We continue to test the Mosaic filter adaptors to use narrow band (and other) filters, where in the center we place the filter of interest, and above the outside detectors there are cut outs to allow guiding the telescope. The cutouts are covered with clear glass to ensure they are parfocal with the science target area.
The dilemma is now that the clear glass lets so much more light pass than a narrow band filter, and scattered light can make its way to the central detector array. The image below shows a flat field simultaneously using the small H-alpha and the large ODI z'-band filter. Why two filters? By combining a z' and an H alpha filter, the central area under the narrow band filter should not receive any direct light. The outer detectors with clear glass in the filter module should see the equivalent of a flat field taken in the z' band.
|Flat field simultaneously using a large ODI z' band filter and a small Mosaic H alpha filter.|
|Zoom into the central area of the flat field.|