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difx:difx2.0specselect [2009/10/29 09:09]
adamdeller created
difx:difx2.0specselect [2009/10/29 09:23]
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-====== Spectral selection and averaging in DiFX2.0 ======+====== Spectral selection and spectral ​averaging in DiFX2.0 ====== 
 +DiFX2.0 has introduced a lot more flexibility in the way you can treat spectral points inside the correlator. ​ In DiFX1.5, whatever came out of the FFT was fixed - you cross-multiplied it and wrote it out to disk.  Any fiddling was deferred to FITS-building time.  DiFX2.0 lets you both select and average channels, or both, inside the correlator. 
 +===== Spectral selection and matching ===== 
 +DiFX2.0 lets you define addition frequency bands (IFs, if you like) which fall within the recorded frequency bands. ​ I call these "​zoom"​ bands. ​ These zoom bands are obtained by positioning a pointer to somewhere within a recorded band - allowing a subset of the spectral points to be correlated, and the rest thrown away.  The following image illustrates the idea:
 {{spectral_select.png?​800}} {{spectral_select.png?​800}}
-{{spectral_average.png?800}}+This is obviously useful if the recorded band is much wider than a spectral window of interest - something that frequently comes up for narrow masers, for example. ​ But it gets better. ​ Say you have a 16 MHz band, which encompasses two narrow regions (~50 kHz) of maser emission 10 MHz apart. You want 1 kHz spectral resolution. ​ You could just correlate the lot with 16,000 spectral points. But then you are sending around and writing to disk data which is mostly garbage. ​ (This is what was done in DiFX1.5). ​ With the zoom bands in DiFX2.0, however, you can place one on each area of interest and throw the rest of the junk away. 
 +Another use of spectral selection is correlating mismatched bands. ​ Say one antenna recorded a single 16 MHz band and another antenna covered the same region of spectrum with two 8 MHz bands. ​ By defining two 8 MHz "​zoom"​ bands for the antenna which recorded the 16 MHz band, you can cross-correlate the two antennas, getting two 8MHz bands out as a result. ​ This couldn'​t be done in any way in DiFX1.5. 
 +===== Spectral averaging ===== 
 +Most continuum experiments want a very small number of spectral points per subband. ​ Getting this directly using a very small FFT, though, is rather inefficient. ​ So in DiFX1.5, continuum experiments would typically be correlated with 128 spectral points, and difx2fits would average down to the required number. ​ This is a bit wasteful of disk space and correlator interconnect,​ however. ​ So in DiFX2.0, in-correlator averaging has been implemented. As described in the [[difx2.0multipc|multiple phase centre]] documentation,​ this feature is crucial for the efficient implementation of multiple phase centres. ​ An image below illustrates how it works. 
difx/difx2.0specselect.txt · Last modified: 2015/10/21 10:08 (external edit)