tokens: al. 2012 ; E. Torresi et al. 2018, in preparation) to estimate a number of physical parameters, such as the number of ionizing photons, covering factors, and mass of ionized gas, with the aim of better understanding the nature of the ionizing source and the properties of the ionized gas itself. We organize this paper as follows. Section 2 presents a description of the sample and the corresponding observations, while the reduction process and calibration of the data are presented in Section 3 . Section 4 shows the catalog of H α +[N ii ] and [O iii ] images and the corresponding flux measurements. In addition, we show [O iii ]/H α +[N ii ] emission-line ratio images. Section 4 presents the results of measuring the physical parameters mentioned before. In Section 5 , we discuss all of the previous results in light of the two AGN classes (LEGs and HEGs). A summary and potential future observations are presented in Section 6 . Throughout this paper, we use H 0 = 71 km s −1 , Ω M = 0.27, and Ω Λ = 0.73. 2. Sample Selection and Observations The sample selection and observations are described in detail in Tremblay et al. ( 2009 ). To summarize, we present a sample of 19 3CR radio sources from the low- z ( z 0.3) extragalactic radio-source Revised Third Cambridge Catalogue (3CR; Bennett 1962a , 1962b ; Spinrad et al. 1985 ). The sample comes from the original HST proposal for 98 3CR sources that was awarded for observations prior to the ACS side 2 electronics failure in 2007 January. Although only 19 sources were observed (Table 1 ), these objects represent an unbiased sample in selection due to the nature of the HST snapshot mode (to fill gaps in the HST schedule, sources are randomly picked from the target list). Moreover, the observed 19 objects span almost the whole redshift range (0.0075 z 0.224) of the original sample and show a wide variety of emission-line ratios, radio power, or spectroscopic properties. With regard to their radio morphologies, the sample consists of four FR Is and 14 FR IIs, taking the double-lobed 3C 314.1 apart (Table 2 ). In terms of their optical spectroscopic classification, eight of the objects in our sample are classified as LEGs and 10 are HEGs, of which four have broad lines, named broad-line objects (BLOs). Of particular interest is 3C 314.1, an RG whose spectral classification is extremely LEG (Capetti et al. 2011 ) interpreted as relic RG (Capetti et al. 2013 ). Table 2 summarizes the radio and spectroscopic classification of the sample. This table also collects the total radio luminosities at 178 MHz, the radio core powers (at 5 GHz), and the X-ray luminosities for the sample. The uncertainties on the radio luminosities are negligible with respect to those of the X-ray luminosities, which amount to ∼0.2 dex. Table 1 Observing Log Note. Log of the HST /ACS WFC observations (Program 10882, PI: W.B. Sparks). Col. (1): 3C source name. Col (2): redshift. Cols. (3)–(4): R.A. and decl. Cols. (5)–(6): ramp filters used
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