HCV Explorer Help


The HCV Explorer is a web visualisation tool to explore the Hubble Catalog of Variables (HCV) and offers interactive and connected plotting of the HCV catalog in the sky, in light curves, in plots and in a results table. The following help describes the catalog, how to use the HCV Explorer tool via the user interface and via programmatic access, and provides some example use cases: 

If you can not find the information you need within these help pages, or you want to provide us with feedback or suggestions, please contact our helpdesk or email technical support at hsthelp 'at' sciops.esa.int

When accessing the HCV Explorer, we recommend using any of the following latest browser versions:

  • Google Chrome
  • Apple Safari
  • Mozilla Firefox
  • Microsoft Edge

JavaScript and cookies will need to be enabled on any browser in order for the tool and its features to work properly.

If you need further assistance, please email technical support.


About the Hubble Catalog of Variables

The Hubble Catalog of Variables (HCV) is the first full, homogeneous, catalog of variable sources found in the Hubble Source Catalog (HSC), which is built out of publicly available images obtained with the WFPC2, ACS and WFC3 instruments onboard the Hubble Space Telescope. The HCV is the deepest catalog of variables available. It includes variable stars in our Galaxy and nearby galaxies, as well as transients and variable active galactic nuclei. In total, the HCV includes 84,428 candidate variable sources (out of 3.7 million HSC sources that were searched for variability) with V ≤ 27 mag; for 11,115 of them the variability is detected in more than one filter. The data points in a light curve range from 5 to 120, the time baseline ranges from under a day to over 15 years, while ∼8% of variables have amplitudes in excess of 1 mag.

HCV column descriptions
Column Name Units Description
  Source - The source identifier (or match ID of the source; MatchID in HSCv3, details here).
  GroupID - Unique identifier for the image group of overlapping images that is taken as the ImageID of some white light image within the group (GroupID in the HSCv3 Groups table).
  Subgr. - Subgroup identifier. Very large groups have been split into subgroups. A value of "-5" denotes that the group was processed as one subgroup.
  R.A.  degrees  Right ascension of source (J2000)
  Dec.  degrees  Declination of source (J2000)
  P.cl. - The pipeline classification. "1" corresponds to a single-filter variable candidate (SFVC), while "2" corresponds to a multi-filter variable candidate (MFVC).
  E.cl. - The expert classification flag: "0" indicates a variable candidate "not classified by the experts", "1" indicates a "high confidence variable", "2" indicates a "probable variable", "4" indicates a "possible artifact".
  #Filters - Number of filters in which the source has been observed.
  Filter - The instrument and filter name (Instrument and Filter in the HSCv3 DetailedCatalog table).
  nLC - Number of points (epochs) in the light curve.
  VarQual - Five letter quality flag for the source variability. Quantifies the deviation of each parameter (CI, D, MagerrAper2, MagAper2-MagAuto, p2p) from the average behaviour within the GroupID. Each letter can obtain the values A (highest quality), B, or C (lowest quality).
  VarFil - Number flag for the variability in this filter (0 = non-variable, 1 = variable). 
  <m_HCV> mag Mean of the light curve's corrected magnitudes (if no corrected magnitude is available, this will be the mean of the light curve's HSC magnitudes).
  <m_HSC> mag Mean of the light curve's HSC magnitudes.
  MAD - The median absolute deviation (MAD) of the magnitude.
  χ2red - Value of the reduced χ2 for the null-hypothesis of the source magnitude being constant.


The HCV can be accessed and downloaded from a number of services such as the HCV Explorer, the MAST Archive (HLSPs), the ESA Hubble Science Archive and VizieR.


How to use the HCV Explorer

Getting Started

The HCV Explorer visualisation tool, developed by the ESAC Science Data Centre (ESDC), has been designed to aid the exploration of the HCV in an intuitive and visual manner, with explorartion in a sky view, easy visualisation of source light curves in various filters, simple access to tabular results and downloading of data, and additional plotting features.

The user interface provides the following functionalities:


Watch the video below to get familiar with the HCV Explorer:


Search field

The search field is located at the top of the interface and accepts coordinates or an object name that can be resolved by SIMBAD. Currently only equatorial coordinates are accepted. Type the name, or coordinates, and click the search button, or click return, and the tool centres on this target in the sky view (top left of the interface) with a default field of view (FoV) of 0.6 degrees. HCV sources found in the FoV are displayed in the sky view and table view (top right).


Sky view

The sky view is located in the top left of the interface and uses AladinLite, created by CDS. When a target is searched for, the sky view updates and centres on the object and any HCV sources found in the FoV are displayed in the sky view (and table view to the left).

