XMM/WFI Survey Release

30 August 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 fields
Number of Fields ........... 12
Passbands .................. B; V; R; I
EIS Release Number ......... 19
Version .................... 0.5
Total Data Volume .......... 79.9 Gb
Release Date ............... August 2004
Release prepared by ........ EIS/PSG team, J. Dietrich, A. Schwope

Product Type ............... Nightly Products
Number of Reduced Images ... 146
Data Volume ................ 79.9 Gb




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 March 24, 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 data from the above programs accumulated until October 16, 2003, corresponding to a total of roughly 130 and 720 science OBs and exposures, respectively, and 80 hours on-target integration. About 15% of the B-band and 85% of the R-band data are from the contributing programs.

With the present release 83% of the data accumulated as part of the Public Survey over 15 fields (roughly 3.75 square degrees) have been reduced. On the other hand, the observations for the XMM survey correspond, at the time of writing, to less than 35% of the original goal of 40 fields.

The 720 raw exposures were converted into 160 fully calibrated reduced images, of which 146 are being released. Of the remaining 14, 10 were observed with wrong coordinates, three (XMM-05 (R), XMM-06 (I), XMM-12 (V)) were rejected after visual inspection and one (XMM-12) was discarded due to a very short integration time (73 sec), associated to a failed OB. From these numbers one finds that the volume of data can be reduced by a factor of at least 5 by converting raw into nightly products and by 16 when final products (stacks) are produced. This information is of interest when dealing with data gathered by wide-field imagers.

The accuracy of the astrometric calibration is estimated to be better than 0.2 arcsec. The photometric solutions obtained by the automatic photometric pipeline, a sub-system of the EIS Survey System, are in excellent agreement with the one computed by the WFI team at La Silla. The estimated error of the absolute calibration, based on the scatter of the available solutions, ranges from 0.05 to 0.08 mag, with passbands B and R having the largest errors. However, these values are still poorly determined due to the relatively small number of nights available. Other effects, associated with a multi-chip detectors, may also impact the error budget. Improvements will require more extensive tests. There are also three anomolous night solutions in three distinct passbands (V, R, I), with the most significant (> 3 sigma) being in I (0.5 mag), with the R and I likely to be affected by fringing.

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 first 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 146 fully calibrated ESO/MPG 2.2m WFI images in B- (36), V- (32), R- (43) and I- (35) passbands for the 12 fields listed below at high (8; XMM-03 through XMM-10) and low (4; XMM-01, XMM-02, XMM-11 and XMM-12) galactic latitude. The table below gives: in column (1) a sequence number; in column (2) the EIS field name; in column (3) the name normally used to identify the field, and in columns (4)-(7) the number of fully calibrated images (and in parenthesis the number of independent nights on which they were observed) being released in each of the passbands.

# EIS Field Name Other B V R I
1 XMM-01 RX J0925.7-4758 3 (1) 3 (2) 3 (1) 3 (3)
2 XMM-02 RX J0720.4-3125 3 (1) 3 (1) 3 (1) 3 (1)
3 XMM-03 HE 1104-1805 3 (1) 3 (2) 5 (3) 3 (1)
4 XMM-04 MS1054.4-0321 3 (1) 3 (2) 4 (2) 3 (2)
5 XMM-05 BPM 16274 3 (1) 3 (2) 5 (1) 3 (2)
6 XMM-06 RX J0505.3-2849 3 (1) 3 (2) 6 (4) 3 (2)
7 XMM-07 LBQS 2212-1759 3 (2) 3 (2) 6 (4) 3 (2)
8 XMM-08 NGC 4666 3 (1) 3 (1)   3 (2)
9 XMM-09 QSO B1246-057 3 (1) 3 (2)   3 (2)
10 XMM-10 PB 5062 3 (1)   5 (3)  
11 XMM-11 Sgr A 3 (1) 3 (2) 3 (1) 5 (4)
12 XMM-12 WR 46 3 (1) 2 (1) 3 (1) 3 (2)

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 (involving reduced images only) 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. Observed MJD;

  3. Date (YYYY-MM-DD) at the start of the night;

  4. the EIS standard field/region name with an associated hyperlink to a JPEG format file. 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);

  5. Passband;

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

  7. Declination (Dec) in J2000.0;

  8. Total integration time (in seconds) contributing to the final product;

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

  10. 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;

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

  12. Hyperlink to a comprehensive descriptive log of the product. (not available for this release);

  13. 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.

