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Appendix 2 HST Specific Information

The information in this appendix is specific to HST data. Note: All discussions of StarView capabilities assume the reader is aware of the syntax. StarView details are found in the StarView chapter.

A2.1 Documentation on HST, Its Instruments and Data

A wide range of technical documentation on HST, its instruments, and data is available on the web at http://www.stsci.edu/hst/HST_overview/documents. In addition, this site allows access to an on-line form, which is used to request a paper copy of any document from the STScI help desk. The help desk serves as a central contact point between the scientific community and STScI and central clearinghouse for hotseat questions.

Some of the documentation available includes the following:

  • HST Archive Manual
  • HST Data Handbook (I & II)
  • STSDAS User's Guide
  • ACS Instrument Handbook
  • COS Instrument Handbook
  • FOC Instrument Handbook
  • FOS Instrument Handbook
  • GHRS Instrument Handbook
  • NICMOS Instrument Handbook
  • STIS Instrument Handbook
  • WF/PC Instrument Handbook
  • WF/PC 2 Instrument Handbook
  • WFC3 Instrument Handbook
  • FGS Instrument Handbook
  • OTA Handbook
  • Calibration Status and Plans
  • The GO and GTO Observing Programs
  • A2.2 ASCII Catalogs

    To assist in searching the HST Archive and in preparing proposals, ASCII catalogs are available which list completed and planned proposals and exposures. The easiest way to access these catalogs is through the World Wide Web at: http://archive.stsci.edu/hst/catalogs.html

    One catalog is available that summarizes proposals which have been accepted.

    • The Proposal Abstracts Catalog lists abstracts from completed and planned proposals. It is sorted by proposal ID.
    Two catalogs have been created which provide a summary of completed and planned exposures. For each catalog, there are separate files for the entire catalog, sorted by RA and Dec, and for solar system targets only, sorted by target name.

    • The Planned and Archived Exposures Catalog (PAEC) lists completed and planned exposures. For planned exposures, conflict types are indicated. The PAEC is appropriate for observers who are planning to submit proposals for HST.
    • The Archived Exposures Catalog (AEC) lists exposures which have been completed and have been archived into the HST Archive. The AEC is useful for Archive researchers.

    The AEC is updated monthly. It contains information such as the target name, position, instrument, mode, and date on which the data become non-proprietary (publicly accessible). Information provided in the AEC is a selected subset of the keyword information stored in the HST catalog. The AEC is produced through an automated search of the catalog. The AEC is also available at http://archive.stsci.edu/hst/aec.html.

    The PAEC, which is produced yearly, is an ASCII listing of all HST targets approved for observation including all general observer (GO) and guaranteed time observer (GTO) observations. Thus, the PAEC includes information for all executed and planned HST observations. The PAEC is also stored as a table in the HST archive database and can be examined from within StarView using the <Duplication Check> search screen.

    Target RA and Dec in the AEC and PAEC are in J2000 coordinates.

    A2.3 Search Strategies for HST data

    Two frequent types of archival searches are:

    • Searching for observations of a specific source (e.g, 30-Doradus or the jet of M87)

    • Searching for observations of a given class of source (e.g, all observations of SNR or O stars)

    Since an astronomical source typically has many names, to understand how best to constrain 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.1 See the HST Phase II Proposal Instructions for details on target naming conventions.

    • Fixed Targets. For targets with common catalog names, it is usually best to use the NED or SIMBAD resolver to find all instances of that target in the HST catalog. This is a positional search.

    • Solar System Targets. PIs provide target names for solar system targets by either using the standard list of solar system targets given in the Proposal Instructions or creating a name. As with fixed targets, qualifiers can also be appended to the name (e.g., MARS-FEATURE1). Therefore, to find all observations of Mars, one must embed Mars in wild card delimiters: *MARS*.

    • Generic Targets. Generic targets are usually given names by proposers that describe either the astronomical characteristics of the targets (e.g., SUPERNOVA or COMET) or the general location of the targets on the sky (e.g., HIGH-LATITUDE). Though observations of specific targets are the final outcome of such proposals, the proposed observations themselves do not include specific target names or specific target coordinates. Targets of Opportunity and Parallel target observations are typical examples of generic targets. It is therefore not generally useful to search for observations of generic targets in the HST catalog using specific target names. It is better to search for such targets either by using the target category or target description keywords (see the next section) or by searching for observations in an area of the sky that is likely to contain HST targets of interest to you.

