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WIRCAM Y-band CFHQSIR Data Statistics

Table of contents:

1. Image quality

The following diagrams are built using image quality measurements from all data collected through the first two semesters of QSO operation (10B+11A) for CFHQSIR. Only the validated exposures (all 75 seconds long) were considered. Note that the exposure time is long enough to ensure that the natural seeing is well integrated. The image quality per image (metric: FWHM isophotal) is an average over the four detectors of the mosaic.



Description :
For the entire CFHQSIR and each of the four Wide patches, the histogram gives the number of images obtained in the given image quality bracket. Each bar is 0.05 arcsec. wide.

Discussion :
The mode of the histogram is the same for all five data set: 0.60". No seasonal signature is apparent, e.g. winter field (W2) vs summer field (W4). Because the QSO image quality constraint is 0.80" for this observing program, the histograms are by design truncated at that bracket. Since the CFHQSIR program uses a large chunk of telescope time, these plots indicate that the median image quality at CFHT in the Y-band is 0.62" (see Global Statistics section) at a median airmass of 1.2: this is comparable to the MegaCam z' value of 0.64" (Cuillandre & Racine 2011).

2. Image quality versus airmass



Description :
This diagram gives the average image quality for a given airmass domain:
  • 1.03 = 1.00 to 1.06 (00 to 20 deg. from zenith)
  • 1.11 = 1.06 to 1.15 (20 to 30 deg. from zenith)
  • 1.22 = 1.15 to 1.30 (30 to 40 deg. from zenith)
  • 1.40 = 1.30 to 1.50 (40 to 50 deg. from zenith)
  • 1.77 = 1.50 to 2.00 (50 to 60 deg. from zenith)

Discussion :
Showcasing a behavior similar to MegaCam's z' data, the image quality degrades only slightly at increasing airmasses. The median airmass of the CFHQSIR data set is 1.2, the optimal domain to achieve the best image quality. The slight degradation detected here in the zenith bracket (airmass ~1) is due to thermal imbalances in the dome that create a cheminee effect (this trend should disapear once CFHT gets a vented dome).

3. Sky Level

The following diagrams were built using image quality measurements from all data collected through the first two semesters of QSO operation (10B+11A) for CFHQSIR. Only the validated exposures (all 75 seconds long) were considered.



Description :
For the entire CFHQSIR and each of the four Wide patches, the histogram gives the number of images obtained at the given sky background level. Each bar is 5 ADU/sec wide.

Discussion :
The CFHQSIR QSO constraint for sky background is low to medium. As a consequence these histograms exhibit a fairly large spread. The sky background level will be rather uneven across the survey and since the image quality is also different, the final depth of the observations (300 seconds total integration) will vary greatly over the surveyed area.