Typically, the product logs consist of seven distinct sections represented in the rendering of the HTML as separate tabs. These sections consist of:

  1. summary - provides information about: user; date; workflow; executed processes; product identification; links to derived products (e.g. catalogs), if available. In addition, it provides relevant information about the instrument, pointing and calibration;

  2. product attributes - provides plots and statistics regarding the seeing, the PSF, sky brightness, limiting magnitude and information about the grade assigned by visual inspection;

  3. calibration - provides information and plots pertaining to the astrometric and photometric calibration;

  4. groups - provides information describing the exposures that make up an observation block (OB) or a collection of OBs. The section includes plots showing the variation of seeing (either from the seeing monitor or as measured on the image, depending on the process phase), background noise and airmass;

  5. process blocks - same as group but after the application of constraints (e.g. maximum amplitude of the seeing, maximum background noise) relevant to the preparation of Process Blocks, in the present case, reduction blocks (RBs);

  6. verification - whenever possible a direct comparison with previous versions, and with results obtained by other authors;

  7. quality control parameters (QCP) - 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;

For the time being, a preliminary implementation of this infrastructure is available for the single chip instruments. Work is under way to upgrade and generalize this infrastructure to support all instruments.


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).

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 Survey/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. The 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 reduced 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 ``reduced'' images, thus for given field and filter there may be more than one image product either from the same night (resulting from differently grouped raw images or several similar OBs) or from different nights.

Comparison to Previous Release(s)

  1. It is worth emphasizing that all public survey raw data are available immediately after their arrival in the ESO Science Archive, In fact, in the period February 20, 2003 to August 2, 2004, 41 requests for a total of 2437 raw images (328 Gb) have been made for the XMM public survey alone (program-id 170.A-0789).

  2. Although no reduced data for this survey have been previously released, the reduction steps should be compared to those adopted in Arnouts et al. (2001). Since then several improvements have been introduced in the code.

  3. Major improvements were made in the ``observation-report' pages (run and night summary) of the WEB. It now has the correct range of seeing, airmass, OB assignment, data acquisition rate, and better overall presentation. It is important to note that in service mode no calibration frames are included in the observation-report statistics.

  4. This release also marks the first use of the newly developed reporting infrastructure which allows easy access to the necessary information required for a detailed and precise description of the data reduced, rejected and released, as well as their properties. Among the new features are plots of completeness and different ways of visualizing the attributes of the reduced images (see above). The reporting facility also makes it possible to provide comprehensive statistics for requests of all survey products (raw, reduced, software) since May 2001.

Data Reduction

This release represents a first attempt to photometrically calibrate WFI data using the EIS Survey System. A proper calibration involves correction for CCD-to-CCD gain variations, illumination correction and absolute zeropoint determination. For the current release no illumination correction has been applied and the verification of the zeropoint determination has been compared with values provided by the 2p2 telescope team. On the other hand, relative photometry and astrometry have been compared to results obtained by the Bonn group (Schneider et al. ) using independent software.

The WFI data were reduced using the EIS/MVM image processing library. This software package is publicly available and can be retrieved starting from the EIS survey release page.

Each field is the co-addition of a number of sky-subtracted frames (the TOT_IMAG keyword in FITS image header) grouped in a number of "Reduction Blocks" (RB). The basic reduction methodology is described in Vandame (2004). Significant improvements have been made among which:

  1. Improved warping technique using a more accurate kernel (Lanczos order 4) which allows sub-pixel warping in contrast to the nearest-neighbor approach adopted by Arnouts et al. (2001).

  2. New de-fringing procedures. In some cases the fringing map used was generated from the data within the RB being reduced. For more crowded fields this proved to be inadequate and ``external'' fringing maps, generated from different RBs on the same night were used instead.

  3. a new procedure to homogenize the chips was adopted by computing median background values sampled in sub-regions bordering adjacent CCDs and using the ratio of the amplitudes to represent differences in relative gain.

