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Instrument and data characteristics
The table below lists the magnitudes measured to correspond to 1 ADU/s for the given filters, on the given dates. Measurements will change as the numerous optical surfaces deteriorate and are cleaned.
Throughput table for INGRID with OSCA
With OSCA the field of view on INGRID is flipped as compared to NAOMI+INGRID alone. This means in the case of NAOMI+OSCA+INGRID in GRACE that North is DOWN and East is to the right. It has been verified that the plate scale on INGRID stays the same with and without OSCA.
As coronography increases the dynamic range compared to non-coronographic observations, the data obtained are also more sensitive to instrumental features/artifacts. These can lead to false detections and/or a significant decrease in sensitivity. The main features seen in OSCA data obtained during commissioning are "blobs" along the x and y axes (see image below). These are caused by diffraction on the edges of the segments of the deformable mirror (DM) of NAOMI. They are not a feature of OSCA but with OSCA these effects become more obvious. Since these regions are not useful for analysis, the importance of obtaining images with different sky PAs (rotational dithers) must be emphasised. The rotation of the field will not change the orientation of the DM pattern.
The above figure shows a bright (H ~ 4 mag) behind the 0.65" OSCA mask, scaled to exaggerate the faint features. The blobby structure (DM pattern) along the x and y axes is indicated. The faint lines seen at 45 degrees direction are caused by the secondary support spider. Note that in this image the OSCA Lyot stop was not installed. The Lyot stop vanes will remove most of the diffraction spikes by the secondary spider seen here. Note that while changing the sky PA rotates all features internal to NAOMI, the diffraction pattern from the secondary support spider will stay in the same position (although it does move rotate relative to the N-S axis as the sky rotates). Note also the quadrant boundaries of the INGRID array. Rotational dithers will ensure that these areas are recoverd as well.
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