About the Image Viewer

Select a target and then move and/or click the mouse in the navigation image window and see a high resolution view of the region under the mouse in the window above. This image viewer only works with browsers which support javascript image swapping such as Netscape v3+, Internet Explorer v4+.

The control panel in the lower left frame allows you to choose the target field, the update mode, the navigation image resolution, and the help text.

The default update mode is to update the high resolution image on mouse clicks only. This is best for slow connections. With a fast connection you can chose to have the high resolution image update continuously, or you can have the update take place only after the mouse has remained still for about 1 second. Image sizes vary somewhat with content, but are on the order of 50 kbytes per 600 x 600 high resolution image.

Celestial coordinates (J2000) of the field center are shown under the high resolution image.

The default navigation image size is 600 x 600, but you can choose a higher resolution of 1200 x 1200 in which you can scroll around. The high resolution image size is fixed at 600 x 600 pixels. The default image size works well with a 1000 x 1260 monitor if the browser is expanded to fill the screen.

The viewer uses a JavaScript 'client side image map'. The image map is a grid of N x N cells (with N typically about 30) underlying the navigation image, and the full high resolution image data is stored as a set of N x N square image sections each of size 200 x 200 pixels stored in jpeg format. When the mouse is clicked in (or moved across according to update mode) a grid cell, the corresponding set of 3 x 3 high resolution image segments are swapped into a 3 x 3 table of images in the right hand display panel. The image swapping approach is convenient since the high resolution image data is only uploaded on demand. This may be painfully slow over a low bandwidth connection, but the image segments should be cached by the browser, so if you revisit a location the image should refresh very quickly.

Nick Kaiser, Dec 1998