The goal of the HLA is to optimize science from the Hubble Space Telescope by:
The FAQ gives a detailed description
of the types of data that are available in the HLA. Currently we have
enhanced image products for ACS, WFPC2, and NICMOS, extracted
spectra from NICMOS and ACS grism
observations, and source lists from ACS and WFPC2 images.
The NICMOS imaging, ACS grism spectra,
and WFPC2 source lists are all new in the DR3 release.
Community-contributed
high-level science products such as GOODS,
COSMOS and the UDF are also available.
The enhanced data products were generated from the standard HST pipeline products. The ACS and WFPC2 images have been combined using multidrizzle, are aligned north up, and have been astrometrically corrected when possible (for approximately 80% of the cases).
Proprietary data and other data not yet processed by the HLA (including
data from STIS, FOS and GHRS) are also accessible through the HLA, although
with more limited functionality. These non-HLA data may be collected in the
shopping cart and submitted
directly to the MAST/DADS interface
for retrieval (also new in DR3).
Browser requirements: Currently the HLA interface is supported on Firefox, Safari (version 3), and Internet Explorer (version 7). To fully realize all of the HLA's functionality, one must have cookies and popups enabled.
Type the name or coordinates of an object of interest into the "search box" at the top of the page and click on the Search button. If a name is used, the request is sent to the name resolver at NED (or to other services such as SIMBAD when necessary), which returns the coordinates and an image size that are used for the search. Some examples:
| M101 | Most objects have a default search radius from NED |
| M87 r=1.0d | You can explicitly change the default search radius |
| Antennae | The resolver recognizes many names |
| 14h03m12.6s +54d20m56.7s | If no radius is given a default of 0.2 degrees is used |
| 14 03 12.6 54 20 56.7 | Units are optional; equinox must be J2000 |
| 14:03.210 54:20.945 | Many formats are accepted |
| 180.468 -18.866 r=0.4 | Default units are degrees in this case |
| Virgo Cluster | The object can be quite extended |
| 0 0 r=180d | An all-sky search. Some restrictions exist. |
There is an advanced search button to the right of the search box that reveals more options for selecting the data of interest, e.g., by specifying the instruments, proposal ID, spectral elements, or moving target flag. You can also select the desired data product level, including exposures (level 1), single-visit combined images (level 2), multi-visit deep images or mosaics (level 3), color images (level 4), or community-contributed high-level science products (level 5). The default best available level selects the data products that are generally the most useful (e.g., a combined image, if it exists, rather than the single exposures that were used to create that image). The Image Preferences button exposes options can be used to increase the size of the small preview images and to view image cutouts centered at the search position rather than the default full-field previews of the entire image.
Also available in the advanced search options is the
Position List File Upload,
which can be used to search at a list of
target positions provided in a file. The RA/Dec columns in
search lists can be in various formats (new for DR3), with a selection
of column delimiters (whitespace,
commas, tabs, etc.) and positions that are given either in
decimal degrees or in sexagesimal format (hr min sec deg
min sec). The image cutout
option is especially useful with source lists.
See the FAQ
for more information and examples.
The tabs just below the search box provide three basic ways to view the results from the search:
INVENTORY (the default) - Shows the results in a table format. The Display link at the beginning of each row opens the interactive display for that image, while the FITS link adds the dataset to the shopping cart for later retrieval. When FITS images are not available in the HLA, the cart link will be named DADS or TAR, depending on the data source, and the Interactive Display is replaced by a link to the MAST or ST-ECF preview page for the observation.
Click on a table column heading to sort by that value (e.g., "ExpTime"). Clicking again on the same heading reverses the sort order. If there are more than 20 images found, there are controls to go through the table by pages or to jump to the first or last page. You can also change to 10, 50, or 100 results per page.
Brief definitions of the table columns are provided at the bottom of the page; you can also get tool tips by placing your cursor on the column heading. By default not all of the (many) table columns are displayed. See instructions on the bottom right to add, remove, or reposition columns.
A subset of the inventory can be selected using the empty boxes under each column heading to filter out certain rows of the table (e.g., *HRC* will select only ACS/HRC images). Numerical columns support filtering using specific values, ranges, or limits (using > and <). Columns with character strings support exact matches or wildcard matches. An exclamation mark "!" at the start of the pattern indicates that only rows that do not match should be retained. Move the cursor over an empty box for instructions on the filter formats.
Table rows can be selected by clicking on them, and selected rows are
highlighted. Clicking a selected row toggles the selection, so the
row becomes unselected (and unhighlighted).
Datasets can also be selected in the
images and footprints
views described below; datasets selected in any view are also
selected and highlighted in the other views. The
Add selection to cart button (new for DR3)
can be used to
add the selected rows to the shopping cart for
downloading. Note that if the table has been
filtered, only selected rows included in the filtering are added
to the cart.
The Reset selection button unselects
all the selected rows.
The row selection can also be used for sorting and filtering the
table (also a new DR3 feature).
By default the selected rows are mixed in with the unselected rows, with the
sort order determined by other parameters, but there are four possible
choices:
When the selected data are shown first and the table is sorted by a column, each group of rows (selected and unselected) is sorted separately.
When rows are selected, they are highlighted but do not immediately move (e.g., to the top of the table when the selected rows are shown first). But when you move to a new page, the selected rows do get resorted and moved. So when selected rows are being shown first, you may notice that when you click a row, move forward a page, and then move back, the selected row has moved. This is even more noticeable when only selected or non-selected rows are shown; then rows may disappear altogether after paging forward and back. If you find this behavior disconcerting, you can use the default "Mixed" view, where whether a row is selected does not affect its position or visibility in the table.
Dataset selection is integrated across the Inventory, Images and Footprint views, so datasets selected in one view are highlighted in all views. The Footprints view has its own table that always shows selected rows at the top.
IMAGES - Shows preview images of the search results, using the same sorting, filtering, selection, paging, etc. The Images and Inventory views always match, so the top row in the inventory table appears at the top of the corresponding images page. Each image is annotated with some of the relevant information from the inventory table, including the target name, instrument, filter, etc. There are also links to add the datasets to the shopping cart and to bring up the interactive display. MAST previews are used for datasets without HLA enhanced products.
The Images view offers a few additional options for data access. The image can be downloaded either as a simple FITS file with only the science image data (named FITS-Science) or the full multiextension FITS file (named FITS-MEF) created by multidrizzle that contains the science image in extension [1], the weight array in extension [2], and the context image in extension [3]. When source lists are available, they can also be added to the cart. There are two types of source lists; DAOPHOT (primarily for point-like objects) and SExtractor (for both point-like and extended targets). There is also a plot link that pops up a window showing some properties of the catalogs.
The "More..." option in the bottom right provides access to a wide range of additional information, including a list of the exposures that went into the combined images, a link to the original HST proposal that collected the data, and links to the catalogs and to a large number of working files that are briefly described in the FAQ.
FOOTPRINTS - Footprints are the outlines of the region of the sky associated with each data product. In this view, footprints are shown overlaid on a Digitized Sky Survey (DSS) image, as well as listed in tabular form. Please note that the footprint view requires cookies to be enabled.
Selectable options for the footprint page include:
A table with various information about each image is provided below the footprint image. You can either click on a table row to see the corresponding footprint, or on a footprint to highlight the corresponding table row. When one or more footprints are selected in the graphical view, the table is resorted to bring the highlighted rows to the top. Selected datasets can also be highlighted through the Inventory and Images views, and will remain highlighted in all three views.
Note that filtering in the Inventory view does not affect the datasets shown in the footprints view. Also, when a file with a list of positions was used for the search, the footprints view is not useful. We plan to rectify these shortcomings in a future release.
To see the smallest apertures (e.g., FOS, GHRS, ACS Grism or NICMOS Grism) you may need to increase the scale. This is done by changing the search radius, either by adding "r=value" in the search box or by changing the radius box value in the advanced search options. The minimum radius used in the footprints view is 0.01 degrees. The background DSS image is not particularly helpful for very small fields, but the ability to see how the HST observations relate to one another can still be helpful.
An important point to keep in mind when using footprints: While we try to make the footprints as complete as possible, they are not as complete as the HST duplication-checking tool. For example, the HLA does not include planned observations. Proposers must therefore still use the duplication tool to check for duplications.