  4. Improved sensitivity to the automatic removal of satellite tracks.

  5. Robust method for cosmic ray detection and removal.


The astrometric calibration was derived using the GSC2.2 reference catalog and a distortion model described by a polynomial of second order. Comparisons yield typical rms scatter in astrometry of 0.2 arcseconds (Vandame 2004). The WCS in the image header is reported using the CD-matrix notation. The projection adopted is TAN and the orientation is north up and east to the left.


Photometric Solutions

This release is the first for which photometrically calibrated WFI images are being provided. It is important to stress that the photometric pipeline being used to automatically extract catalogs, measure fluxes at different apertures, identify those corresponding to known standard stars, and derive from linear fits to these measurements zeropoints, extinction and color terms (if applicable) was originally developed to deal with single chip imagers - only recently systematic tests have been conducted for multi-chip cameras. A number of improvements to the code and more experimentation with the many configurable parameters involved are still possible. Therefore, the solutions presented in this 0.5 release should not be considered final.

In accordance with the established EIS standard procedures, photometric calibration of the science fields is performed on the Vega magnitude system. Zeropoints are obtained from linear fits to flux measurements of standard stars in Landolt (1992) fields taken at the same night as the science frames. These fits were obtained using from one (taking some specified values for the extinction and color term) to three free parameters depending on the available airmass and color coverage provided by the Telescope Team calibration plan (since the XMM survey observations were conducted in service mode).

For the present release photometric solutions were attempted for the 37 nights with observation of standard fields, of 41 with science observations. The four nights without standards are: February 2, 3 and 4, 2003 (Public Survey); and November 8, 2002 (contributing program)

A total of roughly 1000 standard OBs, nearly all consisting of one image per OB, have been reduced and measured. While this amounts to slightly more than 7 hours on-target (or 10% of the science observations in terms of time), it exceeds the volume of science frames. Depending on the night the number of measurements can range from a few to over 300, covering from 1 to 3 Landolt fields. As mentioned above, the period of observations extend over one year.

In practice, the photometric pipeline computes photometric parameters for all possible types of fits (one to three-parameters) and assigns the solution with the smallest zeropoint scatter, to be the ``best solution'' for the night. A night is considered photometric if this scatter is less than a pre-defined value, which at the present time is taken to be below 0.1 mag. If none of the solutions satisfy this criteria and/or the solution found yields unrealistic results (e.g. negative extinction) then the night is considered non-photometric and a default value for the zeropoint is adopted and its error set to -1.

For homogeneity, the default value normally adopted is the median of the zeropoints reported in the trend analysis kept by either the telescope team (depending on the instrument) or the internal EIS database. In the case of WFI, only one solution is currently reported in the WFI WEB pages. It is important to emphasize that EIS has and still is conducting several surveys (e.g. Pilot, DPS, Pre-Flames, XMM and Galex) using WFI, covering its entire operational lifetime. Therefore, in the future a trend analysis of the photometry will become naturally available.

The table below summarizes the number of nights with standard observations and type of ``best solution'' obtained by the photometric pipeline for each passband.

Passband default 1-par 2-par 3-par total
B 0 3 4 3 10
V 0 8 3 5 16
R 0 8 3 3 14
I 4 8 2 5 19

For three nights (March 26, 2003; April 2, 2003; August 6, 2003) the solutions obtained in the passbands V, I, R, respectively (either 2- or 3-parameter fits) deviate from the median by -0.26, -0.5, -0.25 mag. Of those only the I-band zeropoint obtained for April 2, 2003 deviates by more than 3 sigma.

The preparation of this release has shown the need for a number of improvements to be made to the photometric pipeline. Among those either under development or being considered are:

It is important to recall that wide-field multi-chip cameras are complex systems and to obtain a proper calibration depends upon a full understanding of the different effects outlined above. This requires extensive tests which can only be effectively done using automatic procedures as well as versioning infrastructure similar to the ones available in the EIS survey system. In order to carry out these tests efficiently and in a systematic way, a re-factoring of the code, and the implementation of suitable workflows and test-environment are currently underway. This study will be of great importance for defining calibration procedures to be adopted not only for WFI but for other wide-field cameras to be used in large imaging surveys

Image Product Calibration

These photometric solutions were used to calibrate the reduced images. The ZP in the header of a reduced image is given by

ZP = ZP' + KX

where ZP' is the zeropoint at zero airmass (determined from the linear fit), K is the extinction coefficient and X is the observed airmass. 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 reduced 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. It is important to stress that the history of the image calibration process can be retrieved independently of the solutions found for a night.

