CHAPTER 6: Archive Search Strategies In This Chapter... Planned Versus Executed Information / 61 Choosing the Right STARCAT Screen / 62 Search Strategies / 65 Assessing Data Quality / 77 The AEC and the EC / 82 This chapter describes the types of information provided by STARCAT and approaches that can be used to quickly find information and datasets in the HST archives that are of interest to you. Planned Versus Executed Information The science tables of the DMF catalog contain information about both the parameters of each HST observation as they were executed (the so called exposure level keywords) and about the observational parameters as they were planned (the so called proposal level keywords). As described below, in the section on choosing a STARCAT screen, the STARCAT screens are organized within the menu structure so that screens that principally report either exposure level or proposal level keywords are easily distinguished. In addition, there is one screen (PLANNED-VS-DONE under the POST-OBS menu) that allows a comparison between the planned and executed values of several observational parameters. If you need to determine whether a field refers to an exposure level or proposal level keyword, refer to the online documentation describing each screen, and to Chapters 9 and 11 of this manual. Most fields displayed on STARCAT screens correspond to keywords in the headers of HST data files. Choosing the Right STARCAT Screen STARCAT includes many pre-defined screens with which to query the HST catalog. STARCAT screens are organized in a menu structure that groups together screens that provide a particular type of information. Below, a brief outline of the menu structure and a brief description of the contents of each menu are presented. A description of the GENERAL screen is also provided here, since the GENERAL screen will typically be the starting point for most archival searches. Refer also to Chapter 9, where more detailed documentation on the purpose and contents of each screen is provided. GENERAL Screen The GENERAL screen is the all-purpose search screen for archival research. It is usually the best screen to begin with when searching for archival data. This screen provides a window on a mix of both exposure level and proposal level information. It can be used to search for observations at a given position in the sky (using Target RA and DEC), for a specific target (using Target Name), and for observations of a given type of source (using Target Description). The GENERAL screen can also be used to search for observations taken with a given instrument, with a given instrumental configuration, or with a given set of filters or gratings. The screen provides exposure level information, such as the exposure start time and duration, the fgslock employed, the exposure flag, and the PODPS Data Quality value and comments. This information can be used to judge the usefulness of the exposure for a given line of research (see also "Assessing Data Quality" on page 77). The date at which the data set will become (or became) public (i.e., available for archival research) is displayed on the general screen (as the Release-Date). Note that the GENERAL screen can only be used to search for science data with an archival class of CAL (i.e., astrometry data can not be retrieved with this screen). PRE-OBS MENU The PRE-OBS menu lists screens that display proposal level information-information that was provided by the observer during phases 1 and 2 of the proposal submission process. Most of the screens within this menu provide information about both completed HST observations and about HST observations that have been approved for execution, but that have not yet been taken. However, the GTO, OBS, OPPARM and PROP-OBS screens provide proposal-level information only for executed HST observations. Obviously, you cannot retrieve data for an observation that has not yet been executed. POST-OBS MENU The POST-OBS menu lists screens that display exposure level information about executed HST observations. Some of these screens also display proposal level information about each exposure. In addition to the screens within the INSTRUMENT menu, these screens provide the main mechanism for learning about the actual executed parameters of each exposure. TARGETS The TARGETS menu lists screens that display observer-entered information describing the target, such as alternate names, target description, redshift, magnitude, or position. Most of these screens display one record for each target within a given proposal ID. These