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The PF-QHY camera (centre-right) mounted on-axis at the prime focus of the WHT during pre-commissioning tests of the new corrector for WEAVE. The second QHY camera (seen off-axis at left) is not offered for imaging observations.
During the integration of WEAVE at the WHT, an opportunity has arisen to offer the community some service-mode imaging time with a camera. This PF-QHY camera is a test imaging camera mounted behind the new prime-focus corrector for WEAVE. It is available for service-mode science observations during a short period in 2021. The PF-QHY has been characterised and tested by ING, but users should understand that it is not a fully-commissioned common-user instrument.
The QHY camera (model QHY6000L) is based on a back-illuminated scientific CMOS detector, with 9576×6388 3.8-μm pixels, giving a field of view of 10.7'×7.1', at a scale of 0.067"/pixel.
At sky position angle PA = 0, North is left and East is down, when the image is displayed in the usual way in DS9, thus the DS9 needs rotation +270 deg counter-clockwise to match the normal sky orientation.
The filter wheel holds up to five 50-mm circular filters. Initially, the filters will be Sloan u, Sloan g, Sloan r, Sloan i, Sloan z, the broad-band ('clear') luminance filter L-1, and possibly Astronomik B and Astronomik R. Enquiries about the filters should be addressed to the instrument specialist.
The QHY600 quantum efficiency is close to 90% around 550 nm. The net throughput of the PF-QHY camera is probably not very different from that of the old PFIP prime-focus imager with the EEV detector, so a rough idea of the expected SNR can be obtained by running the ING's SIGNAL exposure time calculator for the previous WHT prime focus camera (PFIP) whose pixel size and characteristics are similar to PF-QHY binned 4x4.
The PF-QHY readout noise is 11 electrons for binning 4x4 and 6 electrons for binning 2x2.
The detector can be windowed, and binning options 4x4 and 2x2 are available (binning 1x1 and 3x3 are not offered), with binning 2x2 being the default in command line mode. For most science aims observed in average or good seeing (0.5-1.0"), we recommend binning 4x4 (0.267 "/pixel). Requests for 2x2 binning (0.134 "/pixel) should be justified in the service proposal. The camera saturates around 60K but the linearity goes to 40K.
The following plots describe the linearity of the PF-QHY camera, based on recent ING WFC2 dome flat tests corrected by bias, using gain 30, binning 4x4 and a stable lamp. PF-QHY is linear below 55 KADU where the camera shows deviations up to 1%. The QHYCCD company linearity plot made for gain=0 looks also linear up to 70-80 Ke (units not directly comparable with our ING plot).
Based on internal calibration lamps using no filter, the PF-QHY camera shows some fringing patterns (ThArCr lamp plot and He lamp plot), but the QHT lamp does not show any fringing.
Darks are strongly recommended to be used for image reduction. Dark images must be taken at night using the blank filter, with the dome closed, no lights, and petals closed. Darks taken during the day with dome closed and lights off show lots of counts and look similar with flats fields - samples: 10s dark taken during day with Sloan i in front of the camera, compared with 20s dark taken at night facing R filter. For longer exposures (above 5 minutes), the dark could generate saturated pixels. To correct them, the PF-QHY deputy Richard Ashley created a dark mask for binning 4x4 where hot pixels are marked with a '1' other regular pixels with a '0'. This mask was generated from a set of 3x600s exposures, flagging all pixels that showed 10xsigma above the median ADU pixel value in all three exposures.
The full-frame readout time when binning 4x4 is 9 sec. If a multiple exposure is requested, the readout time for exposures after the first is 5 sec. For 2x2 binning, the times are slightly larger: 10 sec for the first exposure, and 6 sec for subsequent exposures. Windowing does not (currently) save much readout time - e.g. if the detector is windowed 500x500, the readout time with 4x4 binning reduces by only 1 sec. In future, it's possible that faster readout modes will be commissioned, but this depends on how long the camera is available, and on how much interest in faster readout is expressed by potential observers.
Autoguiding is provided by a standard ING autoguider head mounted at radius 0.9 deg in the field of view. Because of differential atmospheric refaction, the large radius has implications for the accuracy of autoguiding over long periods, more details will be provided shortly.
Non-sidereal tracking is also available (but not with autoguiding).
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