# Hubble Space TelescopeWFPC2 Scientific Rationale:

## WFPC2 Parallels

### WFPC2 Scientific Rationale:

WFPC2 UDF parallels are being obtained in F300W. The principal science goal is to enable an increase in the number of reliable U-band dropouts at z ~2.5.

### WFPC2 Pointing:

The WFPC2 fields overlap far more than most of the other parallels. The pointings for these two pairs of overlapping fields are essentially as follows, with the first line of each pair being an approximate position of the WFALL-FIX reference aperture, and the second line of each pair being an approximate position of the intersection of the 4 WFPC2 CCD chips.

```Epoch 1, ORIENT 310  RA (J2000)= 03 32 37.1    DEC (J2000)= -27 41 27.3 ~WFALL
03 32 36.6                 -27 41 23.5 ~Apex

Epoch 1, ORIENT 314  RA (J2000)= 03 32 39.0    DEC (J2000)= -27 41 26.2 ~WFALL
03 32 38.5                 -27 41 22.5 ~Apex

Epoch 2, ORIENT  40  RA (J2000)= 03 33 06.3    DEC (J2000)= -27 47 03.2 ~WFALL
03 33 06.5                 -27 46 56.8 ~Apex

Epoch 2, ORIENT  44  RA (J2000)= 03 33 06.3    DEC (J2000)= -27 47 28.7 ~WFALL
03 33 06.6                 -27 47 22.3 ~Apex
```

F300W (U)

### WFPC2 Depth:

Although we could not a priori know exactly the amount of exposure time we would receive at either ORIENT, nor the combined exposure time in the overlap region of both pairs of fields, we could estimate some depths which we hoped to achieve based on a given number of orbits and exposure time.

In 100 orbits (200 ksec), we could reach a 10 sigma limit of about AB=29.5 at 300nm, assuming full orbit integrations. If we split into two exposures per orbit, we'd instead reach AB=29.2 or 29.3. This assumes a source with size 0.25 square arcseconds. This may be a bit over-optimistic on the total integration time, given some inefficiencies in scheduling, but we might get around 75 ksec of WFPC2 exposure at the "minor" UDF roll angles and 225 at the "major" ones.

For comparison, the HDF-N had about 125 ksec of F300W exposure, so we would be getting two fields clearly deeper than the HDF-N and two a bit shallower. This would yield a sample of order 400 dropout galaxies, scaling off an earlier estimate of a bit under 150 F300W dropouts in the HDF-N. (Note, however, that the fields of view at the two ORIENTs within each of the two epochs largely overlap, so most of the exposure time will go into the same area in the images taken at both ORIENTs for a given epoch. Thus, the area will be smaller, but the depth will be greater than if two separate or mostly non-overlapping fields at each epoch were being imaged.)

### WFPC2 Status:

All data for both Epochs have been obtained.