We present deep VI images obtained with the Advanced Camera for Surveys on board the Hubble Space Telescope , covering three fields in the north-east side of the edge-on disk galaxy NGC 891 .
The observed fields span a wide range of galactocentric distances along the eastern minor axis , extending from the plane of the disk to 12 kpc , and out to \sim 25 kpc along the major axis .
The photometry of individual stars reaches \sim 2.5 magnitudes below the tip of the red giant branch .
We use the astrophotometric catalogue to probe the stellar content and metallicity distribution across the thick disk and spheroid of NGC 891 .

The colour-magnitude diagrams of thick disk and spheroid population are dominated by old red giant branch stars with a wide range of metallicities , from the sparsely populated metal-poor tail at \mathrm { [ Fe / H ] } \sim - 2.4 dex , up to about half-solar metallicity .
The peak of the metallicity distribution function of the thick disk is at -0.9 dex .
The inner parts of the thick disk , within \sim 14 kpc along the major axis show no vertical colour/metallicity gradient .
In the outer parts , a mild vertical gradient of \Delta ( V - I ) _ { 0 } / \Delta|Z| = 0.1 \pm 0.05 kpc ^ { -1 } , or less than 0.1 dex kpc ^ { -1 } is detected , with bluer colours or more metal-poor stars at larger distances from the plane .
This gradient is however accounted for by the mixing with the metal poor halo stars .
No metallicity gradient along the major axis is present for thick disk stars , but strong variations of about 0.35 dex around the mean of \mathrm { [ Fe / H ] } = -1.13 dex are found .
The properties of the asymmetric metallicity distribution functions of the thick disk stars show no significant changes in both the radial and the vertical directions .
The stellar populations situated within the solar cylinder-like distances show strikingly different properties from those of the Galaxy populating similar distances .
This suggests that the accretion histories of both galaxies have been different .

The spheroid population , composed of the inner spheroid and the halo , shows remarkably uniform stellar population properties .
The median metallicity of the halo stellar population shows a shallow gradient from about -1.15 dex in the inner parts to -1.27 dex at 24 kpc distance from the centre , corresponding to \sim 13 r _ { eff } .
Similar to the thick disk stars , large variations around the mean relation are present .