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35.1 Contents of Delivered Data
Most HST data are retrieved over the Internet, but it is still possible to get a tape. In either case, you will receive various FITS files containing science data and other information. StarView gives you the option of obtaining sets of uncalibrated, calibrated1, data quality, observation log, and used and best reference files. In this section we provide a description of the information stored in each file. The STSDAS routine for unpacking your data is strfits. The resulting files will have default suffixes, as described in Table 35.1 below.
The suffixes for the headers of the raw science data images you will have after running strfits are listed in the first column in Table 35.1; they are the output of the Routine Science Data Processing (RSDP) pipeline generic conversion process and are discussed in more detail below. You generally need all of these files in order to calibrate GHRS data.
The third column in Table 35.1 lists the header suffixes for the calibrated data created by running calhrs, as well as some additional engineering files available from the Archive. The RSDP pipeline usually runs calhrs on all GHRS data when it is taken, but due to updates to reference images and tables, we strongly recommend that you recalibrate the data.
File Name Suffixes
35.1.1 Uncalibrated Raw Data
Science Data (.d0h and .q0h)
The science data (.d0h image) contains the single-precision floating point values representing the number of detected counts accumulated for each diode, just as it comes from the spacecraft. Depending on the stepping pattern (STEPPATT) used for the observation, there may be only one, or up to six groups of science data per exposure. The default STEPPATTs are listed in Table 35.3 and the implications are discussed more in "File Sizes" on page 35-5. The science data are read out from the 500 science diodes (see the GHRS Instrument Handbook).
The data quality image (.q0h) associated with the science data records whether there is "fill" data due to technical problems with the observation or due to problems in transmitting the data from the telescope. The .q0h file contains zeroes where the data are good, and ones for Reed-Solomon errors (related to data transmission) and 16 for fill data (see "Calibration Quality Files" on page 37-22 for a more complete explanation).
Return-To-Brightest and Small Science Aperture ACQ/PEAKUP (.d1h and .q1h)
Most acquisitions of point sources since mid-1993 used the return-to-brightest algorithm, in which the aperture (LSA or SSA) was rastered in a 3 x 3 or 5 x 5 pattern, and the pointing with the most counts was returned to for peakup and observation. The return-to-brightest (RTB) target acquisition record is in the .d1h image and also in the trailer file (.trl). This image contains the total counts at each dwell point in the spiral search performed by the RTB acquisition algorithm.
SSA ACQ/PEAKUPs use the same algorithm as RTB target acquisitions. Therefore they will also have a .d1h file whose header file contains information on which dwell point (MAPFND) contained the largest flux (FLUXFND).
The data quality image (.q1h) associated with the RTB data records whether there is fill data due to technical problems with the observation or due to problems in transmitting the data from the telescope.
Extracted Engineering Data (.x0h)
For ACCUM and RAPID mode observations, the extracted engineering data image (.x0h) consists of 24 pixels which contain the values of the twelve special diodes (i.e., focus, background monitor, and radiation monitor diodes; see the GHRS Instrument Handbook) in the detector and twelve pixels of engineering trailer and flight software data relevant to each pattern execution (see Table 35.2). The data quality file (.xqh) records whether any of the data represent fill due to technical problems with the observation or telescope during the observation. These data are sometimes used for determining the background count rate.
Standard Header Packet (.shh)
The standard header packet (SHP, image suffix .shh) contains the telemetry values from the engineering data and some GHRS-unique data. The engineering data includes temperatures, currents, and voltages at various points in the instrument. The header packet also contains information used in the operation of the spacecraft, such as target name, position, and velocity of the telescope, the right ascension and declination of the target, Sun, and Moon, and other proposal information used in the observation which was provided in the Phase II part of the proposal. There is one group of .shh data per pattern used in the observation. Various items from the SHP are used in various steps of the reduction process.
