What is a MOC?

MOCs, originally developed at the Centre de Données astronomiques de Strasbourg (CDS) and adopted as recommendation by the International Virtual Observatory Alliance (IVOA), are scalable descriptions of arbitrary sky regions based on the HEALPix sky tessellation algorithm. The resolution of MOCs is determined by the maximum value of the HEALPix level used to define a region.

The Chandra Data Archive produces updated MOC maps of all public Chandra observations every week. Chandra MOCs include: all public data except for ACIS observations in Continuous Clocking (CC) mode; a set of MOCs for gratings (LETG and HETG) observations; MOCs of all Chandra observations included in all versions of the Chandra Source Catalog (CSC); and Chandra MOCs for all public observations for MOC orders from 10 to 13, corresponding to average resolutions for the cells in the highest HEALPix level (ranging from ~3.4' to ~25.7’’).

Note: CSC MOCs are only provided for MOC order 13.

All Chandra MOCs may be downloaded as fits files below.

How can I use the Chandra MOCs?

In this section, we provide two examples of usage of Chandra MOCs that take advantage of tools that will be described more in details below.

A) The Chandra MOC files can be downloaded and displayed locally using the Aladin Desktop client. The following scenario (based on Aladin Desktop v10.0 for Mac) explains how to visualize a Chandra MOC, load images and MOCs obtained from XMM observations and calculate the MOC representing the overlap between the two MOCs:

• Click on the link of the Chandra Archive MOC file for a given cell resolution, and save the file to any path on your computer
• Start Aladin Desktop on your computer. Click on the "File" menu in the command bar and select "Load local file...". In the window, find the downloaded MOC file and double-click on it.
• The Chandra MOC will appear as a new "plane" in the Aladin plane stack, and will be displayed in the central panel
• Images and MOCs from other observatory to be displayed with the Chandra MOC can be found in the "Available data" pane (left side of the Aladin window). For example: expand the "Image" folder and select "X", then "XMM" and finally click on the item "X-ray images on band 0.5-1 KeV". A new window will appear: check the boxes next to "progressive" and "coverage" and click "Load".
• Two new planes, containing the XMM HiPS image and MOC in the 0.5-1 KeV band, will be displayed with the Chandra MOC
• Click on the "Coverage" menu item, then on "Logical operations". In the "MOC operations" window, select the Chandra and the XMM PN colored MOCs as first and second planes, respectively. Check that intersection" is selected and click on "CREATE"
• A new plane showing the footprint of the overlap between the Chandra and the XMM MOC will appear in the plane stack
• The sky area/fraction covered and the maximum angular resolutions of the MOCs can be accessed by right-clicking (or control-clicking) on the plane associated to any MOC, and then selecting "Properties"

B) The Chandra MOC files can be also employed to filter a list of positions based on whether they are located within the Chandra footprint, using TOPCAT (v4.5 for Mac), as described in the recipe below:

• Click on the link of the Chandra Archive MOC file for a given cell resolution, and save the file to any location on your computer
• Start TOPCAT on your computer. Load the catalog containing the list of positions that you want to compare to the Chandra coverage by clicking on the "Load New Table" icon (top-left of the main TOPCAT window), then select the format of the table, click on "System Browser" and locate the file containing the table. Double click on the file and the table will appear in the "Table list" window
• Click on the "Display Row Subsets" icon (seventh icon from left), and click on the "plus" icon to add a new subset. In the "Define Row Subset" window, type a name in the "Subset Name" field ('within_chandra_coverage'). Then, click on the left-most icon to show a list of functions that can be used to define new subset analytically
• In the "Available Functions" window, expand the "Coverage" folder and click on "inMoc" function. A short explanation of the arguments passed to the function will appear in the right side of the window.
• Close the "Available Functions" window and type the following definition in the "Expression" field of the "Define Row Subset" window:

  inMoc("local_path_to_the_Chandra_MOC_file", name_of_ra_column, name_of_dec_column)

  where 'local_path_to_the_Chandra_MOC_file' is the path of the Chandra MOC file, 'name_of_ra_column' and 'name_of_dec_column' are the names of the columns containing the R.A. and Dec in the catalog
• Click on "OK". A new subset containing only the coordinates in the catalog within the Chandra coverage will be defined. You can save this subset to a file and use the Footprint Service to retrieve the properties and data products of the Chandra observations that overlap each distinct position

MOC-compatible software

Chandra MOCs may be visualized and manipulated with a number of software tools

• Aladin Desktop: GUI tool for the visualization and analysis of astronomical images. Supports interactive exploration of MOCs and simple operations on them
• TOPCAT: GUI tool allowing interactive manipulation and visualization on catalogs and tables. TOPCAT has recently introduced new functions that select positions located within a MOC. Similar capabilities have been implemented in STILTS, the command-line counterpart to TOPCAT
• MOCPy and PyMOC: these Python libraries allow reading, writing, visualization and manipulation of MOCs, in both FITS and JSON serializations

Note: DS9 does not support MOCs.