    A2.3.1 Searching for Specific Sources

    There are two ways to search for observations of a specific source:

    • Search on positional coincidence (i.e., coordinates)

    • Search on the basis of source name
    The source or target name is an observer-entered designation. Therefore, for fixed targets, searches based on positional coincidence are much more reliable than searches based on source name. For parallel observations with no specified targets, searching by coordinates is the only way to check for conflicts and duplications.

    A2.3.1.1 Positional Searches

    There are three ways to search for positional coincidences with observations in the HST catalog using StarView or MAST web. You can constrain the target RA and Dec, specifying a range of permissible values. This defines a search rectangle. You can also specify an RA, Dec, and search radius (i.e., a cone search). You can also specify a common name for the target, let SIMBAD or NED provide the target position, and use the default search radius.

    A2.3.1.2 Searching on Source Name

    The MAST search form and most of the StarView screens have a target name field. In addition, Starview's <Fixed Target> screen contains both the target name and target alias, while the MAST search form contains the target description and allows the user to add the broad category. The archival researcher can constrain and search on the basis of source name using these fields. When searching for a given source name, you must perform two separate searches to ensure that all results for a given target name or alias are returned. That is, search for the target, then search for the alias.

    1. For the first search, constrain the target name. If using MAST web, turn off the resolver by selecting Don't Resolve. Note the results.

    2. For the second search, constrain only on the alias. Clear the search constraints on target name.

    When constraining, the source name (target or alias) should always be embedded in wild card delimiters (e.g., *MARS* or *NGC1068*) because the name will frequently be embedded in additional text.

    A2.3.2 Searching for a Class of Objects

    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:

    • Search on the target description.

    • Search on proposal title, proposal keywords, or proposal abstract.

    • Cross-correlate the contents of the HST catalog with a catalog or personally created list of sources on the basis of position.

    A2.3.2.1 Searching on Target Description

    Searches can be done on descriptions entered by the original observers during the Phase II (planning) process. These fields allow an archival researcher to search for HST observations of classes of objects. The target categories (aka Broad Category) appears on the Staview <Fixed Target> screen. See the StarView chapter for instruction on adding the target description (tak_keyword_test) from the target_keyword table. The MAST search form contains the target description and allows the user to add the broad category. The target description is the target category with additional descriptive words appended, so the target category is somewhat redundant.

    To search for all observations of nova stars within our galaxy, use the <Fixed Target> screen, constrain "Target category" as star, and constrain "Target description" as *nova*. Note the use of the wild card delimiters (*), since the word "nova" will likely be embedded in a longer text. Alternatively, since the target description in StarView is the Phase II target category plus the Phase II target description - in that order - one could simply use the <Quick Search> and <General Search> screens and constrain the "Target description" field to *star*nova* or *star* & *nova*.

    The observer assigns the target description as part of the Phase II proposal process, using the guidelines in the Phase II Proposal Instructions.2 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.

    The categories and corresponding keywords that the observer can use to describe fixed, generic or solar system targets are given in the Phase II Proposal Instruction. See the sections on target category and target description. Obviously, searches based on target description will be most successful if archival researchers use the same terminology used by observers to describe the targets. 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 constraining a target description. If you are looking for a complete list of all HST observations of a given type of source, you are encouraged to search also on the proposal title, abstract, and abstract keywords and to cross-correlate positional lists of sources with the archive. Furthermore, parallel observations usually are not well described by such keywords.

    A2.3.2.2 Searching on Proposal Title and Abstract

    Besides searching for individual HST observations of a given source class based on the target description, you may search for HST observing proposals that concentrate on, or include, observations of a certain class of source, using Starview or Mast web. Refer to our text search strategy at the beginning of this section. In the results, the proposal ids are links that allow the user to view the data and resulting publications.

    For Mast web, see the HST Abstract Search page at http://archive.stsci.edu/hst/abstract.html.