screens are useful when searching for targets having certain shared properties (e.g., optical magnitudes greater than some value, or redshifts greater than some value) or when searching on source name or source type (see also "Search Strategies" on page 65). In particular, the TARGSEARCH screen was designed to make it easy to conduct searches for a particular source based on source name or for a class of sources based on the target description, or proposal title (see "Search Strategies" on page 65). However, you should keep in mind that the target-related information displayed in these fields was entered by the observer and for some of the fields displayed on these screens, the observer is not required to have filled in a value during the proposal process. Thus, the information displayed will be both incomplete and heterogeneous. INSTRUMENTS The INSTRUMENTS menu lists screens that display instrument- specific information about HST exposures. There is at least one screen for each HST instrument. More detailed information on the instrumental setup and observing parameters used for an observation are shown on these screens than on the more general screens in the POST-OBS menu. These screens are useful for searching for all observations with a given instrument in a specific instrumental configuration and for determining more precisely the instrumental parameters employed in a given observation. ENGINEERING The ENGINEERING screens provide a mechanism for finding non-science data files, such as processor dump data, science instrument engineering data, engineering subset data, guide star position data, science mission schedule data, etc. By and large, each screen searches for one type of file (i.e., a file with a given extension). The documentation for each screen explains the naming convention for the files it retrieves. ARCHIVE The ARCHIVE menu contains screens that display accounting information about the data sets and files contained on the optical disks that comprise the HST archive. In general, archival researchers should not need to access these screens, unless they are interested in knowing if a particular type of file has been created for a given data set, or if they need to determine the size of a given file. ADMIN The ADMIN menu contains administrative screens used by the Data System Operations Branch of the Space Telescope Science Institute to monitor STARCAT accounts. Search Strategies Overview There are some standard types of searches that an archival researcher might be interested in doing. Among these are: o Searching for HST observations of a specific source (e.g, 30-Doradus or the jet of M87) o Searching for observations of a given class of source (e.g, all observa- tions of SNR or O stars) o Searching for observations of targets which possess certain shared prop- erties (e.g., a B magnitude greater than 16 or a redshift greater than 0.01). Suggested strategies for carrying out such searches are described below. Searching for Specific Sources There are two ways to search for observations of a specific source: o Search on positional coincidence o Search on the basis of source name Since the source or target name is a generally an observer-entered designation, for fixed targets, searches based on positional coincidence will prove much more reliable than searches based on source name. Positional Searches There are two ways to search for positional coincidences with observations in the HST catalog using STARCAT. You can either qualify on target RA and target DEC, specifying a range of permissible values (this defines a search rectangle-see the example on page 28), or you can use the Center command to specify a RA and DEC and a radius around that position within which to search (this defines a search circle); this is described in "Searching an Area Around a Centered Position" on page 89. The Center command has the added advantage that it allows you to specify the coordinate system and epoch in which the search will be done (e.g., it supports epoch 1950 or 2000 and allows searches based on Galactic coordinates). Searching on Source Name Most of the STARCAT data screens have a field for the target name (often called targname). In addition, the TARGSEARCH screen within the TARGETS menu, contains both the target name and alternate names or aliases. The archival researcher can qualify and search on the basis of source name using these fields. Note that when using the TARGSEARCH screen to search for a given source name, three separate searches must be done: 1. First, qualify on target name 2. Then clear