XMM/WFI Survey Release (#24)

1 December 2004

Technical Summary

Survey ..................... XMM
Telescope .................. ESO/MPG 2.2m
Instrument ................. WFI
Program IDs ................ 170.A-0789; 70.A-0529; 71.A-0110
Origin ..................... ESO/EIS
Number of regions .......... 8
Region ..................... Selected XMM Serendipitous Survey
Number of Fields ........... 8
Passbands .................. B; V; R; I
Number of Filters........... 4
EIS Release Number ......... 24
Version .................... 0.9
Total Data Volume .......... 212 Mb
Release Date ............... December 2004
Release prepared by ........ EIS team, L. F. Olsen, C. Benoist

Product Type ............... Catalogs from Stacked Images
Number of catalogs ......... 28
Data Volume ................ 212 Mb

Origin ..................... ESO/EIS
Number of regions........... 12
Region ..................... Selected XMM Serendipitous Survey
Number of Fields ........... 12
Passbands .................. B; V; R; I
EIS Release Number.......... 22
Version..................... 0.9
Release Date................ 04 November 2004

Product Type................ Image Stacks
Number of Stacked Images.... 44


Based on ideas submitted by the ESO community and evaluated by ESO's Survey Working Group (SWG), the XMM survey (administered by the ESO Imaging Survey (EIS) and the Public Survey Group (PSG) teams) consists of follow-up optical observations of the XMM-Newton Serendipitous Sky Survey (XSSS) using the ESO Wide Field Imager (WFI) at the ESO/MPG 2.2m telescope at the La Silla Observatory. This survey seeks to obtain WFI images in the B-, V-, R- and I- passbands (for minimum spectral discrimination and photometric redshift determination) of XMM fields publicly available in the XMM archive. The fields were selected and prioritized by a collaboration of interested parties from the XMM Survey Science Centre (SSC), the Bonn group (led by P. Schneider) and an appointed committee of the SWG. The observations were originally proposed to cover a total area of approximately 10 square degrees (40 fields) to a limiting magnitude of 25 (AB, 5 sigma, 2 arcsec aperture, see original proposal presented by the SWG to the OPC).

Data for the XMM survey are being provided by:

  1. the ESO Large Programme 170.A-0789(A) (Principal Investigator: J. Krautter, as chair of the SWG) which has accumulated data from January 27, 2003 to October 17, 2004 at the time of writing.

  2. the contributing programs 70.A-0529(A); 71.A-0110(A); 71.A-0110(B) with P. Schneider as the Principal Investigator, which have contributed data from October 14, 2002 to September 29, 2003

The present release consists of an initial version of science-grade, single-passband (SPB) object catalogs extracted from ``final'' stacked images produced for 8 XMM fields ( EIS release number 22) located at galactic latitudes |b| > 15 degrees. The catalogs were extracted using the SExtractor program (Bertin & Arnouts 1996), graded and the results of preliminary statistical analysis compared to those obtained by other authors, in order to characterize and validate them. These results are presented, whenever possible, in the product log associated with each catalog. These logs are accessible from the release WEB page.

For more information about the terminology and conventions used in this document refer to the WEB README pages.

Contents of this Release

This is the third official release of reduced data (not including revisions) for the XMM-Newton follow-up survey, and follows earlier releases of night products and final stacked images.

The present release consists of 28 science grade (S/N > 5), extinction corrected catalogs extracted using SExtractor from fully calibrated ESO/MPG 2.2m WFI stacked images in B-, V-, R- and I- passbands for 8 out of the 12 fields released so far, located at galactic latitudes |b| > 15 degrees. Details about the images can be found in release number 22.

The catalogs being released were extracted from images similar but not identical to those released in release number 22. The images were re-created because, as explained in the COMMENTS of that release, the parameter for the cosmic ray rejection was inadequate, leading to a very noisy weight map. However, as also explained there, the impact on the catalogs is negligible.

Retrieving EIS Products


Official EIS products can be retrieved via two alternate routes, both originating at the EIS home page. These procedures are described elsewhere (see Retrieving EIS Products). It is worth reiterating that to request data users have to be registered with the ESO Science Archive.