    For Starview, the <Proposal> or <Proposal Abstract> screens contain a summary of information about each accepted proposal, including the proposal title and proposal ID. The proposal abstract is available on the <Proposal Abstract> screen. To find, for example, proposals for observations of Seyfert galaxies, constrain proposal title as *seyfert* on the <Proposal> or <Proposal Abstract> screen. To expand the search to proposals that pertain to Seyfert galaxies, repeat the search, using <Proposal Abstract>, clearing constraits for proposal title, and constraining the abstract text as *seyfert*. Having identified and recorded specific proposal numbers of interest, you can then return to the <Quick Search> or <General> screens, list the relevant proposal numbers separated by commas, and bring up all observations that were taken for such proposals.

    A2.3.2.3 Cross Correlation Searches

    The VizieR/MAST or the NED/MAST Cross-correlation tools may be used to find classes of objects within the HST archive. There's also a File Upload Form option on the HST Search Form and the NED/MAST form.

    To use the File Upload Form option on the HST Search Form, the user must have prepared a file, or downloaded a catalog, that contains RA and Dec, and/or target name and/or a data_id. The order of the information is not important, as long as it appears in one of the first 10 columns. The column delimiter must be one of Tab, |, comma or semi-colon. Clicking on the File Upload Form in the HST Search Form will bring up an HST Search Form with fields to provide the name of the file containing the list or catalog, the delimiter used in the file and the column(s) number(s) in the file that contains the coordinates or target name or data_id. The remainder of the form is the standard HST Search Form, which should be used to specify the search.

    The VizieR/MAST Cross Correlation Search permits users to search for catalogs held by VizieR and then either search them internally or to use them as input for a MAST cross-mission search.

    First search for a VizieR catalog. When searching for a catalog name, it is useful to search using a wild card (e.g. *Abell*). The result of the search is a list of the catalogs found at VizieR with two options to the left of each found catalog (S and CC).

    • Click on the "S" to Search the catalog itself. This brings up a form with the same basic format as the mission searches. The top section permits entry of target name or coordinate criteria and the bottom section specifies output formats. The middle section gives you the option of qualifying the search using any of the fields present in the catalog. The field descriptions link brings up a page describing the column of the catalog as provided by VizieR.

    • Click on the "CC" to cross-correlate the catalog with any of the MAST missions (except GALEX at this writing). This form has 4 sections. The top section is the usual Target Name/coordinate specification. The second section permits you to add qualifications concerning the catalog. Again use the Field Descriptions link to see the column/field descriptions from VizieR. The third section permits you to cross-correlate with data from specific instruments. The fourth section is the normal output selection criteria. The first 10 records found per catalog entry will be displayed. The total number of records found will be indicated.

    The NED/MAST Cross Correlation Search permits users to search the NED catalog by object name, position, object category etc. and optionally cross-correlate results with MAST mission databases.

    NED search results are retreived as a web service, and the results are either displayed directly, or the retrieved coordinates are used to display MAST mission observations that are within a specified search radius of the retrieved catalog entries.

    See the tutorial section VizieR/MAST Cross Correlation Search (/tutorial/hints.html#vizier) or the section MAST/NED Cross Correlation Search (http://archive.stsci.edu/tutorial/tools.html#ned) for more information. A tutorial was also published in the (/archive_news/2007/01-Jan/archive_news_jan2007.html).

    A2.4 Assessing Data Quality

    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 HST catalog and displayed by StarView screens that can help you to get a feel for the quality of the data in a given dataset. Among these are the following (each of which are described in the following sections):

    • The PDQ keywords and comments
    • The exposure flag (expflag)
    • The fine guidance system lock used to track the target (fgslock)
    • Preview images
    • Observation log files for most data taken after October 20, 1994

    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.

    In addition to the complete science data set, the archive contains previews of all public data sets. The previews can be viewed with StarView or MAST web and thus allow the user to gain an idea of the quality of an image or spectrum by direct inspection. A discussion of the preview capability can be found in the MAST and StarView chapters.

    In addition to the above methods of assessing data quality, for most data taken after October 20, 1994, you can also obtain observation logs for a given dataset. The observation log files contain detailed pointing information (including jitter information) as well as the information on the spacecraft position and attitude. This information is provided at high time resolution, and should be particularly useful in many cases for investigating problems that are revealed by an inspection of the data or data quality comments. The Observation Logs can be retrieved by clicking on the "Observation Log Files" box on the Retrieval Options Page.

