2WoRLDS Reduction Page
This page will document the steps and intermediate results for the
PISCES narrow-band line data taken with 2WoRLDS.
Live coverage and photometric plots:
Click HERE.
Coverage (as of Oct, 2006)
Fully reduced coverage:
Field Selection
The fields were selected using the full 2MASS PSC within +-10 degrees of
the Galactic plane. A composite J-H, H-K CCD using only high
photometric quality 2MASS stars, overplotted with various 2MASS
sub-catalogs, including dwarfs and giants, Carbon stars, Planetary
Nebulae, Young Stellar Objects, etc., along with known and recently
discovered WN/WC stars, is here. Our
selection criteria are in blue, which is apart from the main body of
plane stars, and follows the slightly steeper than reddening WC track.
Applying this selection in 5 armin bins produces the following maps:
Two areas were selected for the July, 2004, covering approximately 2
square degrees. They are illustrated in the following binned map of the
galactic plane from l=20-120 in magenta, with known WRs in red (WC) and
blue (WN). Lines of contant declination are also indicated.
Runs
So far we've had one run with the new filter set (manufactured April,
2004).
July, 2004
This run was the first for the new WR line filters. They were installed
late June by C Kulesa and D. McCarthy, but the standard H2 line filter
which was intended to serve as our red-side continuum could not be
located, so a broader K continuum filter was used instead. The weather
was surprisingly clear for Kitt Peak in July, and only the last two
nights saw any cloud cover. Night 6 was called at 2 am for increasing
cloud cover. See the observing NOTES for
more info.
The log sheets:
The observing method was switched starting night 2 to a method which
makes 45 frames per filter per tile. See the NOTES for more. The standard 3x3, 384" stepped
tiles look like:
Reduction Steps
Still being expanded... see also Rose
Finn's Pisces Reduction Guide.
A raw Pisces frame:
Pre-photometry
-
Use corquad to remove quadrant-mirrored artifacts (dark comets).
From the corquad README:
This routine corrects PISCES images for the negative "shadows" on
all four quadrants due to bright sources. It actually assumes that
any flux in a pixel has a negative effect on all 4 corresponding
pixels in the 4 quadrants at about a 0.002 level, plus a "shadow"
at a much lower level.
Jackie Monkiewicz has updated corquad with new coupling
parameters, and fitting functions for the most recent Pisces data.
Her version is available here, and
inludes a README. It can be used standalone, or through IRAF. An
example before and after image. Note also that
-
Generate updated bad pixel masks using flat field data (look for
local outliers), including the usable circular field, or use the
ones provided by Rose. The masks will be used for subsequent
image combinations, etc. Essentially, these pixels will be
ignored in all subsequent steps.
-
Make flats in each filter for each night using the 7 pairs of dome
flats made with lamp on and off, i.e. lamp_on-lamp_off, using a
trimmed mean, e.g. with IRAF imcombine,
normalized to the images mean.
-
Flat-field data with appropriate night+filter flat field image.
-
Create sky frames by combining, with an aggressively trimmed mean,
all 45 frames in a single 3x3 tile. Monitor for sky variations.
-
Sky subtract data. In crowded regions, we must be very careful of
the sky computed in situ, to ensure it doesn't retain
signal from stars. If necessary we'll need to adjust the trimming
threshold of the trimmed sky mean.
-
Reject individual frames which suffered from pointing jitter
(e.g. this frame) or bad
relative seeing/focus. Poor global seeing will have to be dealt
with.
-
Combine all 5 frames for each filter, which have been dithered
randomly by 10" typically, into a single coadded image. This can
be accomplished with, e.g., IRAF daofind
together with geomap
and geotran.
Another, perhaps easier option would be gross cross-correllation
of the 5 images to find the maximum, most easily accomplished in
IDL. Shift and add should achieve acceptable image quality for
further photometry.
Photometry and Astrometry
-
Use DAOPHOT,
either through IRAF, or as a stand-alone application, to locate
and compute photometry for the stars in our crowded fields. This
is a complicated package: much of our reduction efforts will be
directed to tuning its operation. Correcting for or at least
identifying saturated or non-linearly exposed stars will be
critical in this step, keeping in mind that stars may saturate in
one filter and not another.
-
Select robust photometry from the DAOPHOT lists. Depending on the
frequency of spurious or compromised detections (e.g. incorrect
dark comment subtraction, saturation bloom from a nearby saturated
star, etc.), we may need to be clever here: e.g. reject all stars
located in a certain region with respect to saturated stars. This
selection will be a function of filter (e.g. different bloom
patterns, like this WRCont2 pattern).
-
Match the positions computed by DAOPHOT to an astrometric
reference using 2MASS PSC stars of good quality, and suitably
clipped in K magnitude. The goal here is to match positions from
one step in the tile to the next, and from one filter to the next
filter. This may be complicated by some distortions in the Pisces
field (2-3 pixels).
-
Compute continuum subtracted line indices and select candidates.
Since the sky transmission is quasi-variable, possibly even on the
timescale of the 3x3 tiles, we may need to make use of fact that
most stars in the frame will have feature-free spectra,
such that deviations from locally averaged line-continuum values
are a robust indicator of broad emission line sources.
Last modified: Thu Aug 9 16:43:22 MST 2007