V.E.1 Potential Detections

IRAS Explanatory Supplement
V. Data Reduction
E. Overview of Small Extended Source Data Processing
E. 1 Potential Detections

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To locate potential small extended sources the data stream of each detector was first successively compressed by adding consecutive pairs of data samples. Designating the original data stream of each detector to be of level 0, new data streams of higher levels were generated according to the rule that the jth data sample in the kth level data stream was given by,

X(j, k) = X(j, k-1) + X(j+1, k-1)


Potential detections in any data stream were found by applying a narrow-bandpass, digital filter at each point in the data stream. This filter consisted of an eight-point, zero-sum, square-wave which subtracted the first and last two data points from the middle four (Eq. (V.C.1)). When the filter passed over a source it gave rise to a characteristic peak, and thus picked out structures whose width spanned approximately four sample points. If we denote the spatial sampling frequency of the level 0 data stream by S (i.e. S is the number of data samples per arc min in the data stream of level 0), then the approximate width of an object (in arcmin) detected by the square-wave filtering the kth level data stream is given by,

W(k) = (4/S) x (2k)


The spatial sampling frequency, S, was approximately 4 in the 12 µm and 25 µm wavelength bands, 2 in the 60 µm wavelength band and 1 in the 100 µm wavelength band. Since it was not intended to detect structures greater than 8' in extent, the 12 um and 25 µm detections were retained from levels 1, 2 and 3, corresponding to characteristic square-widths of 2', 4', and 8'. At 60 µm the level 1 and 2 data streams were searched for sources 4' and 8' in size while at 100 µm only level 1 detections were kept for small extended sources.

A peak in the kth level data stream which was picked out by the square-wave filter was accepted as a potential detection if its amplitude was greater than 3 times the local noise appropriate for that data stream. The amplitude of a square-wave peak was defined as

A(k) = E/(2k)


where E is the maximum value attained above zero in the square-wave filtered data stream. The noise was estimated in the same way as described for point sources.

Since a source typically produced detections in more than just a single data stream for a given detector, each candidate detection from a level k data stream was compared with all candidates from the level (k + 1) data stream of the same detector that were within a distance of five level k samples. In such a comparison the detection with the largest square-wave peak was chosen as the best representation of the source, and other detections were rejected. To reject sources larger than 8', a comparison was also made with the amplitude on a scale of 16'. If, as a result of this comparison, the accepted detection had a characteristic size of 16' (corresponding to a level 4 detection at 12 µm and 25 µm, a level 3 detection at 60 µm, and a level 2 detection at 100 µm), then this source was rejected. Similarly, since point sources were also to be excluded a comparison of each extended source detection was made with any level 0 detections (point source) in the neighborhood. If the level 1 detection was found to have a square-wave amplitude less than 0.75 times the level 0 amplitude, then no extended source was identified. The value 0.75 proved to be optimal for the discrimination between point-like and extended sources and was evaluated after processing data from the Large Magellanic Cloud, where many small extended sources could be clearly seen.

Chapter Contents | Introduction | Authors | References
Table of Contents | Index | Previous Section | Next Section