XMM/WFI Survey Release

4 November 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 .......... 12
Region ..................... Selected XMM Serendipitous Survey
Number of Fields ........... 12
Passbands .................. B; V; R; I
Number of Filters........... 4
EIS Release Number ......... 22
Version .................... 0.9
Total Data Volume .......... 24.7 Gb
Release Date ............... November 2004
Release prepared by ........ EIS/PSG team, L. F. Olsen

Product Type ............... Final Stacked Images
Number of Stacked Images ... 44
Data Volume ................ 24.7 Gb

Origin ..................... ESO/EIS
Number of regions........... 12
Region ..................... Selected XMM Serendipitous Survey
Number of Fields ........... 12
Passbands .................. B; V; R; I
EIS Release Number.......... 19
Version..................... 0.5
Release Date................ 30 August 2004

Product Type................ Nightly Products
Number of Reduced Images.... 146


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 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 April 25, 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 final stacked images for 12 XMM fields including the data from the above programs accumulated until October 16, 2003. The image stacks were created from the 146 reduced images released in EIS release number 19. The 146 reduced images were converted into 44 stacked images of which all are being released. From these numbers one finds that the volume of data can be reduced by a factor of at least 3 by converting nightly products into final stacked products. This information is of interest when dealing with data gathered by wide-field imagers.

Of the 12 fields being released nine are covered in all intended passbands. Of these seven have at least the planned exposure time, yielding limiting magnitudes close to those requested in the original proposal.

The accuracy of the astrometric calibration is estimated to be better than 0.2 arcsec. The photometric zeroopoints of the final stacks rely on the zeropoints of the reduced images for which the accuracy was estimated to range from 0.05 to 0.08 mag, with passbands B and R having the largest errors. The quality of the photometric zeropoints of the final stacked images is estimated from the scatter of the computed magnitude difference between the final stacks and the contributing photometric frames. In general, the mean offset are within few hundredths of a magnitude with a scatter of about 0.1 mag.

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 second official release of reduced data for the XMM-Newton follow-up survey. The data set being released was accumulated in service mode during ESO observing periods 70 and 71, and observations are still on-going.

The present release consists of 44 fully calibrated ESO/MPG 2.2m WFI stacked images in B- (12), V- (11), R- (10) and I- (11) passbands for the 12 fields observed so far. The 44 stacked images were created from the 146 reduced images released in August 2004 in EIS release number 19 and more details about them can be found in the related documentation.

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 FITS header of the image);

  2. EIS standard field/region name with an associated hyperlink to a JPEG format file. This file represents a colour composite image obtained from a combination of either BVI (or BVR) images, if available. Gray scale versions can be found in the release of the reduced images. The JPEG format enables image properties such as saturation and intensity to be interactively modified as desired using color editors (e.g. within the "xv" UNIX/Linux utility).

  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) contributing to the final product;

  8. Total number of science exposures contributing to the final product;

  9. Grade (A to D, from good to bad) assigned as an indication of cosmetic quality of the image, during visual inspection of the product by the EIS team;

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

  11. Hyperlink to a comprehensive descriptive log of the product;

  12. 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.

Product Logs

For each survey product the EIS system prepares an extensive ``product'' log from which the process log and the configuration file used can be easily accessed and inspected. This infrastructure provides the means of ensuring the uniqueness of the ingested products. It also provides the information required to understand differences between versions of the same product.

Even though, the product logs are still not in their final form, the current version of the product logs is included in this release to help the users to select suitable products for their specific projects.

Typically, the product logs comprise of seven 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. These sections consist of:

  1. Product Summary - in addition to the Product Identification, this section provides a list of associated products (e.g catalogs) if available; and more details about the product such as: position; number of contributing frames; pixel scale; astrometric reference system adopted; and photometric zeropoint.

  2. Attributes - provides more details about the attributes of the product. These include, among other things: number of contributing frames; number of photometric frames; effective gain; limiting magnitudes (in the Vega system); measured seeing and information about the PSF distortion.

  3. Process (Stack) Blocks - in the current release these are the stacking blocks (SBs) comprising the reduced images to be used to produce a final stacked image. This section provides an overview of the contributing reduced images and their attributes (e.g. airmass, seeing, PSF distortion, astrometric and photometric calibration).

