Distortion in the field of ALFOSC with CCD#8
ALFOSC went through a change of optical camera when
CCD#8 came in
use. The objective of the optics change was to improve the PSF over
the field of ALFOSC. With this new camera the image distortion is
In Dec 2012 we mapped the distortion using the 2.00mm pitched pinhole grid,
made on the Copenhaguen CNC machine.
This is a dome-light image obtained with the grid mask in the wheel:
ALvl130009.obj (IRAF STARFIND results)
When fitting a model grid, with parameters
Pitch, the pixel distance between the measure pinholes,
the grid pitch is fitted as 77.079 pixels corresponding to 2.00 mm on the grid mask.
X0,Y0, a translation offset to account for mask-mounting inaccuracy,
Rotation, a rotational offset to account for mask-mounting inaccuracy,
From the residual distances wrt. the fitted grid, a distortion map can be produced:
the largest vector distance is 2.53 pixel,
the median vector distance is 0.36 pixel (or 0.068 arcsec on the sky).
Note that the largest residual vectors are all placed in the bottom corners
of the ALFOSC field.