    A2.4.1 Data Quality Keywords and Comments

    The handling of data quality evaluation has changed several times since HST was launched in April, 1990. For the first few years, each science dataset was displayed and evaluated after it had been calibrated by the ground system (then PODPS, now OPUS). A keyword, QUALITY, was assigned to each dataset and a file, called the PDQ file, was produced to hold any comments the reviewer had. The PDQ file was archived as part of the dataset. The QUALITY keyword and 3 comments, called QUALCOMs, were placed in the HST catalog. The QUALITY and QUALCOMs can be viewed using StarView or MAST web.

    At the time of the second servicing mission, manual assessment was replaced by an automated system that was used by OPUS staff to assess the procedural data quality of the data. The datasets were not displayed unless there was some indication that a problem had occurred either with the instrument, the FGSs or HST. PDQ files and QUALITY and QUALCOM keywords continued to be produced for all datasets. Albeit, most files and keywords were produced automatically and contain limited information on the quality of the data.

    On July 1, 2002, the automated system used by OPUS personnel was replaced by a new software package, called PROMPT. Responsiblity for procedural data quality assessment had been removed from OPUS in March 2002. The PROMPT system continues to populate QUALITY and QUALCOM keywords, which continue to be placed in the HST catalog. However, PROMPT does not produce PDQ files. In addition, PROMPT primarily reports on failures, such as non-optimal guiding, loss of lock, and instrument or spacecraft safing. Comments on the actual data only occur, after the fact, if the PI reports a problem to the archive help desk.

    In 2006, a project to regularize the QUALITY keyword and provide a numeric value for the quality was begun. A clean up of the existing QUALITY information is part of this project. As of this revision to the Archive Manual (2007), the QUALITY keywords have been revised. The clean up of the QUALITY information continues.

    The PDQ quality keyword and comments can be displayed in StarView using the <General> screen (which displays the PDQ comments). When interpreting the relevance of the PDQ keyword and comments, bear in mind that:

    • Data quality parameterization is inherently subjective
    • 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 for an observation where the target was a bright star and the integration time sufficient that a clear detection was expected may well indicate poor data quality, the same evaluation for an observation where the target was a distant, faint cluster of galaxies (for which co-adding of many exposures is required) indicates little about the quality of these data.

    As described above, the PDQ evaluation is composed of a parameterized keyword and a comment. For early observations, the comment describing the data is fully at the discretion of the operations astronomer. For later exposures, the comment is constrained to describe the problem and, in almost all cases, is fixed. Table A2.1 lists possible values that can be assign to the parameterized PDQ keyword and a brief explanation of their meaning. This list of permissible keyword values dates to 2007. Data processed prior to that date were described with a similar set of values. Most of the listed values have been used since shortly after launch. Note that when data are reprocessed, they are not re-evaluated for data quality. So, some of the older, obsolete keyword values remain for some observations.

    Table A2.1: PDQ Keywords

    Keyword

    Meaning

    ACQ1FAIL

    A problem occurred during a mode I target acquisition of a reuse target offset.
    ACQ2FAIL

    Scheduled onboard target acquisition failed.
    APER_SHUT

    The instrument shutter did not open, likely because of a loss of lock or a guide star acquisition failure.
    BADCHAN

    Quality significantly compromised by dead CCD column(s) or bad diode(s)

    BADLOCATION

    Target is not at the expected location.
    COSRAY

    Compromised by cosmic-ray contamination

    EXPSHORT

    Actual exposure time is less than planned.
    GSFAIL

    Spacecraft guiding was not as commanded.
    GYROS

    Exposure on planned gyro control. Sources may appear trailed.
    INCOMPLETE

    Only some of the expected data groups received

    LOCKLOST

    The FGSs lost the guide stars during the guiding period. Exposures may be shortened or may have occurred on gyro guiding.
    NO-COUNTS

    Zero-level data

    NO-EVAL

    Inadequate time or resources for data evaluation

    NO-SOURCE

    No sources visible in image

    NO_TLM

    No engineering telemetry (or observation log) available for exposure.
    OK

    No apparent problems

    POOR

    Other problems affecting probable scientific utility (used when no other keyword applies)

    RECENTER

    A spacecraft disturbance caused a temporary loss of guide stars. Exposures do NOT stop during recovery. Target may be trailed.
    SATURATED

    Target saturated; counts rolled over, etc.

