Summary
The Elixir project has achieved an important milestone this semester:
the collection of software is ready for regular operations during
CFH12K runs. The Elixir system has been used for the three QSO runs,
and the single non-QSO run, executed as of May 2001 to produce
high-quality detrend data, astrometric and photometric calibrations,
and to evaluate the nightly sky transparency. In addition, the Elixir
software and the growing collection of CFH12K data products in our
databases have been used to identify several areas where the telescope
\& optics are in need of some improvement to enable top-quality
scientific data.
Background
The Elixir project has a wide-ranging set of goals, all related to the
evaluation and manipulation of the science data obtained with the
wide-field imaging system. At present, Elixir is limited to the
CFH12K imager, but in the future will be expanded to include at a
minimum MegaCam, and possibly the wide-field infrared imagers.
{$\bullet$ \em Monitoring} - The Real-Time component of Elixir
provides a mechanism to monitor a variety of parameters about the
images as they are acquired: seeing, sky flux, dome temperatures, etc.
A variety of representations of the data collected are made available
to the observer as the night progresses.
{$\bullet$ \em Detrend Data Creation} - The Elixir project is
responsible for producing high-quality detrend data appropriate to
each CFH12K image acquired during a run. The collection of bias,
dark, and flat-field images obtained during a run are combined into
master detrend images as the run progresses. The Elixir databases
allow the detrend-creation software to pre-select an appropriate set
of input images, rejecting those with gross errors. A user interface
allows the Elixir team to evaluate the residuals for each input frame
and fine-tune the input selection, dividing a run into subsections as
needed if the detrend data varies during the run. The detrend images
created by the Elixir system are registered in a database which makes
it a trivial task to select the appropriate detrend data for a given
science frame.
{$\bullet$ \em Astrometric Calibration} - The Elixir system provides
automatic astrometric calibration of every science image distributed
by the Data Archiving and Distribution System (DADS) system. The
calibration is performed relative to the USNO and HST Guide Star
reference catalogs. The resulting astrometric calibration coefficients
are included in the image headers of the distributed images.
{$\bullet$ \em Photometric Calibration} - The Elixir system performs a
complete analysis of photometric standards obtained during a CFH12K
run. These measurements allows us to refine our system parameters
(zero points, color terms, and airmass terms) and to evaluate the
stability and transparency of the atmosphere on a nightly basis. The
resulting photometric calibrations are stored in a database, and
included in the headers of science images distributed by the DADS
system.
{$\bullet$ \em Science Image Detrending} - CFH12K data which is
distributed by the DADS system is passed to an Elixir component for
automatic detrending. All images have bias, dark, flat, and mask
corrections applied, and the I \& Z filter images are fringe
corrected, while R images are `skyring' corrected. These last
corrections are additive and should not affect the photometric
accuracy of the images. They do, however, improve the background
flatness and ease the process of image combination.
{\bf Real-Time Operations}
In semester 2001A, the Elixir system has reached a level of
functionality at which all of the primary goals discussed above have
been performed in an operational mode. In previous semesters, while
most of the various tasks had been possible, the connection between
various software components did not exist and the processing required
substantial intervention by the Elixir team. By mid-May, all of the
steps needed to go from raw CFH12K images to the detrended, calibrated
images ready for distribution have been defined and implemented. The
result is that these steps can now be easily performed by the Service
Observer, for example, without detailed knowledge of the underlying
software.
The collection of real-time data displayed by the Elixir system has
been somewhat refined. Data available to the observer include the
seeing from each image, plots of the seeing as a function of time, and
the skyflux as a function of time (both plots are helpful in making
assessments of the appropriate QSO condition). In addition, we
provide an improved analysis of the focus images, displaying parabolic
fits to the focus trends in each of four separate chips. We have
discovered that it is important to use several detectors spaced across
the mosaic to minimize sensitivity to variations in image quality
across the field (see below). This semester saw the implementation of
a real-time display tool and the refinement of the tasks which create
these real-time data products.
\begin{figure}
\resizebox{16cm}{!}{\includegraphics{sac3.pics/mkdetrend.ps}}
\caption{ \label{mkdetrend} Sample portion of the detrend-creation
report. This html-tool lets the Elixir team select the appropriate
input detrend images. }
\end{figure}
The Elixir detrend-creation system has been completed this semester.
This collection of software has made the task of flat-field image
creation significantly more sophisticated, and has improved the
collection of the flat-field data. The Elixir system is used to
generate a test master flat-field (per filter) early on in a run. As
new flat-field images are acquired their residuals are determined and
can be examined easily with our evaluation software (Figure
\ref{mkdetrend}). The evaluation shows the QSO team the quality of
the input images as well as how many counts have been integrated and
the typical scatter per pixel induced by the resulting flat-field.