the qualifications for target name and qualify only on alias1 3. Lastly, qualify only on alias2 When qualifying, the source name should always be embedded in wild card delimiters (e.g., *MARS*, or *NGC1068*), because the name will frequently be embedded in additional text (see below). Since an astronomical source typically has many names, to understand how best to qualify on the target name and alias fields, it is useful to know how target names and aliases are assigned. Target names are used to provide unique designations for targets within a given proposal. They are entered by the observer during Phase II of the proposal process. There are three different classes of targets (fixed targets, solar-system targets, and generic targets). Observers are encouraged to use a specific convention when naming fixed and solar system targets. Below is a brief summary of the conventions for each class of target, which should help you design search strategies based on source name. o Fixed Targets. When providing a target name for a fixed target, the observer must specify one catalog name which will become the target name (e.g., HD124897). He or she is encouraged to provide one or two common names which will become the target aliases (e.g., ALPHA-BOO, ARCTURUS). The preferred order for catalogs to be used to designate the target name is summarized in Table 6.1 below. A sum- mary of the convention for naming uncataloged targets is also given. Observers may also append a qualifier to the catalog name to indicate the location in the source where the observation is centered, for exam- ple, NGC4486-JET, LMC-R136A, or NGC224-004012+40548 (where the name of the parent body is followed by rounded J2000 coor- dinates). Finally, a code designating the target purpose must be appended to the target name when a target is observed specifically to be used as an external calibrator (-CALIB), as an astrometric reference star (-REF), or as an offset for target acquisition (-OFFSET), for example: BD+284D4211-CALIB, 3C273-OFFSET Object Type Stars Star Clusters and Nebulae Galaxies and Clusters of Galaxies Quasars and Active Galaxies Preferred Catalog Order Henry Draper Catalog (e.g., HD140283) Durchmusterung (BD, CD, or CPD) (e.g., BD+30D3639) General Catalog of Variable Stars (e.g., RR-LYR) New General Catalog (e.g., NGC6396) Index Catalog (e.g., IC418) Perek-Kohoutek designation (e.g., PK208+33D1) Sharpless catalog number (e.g., S106) New General Catalog (e.g., NGC4536) Index Catalog (e.g., IC724) Uppsala Catalog (e.g., UBC11810) Abell Catalog (e.g., ABELL2029) See Veron-Cetty and Veron, ESO Report No. 7, 1989 (e.g., 3C273) Table 6.1: Preferred Catalogs for Object Classification o Solar System Targets. The target name for solar system targets can either be chosen from the standard list of solar system targets repro- duced in Table 6.2, or a name can be defined by the observer. As with fixed targets, qualifier's can also be appended to the name (e.g., MARS-FEATURE1). Level 1 Objects Sun Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Pluto Level 2 Objects Moon Phobos Deimos Io Europa Ganymede Callisto Amalthea Himalia Elara Pasiphae Sinope Lysithea Carme Ananke Leda Mimas Enceladus Adrastea (1979J1) Thebe (1979J2) Metis (1979J3) Thethys Dione Rhea Titan Hyperion Iapetus Phoebe Ariel Umbriel Titania Oberon Miranda Triton Nereid Charon Janus (1980S1) Epimetheus (1980S3) Helene (1980S6) Telesto (1980S13) Calypso (1980S25) Pandora (1980S26) Prometheus (1980S27) Atlas (1980S28) Cordelia (1986U7) Ophelia (1986U8) Bianca (1986U9) Cressidea (1986U3) Desdemona (1986U6) Juliet (1986U2) Portia (1986U1) Rosalind (1986U4) Belinda (1986U5) Puck (1985U1) Table 6.2: Solar System Target Names Searching for a Class of Object Archival researchers may be interested in finding all (or most) HST observations of a given class of objects (e.g., observations of elliptical galaxies or Wolf Rayet stars). There are several ways to carry out such searches: o Search on the target description o Search on proposal title, proposal keywords, or proposal abstract o Cross correlate the contents of the HST catalog with a catalog or person- ally created list of sources on the basis of position (see the section on the Archived Exposures Catalog, below). Searches on Target Description A target description and category are entered for each target by the observer during Phase II of the proposal process. These fields allow an archival researcher to search for HST observations of classes of objects. The target description and target categories appear on the GENERAL