This primitive obtains the readnoise in electrons for the current frame. It first attempts to find a value for the calibration system. Where there is no value, it tries a header for the value, and if that’s not defined, the primitive assigns a default.

The readnoise is returned through an argument.

• This primitive is suitable for LCO optical instruments.

• The read noise comes from the readnoise calibration, or failing that the header RDNOISE.