# Putting the Jelly in the Space Donut

The second "A"in ATA stands for array, meaning that this instrument is made of many smalldishes. Although each dish is as big as a house, they are small compared to thecomplete telescope: ten city blocks on a side. The bigger the telescope, themore detail you see in the images. By breaking up our collecting area intohundreds of small pieces, we capture detail as if we had a telescope the sizeof a subdivision for the price of a single apartment building.

But detailisn't everything. Consider the following example, based on a well-known imageby Leonardoda Vinci. Starting with the painting (Fig. 1, left), we deconstruct theMona Lisa into two parts: one containing all the details of the image, and onecontaining the coarse structures. The upper right image contains the details,or as we say in radio astronomy, the high spatial frequencies of the image.Most of the beauty of the painting is found here. In case you've never noticed,this image brings out the nature behind Lisa's head, with trees and a flowingcreek.

But thedetail image is gray and blah compared to the original painting. What gives thepainting its punch are the low spatial frequencies, represented at bottomright. This image makes you reach for your glasses, and by itself isuninteresting or even ugly. It is only when you add the two right-hand imagestogether that we see the full complexity and brilliance of the originalpainting.

Gettingback to the ATA [Allen TelescopeArray], when we run the telescope in array mode, we capture all the highspatial frequencies of the night sky, much like the detail image of the MonaLisa in Figure 1. This is done by comparing radio signals two-by-two for eachpair of dishes in the array. With 350 dishes, there are 61,075 distinct antennapairs (calculation left as an exercise for the reader). But pairwise comparisonleaves out a small but critical bit of information in the low spatial frequencycomponent of the image.

We canrecover the low frequency information by pairing each antenna with itself,which we call single dish mode. Single dish mode does not take advantage of thefull size of the array; it is as if there were only one dish. Because one dishis small by the standards of radio astronomy, the image we obtain is blurry andlacks detail.

As part ofthe ATA commissioning, we recently performed single dish observations of the Milky Way galaxy, which is ourhome. Figure 2 shows a partial map of the hydrogen emission over the entirecelestial sphere. The Milky Way has a disk shape, and because of our positioninside of the disk, it appears as a broad stripe encircling the Earth. Theregions that appear black in the image are directions we have not yet measured.The inset shows a Mercator projection of a world map, which is the sameprojection used in the observation.

Hydrogengas is not visible to the naked eye, but it represents the majority of the massin our galaxy (and for that matter, the entire universe). The bulk of thehydrogen is present in galactic disk (horizontal stripe), but smoky wisps ofhydrogen are seen both above and below the disk. The disk is narrower towardthe center of the galaxy and more diffuse when we look behind, or away fromgalactic center. Notice the small blemish on the lower left. This is where the sunblocked our view of the galaxy in the background, so it appears as a dark spot.

Single-dishobservations like these are important for bringing out the full beauty andscience in radio images. When combined with highly detailed interferometer observations,they put the punch in the picture, or the jelly in the donut. Expectmasterpieces soon.

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