Science Verification data obtained with the Wide Field Imager

The period of Dec. 8, 1998 through Jan. 18, 1999, was used for the installation, testing, commissioning and science verification (SV) of the Wide Field Imager (WFI) at the MPIA 2.2-m telescope on La Silla.  The time was shared between ESO and the Max-Planck-Institut für Astronomie in Heidelberg; the equivalent of 4 dark nights was used by the ESO Imaging Survey (EIS) Team.

The ESO part of the SV data (excluding the EIS observations, which will be published separately) is now offered to the ESO community. The scientific objectives and targets were selected from a number of suggestions solicited from astronomers at ESO and the Osservatorio Astronomico di Capodimonte.  Please consult the list of available exposures for more details.

Unfortunately, the scientific power of the data is reduced with respect to the scope of the original suggestions because of the required broad band filters only the U and B filters were available at the time of SV. However, the U filter turned out to suffer from a severe red leak (the filter is currently being repaired), which at large air mass manifests itself in double images.

The SV database contains only raw data. The frames are recorded in FITS format with 8 image extensions, namely one for each of the 8 CCD's of the 8k x 8k science mosaic. With 2 bytes/pixel, their total file size amounts to almost 142 Mbytes. The FITS headers are entirely unedited and, therefore, suffer from a number of errors, omissions and insufficiencies. The most important ones known to date include:

a) RA and DEC refer to a point 15 arcseconds south and west of the northeastern corner of CCD #55.  However, because of problems with the implementation of the pointing model (not of the pointing model itself, which had an rms of about 9 arcseconds up to zenith distances of 60 degrees), RA is towards very southern declinations increasingly off by up to +1000 arcseconds (not: time seconds).

b)  All negative declinations appear with a positive sign.

c)  UTC is incorrect but DATE-OBS contains the correct UTC.

d)  In many files only the slot number in the filter storage ring is given but not the filter name.  The following cross identification table can be used in these cases:

slot number	filter name
2	U
3	B
10	665n
11	676n

The database comprises only very few bias files. This is a deliberate decision, because the bias level must be extracted from the horizontal overscan pixels of each CCD. Because it depends on the exposure level in the corresponding light sensitive rows, increases roughly logarithmically over the first couple of hundred rows, and can jump after every re-start of the control software, dedicated bias frames would be of very little use.

Observations of photometric standards (Landolt selected areas) were often made before the telescope was focussed in order to save time. But this does not affect the validity of the data. A crude photometric calibration of the B-band data is possible using the following relation:

m_B = 2.5 x log (ADUs/sec) + 24.82 - 0.18 x (B-V)

where m_B is the B magnitude, ADU is the number of analog-digital units of the object to be calibrated, and (B-V) is the color of the object.

During the SV run, the 2.2-m telescope suffered from significant astigmatism. This resulted in telescope position-dependent elongations of the point spread function (PSF) of up to 20%. Within each image, the PSF is very homogeneous across the entire field of view.

The above enumeration of imperfections reads rather sobering.  However, although it is as honest as current knowledge permits, it is not the full truth:  The Wide Field Imager does produce superb data, and also the SV observations have a high scientific value.   As a quick, partial preview, have a look at the ESO Public Image Archive which also contains pictures obtained during the SV run

For more details about properties of the WFI and information missing from the FITS headers (e.g., about the CCD characteristics) kindly consult the user manual.

Copies of files can be requested from the ESO Science Archive by selecting them from the ESO archive page. We apologize for the inconvenience of the late release which is mostly due to technical and resource reasons.

The distribution policy is the same as for VLT SV data, i.e. the science verification data is available to all archive users with an affiliation in an ESO member state or in Chile or in Portugal. The data will be publicly available worldwide one year after the observation date.

Except for many calibration exposures, the observations were made by Dietrich Baade (dbaade@eso.org), who should be contacted if further information is needed.