The ESO Imaging Survey

EIS Pre-Flames Survey Release version 0.6

This is the second data release of the so-called Pre-Flames (PF) survey. These data were accumulated in visitor mode by the EIS team over the period 1999-2002 for the science verification and first period of observations of FLAMES. The present release consists of:

• fully astrometrically calibrated (short and long) B, V and I images of 51 selected fields including the bulge, globular and open clusters, and the Large Magellanic Cloud.
• color catalogs (as they become available) produced using DAOPHOT, to help in a preliminary selection of objects for spectroscopic observations.

While still not final, the products being released are meant to illustrate the ongoing work by the survey group and provide data to groups that already have scheduled observations with FLAMES.

Please report any errors/features found with the released data to eis-help@eso.org, indicating the number of the product identification number (PROD_ID) which can be found in the header of each image and catalog.

Notes on Version 0.6 release

PREVIOUS DATA RELEASE (2001-07-06, version 0.5)

This release completely supersedes the data release of the Pre-Flames release 0.5 performed on 06 July 2001 for the following 6 fields: OC03, OC12, OC14, OC99, SMC05 and SMC06. In the present release a new warping algorithm has been applied to the images, de-fringed I-band images are being released and the Guide Star Catalog (GSC) 2.2 catalog has been used as the astrometric reference catalog.

DATA PROCESSING

The reductions were carried out using the C-based pipeline described in Vandame (2002, astro-ph/020823) but outside the framework of the EIS survey system. The latter is still being tested for single-chip instruments. The basic methodology of the reduction has been described in Momany et al (2001, A&A 379, 436), Zaggia et al (2001, The Messenger, No.105, p.25), and Vandame (2002) and is not repeated here. Some improvements worth noting are:

• the present release includes I-band images corrected for fringing; if possible the fringing map has been computed for the same night as the observations;
• the single CCDs have been relatively homogenized photometrically but the global sky background value of the image has NOT been subtracted. This was done to allow DAOPHOT/ALLSTAR to properly detect objects in the image.
• compared to the previous Pre-Flames release 0.5 the final images have been "warped" using a kernel algorithm instead of the nearest neighborhood method, eliminating the discreteness reported earlier.

An important point is that for each passband there are two final frames: a short- and long-integration exposure, respectively of 30 and 480 seconds in total. The long exposure is composed of 2 dithered single exposures of 240 seconds each.

IMAGE FEATURES

In the present release some images still present features that with the present infrastructure could not be removed completely. We note that

• the Hough transformation used to detect satellite tracks has not been used in the present release. Its implementation is not as efficient in crowded stellar fields as it is in sparse fields. Improvements are under consideration. Images with satellite track include: LMC10,11,14,15,16,19,29,32.
• some images present uneven background due to various reasons (nearby bright stars, images taken during twilight), like for example: GC06, GC11, MW11, OC-03, 15, 21, 22, 26.
• in some cases the zero-point uniformization of the images was not properly done for some of the chips (e.g. MW11).
• some I-band images present some remnant of the fringing map due to the absence of an appropriate fringing map for nights when when sequences of DEEP images were not available. This effect is mostly present on the chip #1 (top-left corner) and #6.

ASTROMETRIC SOLUTION

An astrometric solution was found for each field, band and exposure independently and comparing detected star positions with the GSC 2.2 reference catalog. All released images have been inspected for defects and whenever available a final tri-color image (composed from the 3 long exposures) has been checked for astrometric deviations in the three bands. All final images for each field use the "TAN" projection for the WCS and share the same astrometric grid.

PHOTOMETRIC CALIBRATION

Each image has been normalized to 1.0 second of exposure and the image keyword GAIN is set appropriately to the right number of electrons per ADU. The photometric calibration is still preliminary with no illumination correction applied and no individual night solutions.

The photometric calibration in the WFI natural system (WFI) is based on zero-points (ZP) and extinction coefficients (k_ext) obtained earlier in the campaign and as expected by the telescope team they should have an error of at least +/- 0.2 mag. The header of each image lists the keyword ZP which gives the zero-point value corrected at zero airmass, i.e. also including the correction for extinction: so that

mag(Vega)_WFI = -2.5*Log(flux) + ZP,

where flux is the number of counts directly measurable on the images. Finally, since these are formal ZP we do not quote internal ZP errors in the headers (for this reason the keyword ZP_ERR is set to -1).

Calibration to the standard Johnson-Cousins (JC) system can be obtained by applying color term (c_ter) corrections as shown in the table below and should be used as follows in the case of the B mag:

B_JC - B_WFI = ZP + c_ter*(B-V)_JC

similar equations hold for the V and I bands.

The ZP used for each passband have been taken from the following web page of the SCI-Ops group in La Silla:

http://epu.ls.eso.org/lasilla/sciops/2p2/E2p2M/WFI/zeropoints/

where the complete solutions for the color terms can also be found.

The values used for the ZP and extinctions for each passband are the following:

DATA FORMAT

Each FITS file in the release contains two extensions, the first one is the science frame, the second one is the weight map defined as a variance map (e.g. it should be used with SExtractor using the parameters: -WEIGHT_IMAGE weight_map.fits -WEIGHT_TYPE MAP_WEIGHT).

IMPORTANT NOTICE TO USERS:

• When using these data either in publications or proposals please follow the copyright instructions, available at: /science/eis/copyright.html:
  "Observations have been carried out using the MPG/ESO 2.2m Telescope and the ESO New Technology Telescope (NTT) at the La Silla observatory under Program-ID No. 164.O-0561"
• If the data or part of it is used or shown publicly please give proper acknowledgment to the EIS project by referring to the relevant EIS publications, available at: /science/eis/publications.html
• Please report any errors/features found with the released data to eis-help@eso.org, indicating the number of the product identification number (PROD_ID) which can be found in the header of each image and catalog.

Prepared by: S. Zaggia, J. Dietrich, R. Guimaraes, E. Hatziminaoglou, P. Lynam, C. Rite, G. Sikkema, R. Slijkhuis, B. Vandame, and L. da Costa.

Send comments to <eis-help@eso.org>