    SDF_DOWN

    The science data formatter went down, so exposure was not read out.
    SIPROB

    A problem with the instrument affects the data in a negative manner.
    SLEWING

    The spacecraft was possibly slewing during the exposure.
    TDF-DOWN

    The take data flag (TDF), an indicator that the spacecraft pointing is stable, is down. The exposure may or may not continue. The exposure may be shortened. The target may be trailed.
    TM_GAP

    Not all engineering telemetry is available for the exposure.
    TRAILED

    Observation taken on gyros, trailed sources, but otherwise OK

    UNDEREXP

    Observation seems underexposed

    UNKNOWN

    Uncertain of usefulness or quality of data

    A2.4.2 The PDQ Files

    When it exists, the complete OPUS data quality report can be retrieved from the archive. Every PDQ file is archived with the same root name as the observation, a trailing character of 'c' and an extension of _pdq.fits (i.e., u6712709c_pdq.fits). 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 obtain the PDQ files associated with particular datasets, be sure that the [Data Quality] box is selected on the <Retrieval Options> screen.

    A2.4.3 OPUS Observer Comment Files

    Observer comment (ocx) files may help an archive user determine the quality of an HST observation. These are ASCII text files prepared by OSS (later OPUS) personnel who, early in the mission, monitored the health of the telescope and science instruments, and provided observers with a real-time interface. Early ocx files contain updated mission information obtained at the time the observation was executed. Later ocx files primarily contain information related to spacecraft pointing adjustments made in support of Mode I Target Acquisitions. These files are not created for every observation executed. When they exist, the ocx files, like the PDQ files, can be retrieved by clicking the [Data Quality] box on the <Retrieval Options> screen.

    A2.4.4 Exposure Flag

    The exposure flag keyword (expflag) serves as a comment to the exposure time keyword (exptime). It indicates whether the exposure completed without interruption. The exposure flag can be useful in that it allows the archival researcher 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 A2.2 gives a generic description of the possible values of this keyword and their meaning.


    Table A2.2: Exposure Flag Keyword Values

    Keyword

    Meaning

    NORMAL

    EXPTIME was successfully calculated from telemetry information and there is no indication that the exposure was interrupted.

    INTERRUPTED

    EXPTIME was successfully calculated from telemetry information, but there is an indication that the exposure was interrupted.

    INCOMPLETE

    EXPTIME was successfully calculated from telemetry information and is less than the predicted exposure time.

    EXTENDED

    EXPTIME was successfully calculated from telemetry information and is greater than the predicted exposure time.

    UNCERTAIN

    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.

    INDETERMINATE

    EXPTIME could not be successfully calculated from the telemetry, and the predicted exposure time was not available.

    PREDICTED

    EXPTIME could not be successfully calculated from the telemetry, and EXPTIME was set to the predicted exposure time.

    A2.4.5 Fine Guidance System Lock

    The tracking mode requested for each HST observation is given in the keyword fgslock, which can be added to the MAST or Starview search page. NB: The value is the planned guiding mode, not the actual guiding mode. Possible values are listed in Table A2.3.

    Table A2.3: Values of FGSLOCK

    Value

    Comment

    FINE LOCK

    Observations scheduled to use two FGSs.
    FINE/GYRO

    Observations scheduled to use one FGS. There is no active roll control.
    FINE/GY

    Observations scheduled to use one FGS. There is no active roll control.
    COARSE

    Observations scheduled to use two FGSs in Coarse Track guiding mode. This mode was discontinued in late 1993.
    GYROS

    Observations scheduled on gyro guiding. Usually guide stars were dropped after being acquired to remove pointing errors.
    UNKNOWN

    Planned guiding is unknown.
    NULL

    Internal observations so guiding mode is irrelevant.

    Coarse track has not been allowed since late 1993. Most HST observations planned on gyro guiding are for moving targets where the tracking slew would drive the guide stars out of the FGS field of view. Usually a guide star acquisition is scheduled before these exposures begin, in order to remove pointing uncertainties, then the guide stars are deliberately dropped.

    If no guide star acquisition occurs, the absolute error of positioning is ±30". In 3-gyro mode, 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.

