Chandra X-Ray Observatory
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Last modified: 21 February 2014

URL: http://cxc.harvard.edu/csc/googlecat/index.html

The CSC Interface to Sky in Google Earth


Last Update: 03 March 2011 - CSC KML version 1.2 has been updated primarily for infrastructure and cleanup changes. The fields of view of all 'processed' observations have been updated to include an additional 165 observations which were not included in the previous CSC KML version 1.1.

Synopsis: This document introduces the CSC Google Earth Project, a service which allows you to visualize CSC data on the sky with Sky in Google Earth. We believe this venture represents the first use of actual mission data with Google Earth. Searching Sky by source name or coordinates is a quick and easy way to learn if a Chandra observation of a source is included in CSC Release 1.


Contents


Introduction

Amateurs and scientists alike are now able to view the Chandra X-ray universe without access to a satellite telescope - using Sky in GoogleEarth. This feature of Google Earth turns your gaze outward so that you may view images of astronomical objects on the sky as seen through the eyes of some of the great NASA observatories, such as the Chandra X-ray Observatory. The Chandra Source Catalog images and data are now available with this software so that you may scan the X-ray sky, find favorite objects, or search for specific Chandra observations. (Note that the CSC images are separate from the Chandra Showcase images in the Featured Observatories layer of Google Earth.) We believe this venture represents the first use of actual mission data with Google Earth.

The CSC interface to Sky in Google Earth is easy to download and use - simply loading a CSC KML file to your My Places folder brings images from the Chandra Source Catalog just a click away. Field-of-view data are also available for visualizing the footprint of each observation on the sky.

The image outlines representing the fields of view of Chandra observations may be faded in and out of view with the Google Earth transparency slider. The movement of your mouse on the CSC sky highlights individual observations in red, and clicking the edge of one causes an "Observation Identification" box to appear with the name of the object targeted by the observation and the observation identifier (Chandra ObsID).

You can zoom in on images with the +/- navigation bar or a mouse scroll button to see high-resolution, soft-medium-hard "3-color" images from the Catalog. In zoomed-in views, you can access a list of Catalog source properties for an individual source by clicking the yellow "CXO" placemark which is overlaid on it. To find a particular source or observation, you can use the menu along the left side of the application to perform a location search by name or position, or use the My Places folder to choose specific observations. We welcome your feedback or questions to the CXC Helpdesk.

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Viewing Chandra observations with Google Earth's Sky feature

Left: Zoomed-out view centered on Chandra ObsID 5948, with field-of-view lines visible for every Chandra observation in the field.
Middle: Zoomed-out view centered on Chandra ObsID 5948, with field-of-view lines visible for only CSC Release 1 observations in the field.
Right: Zoomed-in, high-resolution Chandra soft-medium-hard 3-color image of Bode's Galaxy (M81) in ObsID 5948, with all field-of-view images hidden and "CXO" clickable placemarks visible.

How to access CSC data with Google Earth's Sky feature

All you need to navigate the Chandra sky is the latest version of Google Earth and the CSC KML.

Downloading Google Earth

The latest version of Google Earth is available for download on Mac, PC, and Linux.

[IMPORTANT]
Important

[New] (20 Feb 2014). There is a problem with Google Earth version 7.1 that obscures the field-of-view boundaries when the display is zoomed out beyond 15 arcmin. The problem is related to the image overlays (for example the default optical all-sky survey) which can be disabled by deselecting the Layers -> Sky Database -> Imagery option. Alternatively, users can download an earlier version of the application. From the Download page, users should select Customize your installation of Google Earth and then select Previous version (7.0).

The Google tutorial page has a tour of its features. For the full list of system requirements, refer to the Google Earth Help documentation.

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Downloading the CSC KML file

CSC data is made available to Google Earth in a KML (Keyhole Markup Language) file, which is a file that can be viewed in the Google Earth geographic browser.

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Google Earth processes KML files as web browsers process HTML and XML files: you can build a KML file with a tag-based structure with names and attributes for specific display purposes in Google Earth, Google Maps, and other applications.

Note: PNG images were used in the CSC KML since this format is supported by the image transparency feature of Google Earth. It is recommended that you download the most recently released version of Google Earth for use with the CSC KML file, version 6.0, since on some platforms, earlier versions of Google Earth do not support PNG.

