This page provides information of interest to the observer preparing WIRCam
observations. Like Megacam, WIRCam is operated in Queued Service Observing
mode. Classical observing is not offered to observers. Observers will receive
their WIRCam data after pre-processing (and astrometric and photometric
calibration on a per chip basis) by the CFHT Elixir analysis system, though
they may request raw data if necessary.
The WIRCam detectors are read out using two SDSU controllers (2 chips per
controller). Each HAWAII-2RG array is read using 32 amplifiers. The total
readout overhead was brought down to 4.8 seconds, BUT the overhead charged to
PIs in their phase II proposal is 10sec (bear in mind that the telescope
- unguided, 14sec - guided) are not charged to PIs. This more than compensates
for the charged readout overhead of 10sec).
WIRCam images are stored as multi-extension FITS (MEF) files containing 4
extensions (1 per detector). Each extension may contain a cube of
images (if you use multiple exposures at each telescope position or/and
micro-dithering), or one single image (if you don't). The data are
stored as 16-bit signed integers, giving a minimum size for a WIRCam
file of slightly above 32 MB. We limit the maximum size to 2 GB. Recent
versions of ds9 (>4.?.?) support viewing cubes within a MEF.
The WIRCam pixel scale is 0.3"/pixel, so that individual images are
only Nyquist-sampled for seeings worse than
0.3 * 2 = 0.6" (i.e. half
of the time, as the median CFHT seeing is ~0.65"). To provide well
sampled images under good seeing conditions, WIRCam implements
optional micro-dithering (or micro-stepping) of its images. In this mode
the Image Stabilizer Unit (ISU) is used to offset successive images
by 0.5 pixel on a 2x2 XY pattern. The individual images remain undersampled
(for good seeing), but a composite image with finer sampling (0.15") can
be constructed from the set of micro-stepped images, using data processing
techniques such as simple interlacing,
drizzling , or
optimal Fourier-plane combination. In the
ideal case where the 0.5 pixel offsets are executed exactly, all 3 techniques
converge to simple interlacing. A number of misconceptions on micro-dithering
have in the past appeared in observing proposals, so we will try here to
clarify what it is, and perhaps more importantly what it is not.
Like the more familiar "macro-"dithering used in most optical and
IR observing sequences, micro-dithering tries to evenly spread the
fractional part of the offsets over the area of a pixel, as needed
for good reconstruction (e.g.
Lauer et al. (1999)). By controlling the offsets more accurately,
it achieves that particular goal much faster than feasible with the random
fractional offsets produced by telescope motion: O(20) random offsets
are needed to achieve as uniform a pixel coverage as a 2x2 micro-stepping
Because the offsets are so small, micro-dithering does no provide
any measurement of the sky, so some amount of larger scale dithering
(or telescope nodding) is always needed in addition. On the other hand
micro-dithering is essentially instantaneous (faster than the array
readout, and done in parallel), while larger scale dithering incurs
the significant overhead of a telescope offset. If you are going to
obtain multiple guided exposures at each offset position to reduce
overheads, micro-dithering comes for free and there is then little
reason not to use it.
Micro-dithering requires guiding, as it uses the guiding stars to control
the offset produced by the ISU. Guiding needs to be optimal on more than 1 star
so this is why we limit the use of micro-dithering to wide-band filters
Micro-dithering is not expected to improve images which are already
Nyquist-sampled because the current seeing is bad. It therefore brings
no gain if the seeing is worse than ~0.8". On the other hand it does
not harm those images either, and could help in the (unlikely) event
that the data are actually obtained with significantly better seeing
than requested during Phase-2.
For a given exposure time, micro-dithered images and non-micro-dithered
images have identical limiting magnitudes.
The measured optical distortion of WIRCam is <0.8% (maximal in the
corners of the field) or ~20 pixels. As a consequence, dithered
full-field images need to be be interpolated to a common grid before
stacking whenever they involve offsets larger than ~60 pixels (a 60 pixels
offset gives a 0.5 pixel misalignment in the corners).
The WIRCam filter collection currently includes 4 broad-band filters (Y, J, H,
Ks) and 9 narrow-band filters (Low OH- 1, Low OH- 2, CH4 On, CH4 Off, W, H2 v=1-0 S(1), K Continuum, Bracket Gamma, CO).
NOTE: the WIRCam filter wheel has space for only 8
filters, and the set of installed filters will be fixed for a given semester.
The filters which are offered for the next semester will be selected after the
national Time Allocation Committees have evaluated the proposals, and conflicts
if any will be arbitrated by the SAC.
Consult the WIRCam filter table for quantitative information on
the wavelengths and bandwidths of the filters.
Detector quantum efficiency
The quoted quantum efficiencies by Rockwell Scientific in J and K are between
70 and 85%. From on-sky tests, it appears that the four science chips have
the same efficiency to within 10%. More can be learned on the throughput here.