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
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) |