screen, the PROP-OBS screen within the PRE-OBS menu, and the TARGSEARCH screen within the TARGETS menu. The target description spans two lines (or fields) which must each be qualified separately in order to completely search the target description for a given string. For example, if you wanted to search for all observations of nova stars within our galaxy, you could use the GENERAL screen, qualify on category as star and qualify on Descrpt1 as *nova*. For completeness, the search should then be redone, unqualifying on Descrpt1 and qualifying on Descrpt2 as *nova*, in order to find all observations with category equal to STAR and the string nova in either the first or second target description line. Note the use of the wildcard delimiters (*), since the word "nova" will likely be embedded in a longer text (see below). The target description is assigned by the observer as part of the Phase II proposal process, using the guidelines in the Cycle 2, Phase II proposal instructions. Each target must be assigned a single primary Category followed by at least one Descriptive Keyword. Observers are encouraged to use as many as five descriptive keywords per category and are allowed to assign up to two categories (this is appropriate, for example, for absorption studies of the interstellar medium using background quasars). The first category is then entered in the category (or targcat) keyword and the two lines of the target description are filled with the complete description in the form: category,keyword,keyword...;category,keyword,keyword... The appearance of the second category is denoted by a semicolon and the second category is entered in the second category field (or targcat2). The categories and corresponding keywords that can be used to describe fixed targets by the observer are given in Tables 6.3 through 6.11. For solar system targets, the descriptive keyword convention is give in Table 6.12. Obviously, searches based on target description will be most successful if the same terminology used to describe the targets by the observers is used by the archival researcher. Keep in mind that assigning target descriptions is an inherently subjective process. There are many ways to describe the same target and different observers will inevitably describe the same target in different ways. You may, therefore, want to apply broad criteria when first qualifying on target description. If you are looking for a complete list of all HST observations of a given type of source, you are encouraged to also search based on the proposal title, abstract, and abstract keywords, and to cross correlate positional lists of sources with the Archived Exposures Catalog. In Tables 6.3 through 6.11, information in parentheses next to the keyword is explanatory only and is not part of the catalog. For example, Table 6.4 has a keyword identified as "LMXB (Low Mass X-Ray Binary)", LMXB is the keyword that appears in the target descriptor, the text is explanatory only. Category Keyword STAR EXT-STAR STELLAR-CLUSTER EXT-CLUSTER GALAXY CLUSTER OF GALAXIES ISM EXT-MEDIUM UNIDENTIFIED CALIBRATION Contents Galactic stellar object Star in an external galaxy Galactic star cluster, group, or association Star cluster in an external galaxy Galaxy or AGN Galaxy groupings, clusters, or large-scale structure Interstellar medium of the galaxy Interstellar medium of an external galaxy Unidentified object Calibration observations Table 6.3: Keyword Categories Descriptive Keywords for Stars (STAR or EXT-STAR) Brown Dwarf Supergiant O WDO B0-B2 III-I BP DC A0-A3 III-I AP F0-F2 G V-IV M V-IV Long Period Variable Dwarf Nova Polar (AM Her Star) FU Orionis Star Interacting Binary RS CVn Star Beta Lyrae Star Binary Pulsar ZZ Ceti Star Supernova Type Ib Wolf Rayet (WC) OE BO-B2 V-IV B3-B5 III-I SDB DZ A45-A9 III-I Horizontal Branch Star F3-F9 G III-I M III-I Irregular Variable Classical Nova Intermediate Polar (DQ Her Star) Shell Star X-Ray Novae LMXB (Low Mass X-Ray Binary) Algol System FK Comae Star Cepheid Supernova Type II Wolf Rayet (WN) SF B3-B5 V-IV B6-B9.5 III-I DB A0-A4 V-IV AE AGB Star FP K V-IV R, N, or S Types Regular Variable Nova-Like Symbiotic Star Eta Carinae Star X-Ray Burster MXB (Massive X-Ray Binary) Barium Star Pulsating Variable Supernova RR Lyrae Star Main Sequence O SDO B6-B9.5 V-IV BE DA A4-A9 V-IV AM Composite Spectral Type Late-type Degenerate K III-I Carbon Star Luminous Blue Variable Recurrent Nova T Tauri Star Pulsar X-Ray Transient W UMa Star Blue Straggler PG1159 Star Supernova Type Ia Planetary Nebula Central Star Table 6.4: