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SPHERE Quality Control:
Known Problems |
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Problems can be related to
Here we report on problems related to the instrument alone.
Start of operations is on April 01 2015.
From 2015-04-01 to 2015-07: IRDIS optical distortion calibrations using the pinhole mask are often blurred resulting in a lower contrast between the pinhole peaks and the background. This can lead to a failure of the pipeline recipe (V 0.15.0) . In some cases are more tolerant threshold parameter helps. The cause of the low image quality is not yet fully understood. Turbulence inside the instrument and a non-optimal shape of the deformable mirror are discussed. (WH 2015-06-30) IRDIS optical distortion raw frame (a bad frame), showing the part of the detector illuminated by a blurred and contrastless pinhole image. The corresponding good raw frame is given here .
Blurred pinhole mask image :
IFS optical distortion raw frame (a bad frame), showing the slitlets illuminated by a blurred and contrastless pinhole image. The corresponding good raw frame is given here . Persistence: From 2015-04-01 until 2017-05, IFS dark frames with DIT=1.650726 (and other of higher DIT) show persistence, in particular the first low-DIT dark exposure of the night is affected, others acquired later during the day-time calibration sequence are not affected. The pattern looks like an imprint of the IFU flat. The persistence is monitored in a dedicated HC plot. It is recommented to use IFS background calibrations for the mentioned period. The following image shows an affected DIT=1.650726 dark, the unaffected DARK with the same setup has been subtracted. Arc-shaped optical ghost: Since 2016-10-01 all IFS detector flats of the YJH prism show a large prominent optical ghost in the upper part of the imaging flat (~5% more counts). The ifs detector flat pipeline recipe generates four products: the differential flat field (DFF), the large scale flat field, the pre-amplifier map and the bad pixel map (see here for examples), of which only the large scale flat and the pre-amplier flat are contaminated by the ghost but these products are not used by the pipeline. The DFF product used in further steps of the calibration cascade does not contain the optical ghost. After an alignment on 2016-10-11 the ghost vanished (WH 2016-10-11) This figure shows the ratio between the large scale product from 2016-09-08 and that from 2016-10-02. The prominent pattern has canceled out, the remaining structure (fringe-like ringes and lines) is below 0.5% and the bright ghost shows an intensity of about ~5% of the flat field intensity. Imaging flat format move: Since 2016-09-15 un-illuminated areas in the lower left and right corner of the IFS detector flat (all setups) continuously increase, meaning the vignetted area becomes larger. The fixed pattern (prominent features) does not move as shown in the product ratio image above. After an alignmernt on 2016-10-11 the format was adjusted (WH 2016-10-11)
dark contamination shutter problem The Fig. shows the median counts of the V-band imaging flats, resolved per plane of the raw frame data cube. The first four planes are not illiminated with a bias level of ~1000 ADU. Only planes 6 to 10 are fully illuminated. Blue points are from channel 1 (CAM1, Callas) and the red points are from channel 2 (CAM2, Bartoli). Since the number of unilluminated planes varies it is not known how the data reduction recipe behaves in this situation. Master flats and modemeff frames are only processed for reasons to monitor the behaviour via the qc_plane_rms QC parameter, but products are discarded thereafter again. The corresponding health check plots are taken off the main page and are available here (WH 2016-04-14): flats imaging mode The problem is solved in 2016-06.
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