to LWP data to put the large-aperture data at the top of the file, 10. corrections for the spatial deviations (cross-dispersion wiggles) for the LWP and LWR low-dispersion data, The low-dispersion SI is stored in the SILO as a 2-D (640 samples x 80lines) primary array, with the y coordinate in pixels and the x coordinatein in Angstroms. Each pixel represents a flux number (FN) scaled up by afactor of 32 for storage purposes. The pixels are coded as 16-bit, two'scomplement integers, with the bits stored in decreasing order ofsignificance. When the image is displayed with the origin in the lower leftcorner, the large-aperture data appears at the top of the file and thewavelengths increase from left to right. The associated pixel quality flagsare stored as an image extension which has the same dimensions as theprimary array. Table 12.8 in the NEWSIPS Manual (Garhart et al., 1997[GARHARTETAL1997]) shows the basic FITS keywords for the main header andthe image extension header. High-Dispersion Resampled Image FITS File (SIHI): The SIHI contains more information than stored in the correspondinglow-dispersion file and, as a result, the FITS format is slightly morecomplex. Overall, the SIHI is comprised of a primary array containing theresampled image, a binary table of wavelengths and both predicted and foundline positions, an image extension of nu flags, and a second imageextension of background cosmic ray flags. The high-dispersion SI data is similar to the low-dispersion SI data exceptthat the high-dispersion wavelength linearization varies with spectralorder, and the entire image is stored in the primary array. Each pixel isresampled to the position determined by the summation of the vectorscomputed for: * shift to photometric correction (ITF) raw space, * shift from ITF space to geometrically-rectified space, * rotation such that orders are horizontal, * wavelength linearization, * adjustment to maintain the echelle orders at approximately the same locations in the file in the spatial direction, * corrections for the spatial deviations (cross-dispersion wiggles) for LWP, LWR, and SWP data, * heliocentric velocity correction, and * de-splaying correction. The high-dispersion SI is stored in the SIHI as a 2-D (768 samples 768lines) primary array. Each pixel represents an FN scaled up by a factor of32 for storage purposes. The pixels are coded as 16-bit, two's complementintegers, with the bits stored in decreasing order of significance. Whenthe image is displayed with the origin in the lower left corner, theshort-wavelength, closely-spaced high order numbers appear at the bottom,and the long-wavelength, low order numbers appear at the top. Within eachorder, the wavelengths increase from left to right. Because the wavelength linearization varies with spectral order, thestarting wavelength and wavelength increment values vary with each order.This information is stored in a binary table extension to the SIHI, whichfollows the primary array. The entire contents of the binary tableextension include: * Order Number, one 8-bit integer. * Starting wavelength, one double-precision floating point number. Heliocentric velocity correction has been applied. * Wavelength increment, one double-precision floating point number. * predicted line position of order centroid, one single-precision floating point number. * line position where spectral centroid is found, one single-precision floating point number. (This is determined by the high-dispersion spectral flux extraction module and written back into the SIHI file retroactively.) The associated nu flags and cosmic ray flags are stored in the SIHIimage extensions with the same dimensions and orientation as thehigh-dispersion SI data contained in the primary array. The pixel qualityflags are stored as unscaled 16-bit integers, and the cosmic ray flags areunscaled 8-bit integers. Table 12.9 from Garhart et al. (1997) shows thebasic FITS keywords for the main and extension headers for the SIHI. Extracted Low-Dispersion Spectra (MXLO):The extracted low-dispersion file uses the binary 3-D table extension withfixed-length floating point vectors to contain the extracted fluxes andassociated data quality flags. Since no primary data are included, theextension header immediately follows the primary FITS header. Each row ofthe binary table includes the following columns: 1. Aperture designation as 'LARGE' or 'SMALL', stored in 5 ASCII characters. 2. Number of extracted points, one 16-bit integer. The number of extracted points is 640. 3. Starting wavelength, one single precision floating point value. 4. Wavelength increment, one single precision floating point value. 5. Net flux spectrum, array with 640 single precision floating point values. 6. Background flux spectrum, array with 640 single precision floating point values. 7. Sigma vector, array with 640 single precision floating point values. 8. Data quality flags, array of 640 16-bit integers. 9. Absolutely calibrated net flux spectrum, array with 640 single precision floating point values. Wavelengths are linearly sampled, and referenced to vacuum. Double aperturelow-dispersion spectra will contain two rows in the above format, with onerow for each aperture. Table 12.10 in the NEWSIPS Manual (Garhart et al.,1997 [GARHARTETAL1997]) shows the basic FITS keywords for the MXLO file. Note: The keyword NAXIS1 in the table extension defines the number of bytesper row in the table. High-Dispersion Merged Extracted Image FITS File (MXHI): The wavelengths, nu flags, and fluxes extracted from the SIHI arestored in the MXHI as a binary table extension using fixed-length floatingpoint vectors. No primary data or additional extensions are included. The binary table contains 17 fields of various data types. All vectors arepadded with zeroes (both before and after the extracted data) to maintaina fixed length of 768 points. Wavelengths are uniformly sampled for eachorder, are measured in vacuum, and have had the heliocentric velocitycorrection applied. The width of each row (i.e., 65 + 22 * 768 : 16961)bytes, and the number of rows (i.e., NAXIS2) is equal to the number ofextracted orders. In this manner, all the information pertaining to onespectral order is contained in