    In 2-gyro mode, which was instituted on August 25, 2005, the drift rate for gyro hold is typically 0.03 degrees/sec, but may be larger depending on the recent HST slew history. The RMS jitter for fine lock is comparable to that from 3-gyro mode. Coarse track is no longer available.

    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.

    A2.4.6 Observation Log Files

    Observation log files for most science datasets taken after October 20, 1994 have been produced in the pipeline and written to the HST archive. The observation log files contain a specialized set of pointing and engineering data associated with each science exposure (including the jitter information). The extent of this information has changed over the years, with more recent observations logs containing less instrument specific information. You can retrieve these log files by first marking the science dataset for retrieval and then selecting the observation log files option on the archive retrieval screen. It is not necessary to retrieve the science data; the observation logs may be retrieved independently. Remember, the observation log files are only available for those data taken after October 20, 1994.

    A2.5 Locating and Retrieving the HST Ephemeris

    Users who need the location of HST during their observations will find the information in the HST ephemeris file, also know as an ORB file or an orbit file. Each ephemeris file covers 3 days, with a resolution of 60 seconds.

    The ephemeris files are archived and available to any user. First the user must locate the ephemeris file(s) of interest using StarView. Retrieval of files may be done via StarView or MASTWeb can be used.

    A2.5.1 Using Starview to Locate HST Ephemeris Files

    Check the StarView website for information about StarView. This site also allows the user to download and install the latest version of StarView.

    • Bring up StarView.

    • Under the Searches item in the top menu bar select HST -> Data Files

      This brings up a search screen. There are 2 parts to the screen. The top portion is where you qualify your search and the lower portion displays the search results.

      Fill in the following 2 fields.

      • Extension: POD

      • Archive Class: ORB

      • Data Start Time: a small range of dates that encompass your observations

        The syntax is YYYY-MM-DD..YYYY-MM-DD. The .. is important. It indicates the input values are a range. For example, 2007-01-01..2007-01-04.

    • Execute the search by clicking on the binoculars button at the top of the page.

    In the Results portion of the screen you will see a list of Dataset Names in a box on the left hand side of the screen and a Data Start Ti... field to the right of the box. As you click on Dataset name, the value in the Data Start Ti... field will change. You want to select the Dataset names whose start times immediately proceed the start times of your observations.

    See below for retrieval via either StarView or MASTWeb.

    A2.5.2 Using StarView to Retrieve HST Ephemeris Files

    This is a continuation of the above section on locating HST ephemeris files. It assumes the results screen from the location stage is available.

    • In the Results portion of the data files screen, select the files to retrieve by clicking on the dataset name in the box on the left hand side of the screen, then clicking on the Mark or All button at the top of the screen. There are many ways to mark datasets of retrieval. Do not be afraid to experiment.

      A StarView retrieval screen will be displayed.

    • In the StarView retrieval screen, confirm that all desired files are listed and have a check in the "Marked" column.

      Files may be added by returning to the results screen and selecting more dataset names.

    • In the StarView retrieval screen, when all desired datasets are listed and marked, click on the Submit button at the bottom of the screen.

      The HST Retrieval Configurations Options screen is displayed.

    • Fill in the fields, specifying either an archive account or anonymous (check the box)

      • Check the "Show override (Expert only)" box in select data type(s) for retrieval on the middle left hand side of the screen.

      • Enter POD in the Extensions box and check "Fetch only listed extensions." Both are at the bottom of the screen.

        The "Calibrated/OTFR" box should be automatically unchecked at this point. If it is not, uncheck it now.

      • On the right side of the screen, Choose a method of data delivery, select a method for data delivery. NB: Anonymous users may use FTP Delivery, but will be prompted to enter a username and password.

    • Click the DONE button at the bottom of the screen

      A Submitting Request dialog box will be displayed. After the request has been delivered, that dialog box is dismissed and another dialog box, containing the Request ID, is displayed. Click OK to dismiss the Request ID dialog box.

    The standard e-mails regarding submission and completion of the request will also be sent.

    A2.5.3 Using MASTWeb to Retrieve HST Ephemeris Files

    If the name of an HST dataset is known a priori, it may be retrieved without going through the standard MASTWeb search pages. This is done through the dataset lookup page. Navigate to this page from the main MAST page by clicking on Hubble on the lower right hand gutter, then, under "Search & retrieval" in the left hand gutter, select "Retrievals by dataset." Or, go directly to http://archive.stsci.edu/cgi-bin/dataset_lookup.