  4. Calibration - this section provides information on the calibrations (astrometry and relative photometry) and their quality. The astrometric calibration is verified using GSC2 and USNO. In case of stacks, the photometric calibration is verified by comparing the magnitudes measured in the stacked image with the ones measured in the individual photometric images. The mean and scatter of the offset distribution are given and can be used an estimator of the final photometric accuracy.

  5. Verification - this section will be included in the comparison section described below.

  6. Comparison - this section will present the results of comparisons between the final products with internal and external data, if available. This section normally includes plots comparing astrometry and photometry, which in the present case are not available due to the lack of similar data.

  7. Quality Control Parameters (QCP) - This section will become available in a future release. It provides a compilation of quantitative parameters that characterize the product, its calibration and quality. These values will be used in the selection of products satisfying user provided constraints on their attributes, which depend on the specific scientific application considered.

Even though not fully completed, the logs available in the present release serve to illustate the type of information the system will be able to provide survey users. In fact, it is envisioned that the logs would be included as part of the images, possibly as an extension.

For the time being, a preliminary implementation of this infrastructure is available. Even though not complete, it is presented here for the first time.


In the upper right corner of the data release WEB page there are two buttons (completeness and properties) which link to plots showing the completeness of the dataset being released relative to the original strategy. It also presents different visualizations of the attributes associated with the images being released (for details see Data Quality Assessment section below). A more detailed description is provided below.

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

Since WFI was first offered as a common user instrument at the ESO/MPG 2.2m telescope in 1999, the available filter set and naming conventions have evolved somewhat. To avoid confusions it is instructive to state explicitly the filter set used when conducting the present survey. The current La Silla Science Operations naming convention assigns all filters which are permanently available at the WFI an ESO identity number. If the filter does not have a special name (U, B, V etc.) then the assigned name is "CWL/FWHM" where CWL is the truncated central wavelength (nm) and FWHM is the truncated full-width at half-maximum (nm). If the filter has a special name, the FWHM of the filter (in nm) is appended to the special name. A full description of the filter names can be found at the WFI Filter Characteristics page of the La Silla Science Operations.

The present survey makes use of the following WFI filters:

Passband Filter Name ESO identity number
B BB#B/123_ESO878 #878
V BB#V/89_ESO843 #843
R BB#Rc/162_ESO844 #844
I BB#I/203_ESO879 #879

Due to the on-going nature of the survey, the passband range, depth and completeness of the products in the present release is not homogeneous. In fact, the completeness of each field may vary, as some fields have not yet been completely observed. This is shown in the completeness plot accessible from the release WEB page. This figure shows for each field two histograms of time versus passband, the latter represented by the ESO filter name convention. The hatched histogram represents the total integration time required by the original strategy (in seconds). The full colored histograms represents the sum of the integration time (TOT-EXPT keyword in the header) of all stacked images in a particular filter. The color code adopted for the filters (passbands) here and elsewhere is as follows: B (blue); V (green); R (red), I (magenta). Note that in some cases the R-band greatly exceeds the originally proposed integration time. This is because of the contributing programs targeting the high-galactic latitude fields.

As explained elsewhere (see EIS Definitions and Conventions) the conventions adopted by EIS to identify a pointing both internally and externally (e.g. OB name, object name, target name, field name) and their graphical representations on the EIS WEB pages have evolved considerably as new procedures were introduced, making it impossible to strictly maintain consistency.

Field names can normally be found in the OBJECT keyword of the FITS header. Since they are, normally, inherited from ``Observation Blocks'' (OBs) which have been prepared by multiple team members, the naming convention may not be homogeneous, and thus it is recommended that the value of the OBJECT keyword should not be, in general, used to rename files.

In the particular case of this survey one should be warned that the contents of the OBJECT keyword do not necessarily correspond to the name convention listed above (e.g. images with OBJECT=SHARC-2 do not correspond to the second listed cluster of the SHARC collaboration but it is instead an internal convention adopted by the Bonn group).

This release consists of ``stacked'' images, thus for each field and filter there is only one image combining all contributing ``reduced'' images.

Comparison to Previous Release(s)

This is the first release of stacked images for this survey so no comparison data are available.

Data Reduction

This release represents a first set of stacked WFI images created systematically using the EIS Survey System infrastructure. The stacked images were produced from the nightly reduced images available through release number 19. These images were astrometrically and photometrically calibrated as described in ``that release''. Here the procedures applied to the reduced images for creating the stacks are described.

