Observations
Four filters were used on the ACS, centered on 435, 606, 775 and 850 nm, with exposure times set to give equal sensitivity in all filters. These wavelength ranges match those used by the GOODS sample, allowing direct comparison between the two. As with the Deep Fields, the HUDF used Directors Discretionary Time. In order to get the best resolution possible, the observations were dithered by pointing the telescope at slightly different positions for each exposure—a process trialled with the Hubble Deep Field—so that the final image has a higher resolution than the pixels on their own would normally allow.
The observations were done in two sessions, from September 23 to October 28, 2003, and December 4, 2003, to January 15, 2004. The total exposure time is just under 1 million seconds, from 400 orbits, with a typical exposure time of 1200 seconds. In total, 800 ACS exposures were taken over the course of 11.3 days, 2 every orbit, and NICMOS observed for 4.5 days. All the individual ACS exposures were processed and combined by Anton Koekemoer into a single set of scientifically useful images, each with a total exposure time ranging from 134,900 seconds to 347,100 seconds. To observe the whole sky to the same sensitivity, the HST would need to observe continuously for a million years.
| Camera | Filter | Wavelength | Total exposure time | Exposures |
|---|---|---|---|---|
| ACS | F435W | 435 nm | 134,900 s (56 orbits) | 116 |
| ACS | F606W | 606 nm | 135,300 s (56 orbits) | 116 |
| ACS | F775W | 775 nm | 347,100 s (144 orbits) | 288 |
| ACS | F850LP | 850 nm | 346,600 s (144 orbits) | 288 |
The sensitivity of the ACS limits its capability of detecting galaxies at high redshift to about 6. The deep NICMOS fields obtained in parallel to the ACS images could in principle be used to detect galaxies at redshift 7 or higher but they were lacking visible band images of similar depth. These are necessary to identify high redshift objects as they should not be seen in the visible bands. In order to obtain deep visible exposures on top of the NICMOS parallel fields a followup program, HUDF05, was approved and granted 204 orbits to observe the two parallel fields GO-10632). The orientation of the HST was chosen so that further NICMOS parallel images would fall on top of the main UDF field.
After the installation of WFC3 on Hubble in 2009, the HUDF09 programme (GO-11563) devoted 192 orbits to observations of three fields, including HUDF, using the newly available F105W, F125W and F160W infra-red filters (which correspond to the Y, J and H bands):
| Camera | Filter | Wavelength | Exposure time |
|---|---|---|---|
| WFC3 | F105W | 1050 nm ± 150 | 16 orbits, 14 usable |
| WFC3 | F125W | 1250 nm ± 150 | 16 orbits |
| WFC3 | F160W | 1600 nm ± 150 | 28 orbits |
Read more about this topic: Hubble Ultra-Deep Field
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