3.2.1 Photometric Observations

43 fields of size $8.7' \times 8.7'$ were observed in the HydraI area, see Figure . The fields were selected with the intention to cover all E and S0 galaxies brighter than $V_{25} = 15\hbox{$.\!\!^{\rm m}$ }5$ within $1.5^\circ$ from the cluster center. V₂₅ is the Johnson V magnitude within the $25 {^{\rm m} /{\rm arcsec}^{2}}$ isophote, as listed in the HydraI catalog of Richter (1989).

37 fields form a contiguous area centered on the cluster center. This area has maximum dimensions of approximately $46' \times 76'$, with an area of approximately 0.63 square degrees. The two holes are fields that could not be observed due to the presence of very bright stars. An additional 6 outlying fields were observed in order to meet the above goal.

All 43 fields were observed in Gunn r and Johnson B. The 5 very central fields (00, 13, 14, 15, & 18) were also observed in Johnson U. The typical exposure times were 300 seconds in Gunn r, 900 seconds in Johnson B, and 1800 seconds in Johnson U.

Note, that we will occasionally (e.g. where space is limited) abbreviate Gunn r, Johnson B, and Johnson U as GR, JB, and JU, respectively.