Pre-FLAMES Survey Release

3 November 2004

Technical Summary

Survey ..................... Pre-FLAMES (PF)
Telescope .................. ESO/MPG 2.2m
Instrument ................. WFI
Program IDs ................ 164.O-0561; 267.A-5729
Origin ..................... ESO/EIS
Number of Fields ........... 48
Passbands .................. B; V; R; I (2)
Number of Filters .......... 5
EIS Release Number ......... 21
Version .................... 0.7
Total Data Volume .......... 142 Gb
Release Date ............... October 2004
Release prepared by ........ EIS/PSG team, S. Zaggia

Product Type ............... Nightly Products
Number of Reduced Images ... 287
Data Volume ..... .......... 142 Gb

Origin ..................... ESO/EIS
Number of Fields ........... 53
Passbands .................. B; V; I
EIS Release Number ......... 15
Version .................... 0.6
Release Date ............... 24 March 2003

Product Type ............... Nightly Products
Number of Reduced Images ... 303


Based on ideas submitted by the ESO community and evaluated by the Working Group for Public Surveys, the Pre-FLAMES survey, administered by the ESO Imaging Survey (EIS) and Public Survey Group (PSG) teams, is a multi-color survey of selected fields consisting of short- and long-exposures in B-, V-, and I-bands taken using the ESO Wide Field Imager (WFI) at the ESO/MPG 2.2m telescope at the La Silla Observatory. The aim was to gather high quality astrometric and photometric data for fields of high stellar density, to produce object catalogues with the required accuracy to provide input target lists for 160 candidate fields foreseen to be ideally suited to studies using FLAMES, a fibre-fed multiple element spectrograph available on VLT Unit Telescope 2 (Kueyen). The candidate fields consisted of Globular Clusters, Open Clusters, regions of the Milky Way bulge and Halo, a selection of Local Group galaxies, the Sagittarius Dwarf galaxy, and both the Large and Small Magellanic Clouds (LMC and SMC).

The data contributing to the present release were obtained as part of the ESO Large Programme: 164.O-0561 (Principal Investigator: J. Krautter, as chair of the SWG) and DDT programme 267.A-5729 over ESO's observing periods 64 through 68.

The present release consists of data from the above programs accumulated in observations covering 2 years, five semesters (Periods 64-68), from March 31, 2000 through March 23, 2002, corresponding to 450 science exposures, and 21 hours of on-target integration, during a period of 23 nights.

The 450 raw WFI exposures were converted into 291 fully calibrated reduced images, of which 287 are being released, covering more than 12 square degrees in typically three passbands. Of the remaining four, one was observed at the wrong position, two were observed at twilight while the sky was very bright, and one presented elongated images because of telescope tracking problems.

Combined with previous two releases the Pre-Flames survey is roughly 70% complete covering 21 globular and 20 open clusters, mini-mosaics as well as individual pointings covering different regions of LMC (20 pointings), SMC (5 pointings), Sagittarius (17 pointings), and the Milky Way (14 pointings).

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 release of reduced data for the Pre-Flames Survey. The present release consists of 287 fully calibrated, final ESO/MPG 2.2m WFI short- and long-integration time images (typically 30 and 480 seconds, respectively) of 48 selected fields listed in the table below. The images being released include 9 globular clusters, 4 pointings of a mosaic covering the bar of the Large Magellanic Cloud (LMC), a mosaic of 13 pointings covering the central parts of the Milky Way (MW), 2 open clusters, 17 pointings of mosaics covering four distinct regions of about 0.6 square degree each of Sagittarius (SG), and 3 pointings covering 0.75 square degrees of the Small Magellanic Cloud (SMC). These images were obtained in the following passbands:

  1. B: 95 images;

  2. V: 93 images;

  3. R: 2 images;

  4. I: 97 images - 8 obtained using the I filter #879 with a mean wavelength of 8269 Å (3 nights in the period from March 9, 2002 to March 13, 2002) and the remaining 89 using the filter #845 with a central wavelength of 8643 Å and FWHM = 1387 Å (25 nights in the period from April 6, 2000 to March 9, 2002).