Each field is the co-addition of a number of reduced images grouped in a number of "Stack Blocks" (SB). The SBs were constructed based on the pointing, instrument, and filter of the reduced images. Further consistency checks includes the astrometric reference grid as detailed below.

The production of the stacked images consists of the following steps:

  1. Grouping of images: Reduced images are grouped according to position (with a minimum distance between centers of 0.25 times the field-of-view) and filter.

  2. Validation of reference grid: It is required that all images in a group share the same reference grid given by the projection, reference position, pixel scale and orientation.

  3. Validation of groups: Each group of images is then validated according to a number of rules regarding seeing, exposure time, level of PSF distortions and grade. In this release images with grade D were rejected and none of the other constraints led to the rejection of images from the stacks. In addition, the contributing images to an SB, or their original RBs, are checked to prevent the repetition of raw exposures.

  4. PSF homogenization: In order to facilitate the star-galaxy classification the PSF is homogenized among all group members. This is done by convolving each image by a Gaussian with a FWHM computed so that the resulting seeing matches the largest one found among the contributing images.

  5. Exposure time homogenization: Images in the SB are checked to ensure that their flux scale has been properly normalized to 1 sec, and if not they are re-normalized accordingly. This is just a redundant safety measure.

  6. Flux scale determination: Before the images can be co-added, which is done using a weighted mean procedure, it is necessary to have all images at the same flux level. This is done by scaling photometric frames to zero airmass using the extinction coefficients from the photometric solutions. The non-photometric frames are scaled to the zero airmass level of the photometric frames. More details about this procedure are given below.

  7. Co-addition: The contributing images are co-added using a weighted mean. The weight images reflect the exposure times of their associated science image. Therefore, the weight of each image is a combination of expoure time and noise. However, the weight images also contain information about the location of bad pixels and masks which are set to zero weight and thus do not contribute to the final image. In the co-addtion process, performed by the routine add-mosaic of the EIS/MVM library, a sigma-clipping procedure is employed to remove cosmic ray hits.

Inspection of the images after stacking indicated that in one case the final stacked image was significantly degraded by the inclusion of an image (graded B) with high noise amplitude. Therefore, this image was not included in the production of the released stack. The reason for this problem is being investigated and may lead to the inclusion of additional constraints for the automatic rejection algorithm.


The astrometry for the SBs completely rely on the astrometric calibration of the reduced images. As described above, in the creation of the SBs a number of conditions are applied to ensure consistent astrometric calibrations for all contributing images.

The astrometric conditions for including an image into an SB is that the distance between the center of the reduced image relative to the others is smaller than 0.25 times the field-of-view. Furthermore, the adopted reference grid (reference coordinate, pixel scale, orientation, and type of projection) has to be the same for all the contributing images.

The WCS of the stacked images is reported in the image header using the CD-matrix notation. The projection adopted is TAN and the orientation is north up and east to the left.


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 below.

Stacks with one photometric frame

For SBs where only one of the input images were taken in a photometric night, this photometrically calibrated image is used as a reference. Firstly, the flux in this reference image is scaled to the flux level that would be obtained at zero airmass based on the extinction found in the photometric solution. Then the other contributing images are scaled to that same flux level corresponding to that at zero airmass. The scaling factor is obtained by comparing the object magnitudes. After this scaling all the scaled images are averaged as described above. The zeropoint of the resulting stack is the zeropoint at zero airmass of the photometric reference image.

Stacks with more than one photometric frame

For SBs with more than one photometric frame a reference image is created by first scaling all the photometric frames to the flux level at zero airmass based on the extinction for their respective photometric solutions. Then, by computing the weighted average of all of these scaled images. All input images (photometric and non-photometric) are scaled to the same flux level as the reference image. These scaled images are averaged (weighted) to yield the stacked image. The zeropoint of this image is the weighted average of the zeropoints at zero airmass of the photometric input images.

Stacks with no photometric frames

Occasionally, none of the reduced images in an SB are photometric, possibly for different reasons (no standards in the night, observations in non-photometric nights). In such cases, all images are scaled to the flux level of an arbitrarily chosen reference image within the stack. The zeropoint of the final stacked image is then the zeropoint of the adopted reference image. For these cases, the convention adopted is same as that of the non-photometric reduced images.