In the case of stacks, the actual value adopted for the default zeropoint of the reduced images is irrelevant, since those images with default zeropoints have to be re-scaled anyway to match the flux-scale of those images properly calibrated. The final photometric accuracy, however, will depend on the type of solutions and number of independent nights contributing to the final stack.

As a preview, the table below shows for each pointing the best type of solution available for each passband.

# 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

The following remarks can be made concerning the calibration of the image products being released:

  1. XMM-01 (R) - this field was observed as a single OB on February 3, 2003 when standard stars were not observed. Since this is a galactic field there are no complementary observations from the contributing program, and therefore these observations cannot be properly calibrated with the accumulated data.

  2. XMM-02 (R) - The observations for this pointing and filter were done with one OB in R (5 exposures) on February 2, 2003 for which no standard measurements were carried out.

  3. XMM-02 (I)- The observations for this pointings and filter were done with two OBs of I-band (10 exposures each) on February 2, 2003 for which no standard measurements were carried out.

  4. XMM-03 (V) - The V calibration on the night of March 26, 2003 yields a 3-parameter fit that deviates from the median of the solutions by roughly 0.26 mag (less than 3 sigma).

  5. XMM-07 (R) - This field was observed on four nights (August 6, and September 23, 27, and 28, 2003) as part of the contributing program. For the night of August 6 a 3-parameter fit solution was obtained. However, this solution deviates by roughly 0.25 mag relative to median of all R-band solutions.

  6. XMM-08 (I) This field was observed using 3 OBs (which in this case also correspond to 3 RBs) on two nights (March 30, 2003, one OB and April 2, 2003, two OBs). On the night April 2 a 3-parameter solution was obtained for which the ZP determined deviates significantly (more than 3 sigma) from the median of all solutions, even though the conditions of the night seem to have been adequate. The reason for this poor solution is at present unknown. Poor fringing correction is a possibility but needs to be confirmed. The zeropoint for the two reduced images taken on this night has been replaced by a default value (in the current release this value differs by 0.03 mag from the one set by the photometric pipeline because different settings were inadvertently used in estimating a default value - since then this problem has been fixed. Note that this has no impact at all in the results as explained in this README).

  7. XMM-10 (R) - This field was observed in the nights of August 6, and September 23 and 29, 2003 as part of contributing program. Note that as in case of XMM-07 the solution for August 6 deviates somewhat from the median.

  8. XMM-11 (V) - see XMM-03 (V)

  9. XMM-12 (V) - see XMM-03 (V)

These results imply that currently the stacks for field/filter combination of XMM-01 (R), XMM-02 (R) and XMM-02 (I) cannot be properly calibrated.

Other Effects

For detectors formed by a mosaic of individual CCDs, in order to correct for CCD-to-CCD gain variations, median background values sampled in sub-regions bordering adjacent CCDs are used to harmonize the gain to a value common for all CCDs in the mosaic. This has been applied to both science and standard exposures.

It is also known that large-scale variations due to non-uniform illumination over the field of view of wide-field instruments exist. The significance of this effect is passband-dependent and becomes more pronounced with increasing distance from the optical axis (Manfroid et al. 2001; Koch et al. 2004; Vandame et al. 2004). Automated software to correct for this effect has been developed but due to time constraints it has not yet been applied to these data.

Cosmetic Features

In order to optimize the removal of cosmic rays using a sigma-clipping algorithm, the original Observation Block (OB) has been split into at least three reduction blocks (RB). Tests have shown that by so doing there is a significant decrease of cosmic rays in the general region of inter-chip gaps for the final stacked image, which will become available in a forthcoming release.