Viewing CSC data with Sky in Google Earth

If a Google Earth window does not automatically open when you download the CSC KML to your computer, and you have downloaded and installed the full Google Earth application, please do the following:

After downloading the CSC KML to your computer, open the Google Earth application and select "Open" in the File menu to load the CSC KML file to your My Places folder, which is called 'cxo_1.2.kml'. If you are using a recently released version of Google Earth, switch to the Sky view when prompted. If you are not prompted to do so, click the Saturn icon at the top of application and then select 'Sky' to view Chandra images on the night sky.

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Earth view in Google Earth before switching to Sky view

Note: It is not recommended that you run the Google Earth software from a web browser with the Google Earth plug-in, as the CSC KML file is too large to use with this plug-in. In order access CSC data in Google Earth, you will need to disable the plug-in from your browser and invoke Google Earth as a full application downloaded and installed to your computer.

My Places -> Chandra Science Data

Once the CSC KML is loaded and you have entered Sky in Google Earth, you will see a Chandra Science Data folder in the My Places folder on the left side of the application; clicking the title expands the folder tree to reveal two sub-folders called Chandra Source Catalog and Fields of View for ALL Chandra Observations.

My Places -> Chandra Science Data -> Chandra Source Catalog

Within the Chandra Source Catalog folder you will find the Master Sources, Catalog Fields of View, and Images sub-folders. The Master Sources folder contains the list of CSC Release 1.1 master sources that are currently displayed on the screen. Yellow "CXO" placemarks are displayed in association with each master source when the view is zoomed in to approximately 2 degrees. Clicking on a source placemark expands a balloon containing Catalog property information, such as name, position, hardness ratios, source detection flags, and variability indices; you can learn about all the source properties contained in the CSC here. Zooming out to display more of the sky than two degrees will eventually make the placemarks disappear.

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"CXO" placemarks are overlaid on CSC master sources in zoomed-in views;
clicking one displays a list of associated source properties.

The Catalog Fields of View and Images sub-folders contain the same list of numerically ordered Chandra ObsIDs, the former with field-of-view indicators, the latter with corresponding 3-color images based on instrument-specific energy bands. Checking the box next to Catalog Fields of View will produce a visible outline around the images listed in the Images section. Clicking once on an ObsID link brings up an information box for that particular observation, containing the ObsID, OBI, and source name (see images below for an example); clicking twice brings you to the corresponding location on the sky. Searching the sky by source name or location is a quick and easy way to learn if a Chandra observation of a source is included in CSC Release 1.

My Places -> Chandra Science Data -> Fields of View for ALL Chandra Observations

The Fields of View for ALL Chandra Observations folder contains the footprint of every processed Chandra observation on the sky - all observations from the start of the mission through the frequently refreshed "as of" date listed in that folder. The box next to this folder is unchecked by default; selecting it will render the fields of view for all Chandra observations visible using the following color code:

Green = ACIS + no gratings
Yellow = ACIS + gratings
Blue = HRC + no gratings
Cyan = HRC + gratings
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Please note that image data for observations which are not included in CSC Release 1.1 are currently not accessible via Sky in Google Earth, as shown in the images below.

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Left: View of Chandra ACIS ObsID 620 with only the CSC Release 1 field of view shown.
Right: View of ObsID 620 with the Chandra Archive field of view shown in green and the overlapping CSC Release 1 field of view shown in white.
Notice that chip 6 was dropped in CSC Release 1, therefore there is no CSC data image within the green square in the image on the right.

Navigation options:

There are 3 ways to search for Chandra images in Google Earth:

  • Navigation widgets: To move across the night sky in Google Earth, use the North/South/East/West arrow keys in the upper-right corner of the application. To adjust the zoom level, use the the +/- vertical zoom bar, also in the upper-right corner of the application.
  • Mouse: You can also navigate using a mouse, by clicking on the sky and holding down the mouse button while dragging the mouse up, down, left, or right. To adjust the zoom level using a mouse, move the middle scroll button in a back-and-forth motion.
  • Menu: To jump to a specific location on the X-ray sky, you can select a Chandra observation in the My Places->Chandra Science Data->Chandra Source Catalog->Images folder in the menu along the left side of the application, or enter a source name or location in the search box at the top of this menu. Both of these options will place the source or observation selected at the center of the screen; clicking on the observation or source will zoom into the image.