The breakdown of these images per region/field is presented in the table below. The table lists: in column (1) a sequence number; in column (2) the EIS region name; in column (3) the EIS field name, and in columns (4)-(10) 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.

# Region EIS Field Name B#842 V#843 R#844 I#879 I#845
1 GC07 GC07 0 (0) 0 (0) 0 (0) 0 (0) 2 (1)
2 GC09 GC09 0 (0) 0 (0) 2 (1) 0 (0) 0 (0)
3 GC13 GC13 0 (0) 2 (1) 0 (0) 0 (0) 0 (0)
4 GC14 GC14 0 (0) 0 (0) 0 (0) 0 (0) 4 (2)
5 GC19 GC19 0 (0) 0 (0) 0 (0) 0 (0) 4 (2)
6 GC22 GC22 4 (2) 4 (2) 0 (0) 0 (0) 2 (1)
7 GC29 GC29 2 (1) 2 (1) 0 (0) 0 (0) 2 (1)
8 GC30 GC30 2 (1) 1 (1) 0 (0) 0 (0) 2 (1)
9 GC32 GC32 4 (2) 4 (2) 0 (0) 0 (0) 4 (2)
10 LMC Bar LMC01 2 (1) 2 (1) 0 (0) 2 (1) 0 (0)
11 LMC Bar LMC02 4 (2) 4 (2) 0 (0) 2 (1) 0 (0)
12 LMC Bar LMC07 2 (1) 2 (1) 0 (0) 2 (1) 0 (0)
13 LMC Bar LMC09 2 (1) 2 (1) 0 (0) 0 (0) 2 (1)
14 MW-B MW05 2 (1) 0 (0) 0 (0) 0 (0) 0 (0)
15 MW06 MW06 2 (1) 0 (0) 0 (0) 0 (0) 0 (0)
16 MW07 MW07 1 (1) 1 (1) 0 (0) 0 (0) 2 (1)
17 MW08 MW08 2 (1) 2 (1) 0 (0) 0 (0) 2 (1)
18 MW09 MW09 2 (1) 2 (1) 0 (0) 0 (0) 2 (1)
19 MW10 MW10 2 (1) 2 (1) 0 (0) 0 (0) 2 (1)
20 MW12 MW12 2 (1) 2 (1) 0 (0) 0 (0) 2 (1)
21 MW13 MW13 2 (1) 2 (1) 0 (0) 0 (0) 2 (1)
22 MW14 MW14 2 (1) 2 (1) 0 (0) 0 (0) 2 (1)
23 MW15 MW15 2 (1) 2 (1) 0 (0) 0 (0) 2 (1)
24 MW16 MW16 2 (1) 2 (1) 0 (0) 0 (0) 2 (1)
25 MW17 MW17 2 (1) 2 (1) 0 (0) 0 (0) 2 (1)
26 MW18 MW18 2 (1) 2 (1) 0 (0) 0 (0) 2 (1)
27 OC11 OC11 4 (2) 4 (2) 0 (0) 2 (1) 2 (1)
28 OC31 OC31 2 (1) 2 (1) 0 (0) 0 (0) 2 (1)
29 SG-A SG00 2 (1) 2 (1) 0 (0) 0 (0) 2 (1)
30 SG-A SG01 4 (2) 4 (2) 0 (0) 0 (0) 4 (2)
31 SG-A SG02 2 (1) 2 (1) 0 (0) 0 (0) 1 (1)
32 SG-A SG03 2 (1) 2 (1) 0 (0) 0 (0) 2 (1)
33 SG-A SG04 2 (1) 2 (1) 0 (0) 0 (0) 2 (1)
34 SG-B SG05 1 (1) 2 (1) 0 (0) 0 (0) 2 (1)
35 SG-B SG06 2 (1) 2 (1) 0 (0) 0 (0) 2 (1)
36 SG-B SG07 2 (1) 1 (1) 0 (0) 0 (0) 2 (1)
37 SG-B SG08 2 (1) 2 (1) 0 (0) 0 (0) 2 (1)
38 SG-C SG09 2 (1) 2 (1) 0 (0) 0 (0) 2 (1)
39 SG-C SG10 2 (1) 2 (1) 0 (0) 0 (0) 2 (1)
40 SG-C SG11 2 (1) 2 (1) 0 (0) 0 (0) 2 (1)
41 SG-C SG12 2 (1) 2 (1) 0 (0) 0 (0) 2 (1)
42 SG-D SG13 2 (1) 2 (1) 0 (0) 0 (0) 2 (1)
43 SG-D SG14 2 (1) 2 (1) 0 (0) 0 (0) 2 (1)
44 SG-D SG15 1 (1) 2 (1) 0 (0) 0 (0) 2 (1)
45 SG-D SG16 4 (2) 4 (2) 0 (0) 0 (0) 4 (2)
46 SMC SMC01 2 (1) 2 (1) 0 (0) 0 (0) 2 (1)
47 SMC07 SMC07 2 (1) 2 (1) 0 (0) 0 (0) 0 (0)
48 SMC08 SMC08 2 (1) 2 (1) 0 (0) 0 (0) 2 (1)

