ACS/HRC Parallels

The ST-ECF is managing the processing of ACS HRC Parallels. The two pairs of overlapping fields both fall mainly in the GOODS-South field.

Epoch-1 Epoch-2
RA (2000.0) 03 32 54 03 32 31
DEC (2000.0) -27 49 17 -27 50 49
T(F220W) 62 ks 52 ks
T(F555W) 63 ks 57 ks
T(F892N) 169 ks 135 ks
T(G800L) 164 ks 135 ks
Grism Spectra 75 107

ACS/HRC Scientific Rationale:

The ACS High Resolution Channel (HRC) offers the possibility to obtain auto-parallels when the WFC is prime. Since the Wide Field Channel (WFC) and the HRC share the same filter wheels, the filter in use by WFC dictates the HRC filter. For the HUDF observations HRC auto-parallels have been obtained with the F220W, F555W and F892N filters and the G800L grism. The scientific aim is to obtain complementary deep images at a spatial resolution about twice that of the WFC over a smaller field. The pixel size is 0.027arcsec so the sensitivity to compact sources is higher than for WFC. The F555W images will provide a deep V band image; the F220W although not very sensitive will provide a UV colour which may be interesting for the UV morphology of nearby galaxies and low-moderate redshift objects (z~<2). The F892N filter is narrow (150A) but is of interest for detecting emission line objects in a narrow range, such as [O II] emitters 1.37<1.41 and Ly-alpha objects 6.27<6.40 and very red continuum objects. The G800L grism provides slitless spectra at a dispersion of 24A/pixel but the resolution depends on the size of the dispersing object. The coverage is 5500-10500A with peak sensitivity about 7500A. The continuous wavelength coverage in the red makes it very competitive for deep spectroscopy compared with the ground and the small pixels make grism spectroscopy with HRC relatively more sensitive than with the WFC. In 130ks of exposure, useful (3 sigma per resolution element) spectra of continuum objects with V as faint as 27.0 have been obtained.

ACS/HRC Pointing:

Approximate coordinates for the 2 pairs of ACS HRC parallels are:

Epoch 1, ORIENT 310  RA (J2000)= 03 32 54.4    DEC (J2000)= -27 49 07.3 ~center

Epoch 1, ORIENT 314  RA (J2000)= 03 32 53.8    DEC (J2000)= -27 49 21.8 ~center

Epoch 2, ORIENT  40  RA (J2000)= 03 32 31.6    DEC (J2000)= -27 50 52.1 ~center

Epoch 2, ORIENT  44  RA (J2000)= 03 32 30.5    DEC (J2000)= -27 50 45.0 ~center

ACS/HRC Filters:

ACS/HRC autoparallel filters are automatically determined by the ACS/WFC primary filter in use. The UDF filter pairings are below:

Primary Observation




F435W F220W
F606W F555W
F775W F892N
F850LP G800L

ACS/HRC Depth:

The limiting magnitude estimates below were calculated using the ACS ETC. As specified for the ACS/WFC exposures, the calculations are for a S/N of 10 within a 0.2 sq arcsec aperture (15x15 HRC pixels) and average zodiac and earthshine.

Limiting AB magnitudes for S/N = 10 in 15x15 pixels aperture.

  Orient 310 Orient 314 Overlap
F220W 25.1 25.8 25.9
F555W 26.8 27.3 27.5
F892N 23.9 24.6 24.8

The limiting magnitude estimates in the above table should be considered crude guidelines only. Estimates of limiting magnitudes are complicated by the uneven exposures in different parts of the images, different angular sizes of the objects etc. The faintest objects detected in the F555W frames in the Epoch-1 and Epoch-2 data have AB magnitudes of 28.5 and 28.3, respectively. Histograms of the magnitude distributions of objects in the two HRC fields show a rapid decline below ABmag(F555W) = 28, suggesting that significant incompleteness sets in around this limit.


The Hubble Ultra Deep Field ACS HRC auto-parallel data were reduced by a dedicated team from the ST-ECF composed of:

Martin Kuemmel
Soeren Larsen
Jeremy Walsh
with extra help from:
Richard Hook

This page was developed and originally maintained by the ST-ECF. Now please report any problem to