Fixed Target Keywords for Describing Stars (Galactic Stellar Objects) and Stars in External Galaxies Descriptive Keyword for Stellar Clusters (STELLAR CLUSTER or EXT-CLUSTER) Globular Cluster OB Association Open Cluster T Association Table 6.5: Descriptive Keywords for Stellar Clusters (Galactic Star Clusters, Groups, or Associations) or Star Clusters in an External Galaxy Descriptive Keywords for Galaxies (GALAXY) Spiral (Spiral Galaxy) Magellenic Irregular BCM (Brightest Cluster Member) QSO (Radio Quiet) Liner Lyman Alpha Cloud High z Gal (Redshift Greater than 0.5) Lenticular (Lenticular Galaxy) Amorphous Irregular BGM (Brightest Group Member) Quasar (Radio Loud) Starburst Protogalaxy Elliptical (Elliptical Galaxy; not a Dwarf Elliptical) Dwarf Compact (Dwarf Compact/HII Galaxy) LSB (Low Surface Brightness/HI Rich Galaxy) Radio Galaxy Ultraluminous IR Gal Gravitational Lens Dwarf Elliptical Dwarf Spheroidal Seyfert BL Lac (BL Lac or BLAZAR) Interacting Galaxy Einstein Ring Table 6.6: Descriptive Keywords for Galaxies (or AGNs) Descriptive Keywords for Clusters of Galaxies Supercluster Poor Cluster Gravitational Lens Blank Sky Void High Redshift Cluster (z>0.5) Galaxy Pair Group BCM (Brightest Cluster Member) Interacting Galaxy Rich Cluster BGM (Brightest Group Member) Einstein Ring Table 6.7: Descriptive Keywords for Clusters of Galaxies (Galaxy Groupings, Clusters, or Large Scale Structures) Descriptive Keywords for Interstellar Medium (ISM or EXT-MEDIUM) Herbig-Haro Object Dark Cloud High Velocity Cloud Molecular Cloud IGM Planetary Nebula SNR (Supernova Remnant) Intermediate Velocity Cloud Bipolar Outflow ICM HII Region Ring Nebula (Shock Ionized) IRAS Cirrus Absorption Line System (Extragalactic) Coronal Gas (105-106K) Reflection Nebula HI Cloud Cometary Nebula Damped Lyman Alpha Cloud (Extragalactic) Hot Gas (107-108K) Table 6.8: Descriptive Keywords for Interstellar Mediums (of the Galaxy or of an External Galaxy) Descriptive Keywords for Unidentified Objects Radio Emitter Ultraviolet Emitter Infrared Emitter X-Ray Emitter Optical Emitter Gamma Ray Emitter Table 6.9: Descriptive Keywords for Unidentified Objects Descriptive Keywords for Calibration Observations Astrometric Point Spread Function Aperture Location Photometric Flat Field Wavelength Polarimetry Table 6.10: Descriptive Keywords for Calibration Observations Discrete Features of Objects Corona Wind Hotspot Bulge Shell Cooling Flow Filament Shock Front Ring Accretion Disk Nucleus Polar Ringh Tidal Tail Emission Line Nebula Ejecta Ionization Front Ansae Jet Halo Dust Lane Bar BLR (Broad Line Region) Knot Conduction Front Protoplanetary Disk Lobe Disk Spiral Arm Multiple Nuclei NLR (Narrow Line Region) Star Forming Region Table 6.11: Discrete Features and Keyword Descriptors for All Categories Keyword PLANET SATELLITE COMET ASTEROID FEATURE OFFSET RING TORUS OTHER Comments If observation is centered on planet, may be followed by planet name, e.g., PLANET JUPITER If observation is centered on object, may be followed by name of object, e.g., SATELLITE GANYMEDE If object is centered on comet's nucleus, may be followed by name or catalog designation, e.g., COMET 1979X If target is asteroid center, may be followed by common name or catalog number, e.g., ASTEROID CERES Surface features are followed by the name of the parent body, e.g., FEATURE IO May be followed by name of parent body, e.g., OFFSET COMET HALLEY Will be followed by name of parent body, e.g., RING SATURN Will be followed by name of parent body, e.g., TORUS SATURN May be followed by description of observation type, e.g., ASTROMETRIC REFERENCE or ZODIACAL LIGHT Table 6.12: Solar System Keywords Searching on Proposal Title and Abstract Besides searching for individual HST observations of a given source class based on the target description, you may want to search for proposals that concentrate on, or include, observations of a certain class of source. The PROP_ABSTRACTS screen within the PRE-OBS menu contains a summary of information about each accepted proposal, including the proposal title, and the keywords from the proposal abstract, as well as the proposal abstract itself. To find, for example, proposals for observations of Seyfert galaxies, qualify on proposal title as *seyfert*. To generalize the search to proposals that pertain to Seyfert galaxies, repeat the search, unqualifying on proposal title, and qualifying on the abstract keywords as *seyfert*. Having identified specific proposal numbers of interest, you can then return to the GENERAL screen, qualify on proposal