From: Kyle Dawson (kdawson@lbl.gov)
Date: Fri Feb 15 2008 - 19:12:02 PST
Hello All,
After some discussion with the GTO team, we decided to re-run the image
processing one more time. These new data should be consistent with the
published data and analysis techniques (i.e. Jee et al, 2007). Below is
a summary of the reduction:
Feb 15, 2008 by Nao Suzuki (LBNL)
1) Summary
We post a new stacked image, v98, which is drizzled by lanczos3 kernel
following the procedure of the GTO team. This is the same reduction
used for analysis of the red-sequence scatter in high z clusters and the
same reduction used by James Jee in his weak lensing analyses. In
addition, James has generated a set of tools that can be used to create
a pixel-by-pixel PSF across the full ACS FOV for images created with the
new multidrizzle parameters.
As a summary, now we have the followings stacked image posted on the web
which can be used for different scientific purposes.
Ben Koester of University of Chicago has a program to generate the PSFs
from the v98 data. Please contact him if such a PSF model is required
for your science objectives. The model has previously been applied to
F775W data for weak lensing measurements (Jee et al).
Ben Koester : bkoester@oddjob.uchicago.edu
Advantage and disadvantage of the choice of kernels are discussed in
the following papers.
v98 : 0.05"/pix sampling with lanczos3 kernel, pixfrac=1.0
M. J. Jee et al. : astroph/0710.5560
v46 : 0.03"/pix sampling with gaussian kernel, pixfrac=0.8
J. D. Rhodes et al. : 2007 ApJS, 172, 203
v45 : 0.05"/pix sampling with default STScI multidrizzle parameters
STScI hand book
Stacked image are posted here:
http://supernova.lbl.gov:8083/HSTclusterSN/(cityname)/stacked/README
http://supernova.lbl.gov:8083/HSTclusterSN/(cityname)/stacked/v45/
http://supernova.lbl.gov:8083/HSTclusterSN/(cityname)/stacked/v46/
http://supernova.lbl.gov:8083/HSTclusterSN/(cityname)/stacked/v98/
WCS fixed and sky subtracted images can be found in the following
directory so that you can reproduce the same results.
http://supernova.lbl.gov:8083/HSTclusterSN/(cityname)/wcsfixed_flts/v46/
http://supernova.lbl.gov:8083/HSTclusterSN/(cityname)/wcsfixed_flts/v46/drz_script_v98.py
Note: Basic process such as alightment, bias jump correction,
sky subtracion are the same which you can find reports in
earlier versions. (python 2.5)
2) What's new
a) Why lanczos kernel?
b) Zeropoint offset adjustment
c) Issues of distortion correction
d) Issues of photometric zeropoint
a) Why lanczos kernel?
M.J. Jee et al. reports that lanczos3 kernel produces a sharp PSF
without introducing correlated noise, and he made PSF models publically
available. Once you measure star PSFs from an image, we can deduce PSF
at any given pixel pisition using M.J. Jee's PCA-PSF. We expect this
would be useful for weaklensing study or precise photometry of galaxies.
However, lanczos3 is known to introduce ringing and cosmic ray rejection
needs extra masking than others, and it destroys the core of the bright
stars. We exchanged info with STScI people and believe we have
minimized these side-effects. It may be useful to cross-compare
photometry on several bright stars across several multidrizzle kernels
as a sanity check.
See a figure on the same image but different kernel:
http://supernova.lbl.gov/nsuzuki/acs/lanczos/
The following is a quote of e-mail exhange with STScI help desk.
>One thing I have found from drizzling with the lanczos
>kernel as opposed to the square kernel (in the final step)
>is that it creates a narrower PSF, but it also introduces
>a lot of negative pixels. I have also heard that it
>can cause a 'ringing' structure around cosmic rays.
b) Zeropoint offset adjustment
ACS zeropoint has moved because of the CCD temperature has changed
on July 4th 2006.
http://www.stsci.edu/hst/acs/analysis/zeropoints
Please note the number found on the web above sometimes changes.
The followins is a snap shot of what we found in Dec 2007.
Before July 4th 2006 Temperature=-77C
AB Vega
F775W 25.678 25.291 <= This is what we use
F850LP 24.867 24.347 <=
After July 4th 2006 Temperature=-80C
AB Vega
F775W 25.665 25.277
F850LP 24.842 24.323
Since the majority of our 2005-06 observation was taken before
July 4th 2006, we have rescaled the images to match 'before 7/4/06'
zeropoint for a few clusters with data after 7/4/06. The -77C
zeropoints should therefore be correct for all of the data.
Please use Temperature=-77C zeropoint for your photometry.
c) Issues of distortion correcion
We have reported the image offsets of order of 0.2-0.3pix in the
past, although we didn't know the reason. We have investigated
47Tuc which is a dense stellar field for calibration and found
it is a function of rotation. The structure of distortion correction
error shows a skewness and similar to velocity aberration. I expect
slight adjustment of velocity aberration can potentially fix the
problem. However, this adjustment has not been applied to the data and
we do not plan to make this
correction in the near future.
http://supernova.lbl.gov/nsuzuki/acs/distortion/
d) Issues of photometric zeropoint
COSMOS projects report there exists a photometric offset between
ACS and ground based data such as SDSS.
See Capak et al. 2007 ApJS, 171, 99
http://adsabs.harvard.edu/abs/2007ApJS..172...99C
We began to investigate if this is happening to our filters,
F775W and F850LP. If anybody has a good photometric field between
HST ACS or NICMOS which can be tied to ground based data, please
let us know.
For questions or requests, please contact:
Nao Suzuki
nsuzuki@lbl.gov
(510)486-5218
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