Image Viewer V0.6


The image viewer provides a graphical means for interacting with NOAO Science archive imaging data. The entire field may be displayed, or one may zoom and pan on any portion of the field. Functions are provided for extracting science-grade image cutouts.

The image viewer represents an experiment in what is possible in terms of interactive image display using standard Web browsers to access Web services provided by a remote archive server. Web technology is evolving rapidly and the image viewer may not work properly, or at all, with some browsers. It is known to work with Netscape (v4) and Mozilla browsers on Unix-based platforms (e.g., Solaris, Linux, FreeBSD), and with Netscape 4 and Internet Explorer 5.5+ on Windows platforms. Others, including lesser-known or older browsers are not compatible. Better standards for Web browsers are coming however, and this situation should improve in the future.


Viewer Controls

Main Image Window Displays a region of the survey field at the current viewport center and scale.
Pan Window Displays the entire field at a greatly reduced scale. Click anywhere in the pan window to pan the main image window to that point.
Control Panel The control panel provides a manual Reset button, Zoom In/Out buttons, a Digital Panner controlled by the Pan Step tool, and a Scale tool. Image selections can be made by manually entering image coordinates into the text fields provided. Entry into the WCS boxes are planned for the future.
  • Reset Button: Reset the image and any selections made on the image.
  • Zoom In/Out: Zoom in or out at fixed repeatable amounts.
  • Digital Panner: Click on one of the eight points of the compass to pan the image in the direction indicated. The amount the image moves is determined by Pan Step, which specifies the percentage of the visible field shifted in each step. Panning using the digital panner has the advantage that the steps are precisely repeatable. The browser caches the images, allowing a speedy redisplay of previously viewed images.
  • Scale tool: The scale is the fractional scale of the displayed image relative to the full resolution (1/1) image. Changing the scale manually zooms the image. Due to time and disk space considerations, the scales of the image and fits files are different.
  • Image Selection: Enter coordinates into the Upper-Left (UL) and Bottom-Right (BR) corner text boxes to select a region of interest. These boxes also display the coordinates of the regions chosen with the mouse. X-Y Coordinates in the boxes are the actual FITS coordinates, which differs from the viewable graphic. The Reset button in this context keeps the position of the image, but deletes the selected region of interest.
Image Click Functions The Image Click Functions are represented by the following set of buttons. Each button determines what happens when a point is clicked in the Main Image Window. The zoom and pan buttons take the point clicked as the center of the image. The select function chooses the center of the selected region of interest as the new field center. Clicking in the image when the cutout mode is active generates a request to the cutout script. The cutout script produces an image specified by the coordinates shown in the coordinate boxes in the Control Panel.
  • Help! Click this button to get this help page.
  • Click this button to initialize the zoom out function, then click on the desired portion of the image to zoom.
  • Click this button to initialize the zoom in function, then click on the desired portion of the image to zoom.
  • Click this button to initialize the pan function, then click in the image to pan around the image.
  • Click this button to initialize the select function, then make the desired selection by clicking on the image, or entering the appropriate (x,y) coordinates in the text boxes which will be provided.

      The select function accepts the initial mouse click on the image, as the Upper Left (UL) corner of the selection, and displays the coordinates in the coordinate boxes in the Control Panel window. The second click finishes the selection by filling the Bottom Right (BR) corner coordinate boxes, and reloading the image with the selection overlayed on the image. The radio buttons to the left of the X's indicate which corner can be moved. Simply choose the corner you would like to move by selecting the appropriate radio button, then click the desired location on the image.

  • Click this button to initialize the cutout function, which begins downloading the cutout selection created by the "Select Region" tool. Note: this button will not work if you have not made a selection.
  • Click this button to add the desired selection to the selection cart, which provides a method of downloading several images, including cutouts, all at once.
  • Click this button to update the current image in the selection cart. If the image does not currently have a selection, the shopping cart will be updated to reflect the change to a cutout selection. If the image does have a cutout selection, the selection as shown in the cart will be change to the new selection.
Real-Time WCS Tangent Plane Display Moving the mouse in either the Main Image Window or the Pan Window causes the Right Ascension and Declination and the (x,y) position of the mouse to be displayed in real time in the status line area of the browser window. Exactly where this is depends upon the browser type and may differ from what is shown above. The FITS file prefix is also shown.


The image viewer should work with any recent-vintage Web browser - provided the viewer has been "ported" to that particular browser. Browsers differ in the functionality provided and the way the functionality is packaged, sometimes drastically. Display may be different for each browser.

The technology used in the client-side image viewer is mainly Javascript-based, using frames for the different regions of the viewer. The server-side Web services are provided by a PC server running FreeBSD, Apache, and PHP. The Web services are implemented as PHP scripts calling various standard libraries or external modules. IRAF is used to generate the image cutouts and to render FITS files for display. The images displayed by the viewer (both the main image window and the pan window) are not simple graphics images but rather image documents, including image properties, methods (e.g., for the real-time WCS display), and references to the graphical elements. The getImage Web service dynamically generates the imageDoc each time the image is refreshed.