number, and bring up all data sets that were taken for that proposal. Searches Based on Target Properties The screens under the TARGET menu can be used to search for observations of targets that share certain properties, e.g., magnitudes less than some value, or redshifts higher than some value. This can be useful to the archival researcher who wants to see, for example, the signal to noise produced by an FOS spectrum of a given duration of a star of a given magnitude, or what kind (and quality) of HST images of high redshift galaxies exist in the archive. Some of the screens in the TARGET menu display a target name and proposal identification number and not the dataset name. You can either mark data sets for retrieval directly using these screens, or you can use the screens to obtain proposal ids and target names of interest, and then use the GENERAL or INSTRUMENT screens, qualifying on both proposal id and target name, to obtain more information about the relevant observations. Remember that the target-related information displayed on the screens under the TARGET menu were entered by the observer. For many of the fields displayed on these screens, the observer is not required to have filled in a value during the proposal process. Thus, the information displayed will be incomplete and heterogeneous. Assessing Data Quality The quality of data in the HST archive varies greatly. While it is difficult to assess the quality of the data in a given HST exposure from the keyword information alone (i.e., without actually obtaining and analyzing the data), there are several keywords that are written to the DMF catalog and displayed by STARCAT screens that can help you to get a feel for the quality of the data in a given data set. Among these are: o The PODPS (Post Observation Data Processing System) Data Quality keywords and comments o The exposure flag (expflag) o The fine guidance system lock used to track the target (fgslock) Each of these is described below. When interpreting the relevance of these parameters for a given observation, bear in mind that the importance of these parameters to data quality is relative; it depends both on the characteristics of the source being observed and the purpose to which you will put the data. PODPS Data Quality Keyword and Comments Each science data set is displayed and evaluated for data quality by an operator after it has been calibrated by the Post Observation Data Processing System (PODPS). The operator assigns a parameterized PODPS Data Quality (PDQ) keyword to the data set and writes an optional PDQ comment. The PDQ keyword and PDQ comments are written to the PDQ table of the DMF catalog and can be displayed in STARCAT using the GENERAL screen (which displays the PDQ keyword and the first 68 characters of the PDQ comment) or the PDQ screen (which displays the PDQ keyword and the full PDQ comment) within the POST-OBS menu. When you are interpreting the relevance of the PDQ keyword and comments, bear in mind that: o The keyword and comments are not assigned by instrument scientists o Data quality parameterization is inherently subjective o The relevance of the keyword and comments depends on the nature of the target of the observations Care must be taken to combine a knowledge of the characteristics of the target with the PDQ keyword and comments when evaluating data quality. For example, while an evaluation of no-source when the target is a bright star and the integration time sufficient that a clear detection is expected may well indicate poor data quality, the same evaluation when the target is a distant, faint cluster of galaxies for which co-adding of many exposures is required indicates little about the quality of the data. As described above, the PODPS Data Quality evaluation is composed of a parameterized keyword and a comment. The comment describing the data is fully at the discretion of the operator. Table 4.1 is the list of possible values that the operators can assign to the parameterized PDQ keyword, and a brief explanation of their meaning. This list of permissible keyword values was officially adopted as of February 1, 1992. Data processed prior to that date were described with a similar set of values, however some of the original values were found not to be useful and were dropped from the current list, while other values were added. Most of the values on the list have been used since shortly after launch. Note that while several of the allowed values may apply to a given observation, the operator can assign only one value to the keyword for each