jsFITS

FITS is the Flexible Image Transport System and is widely used in astronomy for holding observation data. There are many FITS I/O libraries available in a variety of languages. This is an attempt to build a FITS library for Javascript to allow FITS files to be displayed in a modern web browser. Given that FITS files have no compression they are quite large so I imagine this library will be more useful when run locally. By displaying the image using in the browser, it will be very easy to save a FITS image as a standard graphics file.

Dependencies

For this library to work it has two dependencies:

• binaryajax.js -- this does the XMLHttpRequest and the bit extraction. It is created by Jacob Seidelin and is only about 6.2 kB;
• excanvas.js -- this is used to allow canvas support on Internet Explorer (41.6 kB).

Limitations

Due to web browser cross-domain security issues, any FITS file you want to read must be hosted on the same domain as the code.

Usage

You need to include the appropriate Javascript files:

<script src="binaryajax.js"></script>
<script src="excanvas.js"></script>
<script src="fits.js"></script>

Following those you can define your FITS object:

var fits = new FITS();

Next we define what happens on load:

fits.bind("load",function(){

	// Display some values
	document.getElementById('bitpix').innerHTML = this.header.BITPIX;
	document.getElementById('depth').innerHTML = this.depth;
	document.getElementById('z').value = 0;

	this.draw("FITSimage")	// Draw the image to the element with id=FITSimage
})

We can also bind some other events (which are chainable) e.g.:

fits.bind("click",function(e){
	e.y = this.height - e.y
	var value =this.image[e.y*this.width+e.x];
	document.getElementById('status').innerHTML ='click=('+ e.x+','+e.y+')='+value;
}).bind("mousemove",function(e){
	e.y = this.height - e.y
	var value =this.image[e.y*this.width+e.x];
	document.getElementById('status').innerHTML ='move=('+ e.x+','+e.y+')='+value;
})

Finally, load the FITS file. We've already defined the load event above:

fits.load("WFPC2u5780205r_c0fx.fits");

FITS files

The example makes use of several FITS files. You can get these at:

• M51 (taken with Faulkes Telescope North operated by LCOGT),
• M108 (taken with Faulkes Telescope North operated by LCOGT).
• NGC 2011 (taken with Faulkes Telescope South operated by LCOGT)
• NGC 2020 (taken with Faulkes Telescope South operated by LCOGT)
• WFPC II 800 x 800 x 4 primary array data cube containing the 4 CCD images (trimmed to 200 x 200 and taken from http://fits.gsfc.nasa.gov/fits_samples.html)

Author

Stuart Lowe works for the Las Cumbres Observatory Global Telescope. LCOGT is a private operating foundation, building a global network of telescopes for professional research and citizen investigations.

Some improvements have been suggested by anonymous contributors.