one row of the binary table. The fields aredefined in the order shown below: * Order number, one 8-bit byte. * Number of extracted points n, one 16-bit integer. * Starting wavelength, one double-precision floating point value. * Starting pixel at starting wavelength, one 16-bit integer. * Wavelength increment, one double-precision floating point value. * Slit height in pixels, one single-precision floating point number. * Line number for found centroid of spectrum, one single-precision floating point number. * Net flux spectrum, 768 single-precision floating point numbers with n extracted data points. * Background flux spectrum, 768 single-precision floating point numbers with n extracted data points. * Noise vector, 768 single-precision floating point numbers with n extracted data points. * nu flags as n 16-bit integers stored in two's complement form. * Ripple-corrected net flux spectrum, 768 single-precision floating with n extracted data points. * Absolutely-calibrated, ripple-corrected net flux spectrum, 768 single-precision floating point numbers. with n extracted data points. * Start pixel for background fit, one 16-bit integer number. * * End pixel for background fit, one 16-bit integer number. * * Chebyshev scale factor, one single-precision floating point number. * * Chebyshev polynomial coefficients for global background correction, 7 single-precision floating point numbers. * Note that unlike the MXLO, SILO, and SIHI, the starting wavelengths listedin the MXHI table do not refer to the first data point in the flux vectors,but rather the starting pixel listed in field four. In this manner, the768-point flux vector can be mapped directly to the 768-pixel widehigh-dispersion SI array. As in low dispersion, since the absolute calibration covers the range of1150-1980 for short-wavelength spectra and 1850-3350 for long-wavelengthspectra, data points outside this wavelength range are set to 0 in theabsolutely-calibrated flux vector. The net, background, and noise vectorsare not affected. (Note that unlike the sigma vector in the MXLO file, theMXHI noise vector is uncalibrated.) Uncalibrated data points are alsoflagged in the nu flag vector with a value of -2. *IMPORTANT NOTE: Several adjustments must be made to the lastfour parameters (fields 14-17) if the user wishes to evaluate the Chebyshevcoefficients in order to reproduce the background fluxes as stored in theninth field of the MXHI extension header. First, the parameters haveinadvertently been stored in the reverse order (i.e., the parameterswritten in the first row of the table should have been stored in the lastrow, the parameters for the second row in the second to last row, etc.).So, for example, in the case of the LWR camera, the starting and endingpixels, Chebyshev scale factor, and Chebyshev coefficients found in row 1(echelle order 127) actually pertain to row 61 (echelle order 67). Second,the true starting pixel is 768 minus the stored ending pixel and the trueending pixel is 768 minus the stored starting pixel. These true pixelvalues must be used to correctly evaluate the Chebyshev coefficients.Third, once the Chebyshev coefficients have been evaluated, the resultantbackground ``fluxes'' must be scaled in the following manner: multiply eachbackground value by both the Chebyshev scale factor and the correspondingextraction slit height then divide this result by 32. Finally, theresultant array of background fluxes which are produced upon evaluation ofthe Chebyshev coefficients must be reversed (i.e., the computed backgroundflux for pixel 1 becomes the background flux for pixel 768 and vice versa).We emphasize that these reversals and scalings are needed only when usingthe Chebyshev parameters in fields 14-17 to reproduce the backgroundfluxes-the background fluxes themselves as contained in the ninth field arecorrect.
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| DATA_SET_RELEASE_DATE |
2001-02-19T00:00:00.000Z
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| START_TIME |
1978-10-15T11:38:36.000Z
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| STOP_TIME |
1983-09-07T10:39:53.000Z
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| MISSION_NAME |
INTERNATIONAL ULTRAVIOLET EXPLORER
SUPPORT ARCHIVES
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| MISSION_START_DATE |
1978-01-26T12:00:00.000Z
2004-03-22T12:00:00.000Z
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| MISSION_STOP_DATE |
1996-09-30T12:00:00.000Z
N/A (ongoing)
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| TARGET_NAME |
140P/BOWELL-SKIFF 1 (1980 E1)
19P/BORRELLY 1 (1904 Y2)
22P/KOPFF 1 (1906 Q1)
26P/GRIGG-SKJELLERUP 1 (1922 K1)
2P/ENCKE 1 (1818 W1)
38P/STEPHAN-OTERMA 1 (1942 V1)
67P/CHURYUMOV-GERASIMENKO 1 (1969 R1)
6P/D'ARREST 1 (1851 M1)
8P/TUTTLE 1 (1858 A1)
9P/TEMPEL 1 (1867 G1)
C/AUSTIN (1982 M1)
C/BRADFIELD (1979 Y1)
C/CERNIS (1983 O1)
C/IRAS-ARAKI-ALCOCK (1983 H1)
C/MEIER (1980 V1)
C/PANTHER (1980 Y2)
C/SEARGENT (1978 T1)
C/SUGA-SAIGUSA-FUJIKAWA (1983 J1)
COMET
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| TARGET_TYPE |
COMET
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| INSTRUMENT_HOST_ID |
IUE
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| INSTRUMENT_NAME |
LONG-WAVELENGTH REDUNDANT
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| INSTRUMENT_ID |
LWR
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| INSTRUMENT_TYPE |
SPECTROGRAPH
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| NODE_NAME |
Small Bodies
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| ARCHIVE_STATUS |
ARCHIVED
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| CONFIDENCE_LEVEL_NOTE |
N/A
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| CITATION_DESCRIPTION |
Grayzeck, E.J., IUE LWR DATA OF COMETS, IUE-C-LWR-3-EDR-IUECDB-V1.0, NASA Planetary Data System, 2001.
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| ABSTRACT_TEXT |
IUE Long-Wavelength Redundant (LWR) observations of comets
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| PRODUCER_FULL_NAME |
EDWIN J. GRAYZECK, JR
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| SEARCH/ACCESS DATA |
SBN Comet Website
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