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).

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. the table entry number (not to be confused with the "Product Identification number" which is reported in the FITS header of the image);

  2. the EIS standard field/region name with an associated hyperlink to a color JPEG format file showing a "RGB" color composite image based the last long-exposure for the cases where BVI images exist. 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. Observed MJD;

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

  7. Right Ascencsion (RA) in J2000.0;

  8. Declination (Dec) in J2000.0;

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

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

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

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

  13. Hyperlink to a comprehensive descriptive log of the product. (Product logs are planned for phased introduction over a series of forthcoming releases);

  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.

NB: note that the assigned grade takes into consideration the type of product considered. For instance, a grade ``A'' stack does not have the same meaning as a grade ``A'' night product associated to one RB

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, when appropriate. 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 in the future the 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 ESO Filter Name ESO Identity Number
B B/99 #842
V V/89 #843
R Rc/162 #844
I BB#I_EIS #879
I Ic/Iwp #845

Products and Completeness

This release consists of final astrometrically and photometrically calibrated ``reduced'' images. For the PF survey there are usually one short-exposure (30 sec) and two long exposures (480 sec) per passband and per field, so as to enable to probe both the bright and faint end of the stellar luminosity function of the systems of interest.

As mentioned earlier the overall completeness of the survey relative to the original proposal is about 70%. Considering the 101 fields included in this and the last release only 8 fields do not have full color coverage in three psasbands. Occasionally, in one of the bands only one image is available.

The completeness in passband and integration time for the data being released can be seen in the figures accessible from the release WEB page. This figure shows for each field two superposed histograms showing 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 of all reduced images (short- plus long-exposures) in a particular filter. The color code adopted for the filters (passbands) here and elsewhere is as follows: B (#842, black); V (green); R (red), I (#879; magenta); and I (#845; yellow). Note that only for a few fields are the R-band (2 reduced images) and the I (#879) (8 reduced images) used. However, since the plots are prepared automatically a place for them appears for every field. Also note that for contiguous pointings the completeness plots are shown as a mosaic of thumbnails. In fact, inspection of this figure illustrates the complex nature of the strategy adopted in this survey, pointing out the need of general systematic procedures to enable an adequate reporting of the various cases.

Name Convention

As explained elsewhere (see EIS Pointing Nomenclature) the convention 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)s which have been prepared by multiple team members, the naming convention may not be homogeneous, and thus the value of the OBJECT keyword should not be, in general, used to rename files.

Comparison to Previous Release(s)

Since the last release of PF data carried out in March 2003, several improvements have been made to the code and are described below in more detail. It is worth mentioning that a correction for illumination variation across the camera is applied for the first time. Furthermore, in the present release proper nightly photometric solutions have been derived leading to fully photometrically calibrated final images.

The fields being currently released complement the earlier release either in the color coverage (e.g. GC07, GC09, GC13, GC14, GC19) or the spatial coverage especially of LMC, and the Milky Way.