Image Product Calibration

According to the above procedures the zeropoints of each stack was determined and the image specific zeropoint was added to the header of the images. Therefore, the Vega magnitude is given by

mag (Vega) = -2.5*log(flux) + ZP

where the flux is the number of counts directly measurable on the images (note that the stacked images are normalized to 1 sec).

In the case of a non-photometric night, a default value for the zeropoint is adopted and the error in zeropoint is set to -1. For nights without observation of fields containing standard stars, a default zeropoint and an error of -2 are assigned to the image. Finally, during quality assessment of the data, calibrated images with zeropoints that deviate significantly from a reference value have the zeropoint in the header changed to a default value and its error set to -3. Note that when a default value is assigned all images will have the same zeropoint in the header regardless of the airmass at which they were observed.

In the case of stacks, the actual value adopted for the default zeropoint of the reduced images is only relevant in cases where no photometric frames are available for a given SB. The final photometric accuracy, however, will depend on the type of solutions and number of independent nights contributing to the final stack.

The table below shows for each pointing the best type of solution available for each passband. From this it can also be seen that three stacks (XMM-01 R, and XMM-02 R and I) no photometric solutions were available.

# EIS Field Name Default 1-par 2-par 3-par
1 XMM-01 R BV   I
2 XMM-02 RI BV    
3 XMM-03   I R BV
4 XMM-04   V   BRI
5 XMM-05   R I BV
6 XMM-06   V BR I
7 XMM-07   I B VR
8 XMM-08   V   BI
9 XMM-09     B VI
10 XMM-10     B R
11 XMM-11       BVRI
12 XMM-12   BR V I

More details concerning the calibration of the reduced images contributing to the products being presently released can be found in the readme of release number 19

Cosmetic Features

The reduced images still showed a number of cosmic ray hits, because the construction of reduction blocks (RB) was optimized for removing cosmic ray features in the final stacks. This priority in most cases led to the creation of 3 RBs per observed field and filter. Therefore, in most cases the stack blocks (SB) consist of at least 3 input frames, allowing for the use of a sigma-clipping procedure to remove cosmic ray hits from the final stacked image. The chosen sigma clipping is adapted to the number of input images, such that for SBs with less than 5 images use a one-sigma clipping; SBs with 5 to 10 images use a two-sigma clipping; and SBs with more than 10 images use a three-sigma clipping. Even with this adaptive sigma clipping factor, the stacks consisting of only three RBs (totalling 5 exposures), mostly B-band images, are still showing some cosmic ray hits. This happens in the regions of the interchip gaps, where fewer images contribute to the final stack.

The performance of the automatic satellite track masking algorithm (using the Hough transform) has been proven to be efficient in removing both bright and faint tracks. The frequency for the appearance of these tracks is about 0.9 satellite-tracks/hour, of which only 1/10 are of the bright type. The most extreme case is 3 satellite tracks of varying intensity in a single exposure. The regions affected by satellite tracks in the original images were flagged in the weight images and thus are properly removed from the stacked image. However, in the regions where a satellite track was found in one of the contributing images the noise is slightly higher, as will also be reflected in the weight image.

Data Quality Assessment


Before being released the images were examined by eye and graded by the EIS team, with the grade range being from A (best) to D (worst). This grade refers only to the visual aspect of the data (e.g. background, cosmetics). In the future the grade will also include a larger set of quality control parameters (QCP) and will be available for each survey product via the corresponding product log.

Out of the 44 stacked images covering the selected XMM fields, 33 were graded A, 10 B, and 1 C, with no grade D being assigned. Thus all the stacked images are being released. In addition to the grade a comment may be associated with the visual appearance of the image. The table below presents all cases where comments were made. The table, ordered by increasing , lists: in column (1) entry number; in column (2) the EIS field name; in column (3) the passband; in column (4) the grade given by the visual inspection; and in column (5) the associated comment.

