2.i: Making an image

2.i: Making an image

Logged on 07/09/2009 05:07:46 PM

Let's make an image. If you've read any of the PURR logs which (loosely) describe the generation of a the simulated Measurement Sets you might already be familiar with this part.

Click the TDL Exec button again and expand the 'Make an image from this MS' menu.

Imaging is handled by software external to MeqTrees, but invoked by the Browser. This will probably be the lightweight imager (lwimager) from casarest, or the imager from that much-maligned and misunderstood butt of all radio astronomy jokes, AIPS++.

The imaging options are set up via the Browser and then passed to the imager in the background. Once its generated the FITS file it should also automatically invoke the specified viewer.

An example set of options is provided in the data products below.

Once you've set them up just click 'Make a dirty image'.

You'll also notice there is the facility to CLEAN the image (i.e. deconvolve the PSF). Once curiosity of the lwimager is that it doesn't seem to want to clean the DATA column of a Measurement Set, so this feature is out of our reach for now...

What you can see below is the dirty image, basically the central point source convolved with the PSF of the Westerbork Array. If you look closely you should be able to see some radial features, reminiscent of the waves in Saturns rings. This is another by-product of the varying receiver gains, an unwanted artefact which we should be able to remove with a bit of calibration.

The 'histogram' is also a very useful plot. It shows pixel counts as a function of brightness. The story that this one tells is that there are a large number of low brightness pixels and a few of very high brightness. The y-axis scaling is normalised when PURR automatically generates the figures, so the high brightness ones barely register, although you should be able to see the blue line hugging the x-axis.

Data products

calico-view-ms-imaging-options.png

calico-view-ms-imaging-options.png

SSSC1_WSRT.MS.DATA.channel.1ch.img

SSSC1_WSRT.MS.DATA.channel.1ch.fits (header)

0-full.png0-hist-full.png
0-hist-zoom.png
data range:-0.169142,5.01046
size:1x1x1024x1024
mean:0.000327547
sigma:0.0890185
clipping:95%
clip range:-0.169142,0.169758