This information can guide the QSO team in deciding which flats to
take and whether the filters can be changed during a run. This
detailed analysis makes the resulting flats of such high quality that
we can distinguish contaminating effects from very small amounts of
cirrus clouds. Even thin cirrus can keep the sky from being truly
flat during flat-field acquisition. The identification of flat-field
images contaminated with light cirrus correlates very well with the
Observer's reports of the sky conditions at twilight.
The photometric zeropoint analysis is now regularly achieving a high
level of accuracy. We can demonstrate the stability of the atmosphere
during a photometric night using the standard star data from that
night. During the first run, when essentially every night was
photometric, the zero point measurements show a 1.5\% scatter per
frame in all filters. During later runs, the photometric nights show
this same level of accuracy, but non-photometric nights betray
themselves with substantially larger scatter. This level of accuracy
is currently limited by residual scattered light problems, as
discussed below.
At this point, the Elixir system includes virtually all of the
components needed for regular operations. The major gap to be
addressed is in the automatic creation and application of fringe and
skyring frames. The necessary software is expected to be completed by
late June. There are a few areas where improvements in the software
functionality will smooth the operation of the Elixir software, but
which are not required to achieve the basic goals of the project.
{\bf Flat-fielding and Scattered Light}
We have discovered that scattered light from structures on the bottom
of the primary mirror covers is contaminating flat-field images taken
with CFH12K. The contamination is at a relatively low level, and is
difficult to detect without careful analysis. Nonetheless, the
contamination introduces errors to photometry which may be as large as
a few percent. The systematic nature of the errors means that
repeated measurements of standard stars does not serve to beat down
the errors to acceptable levels. The high level of detail in
the Elixir analysis of the detrend data and the large number of
photometric standard star data obtained during the QSO runs this
semester made it possible to track down this problem.
We will present a more complete report on this problem in the CFH12K
bulletin, but will provide summary details here. The problem first
came to light when the Elixir team performed the photometric analysis
of the standard star images obtained in the first QSO run. High
quality detrend frames had been produced and applied to the standard
star frames. The quality of these calibrations were such that we
expected 1\% or better photometric accuracy, especially when coupled
with the consistent reports of photometric weather. As a result, when
the standard stars were first analyzed, we were surprised to discover
photometric errors as large as 5\%. After substantial investigation,
we were able to show that there was a high level of consistency in
measurements made of the same stars with different analysis methods,
different versions of the flat-field images (twilight vs night-time
superflats), and different nights. The photometric errors observed
were a strong function of the position on the detector. This implied
that the flat-field was somehow inaccurate. Given the care taken to
ensure the best possible conditions for the flat-field acquisition,
and the consistency between twilight and superflats, we came to the
realization that the flat illumination was not actually flat, ie,
there was a source of scattered light which varied across the field.
\begin{figure}
\resizebox{16cm}{!}{\includegraphics{sac3.pics/primary.ps}}
\caption{ \label{primary} The primary mirror and open mirror cover
petals as seen from the prime focus cage.
}
\end{figure}
We identified the major source of scattered light to be a set of
Teflon strips on the bottom of the primary mirror cover petals. When
the cover is open, these strips are visible from Prime Focus (see
Figure \ref{primary}). Furthermore, they are vignetted at the edges
of the mosaic, causing the necessary variation across the field. We
performed a set of measurements to demonstrate the presence of the
scattered light: we obtained a series of dome flats in which strips of
black cloth were used to cover the underside of the mirror cover
petals, and a second set with the petals exposed. The differences in
these flats show a pattern of scattered light appropriate to the
observed spatial trends in the photometric errors.
We have generated scattered light-corrected flat-field images using
the scattered light frame described above. The application of these
corrected flats to the standard star images shows a marked improvement
in the consistency of the photometric calibration. Before the
correction was applied, it was difficult to determine a useful color
term for any of the filters. After the correction is applied, the
color terms was easy to determine. There are still some significant
outliers among the standard stars, so that the typical scatter per
frame ranges from 2.5\% - 3.0\%, but the bulk of the stars are now
much more tightly constrained than before. The remaining outliers may
be due to remaining errors in the flat-field, especially towards the
corners where the corrections are the largest. Alternatively, some of
the outliers may be from Landolt standards which have long-term
variability not detected by Landolt (1992).
\begin{figure}
\resizebox{16cm}{!}{\includegraphics{sac3.pics/position-dm.ps}}
\caption{ \label{scatter} Standard star residuals as a function of
mosaic position. Top: uncorrected R-band data. Bottom: R-band data
after correction for scattered light.