Data Reduction

The WFI data were reduced using the most recent version of the EIS/MVM image processing library, currently still under test, and which will be available in the near future. A stable version of this software package is publicly available and can be retrieved starting from the EIS survey release page.

For this release an attempt has been made to further improve the photometric calibration WFI data using the EIS Survey System. A proper calibration involves correction for CCD-to-CCD gain variations, illumination correction and absolute zeropoint determination.

In contrast to previous reductions, the background of exposures of stellar field are not removed and only the images with the same integration time are combined. For exposures taken close to twilight, there are strong variations in the background level and the difference between consecutive images is subtracted out before the images are co-added. More details about the methodology employed can be found in Vandame (2004). Other features of the code worth mentioning, some representing significant improvements of previous reductions, are:

  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. New procedure to homogenize the chips was adopted

  4. Improved sensitivity to the automatic removal of satellite tracks

  5. Robust method for cosmic ray detection and removal

  6. Illumination correction using external maps derived from a sequence of exposures of reasonably dense fields. A proper discussion of the method as well as a full report on the tests carried out to validate the procedure will be presented elsewhere.


The astrometric calibration was derived using the GSC2.2 reference catalog and a distortion model described by a second order polynomial. Comparisons yield a typical scatter of 0.2 arcseconds (Vandame 2004). However, comparisons made using exposures of a globular cluster shifted by half a field strongly suggests that the internal accuracy is of about 70 mas. The astrometric calibration is carried out on a chip by chip basis. Possibly, connecting the different solutions may lead to a futher improvement in the accuracy. 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

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 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 the measured instrumental magnitudes 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. Nearly all observations were carried out in visitor mode, by a large number of people, making it difficult to ensure a homogeneous calibration plan.

For the present release photometric solutions were attempted for the 23 nights with observation of standard fields and science observations (including images with grade D).

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#842 4 10 0 5 19
V#843 1 13 2 4 20
R#844 0 1 0 0 1
I#879 0 0 1 2 3
I#845 2 10 1 5 18

From the above table one finds that a large fraction of the solutions rely on one-parameter fits, indicating that the adopted calibration plan for the PF survey was not adequate for highly accurate photometry, as the primary goal was to provide astrometric calibrated images for observations with FLAMES.

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

The photometric solutions obtained as described above 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 airmass in which the image was observed. 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.

The following table shows for each field/filter the type of best-solution available.

# Region EIS Field Name default 1-par 2-par 3-par
1 GC07 GC07       I
2 GC09 GC09   R    
3 GC13 GC13       V
4 GC14 GC14       I
5 GC19 GC19   I    
6 GC22 GC22   B V I    
7 GC29 GC29 B V I    
8 GC30 GC30   V I   B
9 GC32 GC32       B V I
10 LMC Bar LMC01   V   B I
11 LMC Bar LMC02   V   B I
12 LMC Bar LMC07   B V   I
13 LMC Bar LMC09 B I V    
14 MW-B MW05   B    
15 MW06 MW06   B    
16 MW07 MW07   B V I    
17 MW08 MW08   B V I    
18 MW09 MW09   B V I    
19 MW10 MW10   B V I    
20 MW12 MW12   B V I    
21 MW13 MW13   B V I    
22 MW14 MW14   B V I    
23 MW15 MW15   B V I    
24 MW16 MW16   B V I    
25 MW17 MW17   B V I    
26 MW18 MW18   B V I    
27 OC11 OC11   I I B V
28 OC31 OC31   B V I    
29 SG-A SG00   B V I    
30 SG-A SG01   B V I    
31 SG-A SG02   B V I    
32 SG-A SG03   B V I    
33 SG-A SG04   B V I    
34 SG-B SG05   B V I    
35 SG-B SG06   B V I    
36 SG-B SG07   B V I    
37 SG-B SG08   B V I    
38 SG-C SG09   B V I    
39 SG-C SG10   B V I    
40 SG-C SG11   B V I    
41 SG-C SG12   B V I    
42 SG-D SG13   B V I    
43 SG-D SG14   B V I    
44 SG-D SG15   B V I    
45 SG-D SG16   V I   B
46 SMC SMC01 B     V I
47 SMC07 SMC07 B   V  
48 SMC08 SMC08     V B I

From the above table one finds that nearly all pointings/filter combinations have photometric solutions, even though most rely on one-parameter fits. The table also serves as a rough indication of the accuracy of the photometric calibration with, of course, 3-parameter fits being normally the best.

