Home       Data-Access       Data-Processing       NDWFS-Publications       Related-Surveys       Links        Additional-Documentation

Third Data Release (DR3)-- More Complete Description -- Still Under Active Construction

The NDWFS third data release (DR3) consists of the photometrically and astrometrically calibrated FITS images and associated data quality masks and images for all of the Mosaic-1 optical (Bw, R, I) and ONIS K-band imaging obtained for the 27 sub-fields that comprise the Boötes Field of the NOAO Deep Wide-Field Survey. Single-band and matched object catalogues are also available as part of this release. Below we describe the optical imaging, IR imaging, and catalogue construction that provided the data products that form the DR3 release.

Optical (Bw,R,I) Images

All the optical images included in DR3 were obtained with the KPNO Mayall 4m telescope and MOSAIC-1 imager. Some of the R-band images were obtained with an early version of the instrument that was equipped with engineering grade CCDs. The remaining R-band images, all of the Bw-band images, and all of the I-band images were obtained after the original CCDs had been replaced with thinned, more sensitive devices. All of the optical images in this release have been projected to a common scale (0.258" per pixel) and orientation (North up, East left). The same tangent point was used for the projection of all of these images (RA=14:32:05.7120; DEC=+34:16:47.496; J2000). Each of the images covers one of our sub-fields, which are nominally the size of a Mosaic pointing and are offset by approximately 35 arcminutes from cardinal direction neighbors.

While we made every effort to construct a data set homogeneous in its quality, the depth and image quality achieved is variable. Notes on the particulars of the variations of the individual fields are in preparation. Details on the reduction procedures used to process the Mosaic images are available in our guide to the processing of NDWFS Mosaic images.

Astrometric Accuracy

Astrometric solutions were generated as described in our reduction notes using the USNO A2.0 catalogue and USNO astrometric standard fields. A more complete description of these procedures is in preparation and will be linked off of these pages when it is completed and discussed by Jannuzi et al. (2005). The typical RMS of the residuals of these solutions is 0.35 arcseconds.

Photometric Zero-points and Delivered Image Quality of Images

The combined/stacked images (in a given filter) have over-lap regions of approximately one arcminute. This allow us to ensure that their relative calibration is accurate to better than 0.03 of a magnitude. The absolute zero-points have been determined using multiple observations of Landolt standards on photometric nights. The images were either obtained on photometric nights, zero-pointed by separate exposures obtained for that purpose, or tied (using the over-lap regions) to fields observed under photometric conditions. Details will be presented by Jannuzi et al. (2005). The photometry in these images has been referenced to Kron-Cousins B-band, R-band, and I-band. The MAGZERO keyword in each image gives the magnitude of one count in the image (Vega).

The stacked/combined images were made from individual exposures with a range of delivered image quality. We did not try to match or circularize the PSF's of either the final combined images taken with different filters, nor the images that went into the final images. This might limit the usefulness of these images for some studies. Jannuzi et al. (2005) will provide more quantitative information about this issue and will be linked off this web page when it is available.

There is also some variation in image quality across the field, but this is generally quite small. We can roughly characterize the DIQ (delivered image quality) as determined by measurements of the FWHM of stars in the released images (based on measurements made by SExtractor of unsaturated stars).

In the table linked below, we list the photometric zero points for each image derived on the Vega system. Note in particular that while Bw observations of spectrophotometric standards indicate that the AB_zeropoint and Vega_zeropoint for this filter are very similar, our current values have been referenced to B-band Landolt standards. We also list the seeing (delivered image quality) and other selected information. The image headers contain the same information (and more) as the MAGZERO, SEEING, DEPTH, and DEPTHCAT keywords.

Table of selected properties of the optical images.

IR Imaging

All the near-IR observations for the NDWFS were obtained using the 2.1-m telescope of the Kitt Peak National Observatory over a period spanning five years. The time period over which we conducted the survey witnessed many advances in the efficiency and size of near-IR arrays. As a result, observations of the NDWFS fields were obtained using three different instruments. We began the survey with the Ohio State - NOAO Infrared Spectrometer (ONIS) and observed the NDWFS fields primarily in the K-band starting in 1997 September and ending in 2000 April. We then continued the survey using the Simultaneous Quad IR Imager (SQIID) from 2000 May through 2001 May, imaging the field in J, H and K. The Florida Multi-object Imaging Near-IR Grism Observational Spectrometer (FLAMINGOS) was commissioned in late 2001. Its 20 arcmin field of view greatly improved our observing efficiency. We completed the IR portion of the survey with this instrument.

The FLAMINGOS and ONIS data together provide K or Ks-band coverage across the entire Boötes field. The DR3 release includes all of the ONIS K-band data of the Boötes field. The FLAMINGOS images are still be prepared for release. The ONIS instrument uses a 512x1024 InSb array and provides near-IR imaging over a field of view of approximately 2.9'x5.8' sampled at a pixel scale of 0.3407 arcsec/pixel. We observed the Boötes NDWFS fields through a K-band filter using ONIS at the f/8 cassegrain focus of the 2.1m telescope over 50 nights, spread out over a period of three years starting in 1998 March and ending in 2000 April.

On each night, we typically observed in a single strip of roughly constant declination, starting at the west end of the field. Each position was observed for 60 sec (typically split into 8 individual readouts of 7.5 sec each) and the telescope was then offset to the east by 17 arcseconds. As a result of the significant frame-to-frame overlap, each stacked pixel has a total exposure time of roughly 10 min. On a typical clear night, we obtained about 300 images covering an area of approximately 1.4 degree times 5.8 arcminuts, the latter number corresponding to the height of the array. Observations were restricted to times when the target field was at an airmass less than 2.0. On each photometric night, observations were also obtained of photometric standards.

The delivered image quality of the ONIS observations varies significantly across each strip, due not only to seeing variations, but also to focus drifts (which are notoriously common at the f/8 focus of the 2.1m) and an intermittent oscillation in the RA drive which resulted in an east-west elongation of the image of about 1\arcsec in some cases. As discussed below, the final stacked images combine observations from different nights and therefore show significant spatial variations of the point-spread function.

The astrometry and photometry of the ONIS data were verified by comparison to 2MASS and to our optical images (for astrometry). Details will be provided on this page in the next few weeks and by Dey et al. (2005).

The released ONIS images cover portions of the 20 sub-fields discussed previously and were constructed from the original observations to match, approximately, the distribution of the optical sub-fields. As a result, the image quality and other properties of the data can change significantly as a function of position in the released images in DR3. The header keywords for the DR3 IR images are therefore approximations or mean values for the data in the image. In particular, that is why we have used a single DEPTHCAT value for all of the DR3 K-band images. Note that all the ONIS IR images have the same MAGZERO by construction.

NDWFS Catalogues

Access to the NDWFS DR3 catalogues.