Chandra Image Data

Available images:

  • Field-of-view images for ALL Chandra observations: outline image of the angular area viewed by Chandra for each observation
When the mouse cursor moves over one of the field-of-view line segments, the outline of the region changes color (to red); when it is clicked, information about the ObsID is displayed in a balloon. When the Fields of View for ALL Chandra Observations folder is checked, the field-of-view images are color-coded to differentiate the various types of Chandra observations, as described above in the section Viewing CSC data with Sky in Google Earth; when it is not selected, the Catalog field-of-view images are visible in white.
  • X-ray 3-color images ONLY for CSC Release 1.1 data: ACIS soft-medium-hard and HRC wide CSC energy band color-coded images, provided at various blocking factors of increasing resolution (resolution increases with zoom level)
There are three PNG format images for each ObsID listed in the Images directory, one for each of three zoom levels. At the highest zoom level, the resolution of ACIS images is 1 arcsecond/pixel (blocking factor 1), and the two lower zoom levels correspond to blocking factors 2 and 4; for HRC images, the blocking factors are 2, 5, and 12.
This resolution was chosen to cut the data load time by a factor of ~3. The images at lower zoom levels utilize true color visuals, while the highest blocking factor images are pseudo-color with 256 intensities. This was also done to speed up load time; the difference between the two types of images is barely detectable to the eye.
  • CSC logo image: CSC logo, appears in upper-left corner of application

Everything outside of the CSC PNG images in the Sky background has a value of "NaN" ("Not a Number", i.e. no data), and comes from either the Sloan Digital Sky Survey (SDSS) or the Digitized Sky Survey (DSS). You can learn more about the contents of Sky on the Google Earth Resources page, which links to help documentation and other useful information.

The scroll bar in the middle of the menu on the left of the application can be used to adjust the transparency of the Chandra images, to reveal the corresponding SDSS or DSS images in the Sky background underneath. The Chandra fields of view, images, and logo image can be faded in and out of view by highlighting the corresponding line of text in the menu (e.g. "Fields of View") and then sliding left to fade and right for a brighter image.


Caveats

CSC KML:
* Image intensities are not normalized across the sky; each image has its own normalized intensity.
* CSC source positions are slightly offset from those of the SDSS and DSS in Sky. The shift with respect to the SDSS is barely noticeable, however there is a more significant error in alignment with the DSS.
Applications:
* Within 10 degrees of the poles, the map projection used by Google Earth produces highly distorted images. In these regions, only the Fields of View should be considered reliable.
* There is a Google Earth plug-in available which allows the software to run from a browser, without having to install the full application; this only works in Firefox and Internet Explorer 6/7 on Windows. It is not recommended that you use the Google Earth plug-in for use with the CSC KML, as this will prevent you from gaining full access to CSC data (the CSC KML is too large for use with this plug-in).
* Due to a bug in Google Earth on the Macintosh, spurious scrollbars are occasionally added to balloons such as the information boxes that pop up when the outline of a Field of View is clicked.
* PNG format images were used instead of JPG because the latter is not compatible with the image transparency feature of Google Earth. This places a platform-specific lower limit on the version number of Google Earth which can be used with the CSC KML, because not all versions of the software support PNG format images. It is recommended that you download the most recently released version of Google Earth for use with the CSC KML file, version 6.0.
* If the CSC KML is loaded into Google Earth from a web page - as opposed to from a locally stored file - the "CXO" placemarks overlaid on master sources may not refresh after their initial display, e.g. after panning the sky. This issue only occurs if the file is opened from the URL; if the file is first downloaded to the system running Google Earth (as this document instructs), the placemarks will refresh as expected.
If you would prefer not to download the CSC KML first, you can work around the refresh issue by going into the My Places panel on the left side of the Google Earth application and deselecting and then reselecting the CSC Network Link option under My Places -> Chandra Science Data -> Chandra Source Catalog -> Master Sources.
After reselecting CSC Network Link, panning or zooming should result in placemarks refreshing after the movement stops.
* [New] (20 Feb 2014). There is a problem with Google Earth version 7.1 that obscures the field-of-view boundaries when the display is zoomed out beyond 15 arcmin. The problem is related to the image overlays (for example the default optical all-sky survey) which can be disabled by deselecting the Layers -> Sky Database -> Imagery option. Alternatively, users can download an earlier version of the application. From the Download page, users should select Customize your installation of Google Earth and then select Previous version (7.0).

Last modified: 21 February 2014
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