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ESO - A number of OmegaCAM filters show vignetting in their central regions. What are the causes for this and what are the dimensions of these features?
 
 

Data Reduction Frequently Asked Questions

A number of OmegaCAM filters show vignetting in their central regions. What are the causes for this and what are the dimensions of these features?

A number of OmegaCAM filters are not monolithic, but consist of four segments (each covering 8 detectors) that have a central cross-shaped support structure. This support creates a vignetting effect that will be visible in 20 detectors which is significantly larger than the gaps present between adjacent CCD's. The affected filters are:
    B_JOHN
    V_JOHN
    H_ALPHA
    NB_659 NB_852
and the effect looks like this (for the H_ALPHA filter):

When using these filters, one needs to take the vignetting into account when chosing a dither pattern. A conservative accounting for the total size of the vignetted area can be summarized as:

Filter North/South Vignetting East/West Vignetting
B_JOHN 1405 pixels (300 arcsec) 1405 pixels (300 arcsec)
V_JOHN 1430 pixels (306 arcsec) 1410 pixels (302 arcsec)
H_ALPHA 1418 pixels (305 arcsec) 1407 pixels (302 arcsec)
NB_659 1418 pixels (305 arcsec) 1407 pixels (302 arcsec)