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What is a Baseline?

A baseline is the distance between one pair of dishes in a radiotelescope array. The number of baselines equals the number of pairs of dishes. So an array with three dishes has three baselines. An array with 6 dishes has 15 baselines. A 10 dish array has 45 baselines while a 12 dish array has 66 baselines.

Baselines: 6 dishes; 12 dishes

JPEG Image (30K)

Baselines In Interferometry

Optical telescopes produce a image by focusing the light collected by the objective lens through an eyepiece. The image is formed a short distance beyond. Radiotelescopes, on the other hand, possess neither objective lenses nor eyepieces. Instead, radiation is collected in a receiving dish, then focused onto a sensing device. Image formation normally requires intensive data processing.

An array comprising of several dishes can emulate a much larger, single-dish telescope. By combining the incoming signals, astronomers can generate high quality images. This technique is called radiosynthesis interferometry and is illustrated in the following movie.

Correlation of Two Signals
Correlation Movie button
QuickTime (1.7 MB); MPEG (943K); Sound (1.3 MB); Thumbnail (21K); Text

As its name implies, the technique relies the phenomenon of interference between the incoming radiowaves due to their slightly different arrival times at each of the array's dishes. The rich information contained in the patterns of interference must be extracted with the aid of powerful computers.

Baselines and Spatial Resolution

Interferometry relies heavily upon the employment of different baselines. First, the signals from each pair of dishes making up each baseline are correlated, yielding what is called a visibility. All the visibilities are then combined to produce an image. Generally, more baselines translate into more visibilities and superior images.

But the utility of an array depends on more than its sheer numbers of baselines. That's in part because each baseline has a corresponding spatial resolution. Long baselines result in high spatial resolutions while short baselines yield low spatial resolutions.

Although it is generally preferable to use a large number of baselines, it is better to have fewer, variable baselines than many of the same length. After all, each baseline length provides a unique spatial resolution that, in turn, determines the level of detail revealed by that baseline. Consequently a variety of baselines permit imaging at different levels of detail.

For observational purposes, the dishes in the BIMA array are set in only one of three configurations along a T shaped track. Each configuration allows for different spatial resolutions, according to its baseline lengths. Thus the BIMA array can be used flexibly to observe both large and small scale structures in the same object.

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NCSA. Last modified 11/12/95