It is important to note that although not forming part of this release the methodology adopted for cosmic ray rejection had as its main goal the optimization of the quality of final ``stacked'' images. Therefore, the reduced images (which are in reality intermediate products) being released may still have a high frequency of cosmic rays which do not reflect the quality of the final ``stacked'' image. From visual inspection of the images, the occurrence of cosmic rays is higher in B- and V-bands, being of the order of 2-3 cosmic rays/arcmin square. This is because, typically the total number of exposures in these bands is smaller, thus affecting the cosmic rays rejection algorithm being used.

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.

Those interested in solar system objects should be aware that for the typical integration times used in the present survey minor bodies (e.g. asteroids) should usually exhibit short tracks in individual exposures and should therefore be unaffected by the Hough transform technique. However, visual inspection of the images reveals that in some cases they are rejected by the sigma-clipping rejection technique used for cosmic rays removal.

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 a corresponding product log.

Out of 150 reduced images covering the selected XMM fields, 104 were graded A, 35 B, 7 C and 4 D. Note that images with grade D are not being released, since they have no scientific value. In the table below the images with grades C and D are presented. The table, ordered by field and date, lists: in column (1) entry number; in column (2) the EIS field name; in column (3) the passband; in column (4) the date when the night started (YYYY-MM-DD); in column (5) the grade given by the visual inspection; and in column (6) the primary motive for the grade.

# EIS Field Name Passband Date Grade Comment
1 XMM-05 R 2002-10-14 D strong stray light contamination
2 XMM-06 I 2003-01-29 D inadequate fringing correction, probably due to improper dithering
3 XMM-12 I 2003-03-29 D very short integration time
4 XMM-12 V 2003-09-27 D out-of-focus
5 XMM-01 V 2003-02-01 C strong shape distortions
6 XMM-07 R 2003-08-06 C stray light contamination
7 XMM-10 R 2003-08-06 C fringing
8 XMM-10 R 2003-09-23 C fringing
9 XMM-10 R 2003-09-29 C fringing
10 XMM-10 R 2003-09-29 C fringing
11 XMM-10 R 2003-09-29 C fringing

As can be seen the success rate is better than 95% and most of the lower grades are associated with observational problems rather than inadequate performance of the software operating in an un-supervised mode. An interesting point is that occasionally R-band images are affected by fringing - for instance, in the nights of August 6 and September 23 and 29, 2003, all from the contributing program. The night of August 6 is one of the nights for which the computed R-band zeropoint deviates from the median. This points out the need to consider applying fringing correction also in the R-band, at least in some cases. The R-band fringing problem accounts for five out of seven grade C images. The remaining cases are due to stray-light and strong shape distortions.

Photometric Calibration

The photometric solutions computed automatically by the EIS Survey System were compared with the ``best solution'' recently obtained by the 2p2 Telescope team. The results of this comparison, which agree remarkably well, are presented in the table below for the general case of 3-parameter fits. The table lists: in column (1) the passband; in columns (2)-(4) the offsets (EIS-Telescope Team) of the zeropoint, extinction and color term.

Passband Zp k color
B 0.00 -0.03 -0.06
V -0.05 0.0 -0.01
R 0.0 0.1 0.0
I -0.04 0.05 -0.02

It is worth emphasizing that, for the present release, the periods of observations of standard stars available to the two teams do not coincide.

Not surprisingly, larger offsets are obtained when 2- and 1-parameter fits are included, depending on the passband and estimator used to derive the estimates for extinction and color term, as applicable. Finally, taking into consideration only 3-parameter fit solutions and after rejecting 3 sigma outliers one finds that the scatter of the zeropoints is less than about 0.08 mag. It is important to emphasize that this number is still uncertain given the small number of the 3-parameter fits currently available, especially in the R-band. This value for scatter is a reasonable estimate for the current accuracy of the absolute photometric calibration of the XMM survey data.

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 the 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), sky brightness, airmass, 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. As a reference, the vertical lines in the sky brightness distribution represent estimated values expected for new moon and full moon (Walker 1987, NOAO Newsletter).

  2. the second figure consists of seven panels showing the time dependence of the following image attributes: grade, limiting magnitude (Vega, 5 sigma, 2 arcsec aperture), sky brightness, airmass, seeing, the rms of the PSF and the zeropoint of the night solution. The color code is the same as before and the night is the number of days relative to October 30, 2002, the first night associated with a released image (images before this night were discarded).

