RPS2 Planning and Observations

     The placement within the WFPC2 field-of-view (the "chevron") of each of the galaxies in our mid-UV survey program, and the choice of the HST orientation in the case of the Cycle 9 survey, may be less than straight-forward to understand. The following examples from the WFPC2 Instrument Handbook v7.0 (Biretta, Lubin, et al. 2002; Sections 1.1.1 and 7.8) may help in the interpretation of the ORIENT, X_POSTARG and Y_POSTARG values in the RPS2 planning of the observations. Click on any of the figures to enlarge them.

WFPC2 Field-of-View (FOV) projected on the sky. The read-out direction is indicated with arrows near the start of the first row in each of the four CCDs. The X-Y coordinate directions are for "POS TARG" offsets. The position angle of the +V3 axis varies with pointing direction and epoch of observation, and is listed in the calibrated science image headers as keyword "PA_V3".

Definition of ORIENT, aperture positions, and CCD alignments. The origin of the (V2,V3) system is at the origin of the plot axes, with V2 and V3 exactly along diagonal lines as marked. "POS TARG"s are offsets measured from the aperture specified on the proposal (e.g. PC1, WF2, WFALL, etc.); their directions are as indicated. All CCDs have pixel (1,1) where the four CCDs overlap.

Example of ORIENT and POS TARG selection (From the WFPC2 Handbook): (a) a galaxy with major axis at PA=60° is to be placed across WF2 and WF3; (b) the aperture is specified as "WF3" and the nucleus is placed near the outer edge of WF3 using POS TARG = (+20,0). To place the major axis across WF2 and WF3 an ORIENT range of 5° to 25° is specified. An ORIENT differing by 180°, namely 185° to 205° would also work.

RPS2 Planning for the Cycle 10 mid-UV SNAPshot survey.