As the cursor is moved around the sky, the coordinates in the top left update. Pan around the sky with your mouse and zoom in and out of the sky with the scroll button of your mouse or with the +/- buttons:

Clicking on the 'Update FoV' button will perform another search on the HCV catalog for the new FoV and will update the sources in the sky and table view accordingly. If the FoV is very crowded with sources, a limit is set to 2000 sources displayed in the sky, for browser performance reasons, and a message appears at the top of the sky 'Showing brightest 2000 sources'. Zoom into the sky and click 'Update FoV' in order to load all the sources. Select the fullscreen button to view the sky component as full screen: Click the 'restore original size' button to go back to the default view:

Clicking on a source highlights it in red and brings up a panel at the bottom of the sky view with a summary on the source: the SourceID number; the number of available filters for that source; the pipeline classification, i.e. single-filter variable candidate (in blue) or multi-filter variable candidate (in green); and the expert classification, i.e. a high confidence variable (purple), a probable variable (light blue), a possible artifact (dark yellow) or that the object has not been classified by an expert (dark green):

Clicking on a source will also highlight the relevant row in the table view (top right in the interface) and will plot the source light curve(s) (bottom left) and median absolute devation plot (bottom right), see more information below.

The default background displayed in the sky is a Hierarchical Progressive Survey (HiPS) of the Digitized Sky Survey (DSS) in colour, more information here. To change the background HiPS, click on the manage layers button:

open up the 'Base image layer' field to select the different HiPS:


Also available in the manage layers menu are the options to show/hide the single-filter variable candidates, multi-filter variable candidates, reticle and a HEALPix grid and to save the view as a png image. 

Note that the HCV/HSC coordinates are currently not aligned with the HST images from the HLA (there is an offset in the astrometry). The current version 3 of the HSC (used to produce HCV v1.0) has absolute astrometry based on the Gaia DR1 catalog, which is more accurate. The next version of the HLA images will also incorporate these corrections. See the HLA frequently asked questions for more details: http://hla.stsci.edu/hla_faq.html


Table view

The results table is shown in the top right area of the HCV Explorer and lists all the HCV sources in the FoV of the sky view (top left). Hovering over the column header displays a summary description for each column. Many of the columns can be sorted by clicking on the column header. Clicking on a source / row will highlight the row and the relevant source in the sky view. It will also trigger plotting the source light curve(s) (bottom left) and median absolute devation plot (bottom right), see more information below. A download button is located at the bottom left of the table, where the results table can be downloaded in various formats.


Light curves

Light curves are plotted in the bottom left of the interface. To load a light curve, click on a source in the sky view or click on a source / row in the results table. All light curves in all available filters for the selected source are plotted. The plot is interactive and can be zoomed in on by clicking and dragging the mouse in the desired area. To zoom out again, click the 'reset zoom' button. Hover over each point to see the data values and the HLA cutout images around the selected source for the brightest point in the light curve, the faintest point in the light curve and the selected point in the light curve. This functionality is using the HLA cutout service from STScI. Click on each point in the light curve to view the HLA cutouts in a larger window. Each cutout can be downloaded and a link to the full HLA image in the EHST archive is provided. Before downloading and working with the catalog it is advised to inspect the HLA cutout images, for example to check that no artifacts are contaminating the source and may have potentially affected the automatic classification (see Bonanos et al. 2019 for a description). Click on the filters in the legend to show and hide the points for the selected filters (by default they are all shown). Click on 'Show errors' to plot the error bars. The light curves for the source can be downloaded in various formats via the download button, located to the bottom left of the plot. Before downloading, the HLA cutout images for each filter around the selected source for the brightest point in the light curve, the faintest point in the light curve and the currently selected point are again displayed. Additionally, the plot can be downloaded in various formats by selecting the button to the top right of the plot.



The bottom right area of the interface displays a plot of the median absolute deviation (MAD) of the magnitude versus the median magnitudes for the selected filter and group (and for some crowded regions, the subgroup). Plotted in green are the HCV multi-filter variable candidates: HCV (MFVC); in blue are the HCV single-filter variable candidates: HCV (SFVC); and in grey are the HSC sources for the selected filter and HSC group. This MAD plot indicates the scatter in a source's light curve, for the selected filter, and can be used as a variability index. As described in Bonanos et. al. 2019, 'The data processing pipeline calculated the median absolute deviation and used a 5 sigma threshold to select variable candidates. These were passed to the validation algorithm, which classified the sources as single or multi-filter variable candidates and assigned a variability quality flag to each source.' Note that some of the variable sources, especially the single-filter variable candidates, may be variable in a different filter to the one being plotted. To change the filter, click on the drop down menu to the top left of the plot and select a filter. Only available filters for the displayed group are listed.

The plot is interactive and can be zoomed in on by clicking and dragging the mouse in the desired area. To zoom out again, click the 'reset zoom' button. Click on the legend to show and hide the HSC, HCV (SFVC) and HCV (MFVC) points (by default they are all shown). The selected source is highlighted in red. Any source can be selected and will trigger the light curve plot to be reloaded, for this selected source, and will highlight the source in the sky and results table. The plot can be downloaded in various formats by selecting the button to the top right of the plot. Further plotting features, such as Colour Magnitude Diagrams, are planned for future releases of the tool. 