CART - The HLA has a (shopping) cart system to help simplify data retrieval. Any item in the HLA with a cart icon next to it can be added to the cart. This includes all files available for download. Note that in order for this to work, your web browser must have cookies enabled.
When items are added to the cart, the text on the cart tab will change
to reflect what is in the cart. For files with known sizes, the total
sum of the request size will be tallied as well as the total file count
and dataset count for MAST-DADS items (new for DR3).
Clicking the cart tab shows what is in the cart. Items from the same HLA dataset are grouped together into a single line for that data group, with each file listed separately. For each group and file, a button is present to allow you to remove a file or group of files from the cart. For (non-HLA) products that are to be retrieved using the MAST/STDADS system, the files are be labeled as "To Be Retrieved from MAST/STDADS".
Below the cart are buttons to allow you to remove everything from the
cart. If data in the cart are to be fetched from the HLA,
a button labeled "Fetch HLA Data" will be found.
Clicking this will begin the download stream of all of the data in a
single zip file. If data in the cart are to be retrieved from the
MAST/STDADS interface (new for DR3), a button with the text "Request DADS Data" will
be found. Clicking this will popup a new window to allow the user to
use the MAST/STDADS interface to retrieve all of this class of data
found in the users cart in a single MAST/STDADS request. In the event
that data are to be retrieved from both sources, both buttons will be
present and must each be clicked.
GRISM SPECTRA -
Spectra for many sources in NICMOS and
(new for DR3) ACS
GRISM observations
have been provided by the Space Telescope-European Coordinating Facility (ST-ECF).
These spectra have been extracted
with uniform procedures and are available directly from the
ST-ECF web page,
which can be accessed through the Grism Spectra (ST-ECF)
link just below the search box. They also appear in the HLA search results.
INTERACTIVE DISPLAY - You can look at an image or 2-dimensional spectrum more carefully, and overlay a source list or other catalogs (SDSS, 2MASS, GSC2, FIRST), by using the "interactive display" (via either the inventory or images view). The image can be moved by either using the arrows in the upper left (< > ^ v) or by dragging the image using the mouse. The o button recenters. The + and - buttons zoom in or out. The Lighter and Darker buttons change the contrast. The Invert button changes from white on black to black on white. SExtractor or DAOPHOT source lists from the image can be overlaid by clicking the appropriate box, as can SDSS, 2MASS, GSC2, FIRST, or GALEX catalogs (very useful to check the absolute astrometry). A readout in the upper left shows the x-y position of the cursor along with either RA/Dec or, for spectra, the wavelength and position along the slit. A line or column plot can be made at the location of the cursor by typing "L" or "C". This is especially useful for spectra.
The advanced contrast controls
(new in DR3) allow finer control over the display than
the Lighter/Darker buttons. The can be used to adjust the
color balance by changing the contrast independently for
each color and to adjust the bright/dark ends of the lookup
table separately for color or grayscale images.
Another useful feature (new in DR3) is that typing 'S' on the keyboard
translates the current cursor position to RA/Dec and copies it into
the search box on the HLA search form. You can use this in conjunction
with the image cutout options
to get quick previews from other HLA images at the same sky position.
See the Help in the upper right of the interactive display for more details.
SOURCE LISTS - Two types of source lists are available via the HLA: DAOphot, which is optimized for point-like objects, and SExtractor, which is optimized for extended objects. In both cases the detection image is a "white light image" (i.e., a combination of images using all available filters within a single visit, also referred to as the "detection"or "total" image). The source lists can be overlaid on an image using the interactive display, and can be downloaded via the Images view. The magnitudes are in the ABMAG system.
Please note that while the HLA images and source lists will be sufficient for many people's science, by necessity these products are developed for general usage rather than being "tuned" for a specific scientific goal. Hence in many cases the optimal science can be achieved by going back to the original STScI calibration pipeline data and processing on a chip-by-chip basis.
All-sky searching: All-sky (0 0 r=180) searches for all instruments has been temporarily disabled since this can lead to very long searches with a large load both on our servers and your browser. Unrestricted searches with multiple instruments are currently limited to search radii smaller than 10 degrees (which can still return very large search results). All-sky searches are allowed (1) if only a single instrument is checked in the Advanced Search options or (2) if the proposal ID, moving target flag, or specific spectral elements are specified.
