IIWI version 1.0 has been released. MORE WILL BE ADDED SOON, THE FOLLOWING
APPLIES FOR THE BETA VERSION OF IIWI
The current pipeline goes through the following steps:
Known issues with the current version:
- Subtracts the bias from each line using reference pixels
- Subtracts the dark
- Divides by the dome flat field
- Masks the bad pixels
- Builds and subtracts the sky background
- Subtracts the common noise pattern (crosstalk)
- Provides a rough ~15 arcsec astrometry
- Matches the 2MASS catalog to measure the absorption and photometry
- No non-linearity correction (of up to 5% for 2005B and 2006A).
- No DC correction of the horizontal bands extending on each sides from the
guide box (does not affect all guided exposures).
- Crosstalk should be removed as the first step. Current method still yields
residues of subtraction. An overall improvement of the crosstalk subtraction is
- No illumination correction of the flat field to account for stray-light or
uneven photometry across the field of view.
It is worth detailing a little more the step of sky subtraction. A
relatively standard method is used. Processed images taken close in time and at
different sky positions are scaled to the same median level and each pixel is
medianed, excluding the current image. The exact number of images, of different
DP positions and the time window used are variable and can be set manually. A
minimum of 4 dithered exposures are used within a 30 minute time window. If
these constraints can not be met, the image is currently discarded. Usually,
about 5-9 images are used.
A mask is applied prior to the medianing. The current implementation of the
masking is a quick and dirty one. All pixels above a 4-sigma threshold are
masked prior to the medianing. It has been noticed on stacked images that sky
subtraction residues exist due to faint sources remaining under that threshold.
A note about flat fielding. We currently use dome flats for our
processing. Altough, twilight flats have been obtained regularly almost every
night. Differences exist between dome and twilight flats and a detailed
investigation to determine which flat is better is underway.
A note about dark subtraction. We currently acquire sequences of darks
at a few exposure times and use the exposure time closest to the actual
exposure time for data processing.
General quality of the preprocessed images. The photometric accuracy
of the current processing is probably close to 5%. The main systematic effect
should be coming from the uncorrected non-linearity. On deep stacks, the quick
and dirty sky subtraction sets the limiting magnitude. A two-step sky
subtraction process has now been put in place at Terapix to better build and
subtract the sky. It uses the fully sky-subtracted images provided by CFHT to
build a first-pass stack. Then it uses this deep stack and uses the a second
set of CFHT preprocessed images (a set without sky subtraction) to build more
accurate masks to build and subtract the sky.
The most important artifacts affecting final stacks are:
- 1) Crosstalk caused by a bright star which produces a repetitive pattern on
all other amplifiers;
- 2) Vertical crosstalk spreading from the guide box vertically in both
directions.It remains on stacks because our guide star selection algorithm
always picks the brightest available star for guiding.
Coming improvements for version 1.0 of the pipeline:
- Improved crosstalk subtraction
- Non-linearity correction