Unique Data Log (.ulh)
The unique data log (UDL, image suffix .ulh) contains the command values (i.e., the Observation Control Table) used to control the aperture, detector, carrousel, Digicon deflections, observing modes, and flux measurements as well as some observation data produced by the flight software. There are two groups of .ulh data per pattern per observation. For images and standard ACCUM mode, a leading and following UDL are readout that bracket the observation.
Trailer File (.trl)
The trailer file (suffix .trl) contains many messages generated by the conversion of the data from what is on-board the spacecraft into STSDAS images. These messages include information on missing or filled packets, RTB information for some acquisitions, and the informational messages produced by calhrs as it is used to calibrate the data.
35.1.2 Calibrated Data
Calibrated Wavelength Solution (.c0h)
The calibrated wavelength solution (.c0h) image contains the wavelengths in Angstroms for each corresponding pixel in the .c1h file. The dimensions and number of groups is the same as the .c1h file. This file is produced when ADC_CORR is set to PERFORM.
Calibrated Science Data (.c1h)
The calibrated science data (.c1h) image contains the calibrated science data, and is always produced by calhrs. The number of groups and dimensions of the spectra depends on the stepping pattern used (STEPPATT, see Table 35.3 and "File Sizes" on page 35-5) in the observation and also which calibration steps are performed. The contents can range from being an exact copy of the raw science data (found in the .d0h image) to a fully flux-calibrated spectrum.
Propagated Statistical Error (.c2h)
The propagated statistical error (.c2h) image contains the propagated statistical error associated with the .c1h science data. The number of groups and dimensions of this file will be the same as the .c1h image. The units of the error will also be the same as the final units of the science data.
Calibrated Special Diodes (.c3h)
This twelve-pixel image contains the calibration of the special diodes whose raw values correspond to the first twelve pixels in the .x0h file (see Table 35.2).
Special Diode Data Quality (.c4h)
The special diode data quality (.c4h) image contains the data quality flags for the special diodes. See Table 37.5 for data quality flags.
Background (.c5h)
The background (.c5h) image contains the actual subtracted background. The units of the background will be either in counts or count rate, depending on the setting of the EXP_CORR switch. This file is produced when a background correction is performed. You have several options for calculating the background rate; these are described in "Calibration Steps Explained" on page 36-2.
Calibrated Science Data Quality (.cqh)
The calibrated science data quality (.cqh) image contains the data quality flags associated with each pixel in the calibrated science data image. The flags are numbers; the higher the value, the worse the data in the corresponding pixel. See Table 37.5 for calibrated data quality flags.
Special Diodes
35.1.3 File Sizes
As mentioned above, the size (number of pixels and number of groups) of your raw science file (.d0h) will depend on the substepping strategy used in the original observations, and that is determined by the setting of the STEPPATT keyword. The default STEPPATTs are listed in Table 35.3. Quarter-stepping, i.e., four output pixels per diode, was ordinarily done with science observations in order to achieve full resolution, but the default background measurement strategy may differ depending on the grating and order being observed. The different STEPPATTs you may encounter with GHRS data are listed in Table 35.3.
A typical raw science file (.d0h) will have a STEPPATT=5 and, like other data from STEPPATTs with quarter-stepping and two interorder background bins, will contain six 500-pixel groups per exposure. The first four groups (or bins) are the science data (each bin shifted by 1/4 diode) and the next two are interorder background measurements, one above the spectrum, one below. Mathematically, the number of raw science groups can be represented by the following formula;
number(.d0h groups) = (spectrum + background bins)*(RPTOBS + 1) * fpsplits
- The number of spectrum and background bins are determined by the -STEPPATT, see Table 35.3.
- RPTOBS and STEPPATT are available in the header.
- fpsplits = 1 (FP_SPLIT=DEF) or 2 (FP_SPLIT=TWO or DSTWO) or 4 (FP_SPLIT=FOUR or DSFOUR).