    • Go to MASTWeb, retrieve by dataset name page:

      http://archive.stsci.edu/cgi-bin/dataset_lookup

    • Enter the Dataset Names in the box titled "Enter a list of datasets."

      Click on "Enter a list of datasets" to get help on options for entering the Dataset names. You may be able to submit an already prepared list of names. Case is important here. The filenames should be in UPPER case.

    • After the list of datasets is entered, click on the "Submit dataset for retrieval" box at the bottom of the page.

      Another form will be displayed. The list of Dataset Names will be displayed along with other information. All should have Class ORB. All Dataset Names should start with a P.

    • If not already checked, check the Mark box (or click on the "Mark all" button). Then Click on the "Submit marked data for retrieval from STDADS" button.

    • The Retrieval Options page will be displayed. Fill the top section as you would for any other HST data.

      You can use anonymous as the Archive Username and your e-mail address for the Archive Password. Using anonymous and requesting ftp delivery will require the entry of a username and password on the right hand side of the page.

    • Unclick the "Calibrated" box under "Science Files Requested:." At the bottom of the page, enter POD in the "File Extensions Requested" box. Click on the "Send retrieval request to ST-DADS" box.

    The usual "Request Sent to ST-DADS" page, listing the Dataset names to be retrieved, will be displayed. The e-mails regarding the submission and completion of your requests will also be sent.

    The files you get will be named like pr150000r.pod. The file can be paged on a unix system.

    A2.5.4 Increasing the Resolution of the HST Ephemeris

    As archived, the HST ephemeris has a resolution of 1 minute of time. Some users need a finer resolution. Although we can not provide the HST ephemeris at a higher resolution than 1 minute, a technique does exist to interpolate between the discrete points.

    Start with the HST state vector from the data headers and do a linear interpolation between HST ephemeris points.

    The HST state vector, that is the X, Y and Z components of HST's position and velocity, in the header should be the state vector at the start of the exposure. For HST data the information is in the shf file for the exposures.

    For example, proposal 10500 produced products named J9CX01010, J9CX01020, J9CX02010 and J9CX02020. The association file, J9CX01010_asn.fits, shows exposures J9CX01CEQ, J9CX01CHQ, J9CX01CKQ, J9CX01CNQ, J9CX01CQQ, J9CX01CTQ, J9CX01CZQ and J9CX01D5Q comprise the product. It is the shf file (e.g., J9CX01CQQ_shf.fits) for these exposures that contain the HST state vectors at the start of each exposure that makes up J9CX01010. Depending on what was requested when the data were retrieved, the shf file may or may not be present. Request uncalibrated data to get the individual member exposures, which will include their shf files.

    Continuing with the same dataset, there will be 8 state vectors, one from each exposure that comprises J9CX01010. Since the data cover about 43 minutes of time, the ephemeris file should yield about 45 data points, with the first point at a time before the start of the first exposure and the last point from a time after the last exposure. A curve can be fit to the data.

    A2.5.5 Decoding Ephemeris Filenames

    The archived version of the HST ephemeris files are named to follow the engineering file conventions (ICD-19), with a first character of 'P' and an encoded time for the start time of the file. The syntax is PYMDhhmm, where P indicates an ephemeris file, Y is the year code, M is the month code, D is the day code, hh is the starting hour and mm is the starting minute. The codes start at 1 and increment through the single digits, then start into the alphabet. For Y, year 1 is 1980. For M, January = 1, December = C. For D, the first = 1, the tenth = A, the 31st = V. For example, dataset 'PNBP0000R' starts in year 'N' = 1980 + 23, month 'B' = November, day 'P' = 25, hh:mm = '0000'.



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    1 Refer to the Phase II Proposal Instructions (accessible from http://www.stsci.edu/hst/programs) for a detailed list of these conventions, including the conventions for naming uncataloged targets.

    2 Although Cycle 0 and 1 observations were not initially described using this set of guidelines, target descriptions for all Cycle 0 and Cycle 1 GO and GTO observations were redefined in the HST catalog to comply with the most recent Phase II Proposal Instructions, ensuring that a consistent set of terms are used to describe the archived observations.