---

7.8 CCD Position and Orientation on Sky

The POS TARG special requirement can be used when a position offset is needed. The target is positioned with offset "POS TARG x,y", measured in arcseconds, from the specified aperture. The approximate directions (within 1 degrees ) of the POS TARG offsets are shown in Figure 7.8. The exact directions of the offsets are parallel to the rows and columns of the CCD on which the aperture is specified. There are small rotations (few tenths of a degree) between the CCDs. (For detailed information see "Dithering: Relationship Between POS TARGs and CCD Rows/Columns" obtainable from the WFPC2 WWW pages or help@stsci.edu.)

It is often useful to explicitly specify the desired rotation of the WFPC2 field-of-view on the sky. This is specified in the Phase II proposal using the ORIENT special requirement. It is defined as the PA (measured from North through East) of the +U3 axis on the sky. Figure 7.8 shows the CCD orientation and aperture locations relative to the U3 axis.

Figure 7.8: ORIENT Definition, Aperture Positions, and CCD Alignments.

Observers should try to specify all possible ORIENTs which would give the desired data, since having a range of values, or several ranges, will make the observation much easier to schedule. Often two ORIENTs separated by 180 degrees will both give useful data. Sometimes ORIENTs separated by 90 degrees will also give similar data.

The ORIENT for any observation can be computed as follows:

1. Obtain the Position Angle (PA) of the source axis on the sky, measured in the standard way, North through East.

2. Look at Figure 7.8 and decide what angle you want, measured clockwise, from the +U3 axis to the source axis.

3. Sum the angles in steps 1 and 2.

4. ORIENT must be between 0 degrees and 360 degrees , so subtract 360 degrees , if necessary. The result is the ORIENT you should specify on the proposal.

Note that the +V3 axis is quite different from the +U3 axis. They are exactly parallel, but oppositely directed. The +U3 axis is used for specifying orientation (ORIENT) in the proposal, while the +V3 axis is used in the data headers to indicate field orientation. Data header keyword PA_V3 gives the position angle of the +V3 axis on the sky.

We now give two examples of how the POS TARG and ORIENT special requirements might be used. The first example (Figure 7.9) shows placement of a 100" long jet along the CCD diagonals in PC1 and WF3 (i.e. along the -U3 direction). The coordinates of the nucleus are given on the proposal. Aperture PC1 together with POS TARG +10, +10 are used to place the nucleus near the outer corner of PC1. We want to rotate the WFPC2 field-of-view about the nucleus so the jet is diagonal on PC1 and WF3. We can thus compute the desired orientation as

ORIENT = (source PA on sky)

+ (desired source angle in field-of-view measured CW from +U3 axis)

= 290 degrees + 180 degrees = 470 degrees - 360 degrees = 110 degrees

On the Phase II proposal we would allow some range in the angle (to ease scheduling), hence "ORIENT 105D TO 115D" might be specified.

Figure 7.9: Example of ORIENT and POS TARG Selection.

The second example (Figure 7.10) shows placement of a galaxy across WF2 and WF3, with the nucleus on WF3 safely away from the vignetted region. Aperture WF3 together with POS TARG +20, 0 is used to place the nucleus near the outer edge of WF3. We want to rotate the WFPC2 field-of-view about the nucleus so the galaxy major axis is across WF2 and WF3. We can thus compute the desired orientation as

ORIENT = (source PA on sky)

+ (desired source angle in field-of-view measured CW from +U3 axis)

= 60 degrees + 315 degrees = 375 degrees - 360 degrees = 15 degrees

On the Phase II proposal we would again allow some range in the angle (to ease scheduling), hence "ORIENT 5D TO 25D" would be specified. Note that "ORIENT 185D TO 205D" is also feasible, and should be indicated in the visit level comments.

Figure 7.10: Example of ORIENT and POS TARG Selection.