}
\end{figure}
This scattered light problem has contaminated all CFH12K data obtained
to date. Observers who have obtained CFH12K image in the past and who
require accurate photometry should consider this issue carefully. The
scattered-light images we have used to correct the flats obtained this
past semester should be applicable to data obtained in previous
semesters as well. We will make the scattered light frames available
to the community, as well as the recipe used to generate the corrected
flat-field images. Alternatively, a correction may be applied to
stellar photometry to remove the observed trend for data which has
already been analyzed. As of 15 May, 2001, we have removed the Teflon
strips which appear to cause the problem so that future CFH12K runs
will not be affected.
{\bf Mosaic-wide Image Quality}
\begin{figure}
\resizebox{16cm}{!}{\includegraphics{sac3.pics/570925o.ps}}
\caption{ \label{570925o} Image quality as a function of mosaic
position. Each cross represents the average image quality in a 500
pixel box. The histogram shows the observed FWHM across this image.
}
\end{figure}
In the detailed photometric and astrometric analysis of each image,
the Elixir system records the observed seeing parameters for each
object independently (FHWM$_X$, FHWM$_Y$, angle). We have used this
collection of data to determine the variation of the image quality
across the mosaic. We create a grid of points on the mosaic, in which
the average of these three statistics are used to generate a
representative star in that grid location. Figure \ref{570925o} shows a typical
image taken in poor seeing. The top part of the diagram shows the
range of observed FWHM values for this image, while the bottom part
shows the average image quality across the field. When the seeing is
as poor as in this diagram ($>0.8''$), the image quality is quite
consistent everywhere in the image.
\begin{figure}
\resizebox{16cm}{!}{\includegraphics{sac3.pics/570914o.ps}}
\caption{ \label{570914o} Image quality as a function of mosaic
position for an image with good seeing.
}
\end{figure}
Figure \ref{570914o} shows the alternative situation. Part of the time, when the
image quality is better than 0.8'', a clear variation of the image
quality across the field becomes apparent. We have been working to
understand the possible causes of such a large variation. One
possibility, a misalignment of one of the optical elements in the
wide-field corrector, appears to have been ruled out. Another
possibility is that there is a mechanical problem which is letting the
camera tilt slightly with respect to the focal plane. This may occur
during a focus operation if there is slack in the focus drive chain.
We are investigating the observations of this effect further and will
attempt to address it when we have a clear understanding of the cause.
{\bf Skyprobe}
Perhaps the most important new tool from the Elixir team is Skyprobe.
This device addresses the difficulty of determining the instantaneous
sky transparency, and provides a way to monitor the conditions without
excessive standard star observations. Skyprobe is a small (50mm)
camera lens mounted on a small (500x700) SBIG-2 CCD. The system is
mounted on the top of the CFH12K cage, and driven by an iOpener
lap-top-sized computer running Linux. During the night, an image is
obtained by this system every minute. The field-of-view is large,
5degrees x 7degrees, and aligned with the CFH12K FOV.
Skyprobe was first installed in the third QSO run, April 12. For this
run, the images were displayed in real-time in a web-browser for the
observer, and also shown in animation. It is possible to see the
presence of cirrus in the images, either by their obscuration of the
stars or by their reflected light if the Moon is sufficiently bright.
This system has already been helpful in letting the observers
determine if there are clouds or not while they observe.
Since that run, we have implemented a quantitative analysis of the
Skyprobe images. Using the Elixir software, it is now possible to
measure the stellar photometry of the stars in the Skyprobe image,
determine their astrometry, and match the stars with stars from the
Tycho database. This collection of stellar photometry covers the
entire sky in the appropriate magnitude range, and provides accurate
photometry for approximately B and V filters. We can therefore
determine a zeropoint for each Skyprobe image relative to the Tycho
photometry. A comparison of this photometry with the nominal
zeropoint provides an accurate, time-resolved measurement of the sky
transparency during the entire night.
There are still some details of the photometry to work out, but the
results so far are very encouraging. Figure \ref{skyprobe} shows the
zero-point residuals for one night during the third QSO run, during
which the observers and the Elixir CFH12K standard star analysis
stated that the night was photometric. The period from 8h to 13h has
a scatter of just over 1\%, and the period between 6h and 8h has a
scatter of 2.2\%.
\begin{figure}
\resizebox{16cm}{!}{\includegraphics{sac3.pics/skyprobe.ps}}
\caption{ \label{skyprobe} Skyprobe zeropoint offsets for images taken
2001/4/23 UT.
}
\end{figure}
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