The .q0h, .x0h, .xqh, and consequently, .c3h and .c4h files will have the same number of groups as the .d0h file. Patterns which use the corner diodes for background measurements may still create measured background bins in the raw data, although they are not used by calhrs.
The size (number of pixels and number of groups) of your calibrated output flux file (.c0h, .c1h, .c2h, .c5h, .cqh) will depend on the switch settings and the substepping strategy used in the original observations. In a typical observation, the four groups of science data are merged (see the MER_CORR description in "Calibration Steps Explained" on page 36-2) and the combined background groups subtracted from that merged spectrum (see the BCK_CORR description in "Calibration Steps Explained" on page 36-2) to create one 2000-pixel group of calibrated science data per exposure. (Half-stepped data would create a 1000-pixel calibrated exposure.) Unmerged data, such as RAPID-mode, remain in groups of 500 pixels. Mathematically, the number of calibrated flux file groups is:
number (.c1h groups) = (RPTOBS + 1) * fpsplits
RAPID-mode data are slightly different and will have a number of raw groups equal to three plus the exposure time divided by the sample (or step) time:
number (raw RAPIDs) = 3 + (EXPTIME / STEPTIME).
The number of calibrated data will be three less than that because the first two and last groups of raw RAPID data are hysteresis and are thrown away.
The pipeline calibration does not combine RAPID, FP-SPLITs, or repeat -exposures.
35.1.4 Observation Log Files
As of February 1995, the observation log files (.jih) and jitter files (.jit), which are provided by the Observatory Monitoring System (OMS), were routinely available. These can give valuable information, such as telescope performance during the observation and the position of the target within the aperture. A so-called jitter ball is a routine portion of all GHRS paper products for which jitter files exist. This plot shows the motions of the FGSs needed to keep the target centered, recorded every six seconds throughout the exposure. See Appendix C in Volume I of this handbook for a description of these files and additional quantities available in the jitter files.
35.1.5 PODPS Data Quality
OPUS (formerly OSS and PODPS) staff performed a quick data quality assessment of the target acquisitions and the science observations. This assessment was performed to identify any target acquisition failures, guide star acquisition failures, other spacecraft anomalies, or instrumental problems. This assessment was recorded in the data quality keywords contained in the PDQ (Procedural Data Quality) file. The PDQ files are archived to data class PDQ and can be retrieved from the HST Archive using StarView. OCX files (Observation Support System (OSS) Observer Comment files) contain information about the success or failure of target acquisitions and science observations. Before April 17, 1992, OCX files were not always archived separately, and, in some cases, were prepended to the RSDP pipeline history file, the .trl trailer file. After February 1995, OCX files were produced only to document real-time activity (interactive target acquisition) in support of an observation. After October 1996, OCX files may contain information about observation execution problems.
Please note that the PDQ files and their comments should not be overinterpreted. These remarks were based on a quick visual examination of the data and the experience of the OPUS staff. The technical remarks are usually sound, but some comments about the "quality" of the observations can arise from an incomplete knowledge of the science goals of the original proposer. Unless the object was significantly miscentered, for example, the achieved signal-to-noise is probably close to what was expected, even if it may appear low in a single exposure. To assess whether or not a low flux is meaningful, you should examine the jitter file or other records of spacecraft performance, or simply look at the total number of counts achieved.
35.1.6 Reference Files
The Archive will also give you the opportunity to retrieve the "used" or "best" reference files that go along with the data you are retrieving from the Archive. The "used" reference files are those listed in the calibration reference file keywords, and are those actually applied to the observations to produce the calibrated files in the Archive. The "best" reference files are meant to reflect accumulated experience on GHRS calibration and performance and so are recommended. For more information, please see "Recalibrating GHRS Data" on page 36-13.
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1
As we noted in the previous chapter, we urge you to recalibrate all GHRS observations in order to take advantage of the latest and best calibrations and knowledge of the instrument. However, it can be helpful to look at the calibrated observations to judge the quality of the data and their suitability for your needs.
stevens@stsci.edu
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