observation. It should also be noted that when data is reprocessed, it is not re-evaluated for data quality. Keyword BADCHAN COSRAY DATA-DROP EDGEPSF INCOMPLETE No-COUNTS NO-EVAL NO-SOURCE NOISY OK POOR SATURATED TRAILED UNDEREXP UNKNOWN Meaning Quality significantly compromised by dead CCD column(s) or bad diode(s) Compromised by cosmic-ray contamination More than about 2% missing, or affecting probable area of interest Got only an edge of the point-spread function Got only some of the expected data groups Zero-level data Inadequate time or resources for data evaluation No sources visible in image High background, low S/N No apparent problems Other problems affecting probably scientific utility (used when no other keyword applies) Target saturated; counts rolled over, etc. Observation taken on gyros, trailed sources, but otherwise OK Observation seems underexposed Uncertain of usefulness or quality of data Table 4.1: PDQ Keywords The PDQ Files The complete PODPS data quality report can be retrieved from DMF. Every PDQ file is archived as ancillary data (class ASA) with the same rootname as the observation and an extension of PDQ. In addition to the data quality keyword and comments, the complete PDQ file contains predicted as well as actual observation parameters extracted from the standard header and science header files. To mark a PDQ file for retrieval, use the FILES screen under the DATAFILES menu, qualify on extension, PDQ, and dataset_name using the full dataset name (e.g. W0340A01T). Press for FindNext and for Mark. OSS Observer Comment Files Observer comment files may help an archive user determine the quality of a HST observation. The SOGS Observation Support System (OSS) monitors the health of the telescope and provides observers with a real-time interface. Observer comment files contain updated mission information obtained from the time the observation was executed. OSS personnel also put keywords and comments into this ASCII text file. However, these files are not created for every observation executed. If an observer comment file exists for an observation, the file may be retrieved from the ancillary data class (ASA) of DMF. The extension for all observer comment files is OCX. The filename follows the standard observation rootname naming convention for the first eight characters, but the last character is either an "x" or a "c". To retrieve the OCX file, you need to mark the file using the FILES screen under the DATAFILES menu. Qualify on extension OCX and dataset_name using the first 8 characters of the dataset name followed by a * (e.g., to search for the OCX file for dataset W0340A01T, qualify on dataset_name = W0340A01*). For data taken prior to April 17, 1992, the OCX file was not always archived separately, and in some cases was appended to the trailer file (extension TRL under archive class CAL). If you cannot find a OCX file for your dataset, and the dataset was taken prior to April 1992, then you may want to retrieve the TRL file to see if it has an OCX file appended to it (use the FILES screen and qualify on the full dataset_name and ext = TRL). Exposure Flag The exposure flag keyword (expflag) serves as a comment to the exposure time keyword (exptime). It indicates whether the exposure completed successfully, without interruption, and whether the actual exposure time was different from the predicted exposure time. (Note here the difference between predicted exposure time and proposed exposure time. The proposed exposure time includes setup time, while the predicted exposure time is only the predicted on-source integration time.) The exposure flag can be useful to the archival researcher in that it allows him/her to determine whether the integration time obtained for a given observation is equal to the integration time the original observer expected for that exposure. The details of how the value of this keyword is set are complex and instrument-dependent. Table 4.2 gives a generic, description of the possible values of this keyword and their meaning. Keyword NORMAL INTERRUPTED INCOMPLETE EXTENDED UNCERTAIN INDETERMINATE PREDICTED Meaning EXPTIME was successfully calculated from telemetry information, is equal to the predicted exposure time, and there is no indication that the exposure was interrupted. EXPFLAG is also set to NORMAL when EXPTIME was successfully calculated and the predicted exposure time was not available. EXPTIME was successfully calculated from telemetry information, is equal to the predicted exposure time, but there is an indication that the exposure was