In the case of the present release the "data release information section" is followed by a section detailing the "contents" of the release, listing:

  1. Table entry number (not to be confused with the "Product Identification number" which is reported in the main HEADER of the FITS format catalog);

  2. EIS standard field/region name

  3. Passband;

  4. ESO's filter number;

  5. Right Ascension (RA) in J2000.0;

  6. Declination (Dec) in J2000.0;

  7. Total integration time (in seconds) of the image from which the catalog was extracted;

  8. Total number of objects in the catalog being released, including those flagged;

  9. Total area (in square degrees) within which objects were extracted;

  10. The smallest value of S/N for an object to be included in the catalog. The S/N is calculated as the inverse of the SExtractor error for the chosen magnitude type (in this particular case MAG_AUTO);

  11. Grade (A to D, from good to bad) assigned to the catalog, from the visual inspection of the product by the EIS team;

  12. Total volume (in Megabytes) of the selected product;

  13. Hyperlink to the product log;

  14. Interactive check-box which enables the individual product to be selected/de-selected before finally submitting the selection to the ESO Science Archive Facility using "Request Marked Products" at the foot of the page.

Note: All data products are subject to revision and update once released. A report of the changes may be found in the ``COMMENTS'' field next to the particular release (in the release WEB page) and/or in the READMEs of the revised release.

Product Logs

For each survey product the EIS system prepares a ``product'' log from which the process log and the configuration file used can be accessed and inspected. Currently, the product log associated to a science-grade, SPB catalog comprise four distinct sections represented in the rendering of the HTML. All sections have a ``Product Identification'' sub-section which, among other things, identifies the user that created the product, the type and main attributes like passband and exposure time. This sub-section is reproduced so as to enable a proper identification of the information if these are printed separately (not currently possible). The sections in the product log are:

  1. Science Grade Summary - in addition to the Product Identification, this section provides an overall summary about the primary catalog (e.g. number of extracted, saturated objects), the science grade catalog; the structure and contents of the FITS catalog, and intrument and filter properties, including the filter transmission function and the final convolved response function of the entire optical system, including telescope, detector and filter.

  2. Primary Attributes - estimates of the completeness, random magnitude errors, a variety of characteristic (``limiting'') magnitudes associated to the catalog, derived based on the errors attributed to them by SExtractor, number and projected distribution of spurious objects as a function of magnitude based on ``negative'' images assuming a symmetric distribution of the noise. Also shown is the stellarity index versus magnitude used for the star/galaxy classification.

  3. Science Grade Attributes - provides information about the attributes of the final science-grade catalogs being released. These include: 1) the parameters used in pruning the catalog; 2) final number of objects, galaxies and stars; 3) smooth density field; 4) the projected point-distribution, indicating objects with different flags set by the system (trimmed, masked); 5) the completeness function of the catalog derived from simulated data created by artificially fading the image but keeping the noise constant

  4. Verification - comparison of the number counts of stars derived from the data with model predictions of the population synthesis model of Girardi et al. (2004) and number counts of galaxies with recent results from other authors.

Even though the product logs are still not in their final form, the logs available in the present release serve to illustrate the type of information the system is able to provide to survey users and is a vital complement to the catalog itself.


In the upper right corner of the data release WEB page for catalogs one finds a button (properties) which link to plots showing the distribution of some of the quantities that characterize the catalogs (e.g. the grade attributed to the catalogs; 80% completeness magnitude, number of objects, galaxies and stars and number density of objects in the science grade catalog)

Since the infrastructure is still under development, currently plots are being produced without adequate description. It is foreseen that these plots will be embedded into HTML files providing captions and statistics.

Comments Specific to this Release

The catalogs being released are in the FITS format, based on the ``Leiden Data Center'' (LDAC) convention originally adopted by the DENIS project and later expanded in the course of the EIS project. It currently consists of a FITS header and the following tables: FIELDS, OBJECTS, MASK and FILTER.

Currently, the FIELDS table consists of 109 columns. These include: 1) basic information set by the LDAC library; 2) keywords taken from the FITS header of the image from which the catalog was extracted; 3) the main SExtractor configuration parameters used; and 4) information computed by the EIS Data Reduction system. The latter includes, for instance:

  1. the diameter of the 10 apertures used, ranging from 1 to 5 arcsec in steps of 0.5 arcsec and a large aperture of 10 arcsec;

  2. the WCS coordinates of the corners of the original image and of the trimmed area;

  3. the amplitude of the extinction correction. This is currently being computed as the average of the value of the extinction in cells of 3 arcmin, distributed over the area of interest. In the future this correction will be computed and applied object by object, with the a new column containing the correction added to the OBJECTS table;

  4. the value added to the original magnitude of the extracted objects in the Vega system to produce the reported magnitudes in the catalog in the AB system;

  5. an estimate for the fudge factor used to multiply the errors reported by SExtractor to correct for the correlated noise;

  6. total and trimmed area.