The above table can also be used to evaluate the completeness of the color coverage for each pointing, except for a few cases where images in other passbands have been previously released (e.g. GC07, GC09, GC13, GC14, GC19)

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 and has been applied for the first time to the present data, except for the R-band. While the accuracy of the correction requires further investigation, tests carried out so far suggest that for the BVI passbands the accuracy of the relative center-to-border zeropoint is better thatn 2%. This after applying corrections for the chip-to-chip variation and illumination effects. It is worth emphasizing that the illumination effect can reach 11%, 14% and 20% for the passbands B, V and I, respectively. Note that the illumination correction has been applied only to the science frames. This is because the reductions of the science exposures were carried out after the standards had been reduced for the DPS survey which shared nights with Pre-Flames.

Cosmetic Features

Given the observed strategy no removal of cosmic rays has been done.

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. Occasionally, the Hough transform technique has some side effects improperly masking bad columns and the edges of the chips.

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 291 reduced images covering the selected Pre-Flames fields, 178 (61%) were graded A, 101 (35%) B, 8 (3%) C and 4 (1%) D.

Note that images with grade D are not being released, since they have no scientific value. In the table below the 12 images with grades C and D are presented. The table, ordered by field and date, lists: in column (1) entry number; in columns (2)-(3) the EIS region and field names; in column (4) the passband; in column (5) the date when the night started (YYYY-MM-DD); in column (6) the grade given by the visual inspection; and in column (7) the primary motive for the grade.

# Region EIS Field Name Passband Date Grade Comment
1 GC18 GC18 B#842 2000-08-03 D wrong pointing
2 GC22 GC22 I#845 2000-08-01 D High-background
3 GC22 GC22 I#845 2000-08-01 D High-background
4 SG-A SG02 I#845 2000-07-27 D problems with telescope tracking. Stars are all elongated.
5 MW09 MW09 I#845 2000-07-28 C Central gap still present in part of the image. Larger trimming than normal. Fringing.
6 MW10 MW10 I#845 2000-07-29 C Central gap still present in part of the image. Larger trimming than normal. Fringing.
7 MW13 MW13 I#845 2000-08-01 C Central gap still present in part of the image. Larger trimming than normal. Fringing.
8 MW16 MW16 I#845 2000-08-01 C Central gap still present in part of the image. Larger trimming than normal. Fringing.
9 MW17 MW17 I#845 2000-08-01 C Central gap still present in part of the image. Larger trimming than normal. Fringing.
10 MW18 MW18 I#845 2000-08-01 C Central gap still present in part of the image. Larger trimming than normal. Fringing
11 OC11 OC11 I#879 2002-03-09 C Central gap still present in part of the image. Larger trimming than normal. Some fringing still present. Bright central object: HD 63032 (c Pup, B=5.34)
12 OC11 OC11 I#879 2002-03-09 C Central gap still present in part of the image. Larger trimming than normal. Some fringing still present. Bright central object: HD 63032 (c Pup, B=5.34)

The table below shows for each filter the grade breakdown. The table gives: in column (1) the passband; in column (2) the ESO filter number; in column (3) the number of images; (4)-(7) the ratio of images in a given grade to the total number of images taken with that filter.

Passband Filter Number of products A B C D
B #842 96 0.78 0.21 0.0 0.01
V #843 93 0.7 0.3 0.0 0.0
R #844 2 1.0 0.0 0.0 0.0
I #879 8 0.63 0.13 0.25 0.0
I #845 92 0.34 0.57 0.07 0.03

The next table shows how images of different passbands contribute to a given grade. The table lists: in column (1) the grade; in column (2) the total number of images with that grade; in columns (3)-(8) the fraction of images of a given grade and filter relative to the total number of images with that grade. It is interesting to point out that the I-band images usually have lower grades.