# Region EIS Field Name Passband Grade Comment
1 XMM-10 PB5062 R#844 C Fringing
2 XMM-01 J0925.7-4758 R#844 B Poor background substraction near bright stars
3 XMM-08 NGC4666 I#879 B Poor background subtraction in the neighborhood of the bright galaxy in the center
4 XMM-10 PB5062 B#878 B Shorter exposure, higher background noise
5 XMM-09 Q1246-057 B#878 B Poor background subtraction in the neighborhood of the bright galaxy in the upper right corner
6 XMM-09 Q1246-057 V#843 B Poor background subtraction in the neighborhood of the bright galaxy in the upper right corner
7 XMM-09 Q1246-057 I#879 B Poor background subtraction in the neighborhood of the bright galaxy in the upper right corner
8 XMM-12 WR_46 B#878 B Poor Background substraction near bright stars in the center
9 XMM-12 WR_46 V#843 B Poor Background substraction near bright stars in the center
10 XMM-12 WR_46 R#844 B Poor Background substraction near bright stars in the center
11 XMM-12 WR_46 I#879 B Poor Background substraction near bright stars in the center
30 XMM-07 LBQS_2212-1759 I#879 A remains of stray light reflections visible in the lower-right corner
34 XMM-04 MS1054.4-0321 I#879 A low level fringing still visible
36 XMM-08 NGC4666 V#843 A slight background variation in the neighborhood of the bright galaxy in the center
40 XMM-06 SHARC-2 I#879 A low level fringing still visible

The comments are mostly related to poor background subtraction either due to very bright stars or extended, bright galaxies in the field. Some improvement may be possible with changes in the observing strategy. The reduction mode for these data was optimized for extragalactic, non-crowded fields, which is not optimal for some of these fields. In one case the comment is related to the fringing in the R-band as was already noted in the release of the reduced images. In two cases the comments report low level residual fringing in the I-band.

Photometric Calibration

In the release of the reduced images the quality of the photometric solutions was assessed. Here the main concern is the accuracy of the zeropoints assigned to the stacked images. Since the zeropoints of the contributing photometric frames is well documented (see EIS release number 19), the comparison of object magnitudes between the contributing images and the ones measured in the stack is used to characterize the quality of the photometric calibration. For each stack the average and scatter of the magnitude offsets between the photometric frames and the finalstack are computed. The distributions of these quantities show the magnitude offsets are in general small and the scatter is of the order of 0.1 mag and comparable to the estimated accuracy of the individual zeropoints.

Image Attributes

An important feature of the EIS survey system is that it attempts to provide a backbone infrastructure to enable the user to have a global view of the dataset being released both in terms of the completeness of the survey as well as the quality of the survey products. This is achieved by the production of plots accessible through the release WEB page and the quantitative information (QCP) available in future product logs, so as to make the selection of products with the desired attributes possible. This infrastructure will be complemented by a proper script that will allow the user to select products based on configurable parameters.

The attributes of the released images are summarized in the following figures, available from the WEB release page:

  1. the first figure shows the distribution of grades, limiting magnitude (Vega, 5 sigma, 2 arcsec aperture), seeing and an estimate of the rms of the PSF distortions. The color code is the same as in the completeness plots. The first panel shown in black is a combination of all filters used primarily for internal purposes.

  2. the second figure displays the correlation of limiting magnitude (Vega, 5 sigma, 2 arcsec aperture) versus seeing and integration time.

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-07 (B): from the three reduced images available only two were used for stacking because of the high amplitude of noise in one of them which greatly affected the final product. The discarded image was originally graded B.

Next Release(s)

Anticipated future EIS/XMM releases include:


This release is the sixth of 2004, and number 22 since March 1998 (of which two were software releases). It is the first release of final stacked products created using the framework of the EIS Survey System which provides the tools required for the administration of the data including versioning capabilities. Some additional data for XMM are available and, conditions allowing, similar products will be prepared and released. In the meantime, the final stacks of DPS are being prepared.

Since May 2001 a total of over 1741 requests (averaging nearly 80 requests/month in 2004 and 30 requests/month over the period in which information is available) have been made for raw (1111) and reduced (630) data and of the EIS/MVM image processing software (73) . These requests comprised 82,400 files (5.8 Terabytes; 3.2 Tb compressed) of raw data and 7936 files (626 Gb) of survey products (reduced images from different instruments such as SOFI, ISAAC, and WFI). Note that the last public data release was carried out in November 2004.

A statistical summary of the requests made to EIS deliverables can be found at the EIS Survey Release page


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

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

Koch, A., Grebel, E. K., Odenkirchen, M., Caldwell, J. A. R., 2004, Astronomische Nachrichten, 325, 299

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

Manfroid, J., Selman, F., Jones, H., 2001 The Messenger 104, 16

Schirmer, M., et al. , 2003, A&A, 207, 869

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

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

eis data account 2004-11-04