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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.
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