MOSAIC updates

[oczoske]

• mIFU bundles have 168 fibres each. This is a regular grid of 169 fibres with one fibre missing on purpose. We currently do not know which fibre is missing. The previous version had 452 fibres based on an old, possibly illustrative, figure.
• Ruben Sanchez-Janssen proposes introducing slight variations in the wavelength solution and/or line-spread function of the sky spectrum with respect to the source spectrum. Rough magnitudes for this variations should be available in the instrument requirements. The purpose is to alert users to the uncertainties in the background subtraction due to the necessarily imperfect data reduction. As a first step, we will describe in the notebook how the user can introduce such variations themselves. Automatic inclusion in Scopesim may follow later.
• The PSF is currently an analytic Gaussian, but it will be better represented by a (double?) Lorentzian. Jay Stephen willl provide parameters. We willl need a new effect in Scopesim, and provide parameters in irdb.