EUROPEAN SOUTHERN OBSERVATORY
Imaging with EMMI can be done with both the RILD and the BIMG modes, respectively for red (lambda > 400 nm) and blue (lambda < 500 nm) observations. The following figures illustrate the light paths.
For the Red CCD, to avoid that the main object falls in the inter-chips gap, the mosaic has been slightly shifted in the dewar so the target will fall in the area of the master chip (right-hand side of the mosaic).
Other effects to keep in mind for the Red CCD are translation, rotation and astrometry.
The slave ship is translated with respect to the master (note: the master chip is Zeus):
x(master) - x(slave) = 27.0 pixels = 4.5''
y(master) - y (slave) = -2.6 pixels = 0.43''This effect is corrected by the software, adjusting the number of pre-scan column on each chip. Joining the 4 sub-images will produce an image geometrically correct within 1 pixel in the central region (y ~ 2000). The geometric error goes up to 2 pixels at the top and the bottom of the gap.
Delta X = 0.90 pixels over 4000 pixels
Delta Y = 0.35 pixels over 4000 pixels
These values correspond to a rotation of 0.22 mrad. This is a physical rotation of the CCD, so no correction is possible.
An estimate of the astrometric transformation has been determined for both chips:
Master chip (right hand side one): d (alpha) = cost + 0.046274x + 0.000144y + 0.000002xy + 0.000024x^2 - 0.000014y^2 d (delta) = cost - 0.000029x - 0.046179y - 0.000044xy + 0.000039x^2 - 0.000019y^2
Slave chip (left hand side one):
d (alpha) = cost + 0.046489x + 0.000139y + 0.000000xy - 0.000018x^2 - 0.000005y^2
d (delta) = cost - 0.000050x - 0.046299y - 0.000009xy + 0.000004x^2 - 0.000003y^2