Observing Strategy

• What filters/gratings to use for each instrument ?
• What exposure time to use for each filter/grating ?
• How to choose the detailed instrumental setup and observing parameters ?
• How to pack the observations into the individual orbits during the campaign ? In CVZ observations, scattered earth light increases the sky background in certain bandpasses on the day side of the orbit. It was therefore essential to tailor the observations for all the instruments to make optimal use of `bright' and `dark' time.

Below please find discussions of the observing strategy for each of the individual instruments. For more detailed information, check the instrument specific technical pages for a description of the data products and the implemented data reduction steps for STIS , WFPC2 , NICMOS and the Flanking Fields . Consult the observing logs or the header information of the data files for additional information. Check the page with warnings and advisories before making any use of the data.

WFPC2

Different areas of the Version 1 output image correspond to variable amounts of exposure time, due to the relatively large pointing shifts necessitated by the multi-instrument observing strategy. The weight images delivered with each image can be used to estimate the noise per unit area in the output images. About 7% of the individual exposures were excluded from the Version 1 images, generally because more processing was necessary either to determine an accurate pointing or to correct some minor image blemishes.

For quick estimates of the image depth, these are the average exposure times used in the Version 1 images and an approximate limiting magnitude for each filter (defined as 10-sigma within a 0.2 square arcsec area, or 125 pixels in the processed image):

NICMOS

NICMOS will observe in parallel with WFPC2 and STIS, with the Pupil Alignment Mechanism (PAM) set to optimize focus for Camera 3, providing the widest available field of view. At present and in the foreseeable future, Camera 3 remains somewhat out of focus even with the PAM set to the end of its travel range. Nevertheless, the image quality is sharp enough to be undersampled by the NIC3 pixels, and there is little doubt that interesting science on faint galaxy images can be achieved. The images will be dithered using the NICMOS Field Offset Mirror (FOM) in order to improve flat fielding, sky subtraction, and detector artifact removal.

Nearly all of the dark-time orbits will be used for broad band imaging with the F110W and F160W filters (J and H--bands, approximately), giving approximately 48 hours of observing in each band. Limiting magnitudes are given in Table 2. It appears that scattered earthlight during the ``bright'' portions of CVZ orbits affects NICMOS imaging, and various options (including K or narrow band imaging) are being considered to take advantage of these periods. Slitless grism spectroscopy is another option during dark time, but its utility may be compromised by the fact that we cannot easily obtain data at multiple position angles during the HDF-S campaign.

STIS

The flowchart shows the STIS observing plan schematically. Most of the MAMA observing time is put into high-resolution spectroscopy. This provides high resolution (10 km/s) QSO absorption-line data in the region from 2650-3200 Angstroms, and low resolution (250 km/s) spectra from 1600 to 1200 Angstroms. The region from 1600-2650 is suppressed by a Lyman continuum absorption-line system at z ~ 1.9.

MAMA imaging provides UV morphologies of galaxies near the QSO, and a measurement of the Lyman break for galaxies as faint as B_AB = 27 at redshifts z ~ 1.7 and 0.5.

The STIS CCD images provide a deep view of galaxies immediately surrounding the QSO. The images are significantly deeper than those with the WFPC-2, and have a higher spatial resolution. Color information is a bit cruder than for the WFPC-2 images, but in the portion of the field with MAMA UV imaging and long-pass filter imaging, there are four bandpasses available for photometric redshifts, and the inclusion of the UV provides greater accuracy for galaxies in the redshift range 0.5-2.5.

Bright time for the CCD was used to obtain a QSO spectrum at a resolution of 30 km/s and with S/N ~50 per resolution element.

This page was last updated on December 2, 1998.
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