Constructing Images - HLA enhanced image products are projected onto a geometrically corrected, rectilinear grid and are oriented with North up. For each observation, the images have been grouped according to the filter in which they were obtained, and also according to which camera was used (eg., obsevations carried out with ACS could be obtained with either the WFC, HRC or SBC camera).
Each set of images (grouped by filter and camera) is then processed through the HLA pipeline using the Multidrizzle software (Koekemoer et al. 2002, HST Calibration Workshop, p.337 [PDF]) which corrects geometric distortion using the "drizzle" software (Fruchter & Hook 2002, PASP 114, 144), removes cosmic rays and combines the images. For the current release, the output pixel scale is set to be similar to the input pixel size, except for NIC3:
| Detector | Pixel Size |
|---|---|
| ACS/WFC | 0.05" |
| ACS/HRC | 0.025" |
| ACS/SBC | 0.025" |
| WFPC2 | 0.10" |
| WFPC2-PC | 0.05" |
| NIC1 | 0.025" |
| NIC2 | 0.05" |
| NIC3 | 0.10" |
Smaller pixel scales were precluded primarily on the basis of diskspace limitations, but are planned for a future release.
Each individual exposure, for all the filters used with a given camera, is first transformed separately to an undistorted output image. These (level 1) exposures are all registered to a common pixel grid (so that objects can be directly aligned on each image, even if dithering was used).
In addition, all the exposures for a given filter/camera combination are combined using Multidrizzle into a final, clean (level 2) image for that filter/camera combination, which involves identification and removal of cosmic rays as well as astrometric alignment and distortion removal using "drizzle".
Finally, if more than one filter was used with a given camera during the observation, then the exposures from all these filters are combined into a single "total" (also called "detection" or "white-light") image, as long as two or more exposures were obtained in each filter so that the cosmic ray masks obtained separately from each filter can be used. These "total" images can potentially be significantly deeper than the images obtained in each filter separately, and are therefore used as detection images for the source lists. Images obtained with grisms or polarizer filters are excluded from the total image, although they are combined for each such spectral element separately.
The HLA provides access to its image data via a
Virtual Observatory (VO)-compatible
Simple Image Access (SIA)
service. The VO SIA protocol allows retrieving image data from a variety
of astronomical image repositories through a uniform interface. A query
defining a region on the sky is used to select candidate images. The
service returns a list of candidate images formatted as a VOTable. For
each candidate image an access reference URL may be used to retrieve
the image. Images may be returned in a variety of formats including FITS
and various graphics formats.
To access the HLA SIA service, use the URL http://hla.stsci.edu/cgi-bin/acsSIAP.cgi?strict=1 with a region expressed as the right ascension and declination of the field center and a search radius, both specified in decimal degrees. For example:
http://hla.stsci.edu/cgi-bin/acsSIAP.cgi?strict=1&POS=40.669,-0.013&SIZE=0.1
This query will return a VOTable of data that overlap the user input region. The client then can access each image via the URL field in the VOTable.
Note that the HLA SIA returns observations for which the actual sky coverage footprint of the observation overlaps the search circle. This differs from other SIA implementations, which usually return images with a reference position that falls within the search box. Specify SIZE=0 to retrieve data only for images that cover the specified RA/Dec point on the sky.
There are additional HLA-specific parameters that can be used to limit the list of data by instrument, processing level, etc. The standard SIA format=metadata query gives information on the other parameters.
- A paper entitled "The Hubble Legacy Archive: General Description and Validation of Products" will be available. (~Summer 2009)
- An advanced search capability will allow you to select classes of objects (e.g., starbursts, irregulars, cepheids, ...).
- Source lists from individual observations will be integrated into an All-HST-Sky Source List.
All refereed publications based on data obtained from the HLA are requested to carry the following footnote:
Based on observations made with the NASA/ESA Hubble Space Telescope, and obtained from the Hubble Legacy Archive, which is a collaboration between the Space Telescope Science Institute (STScI/NASA), the Space Telescope European Coordinating Facility (ST-ECF/ESA) and the Canadian Astronomy Data Centre (CADC/NRC/CSA).
In addition, publications of research supported by an STScI grant must carry the following acknowledgment:
Support for Program number ____________ was provided by NASA through a grant from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Incorporated, under NASA contract NAS5-26555.