Some of the information contained in the FIELDS table is also available in the ``Product Log''. In the future, embedding the product log into the FITS format of the catalog should be considered to avoid duplication of the information and facilitate distribution.

The OBJECTS table reports the parameters characterizing the extracted objects, as computed by SExtractor. Currently, there are 69 columns, some of which represented by vectors (e.g. aperture magnitudes). The choice of apertures and the flags defined are the result of suggestions made by users of EIS data products and they may evolve in time. In addition to the SExtractor flag, the values of which are described in the proper manual, 14 other flags have been are defined to facilitate the filtering of the catalogs. To the this end the following flags have been added:

  1. FLAG_SEX1 - FLAG_SEX128 - 8 flags individually representing the various SExtractor flags

  2. FLAG_SAT - 1 if saturated object. The saturation level is determined from the FWHM and peak flux of the bright objects. The distribution of the FWHM is determined and sigma-clipped to exclude bright galaxies from the sample. Among the remaining objects those with FWHM deviating more than 3 sigma are taken to be the saturated objects. The saturation level is set to the minimum peak value among these objects.

  3. FLAG_TRIM - 1 if inside trimmed area

  4. FLAG_MASK - 1 if inside masked area


  6. FLAG_STATE - 1 if any of the above flags are set

  7. FLAG_STAR - 1 if star, 0 if galaxy

Feedback from users regarding the parameters is welcome.

The MASK table gives the number and coordinates of the vertices of both automatically created masks as well as those drawn by hand using a SKYCAT plug-in. Saturated objects are always masked. In addition, the user has the option of also masking objects brighter than a user-specified magnitude. The size of the mask scales with the major-axis of the object, as computed by SExtractor. The scaling factor is specified by the user. The adopted shape of the mask is a square with one of the diagonals oriented north-south in an attempt to mask the diffraction spikes. The parameters used in the mask definition are reported in the product log.

The FILTER table gives the filter transmission curve and the total convolved optical system response function. In the future, it will also include some of the basic characteristics associated with the filter such as the ESO identification number, effective wavelength, and FWHM of the filter.

Comparison to Previous Release(s)

This is the first release of single passband catalogs for these fields and therefore no comparison data are available.

Data Reduction

This release represents a first set of catalogs extracted from WFI images and created systematically using the EIS Data Reduction/Survey System infrastructure in a fully un-supervised manner. The input images are final stacked images produced from the nightly reduced images. These images were astrometrically and photometrically calibrated as described in release number 22.

Catalogs were extracted using the SExtractor program (version 2.3.2) and a common configuration file, with the option of using the weight-map associated to each image. Some of the critical SExtractor configuration parameters are reproduced in the FIELDS table of the catalog itself. For each object extracted, 69 parameters are given describing the main geometric and photometric properties of the objects. The parameters were chosen as a compromise between the total number of parameters and the most frequently requested parameters from survey product users.

The first catalog to be extracted is a very low S/N catalog, which contains a large number of spurious objects. Starting from this catalog, a final science-grade catalog is produced by:

  1. pruning objects with S/N < 5 (as computed by MAG_AUTO error);

  2. splitting the SExtractor Flags into its 8 components;

  3. setting the FLAG_MASK to 1 for objects within masks;

  4. setting the FLAG_TRIM to 1 for objects outside a border defined by where the weight map on average drops below a certain level, normally taken to be 80% of its peak value;

  5. determining the saturation level and setting the FLAG_SAT to 1 for saturated objects;

  6. setting the FLAG_STATE to 1 for objects with either of the above mentioned flags set;

  7. setting the EISFLAG to the sum of FLAG_TRIM and FLAG_MASK;

  8. setting the FLAG_STAR to 0 (galaxy) or 1 (star) depending on the classification of the object as star/galaxy using the stellarity index criteria;

  9. converting magnitudes to the AB system according to the response function of the optical system;

  10. correcting magnitudes for galactic extinction. Note that at present this correction is applied to the magnitudes of all objects, including stars.

Note that except for objects with S/N less than that required, no object is removed from the catalog. If necessary, these objects can be pruned by the user according to the flags described above. In addition, the default magnitude system adopted for the objects can be changed using the information available in the FIELDS table.


The accuracy of the astrometric calibration relies on the astrometric calibration of the reduced images and the reference catalog used. As described in previous releases, it is typically better than 250 mas.


