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HDF-S Observing Strategy

Observing Strategy

Discussions about the observing strategy for the HDF-S focussed primarily on the questions:

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

The WFPC2 observing strategy was similar to that for HDF-N. We used the same filters, F300W, F450W, F606W, and F814W, and very similar total observing times. During each orbit, the part with elevated background (because of scattered Earth light) was used for F300W, where the background accounts for a relatively small fraction of the total noise. The remaining time was split evenly between F450W, F606W and F814W, with F606W receiving somewhat less observing time than the others.

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):

Table 1. WFPC-2 Filters, exposure times and limiting AB magnitudes for Version 1 images

Filter	Exposure time (ks)	AB mag. limit S/N=10 in 0.2 sq. arcsec
F300W	140	26.8
F450W	101	27.7
F606W	81	28.2
F814W	100	27.7

NICMOS

The discussion of the observing strategy for NICMOS given here is somewhat outdated. It describes general issues, as well as the planned strategy as of July 1998. The strategy used for the actual campaign was somewhat different, due to late changes in the observing strategy. Consult the observing logs or the header information of the data files for more accurate information.

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.

Table 2. NICMOS Filters, exposure times and limiting AB magnitudes

Filter	Exposure time (ks)	Surface Brightness limit (1 sigma over 1 sq. arcsec)	Limiting Magnitude S/N=10 in 0.8 sq. arcsec
F110W	176	6.4 x 10^-8 Jy/arcsec-2	J_AB = 27.0
F160W	174	7.6 x 10^-8 Jy/arcsec-2	H_AB = 26.8
F220M	111	1.0 x 10^-6 Jy/arcsec-2	K_AB = 24.0

STIS

Observations with the MAMA (UV) detectors on STIS are limited to about 60 orbits due to the restriction that they only operate during during SAA-free orbits.

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.

Table 3. STIS Modes and exposure times

Detector	Filter or Grating	Exposure time (s)
CCD	50CCD	155590
CCD	F28X50LP	49768
CCD	G430M (cenwave=3165)	31492
CCD	G430M (cenwave=3423)	25600
FUV-MAMA	G140L	18480
FUV-MAMA	MIRROR	52124
NUV-MAMA	E230M	151074
NUV-MAMA	G230L	18424
NUV-MAMA	MIRROR	22616

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