Grade Number of Images B#842 V#843 R#844 I#879 I#845
A 178 0.42 0.37 0.01 0.03 0.17
B 101 0.2 0.28 0.0 0.01 0.51
C 8 0.0 0.0 0.0 0.25 0.75
D 4 0.25 0.0 0.0 0.0 0.75

Photometric Calibration

As part of the photometric calibration, nights with good airmass and color coverage were used to compute complete photometric solutions, namely those for which all parameters (zeropoint, extinction and color term) can be independently estimated. In the table below we show the results giving: in column (1) the passband/filter; in column (2)-(4) the median zeropoint (Zp), extinction, (k), and color term (color) values for all solutions with three parameters.

Passband Zp k color
B#842 24.54 0.15 0.24
V#843 24.23 0.14 -0.13
R#844 n/a n/a n/a
I#879 23.34 0.04 0.04
I#845 23.12 0.1 0.11

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 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#842 n/a n/a n/a
V#843 0.08 0.03 0.0
R#844 n/a n/a n/a
I#879 -0.03 0.04 0.01
I#845 n/a n/a n/a

The solutions are remarkably similar, despite the fact that they are over two years apart.

It is worth emphasizing that, for the present release, the periods of observations of standard stars available to the two teams do not coincide. Also note that there are two (in the case of the Pre-Flames survey, B and I) filters EIS has used for which no solutions have been reported by the telescope team. For these cases, the differences are listed as not-available (n/a).

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 0.1 mag. This value for scatter is a reasonable estimate for the current accuracy of the absolute photometric calibration of the Pre-Flames survey data.

The quality of the photometric calibration for the dataset being released can be estimated from the table below. The table lists: in column (1) the passband/filter combination; in column (2) the median zeropoint (Zp) including solutions obtained from fits with an arbitrary number of free parameters; in column (3) the estimated rms; in column (4) the largest offset of a night calibration relative to the median; in columns (4)-(7) the same as the three previous columns, except that now the estimate of the median was computed using only 3-parameter fits.

Passband Zp RMS $\Delta$max Zp RMS $\Delta$max
B#842 24.77 0.13 0.27 24.54 0.08 -0.33
V#843 24.09 0.14 0.49 24.23 0.12 0.63
R#844 24.4 0.0 0.0 n/a n/a n/a
I#879 23.32 0.05 0.06 23.34 0.02 0.08
I#845 23.15 0.09 0.23 23.12 0.07 -0.2

From the above table, one sees that considerable work remains to be done to significantly reduce the photometric calibration error which is typically of the order of 0.1 mag.

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 (2 arcsec aperture, 5 sigma, Vega), 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 (2 arcsec aperture, 5 sigma, Vega, 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 March 21, 2000, the first night associated with a released image.

  3. the third figure displays the correlation between limiting magnitude (2 arcsec aperture, 5 sigma, Vega 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.

The color code adopted is as follows: B (#842, black); V (#843, green); R (#844, red), I (#879; magenta); and I (#845; yellow).

Remarks on Individual Products

Inspection of the individual FITS images as well as the color composite JPEG images show that normally the inter-chip gap imprints are visible due to the small number of images used in combination for this survey (e.g. MW10). In other occasions only small holes near the center are noticeable (e.g. GC22). It is also worth noting that in some cases the diffraction spike of bright stars, both horizontally as well as vertically, across a single-chip, are also masked by the Hough technique in at least one of the passbands (e.g. GC30, MW13, MW14, MW16, MW18, OC11).

Next Release(s)

Anticipated future related releases include:


This release is the fifth of 2004, and number 21 since March 1998 (of which two were software releases). 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 October 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.

With present release the Pre-Flames has provided astrometrically and now, photometrically calibrated images in three-passbands for 101 selected fields which include 21 globular and 20 open clusters as well as mosaics covering different regions of LMC (20 pointings), SMC (5 pointings), Sagittarius (17 pointings), and the Milky Way (14 pointings).


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

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

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

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

eis data account 2004-11-03