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LIRIS Data Readout Modes

The rmode command is used to set the number of multiple non-destructive reads (MNDR) to Nr and the number of coaverages to Nav.

Nr is the number of times that the array is read non-destructively prior and after an integration period. The array data from the Nr pre- and post-integration reads, respectively, are averaged independently. Nr is an integer between 1 and 16 (default is 1; i.e. double correlated sampling). Nav is the number of accumulated exposures that are averaged and written as a single output file.

SYS> rmode liris mndr <Nr> <Nav>

We recommend setting Nr = 1 for imaging and Nr = 4 for spectroscopy, since in the latter case the readout noise becomes dominant over the sky background noise. For imaging, the exposures taken get read out just once, and so read-out is faster, but minimal read-noise suppression is achieved (but this is usually enough in imaging mode, due to its inherently high sky background). i.e.:

SYS> rmode liris mndr 1 1  (for imaging)

SYS> rmode liris mndr 4 1  (for spectroscopy)

IMPORTANT: the readout noise value given in the image FITS headers is fixed, and regularly updated, but not derived from each specific image. Observers are therefore recommended to check the readout noise of their images.

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Contact:  (LIRIS Instrument Specialist)
Last modified: 19 January 2014