The reduced images were calibrated to the Vega magnitude system based on observations of Landolt (1992) standard stars as described in release number 19. Based on these photometric calibrations the photometric zeropoints of the stacked images were derived, as described in the release number 22, where more details concerning the accuracy of the photometric calibration can be found.

General Features

  1. The production of catalogs was done completely un-supervised. For the XMM fields considered the data rate ranges from 0.08 to 0.15 Mpix/sec. In other words, it took about 12 minutes per WFI image to produce not only a science-grade catalog but the extensive associated XML product log. Verification and grading of catalogs takes on average some 5 minutes per WFI image. While this could be a very time-consuming procedure for catalogs extracted from night products, this is not the case for stacks.

  2. The XMM selected fields include a broad range of galactic and extragalactic fields of varying density of objects, bright galaxies and stars. In this sense this survey is a useful benchmark for the problems one must deal in extracting science grade catalogs, making it difficult to have a unique set of parameters to cope to the large variety of situations encountered. Finding compromises is the main challenge to enable the automatic, un-supervised generation of catalogs.

  3. The inspection of the projected distribution of objects, strongly suggests that the automatic masking of satellite tracks has worked remarkably well as no prominent linear features, a signature of this type of problem, are seen on the inspected catalogs.

  4. One of the main criteria to assess the quality of a catalog is to minimize the number of false positives. The parameters of extraction adopted in the production of the catalogs being released, in general, work well. However, there certain unavoidable situations, among which: 1) the presence of ghost images near bright stars. Their location and size vary with position and magnitude making it difficult to deal with in an automatic way; 2) the presence of bright galaxies since the algorithm for automatic masking does not work well in this case; 3) residual fringing on the image; 4) the presence of stray light, in particular, associated with bright objects just outside the observed field. The XMM fields is a showcase of these various cases.

  5. Another sensitive parameter is that regulating the de-blending. Experience shows that the effects of de-blending depend on the type of field being considered and vary across the image. Some tests were carried out but it may require further analysis.

  6. A number of tests have been made to find an adequate compromise for the scaling factor used in the calculation of the size of the automatic masks. The size of the mask depends on the magnitude and passband, and will require further investigation. While the masking works, in general, well, it is clear that for precision work, such as lensing studies, masking by-hand is unavoidable with the present technology.

  7. The criteria adopted in the procedure used to define the ``trimmed'' region sometimes overshoots because of the nature of the weight map at the corners and because in some cases the image is slightly tilted. This has as a consequence that large number of spurious objects at the corners of the image are not flagged appropriately.

  8. Occasionally, the masking of saturated stars fails (less than 10% of the cases in 5 out of 28 catalogs). These are likely to be stars just barely saturated. The reason for that is still being investigated.

  9. Catalogs extracted from stacked images created with different settings of the cosmic ray rejection algorithm were compared and as earlier claimed it was confirmed that it has little effect on the properties of the derived catalogs

Data Quality Assessment


Before being released the catalogs overlayed on the images were examined by eye and graded by the EIS team, with the grade range being from A (best) to D (worst). The grade is meant as a subjective indication of the quality of the catalog. Additional information about the validity of the catalog can e found in the verification section of the associated product log.

Out of the 28 catalogs being released, 13 were graded A, 10 B, 4 C, and 1 D. In contrast to the images, the catalog graded D is also being released in order to illustrate the impact of inadequate de-fringing. It is important to note that the catalog graded D was extracted from a grade C image. The table below presents all cases where comments were made. The table, ordered by increasing grade, lists: in column (1) entry number; in column (2) the region name; in column (3) the EIS field name; in column (4) the passband; in column (5) the grade; and in column (6) a comment.

