Point Spread Function

5.9 Ghost Images

Common ghost images result from internal reflections in the filters and in the field-flatteners. Two
filter ghosts, caused by double (and quadruple) reflection inside the filter, are visible below and to the right of the star in Figure 5.10. The position and brightness of these ghosts varies from filter to filter, typically being most obvious in interference filters. The comatic shape of the ghost is caused by the camera optics being effectively misaligned for the light path followed by the ghost. The relative position of these ghosts does not vary much over the field.

An additional ghost is caused by an internal reflection inside the MgF2 field flattener lens immediately in front of each CCD (Figure 5.11). The field flattener ghost is doughnut shaped (image of OTA pupil) in the WFC, but is smaller and more disk-like on the PC. This ghost contains ~0.15% of the total energy of the star. It is positioned on a line through the CCD center and the bright star; the distance from the ghost to the CCD center is 1.25 to 1.4 times the distance from the bright star to the CCD center. This geometry results from curvature of the field flattener lens.

The large ghost image expected to be caused by reflection off the CCD back to the filter and then back to the CCD is not seen. It was deliberately eliminated in the PC by tilting the CCD slightly.

Figure 5.10: Saturated Stellar Image Showing Filter Ghosts.

Figure 5.11: Saturated Stellar Image Showing Field Flattener Ghost on WF2.