Author: Glenn Gombert Date: 970720 Revision: #0 970720 Key Words: photometry, techniques, computation
Introduction
On June second Emmanuel Bertin released a new version of his “SExtractor”
(or Source Extractor) program which includes a number of new features which
should prove quite useful in the reduction of TASS images to Star-Lists.
This technical note details some of these new features and how they might
be used to improve TASS data reduction.
Shown below are some of the new features (from the Readme file) of the June, 1997 release of SExtractor:
(1). Enhanced support for WCS coordinates. This means that you can get
the alpha and delta for objects detected on FITS images which comply with
the WCS standard (see:
http:// www.cv.nrao.edu/fits/documents/wcs/wcs.html)
(2). 2D positional error estimate through ERRA_xxx, ERRB_xxx and ERRTHETA_xxx. Note: these estimates are exact for unconvolved, white-noised images only.
(3). Linear photometric parameters (FLUX_xxx and FLUXERR_xxx in addition to MAG_xxx and MAGER_xxx).
(4). Better outputs: units and vectors in FITS, (limited) backward compatibility with the old V1.0 output format (CATALOG_TYPE FITS_1.0), new detailed ASCII output mode (CATALOG_TYPE ASCII_HEAD).
(5). Handling of gigantic images (up to 2e9x2e9 pixels on 64bit machines).
(6). Online decompression of compressed DeNIS images.
(7). New "vignet" vector output: for instance VIGNET(8,5) in the .param file will write in the output catalog a small 8x5 pixels array around each detection center.
(8). There is now a new "DETECT_IMAGE" parameter which requires a string as argument. If it set to "SAME", it means that the input image will be used for both detection and measurement, as in the old version. If you give it an mage filename (provided that the dimensions of this image are the same as those of the input image), it will be used as a kind of ``detection template'': all sources will be detected and deblended on this image, and will serve as reference for the measurements done on the regular image.
(9). The old THRESHOLD parameter has been now split in two: DETECT_THRESH and ANALYSIS_THRESH. DETECT_THRESH is the threshold of detection, just as before. You may express it in units of sigma of the background noise, or in surface brightness units, associated to a magnitude Zero-Point of the detection frame.
Ex: "DETECT_THRESH 3.0" will set
the threshold at 3.0 sigma,
"DETECT_THRESH 22.0,25.4" will set the
threshold at 2 mag.arcse-for a detection-image whose mag-ZP is 25.4 mag.
(note: the "PIXEL_SCALE" should have been set to the right value). In this
way, the THRESHOLD_TYPE parameter has been eliminated. ANALYSIS_THRESH
is a threshold in the measurement-frame (it can be the same as the detection-frame).
It is used only for estimators that need accurate information about the
object profile in the measurement frame star/galaxy separation and FWHM.
(10). Similarly, the 3 Kron parameters PHOTOM_K... have been replaced by a common PHOT_AUTOPARAMS which requires 2 arguments, equivalent resp. to the former PHOTOM_KPAR and PHOTOM_KMINSIG. PHOTOM_KSIG is now set definitely to 6.0 in the code. This is because the new MAG_AUTO apertures are much more robust to crowding effects than before.
(11). Also, for aesthetic reasons, PHOTOM_APERTURE has become PHOT_APERTURES. There is an S at ApertureS, because the software can now photometer each source simultaneously through up to 4 apertures (see default.param). In default.param, MAG_APER is equivalent t MAG_APER1, and MAGERR_APER is equivalent to MAGERR_APER1.
(12). The MAG_AUTO magnitudes are computed in a slightly different way
than before, leading to a gain of about 20% in photometric precision in
most extragalactic fields, especially in clusters. Obviously MAG_BEST benefits
from this.
Application to TASS Image Reduction:
Of the new features described above several of them have greatly enhanced the use of “Sextractor” for the reduction of TASS images.
* The enhanced support of World Coordinate System (WCS) parameters corrected some problems that I ran into when reducing of some data taken here in Dayton for the June AAS meeting. The astrometric errors (in RA) that I found were 2-3 arcseconds, the errors in Declination were typically larger than this (10 arcseconds or more). This let to only four matched (between V&I band data) for the data that was posted to the TASS Home page for the night of April 8/9. When I ran the same raw data files through the new version of “Sextractor” I found that there was 471 matches of the same data. This indicates that the problems that I had encountered before in the previous version of Sextractor mapping WCS parameters to RA and DeC had been fixed in the new release.
* The new photomertic functions of SExtractor should be of significant help improving the photometric reduction of TASS Images. One new function (#4 above) allows multiple fixed apertures to be calculated at any one time. Also the “MAG_BEST” photometric estimate has been enhanced to provide a much better estimate of the instrumental stellar magnitude than the previous version.