  3. the third figure displays the correlation between limiting magnitude (Vega, 5 sigma, 2 arcsec aperture) versus seeing, sky brightness and integration time. The points lying in the lower left corner of the second panel (shallow and bright) are fields located at low galactic latitude.

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 rays removal, gain-harmonization) were resolved, leading at the end to results in excellent agreement.

Remarks on Individual Products

For some reduced images (e.g. XMM-01, XMM-02 and XMM-12) significant distortions are visible at the corners, especially in the R- and I-bands.

  1. XMM-01: this field is particularly crowded/dense -- which present challenges when trying to select an optimal reduction mode. Reductions tuned to correct for structures such as fringes, by generating corrections from the content of a reduction block, may not always be successful in extremely crowded fields. In this case, results have improved by adopting a fringe map produced using data external to the RB. This technique improves the overall cosmetic result, although some residual fringes may remain in the I-band. One of the V-band images shows, as mentioned above, significant (about 20%) image distortions over the entire WFI field.

  2. XMM-02: the same technique of fringing correction for I-band as XMM-01 is applied.

  3. XMM-05: justifies the use of the GSC2.2 astrometric reference catalog which yields scatter in astrometry of circa 0.2 arcseconds. By comparison, the use of USNO(A) astrometric catalog for this field has scatter in astrometry of circa 0.4 arcseconds. R-band images for this exhibit low-level fringes.

  4. XMM-07: stray light reflections contaminate the V-, R- and I-band images, being most prominent in R.

  5. XMM-08: no R-band data.

  6. XMM-09: no R-band data. XMM-09 exhibits the highest occurrence of asteroid tracks in all passbands.

  7. XMM-10:

    - Raw data for V- and I are not yet available.

    - The R-band image of this field shows relatively strong fringing on a large-scale indicating that it may be necessary to apply fringing correction (see above).

    - Originally, wrong coordinates were provided to the EIS team which entered them in its supporting database. Since OBs and associated finding charts for execution are automatically generated from these tables, the service mode package submitted for observing period 71 contained this error. This resulted in a series of observation blocks being executed at the incorrect position (20:05:09.8, -02:55:18) for the XMM-10 field executed as part of the large programme 170.A-0789 on the following dates 2003-04-04 (B); 2003-04-07, 2003-04-12, 2003-07-290 (V); 2003-04-08, 2003-04-09, 2003-04-11 (I). As mentioned earlier these data, comprising 53 exposures taken in roughly 5 hours covering seven nights, are not included in this release.

    - Data at the correct co-ordinates of (22:05:09.8, -01:55:18) (R- and limited B- passband) were obtained as part of program 71.A-0110 on the following dates: 2003-08-06 (B, R), 2003-09-23, 2003-09-29.

  8. XMM-11: Like XMM-01 and XMM-02 this is a crowded field and uses an external fringing map.

  9. XMM-12: Like XMM-01, XMM-02, XMM-11.

Next Release(s)

Anticipated future EIS/XMM releases include:


This release is the third of 2004, and number 19 since March 1998 (of which two were software releases). Since May 2001 a total of over 1500 requests (averaging 30 requests/month) have been made for raw (919) and reduced (543) data and of the EIS/MVM image processing software (56) . These requests comprised 68,533 files (4.6 Terabytes; 2.5 Tb compressed) of raw data and 6184 files (470 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 May 2004.

Over the past few months significant progress has been made in the development of an entire set of administrative tools enabling users of the EIS Survey System to have easy access to a host of information essential for a controlled data release, ensuring a proper accounting of the destination of the accumulated data and a thorough description of the quality of the survey products, including summary plots now also available to the users of the survey data.. Major improvements were also made to the report-facility responsible for the preparation of the material displayed on the WEB and of the statistics of survey products requests. Most of these improvements are illustrated by the contents of the present release, others will be phased-in in subsequent releases.

Finally, it is worth mentioning that the present release was a necessary intermediate step which will soon be followed by the release of stacks and more advanced products (single-passband and color catalogs).


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

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XMM/WFI Survey Release

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