interrupted. EXPTIME was successfully calculated from telemetry information and is not equal to the predicted exposure time. EXPTIME was successfully calculated from telemetry information and is greater than the predicted exposure time. The Shutter Log Overflow flag was set, which indicates that not all shutter open and close times are available. EXPTIME was calculated from the shutter open and close times that are available. EXPTIME could not be successfully calculated from the telemetry and the predicted exposure time was not available. EXPTIME could not be successfully calculated from the telemetry and EXPTIME was set to the predicted exposure time. Table 4.2: Exposure Flag Keyword Values Fine Guidance System Lock The tracking mode employed during each HST observation is given in the keyword fgslock which is displayed on the GENERAL and EXP screens within the POST-OBS menu. There are currently three tracking modes available during HST observations; coarse and fine lock which use the Fine Guidance Sensors and gyro stabilization. When the HST is stabilized with gyros, no guide star acquisition occurs and the absolute error of positioning is 30". The anticipated guiding accuracies for the three modes are 0.002"/sec drift rate for gyro hold, 0.015" RMS jitter for Coarse track and 0.005" RMS jitter for Fine Lock. Obviously, the guiding accuracy required by the archival researcher in a given observation depends on the purposes for which the data are to be used. The AEC and the EC As described briefly in Chapter 2, there are two ASCII files that are maintained in the exposure sub-directory of the documents directory on the archive computer which may be useful to the archival researcher. These are the Archived Exposures Catalog (AEC) and the Exposures Catalog (EC). The AEC is a list of all the science observations (CAL class data sets) in the HST archive. It is updated monthly. The AEC contains information about the observations including the target name, position, instrument, mode, filters and gratings employed, and the date at which the data become public (i.e., non-proprietary). The information provided in the AEC is a selected subset of the keyword information in the DMF produced through an automated search of the DMF catalog using STARCAT. Note that observations taken with the Fine Guidance Sensors (FGS) are not included in the AEC. Positions in the AEC (and the EC) are in J2000 coordinates. The EC is an ASCII listing of all guest observer (GO) and guaranteed time observer (GTO) HST targets approved for observation. It contains a listing of the proposal level parameters of all planned GO and GTO HST exposures for which positional information has been entered in the Proposal data base. Thus, it includes information for both executed and planned HST observations. The EC listing is produced by the User Support Branch of STScI. The principle purpose of the EC is to allow an astronomer to determine in a straightforward way whether a given object or position in the sky will be observed by the HST as part of an approved HST program. This information is important to astronomers submitting proposals to the HST, as it allows them to search for conflicts with pre-existing programs. In addition to its role as a standalone catalog on the archive host workstation, the EC has been made into a table within the DMF catalog and can be queried by STARCAT using the EXP_CAT screen under the PRE-OBS menu. All of the information contained within the AEC and the EC is fully available through the STARCAT interface, in the former case because the AEC is produced by a STARCAT search of the DMF catalog and in the latter case because the EC is a table within the DMF catalog that can be viewed with the EXP-CAT screen. However, the ASCII listings of these catalogs provide the archival researcher with an added degree of flexibility. First, because they are ASCII files, they can be examined easily, e.g., with an editor, without the need to learn the STARCAT menu system. Second, and perhaps more importantly, they can be used to cross-correlate existing catalogs (e.g., the Abell Catalog, or a catalog of bright stars) with the contents of the HST catalog. Such cross correlations are useful for looking for all observations of a given source type (e.g., all positional coincidences with Abell clusters) or for finding observations that might prove useful for obtaining point spread functions. Cross correlating catalogs with the HST catalog is not a capability currently supported by STARCAT and so can only be done using these ASCII listings. 61