Off-field nearby bright stars immunity
Scattered light from nearby bright stars was a serious issue on CFH12K despite all the efforts to improve the internal baffling of the original CFHT prime focus and wide-field corrector. MegaPrime has been designed taking note of that important constraint and the results are very convincing: nearby off-field bright stars are not an issue anymore.

To run this crucial test, we have placed Sirius at a variety of positions close to the mosaic to test the ability of MegaPrime to reject scattered light from nearby bright source. Sirius was placed a several positions North and South of the mosaic center, ranging from 0.6 degrees to 4.0 degrees (see table) from the center of the mosaic. All exposures were 30 seconds with the r' filter. In 30 seconds, Sirius should deposit roughly 4.5 x 10^12 electrons in a circle 2 arcsec diameter. The table shows the measured median sky background on the various images. Only the 0.6 degree offset images (in any direction), in which Sirius is nearly on the mosaic, show any significant gradient.

PositionMedian (ADU / pixel)Image ID
North 0.6 140.689 675867o.fits
North 1.0 126.432 675868o.fits
North 2.0 122.592 675869o.fits
North 4.0 124.552 675870o.fits
South 0.6 235.002 675871o.fits
South 1.0 164.96 675872o.fits
South 2.0 147.934 675873o.fits
South 4.0 131.337 675874o.fits
East 0.6 322.102 675875o.fits

The example image from Figure 1 shows the result for Sirius placed 0.6 degrees East of the mosaic (0.1 degree off the edge of the mosaic). The blue contour represents 600 counts per pixel and the red represents 1200 counts per pixel. This image is scaled with [white = 50 counts per pixel] and [black = 1300 counts per pixel]. The plot of Figure 2 shows a cut across this image in the X direction giving counts per second per pixel versus the approximate pixel distance from Sirius.

Figure 1. Sirius 0.1 degree off the edge, to the East.

Figure 2. Cut along the X axis from the Figure 1 image.

The following inverted images are scaled with [white = 50 counts per pixel] and [black = 500 counts per pixel]. Note that the low-level squarish structures near the center are the result of internal LED light injection in the camera beam as these frames were taken during the first engineering phases.

North 0.6
North 1.0
North 2.0
North 4.0
South 0.6
South 1.0
South 2.0
South 4.0
East 0.6