# Region EIS Field Name Passband Grade Comment
1 XMM-10 PB5062 R#844 D Fringing leading to severe problems (spurious detections all over the image)
2 XMM-08 NGC4666 V#843 C Missing masks for a few saturated stars. Spurious objects around large central galaxies.
3 XMM-09 Q1246-057 B#878 C Spurious objects around large bright galaxy at the top right corner. Cosmic rays misidentified as real objects.
4 XMM-09 Q1246-057 V#843 C Large number of spurious objects around large bright galaxy at the top right corner
5 XMM-09 Q1246-057 I#879 C Large number of spurious objects around large bright galaxy at the top right corner and due to multiple reflection rings.
6 XMM-05 BPM16274 B#878 B Masks missing around a few saturated stars. Some cosmic rays misidentified as real objects.
7 XMM-03 HE1104-1805 B#878 B Masks missing for a few saturated stars
8 XMM-07 LBQS_2212-1759 R#844 B Spurious objects at the corners caused by insufficient trimming
9 XMM-04 MS1054.4-0321 V#843 B Masks missing for a few saturated stars
10 XMM-08 NGC4666 B#878 B Masks missing for a few saturated stars
11 XMM-08 NGC4666 I#879 B Spurious objects around large central galaxies
12 XMM-06 SHARC-2 B#878 B Slight tilt of the image causes trimming frame to not mask completely the borders
13 XMM-06 SHARC-2 V#843 B Insufficient de-blending near bright galaxies but over de-blending near bright stars
14 XMM-06 SHARC-2 R#844 B Insufficient de-blending near bright galaxies
15 XMM-06 SHARC-2 I#879 B Insufficient de-blending around bright galaxies. Spurious objects caused by bright stars reflection features and stray light reflections

Not surprisingly, there is a strong correlation between the grade of the catalogs and the grade of the images from which they were extracted.

Photometric Calibration

The quality of the photometric calibration of the reduced and stacked images have been assessed in the EIS release number 19 and 22 , respectively, and will not be repeated here.

The magnitudes of the final catalog have been converted into the AB magnitude system and corrected for the galactic extinction using Schlegel et al. (1998).

Catalog Attributes

The attributes of the catalogs are summarized in distribution of grades, the 80% completeness limiting magnitude in the Vega system, the number of objects, galaxies and stars, and number density of objects. The color code adopted for the filters (passbands) here and elsewhere is as follows: B (blue); V (green); R (red), I (magenta). This color coding is consistent with that adopted in previous releases. The first panel shown in black is a combination of all filters.

Comparison with other authors

The reduced data (in the form of stacks) were also compared to results obtained using the GaBoDS pipeline (Schirmer et al. 2003) and Erben et al. (2004) developed by the Bonn group. Extensive and repeated comparisons were made between SExtractor-produced catalogs from the images generated by these two independent systems. Initial discrepancies, resulting from the different techniques used (e.g. cosmic ray removal, gain-harmonization) were resolved, leading at the end to results in excellent agreement.

Remarks on Individual Products

  1. XMM-08 - contains an extended, bright galaxy (NGC 4666) at the center of the image, plus a companion located SE from the bright galaxy. The presence of these galaxies lead to a large number of spurious objects in their surroundings in all bands.

  2. XMM-09 - Very bright galaxy located at NW of the image (right upper corner) leading to the detection of a large number of spurious objects extending over a large area (10 arcmin on the side) in all bands.Even though the object has been automatically masked, the affected area is much larger than that predicted by the algorithm, optimized for stars, thus requiring additional masking by hand.

  3. XMM-10 (R) - the original image was graded C because of observed fringing in the final stack. This has led to a catalog with no scientific value because of the high number of spurious objects detected along the fringing pattern. This catalog is being released exclusively as an illustration.

Next Release(s)

Originally, the following releases were scheduled:

Unfortunately, since the EIS project is scheduled to terminate on December 31, 2004 the EIS team will not be able to complete the work until then.


This release is the eighth of 2004, and number 24 since March 1998. It is the first release of science-grade catalogs extracted from stacked products created using the framework of the EIS Data Reduction/Survey System. Each catalog has an associated product log which provides complementary information about the product as well as the results of some simple analyzes carried out to characterize and validate the catalog. The production of the catalogs was done in a completely un-supervised way, with a throughput ranging from from 0.08 to 0.15 Mpix/sec, a good match to the data rate of other parts of the pipeline. It is important to emphasize that as one goes down the chain of survey products the more complex they become making it difficult to assign a quality for all possible applications. Fine-tuning of parameters may also prove necessary, and may depend on the specific application. Feedback from users would be extremely helpful.


Arnouts, S. et al. , 2001, A&A, 179, 436

Bertin, E. & Arnouts, S., A&AS, 117, 393

Erben, T. et al. , 2004, in preparation

Girardi, L., et al. , 2004, A&A, submitted

Landolt, A. U., 1992 Astronomical Journal 104, 340

Schlegel, D., Finkeiner, D., & Davis, M., 1998, ApJ, 500, 525

Vandame, B. et al. , 2004, in preparation

Vandame, B., 2004, PhD thesis, in preparation

eis data account 2004-12-01