Shown below is a plot of fixed aperture size vs magnitude for a star
that was detected in a recent TASS image. The “MAG_BEST” estimate of this
star (from SExtractor) is 10.8516 magnitude showing how the flux from the
star being measured is recovered with SExtractor’s “adaptive-aperture”
magnitude algorithm. This is even more flux recovered that using a large
(17 pixel) aperture size shown in the plot below:
The automatic aperture routine was inspired by Kron’s “first moment” algorithm (Kron R.G. 1980 ApJS 43,305). A description (from the Sextractor Manual) of how this algorithm works is shown below:
* The ability to save the area around each star that is detected in an image should prove useful when storing TASS images in any type of a database. This allows a circular area around each star to be written out to a FITS format table so that this data is not lost if raw images are not saved to disk.
Arne suggested this option in one of his posts
recently concerning TASS Database structures:
“As far as keeping a pixel archive: George Turner and
I (plus others) have maintained since the beginning that robotic telescopes
should extract their data and then throw away the images. You gain
factors of 100-1000x compression. Test your extraction software carefully,and
then trust it. The few times you need something less than 1sigma will be
extremely rare. You can always save postage stamps around those objects
of interest, or objects that do not appear in the master star list, if
you are concerned about losing information.”
An example of the star-list that is generate (with this option enabled is shown blow) for several objects that are detected in the image "g0483967.fts":
x-pos y-pos
ra
dec
mag flags
109.52 7.49 -1.052273e+000
1.998669e+002 14.0595 000 -65.2861
-46.4941 -49.7139 -24.9443
-100.184 21.542 28.3467
115.139 -4.07813 -59.3057
46.3691 349.186 239.991
34.7861 37.5723 -44.8057
294.023 314.842 121.649
-61.5518 -83.9805 28.8613
59.6924 -39.4873 15.3252
x-pos y-pos
ra
dec
mag flags
474.93 9.35
3.434709e-001 1.998740e+002 11.6518
000 244.931 618.757
538.593 362.438 82.2939
542.822 1596.65 1726.49
896.339 374.196 836.724
2260.56 3610.4 2102.25
652.107 558.637 1554.47
2824.32 2546.17 836.03
326.557 806.395 1044.24
1058.1 515.962
x-pos y-pos
ra
dec
mag
flags
759.01 12.18 1.428556e+000
1.998848e+002 13.2609 000 64.3066
195.654 110.988 108.31
7.61914 114.994 332.347
595.687 247.016 -93.668
171.673 529.031 916.378
587.714 209.038 40.3447
269.709 483.063 638.405
225.737 101.01
102.38 211.74
219.09 106.43
===============================================================
* To detect the effects of crowding on TASS images the “status flags” parameter can be used to detect when a faint star might influence the photometric measurement the star that is being measured. A definition of the SExtractor “flag” parameters is detailed below:
( 1) Flag = 1, The object has neighbors, bright and close enough to enough to bias significantly (delta m ~ 0.1)
(2) Flag = 2, The object was originally blended with another one.
(3) Flag = 4, At least one pixel of the object is saturated (or very close to).
(4) Flag = 8, The object is truncated (too close to an image boundary.
( 5) Flag = 16, Object’s aperture data is incomplete or corrupt.
(6) Flag = 32, Object’s isophotoal data is incomplete or corrupt.
(7) Flag = 64, A memory overflow occurred during deblending.
(8) Flag = 128 A memory overflow occurred during
extraction.
As an example of how this might be useful, shown below is two lines from the star-list that was generated from image "go483967.fts":
x-pos y-pos
ra
dec
magnitude flags
492.74 16.90
1.878284e+000 6.074249e-002 13.1287
002
492.09 21.48
1.875801e+000 7.821706e-002 12.8884
002
The "flags" parameters indicates that these two objects had been origianlly "blended" together and had been "separated" from each other by the star-extraction process. The small segment of the image below shows these two objects from the original image:
These flags could be used to tell which measurements
one might wish to excluse from a data set (to determine specific conditions
upon which a mesurement would not want to be included). (i.e. such as some
of the pixel data was saturated, etc. )
* The ability to generate "check-images" from SExtractor to tell what kind of performance is being experienced by the Extraction program:
(1). Background: interpolated background: useful to adjust the background parameters like mesh-size.
(2). -Background: difference between the image and interpolated background.
(3). Convolved: background -subtracted convolved image. Note: CONVOLVE must be set to Y in the parameter file.
(4). OBJECTS: objects detected above the thrshold.
(5). Segmentation: each pixel of the image is assigned a value corresponding to the object it belongs to. Very useful to adjust deblending parameters.
(6). Apertures: MAG_AUTO and MAG_APER circular apeture limits are superimposed to the background-subtracted image. MAG_AUTO ellipses surrounding objects flagged as "crowded" are dashed.
Shown below is the image from TN#32 "go483967.fts":
Shown below is the "BACKGROUND" plot from the above image that Michael Richmond used in his TN#32.
An example of the "Apertures" plot is also shown for the same image:
The objects that are detected in the image can be
seen and the overall operation of the extraction process can be "fine-tuned"
using the various "check-image" options of SExtractor.
Conclusions:
The above TechNote summarizes some of the more useful
featuers of the SExtractor program for reducing TASS Images. A GUI interface
for SExtractor is underway for Window95/NT and should be available soon.
I will try to add to this TechNot in the future as I discover new features
of SExtractor that might be of interest to the TASS effort.