The full HCV, including all light curves, can be downloaded via the 'Download FULL HCV Catalog' button at the bottom left of the interface. Different formats are available for download, such as csv, votable and fits, and can be chosen in the drop down menu next to the button. Additionally, the results from a search can be downloaded via the button at the bottom left of the table view: (note the number of rows downloaded will be larger than the number of sources summarised at the bottom of the results table view as many sources have been observed in more than one filter). Light curves per selected source can be downloaded via the button at the bottom left of the light curve plots: .  

Before downloading and working on the catalog data it is advised to take a look at the HLA image cutouts of each source of interest. Before downloading light curve data, a preview of each of the filter's brightest, median and faintest HLA cutouts is provided in a window for review.


Programmatic access and Jupyter notebooks


The HCV, and the ESA Hubble Science Archive (EHST) database content, can be queried via the Table Access Protocol (TAP) (see: http://www.ivoa.net/documents/TAP/).

The default query language for TAP is the Astronomical Data Query Language (ADQL), which includes most features of SQL plus some spatial search functions.

TAP services can process synchronous (immediate) or asynchronous (batch job) queries.

The EHST TAP can be accessed via TOPCAT, via command line, or via astroquery.



The following describes how to access the HCV using the EHST TAP via TOPCAT. 

  1. Run TOPCAT in your local environment. If you have Java's WebStart installed, you can install and invoke TOPCAT in one click from: http://www.star.bris.ac.uk/~mbt/topcat/topcat-full.jnlp
  2. Go to the top menu of TOPCAT and select "VO" and there select "Table Access Protocol (TAP) query".
  3. In "Select Service", introduce the following TAP URL at the bottom of the window, in "Selected TAP Service":
    and click on "Use Service".
  4. Alternatively, search for "ESA" in the Keywords field, choose the Hubble TAP, and click on "Use Service".
  5. On the left side of the Metadata panel, select the Tables to be queried. There you will see a drop down menu for the HCV.
  6. Once a table is selected click on "Columns" on the right side of the panel to get information on the table parameters that can be queried. Click on "Table" to get the description.
  7. Introduce query commands in the ADQL Text panel below. The examples provided can be edited (click on the "Examples" button).
  8. When clicking on "Run Query" the selection is sent to TOPCAT.


EHST TAP VIA Command line

The following describes how to access the HCV using the EHST TAP via command line. The structure of a TAP query depends on whether the query is synchronous or asynchronous. For both cases the main query parameters are REQUEST, LANG, QUERY, FORMAT and UPLOAD (to upload a table). See the description of these parameters in section 4.

  1. The full list of public table names and columns can be found via Topcat or by:
    curl -o tables.xml "http://hst.esac.esa.int/tap-server/tap/tables" 
  2. Synchronous queries.

    The retrieved results from a synchronous query is a VO table by default (see the parameters in section 4. to specify a different output format). The results can be saved in a file and inspected using any analysis tool like TOPCAT, for instance.results from synchronous queries.


    Query the HCV for the
    top 1000 sources and return a csv file.
    curl -o top1000.csv "http://hst.esac.esa.int/tap-server/tap/sync?

    Query the HCV centred on M87, in a 15 arcminute circle of radius, and return all columns in a csv file. 
    curl -o file.csv "http://hst.esac.esa.int/tap-server-dev/tap/sync?
  3. Asynchronous queries.
    curl -i -X POST --data "PHASE=run&LANG=ADQL&LANG=ADQL&REQUEST=doQuery&QUERY=select+top+500+*+from+hcv.hcv" "http://hst.esac.esa.int/tap-server/tap/async"

    The response will contain the URL of the job running at the server side (see Location header):
    HTTP/1.1 303 See Other
    Date: Tue, 17 Sept 2019 09:20:34 GMT
    Server: Apache-Coyote/1.1
    Location: http://hst.esac.esa.int/tap-server/tap/async/1461057634887D
    Content-Type: text/plain
    Transfer-Encoding: chunked 

    To obtain the status of the running job run:
    curl "http://hst.esac.esa.int/tap-server/tap/async/1461057634887D" 

    To obtain the results of the job (once the job is finished) run:
    curl "http://hst.esac.esa.int/tap-server/tap/async/1461057634887D/results/result" 
  4. Here are the parameter descriptions for an asynchronous/synchronous query:
    Parameter Value Comments
    REQUEST doQuery Requests to execute the provided query
    LANG ADQL Query language
    FORMAT - votable_plain
    - votable
    - csv
    - json
    Results output format
    QUERY ADQL query Query


Jupyter notebooks

HST Python Tutorial: Hubble Catalog of Variables light curve period analysis

This notebook shows you how to access the HCV using the EHST TAP via astroquery and provides some example use cases such as:

  • Querying the HCV for a particular object or performing a cone search.
  • Plotting the light curves as a function of observation date.
  • Computing the Lomb-Scargle periodogram of the light curve in one band.
  • Phase and plotting the light curve.
  • Downloading an associated HLA image.
  • Plotting sources on an associated HLA image.
  • Visualising cutouts of HLA images.

Download the notebook: