SUMMARY OF PROPERTIES OF WFCAM PHASE-I COMMISSIONING DATA --------------------------------------------------------- Mike Irwin & Jim Lewis 4th February 2005 All of the WFCAM commissioning data covering the period 18th October to 30th November inclusive has been shipped to Cambridge on LTO-I tape and converted to Multi-Extension FITS (MEF) format. This ~2 Tbytes of data resides on RAID disks and has been used as the basis for a series of tests. A brief summary of which is given here. More detailed reports on the commissioning data are available on the CASU WFCAM web site. http://www.ast.cam.ac.uk/~wfcam Darks and Reset Anomaly ----------------------- Tests have been done to assess the stability of the darks/reset anomaly properties. A steep reset anomaly "exponential" ramp is seen in all detectors, save #2, at the readout electronic edges. This ramp is stable throughout the night for a given set of exposure parameters (exposure time, readout mode, number of coadds). For longer exposures (>~ 5 seconds) the ramp is independent of exposure time. The ramp is also stable over a time period of at least a few days. This implies that using master darks accrued over a similar time span can be used to reduce the data, rather than relying on single dark exposures taken contemporaneously with the object data, which could be contaminated with cosmic rays or other transient features. Using dark frames taken with multiple coadds to remove the reset anomaly from similarly observed object frames for some unknown reason results in badly corrected backgrounds. Whereas a CDS dark frame with the correct exposure time and 1 coadd models the reset anomaly ramp much better. A low level rippling effect is seen in the background of all dark, flat and object frames. The structure and scale of the ripples change for each exposure but are at the level of about +/-5 ADUs. The cause of this is unknown, both hardware and software solutions are being pursued. Sky Flats and Fringing ---------------------- Preliminary results from tests on flats show them to be stable over a period of a few nights. This indicates that master twilight or dark sky flats can be used to correct data and removes the pressure to reobserve twilight flats every night in every filter band. Using stacked high count level twilight flats to flatfield stacked dark sky flats fails to reveal any measureable fringing (ie. it is <<1% of the background)) in all of the passbands so far analysed (Y,J,H,K). Linearity --------- All of the detectors show measurable non-linearity (eg. ~1% 5K counts, ~2% 10k to ~4% at 20k counts) over their entire dynamic range. In CDS mode this can be corrected post-acquisition using an algorithm developed by CASU (see http://www.ast.cam.ac.uk/vdfs/docs/). Although the overall dominant determinant here is the general detector characteristics, differences of order +/-10% of the overall non-linearity for a detector can be seen on an individual channel basis. One of the detectors (#4) shows non-linearity of the opposite sign to the others. The good news is there are no measurable pixel-by-pixel differences within a channel. This non-linearity has been characterised using sequences of dome flats and will be corrected as one of the first steps in the pipeline. Astrometry ---------- The 2MASS point source catalog (PSC) has been successfully used in two relatively crowded Galactic Plane regions to categorise the astrometric distortion with very promising results. The proposed WCS projection gives an accurate description of the radial distortion albeit, at 2/3 of the level expected. This is good news for offset restrictions within jitter sequences. The first pass astrometric residuals over the entire field are less than 100mas and the density of suitable 2MASS PSC stars even in high Galactic fields is sufficient to give reliable WCS solutions. The on-sky astrometric solutions have been used to accurately measure the relative positions and orientiations of the detectors to micron precision. Photometry ---------- The 2MASS PSC has also been used to calibrate several MSB sequences of science data, including first pass colour equations, and also demonstrates that frame-by-frame extinction monitoring at the few % level is achievable. The calibration has been used to measure the zero-points in several passbands and shows that WFCAM performance is at least as good as UFTI in K and somewhat superior in the bluer passbands. Other quality control measures such as seeing, image ellipticity (trailing), sky brightness are also giving excellent results. Science ------- A full tile of data giving contiguous coverage of 0.8 sq deg of the Northern Lockman Hole regions in YJHK passbands has been processed to examine the science potential of the phase-I commissioning data. The assorted colour-magnitude diagrams and two-colour diagrams produced show the enormous potential of WFCAM as a survey instrument. Examples can be found in the commissioning reports at the above mentioned web address.