We present HST NICMOS photometry of the resolved stellar population in the dwarf irregular galaxy NGC 1569 .
The color-magnitude diagram ( CMD ) in the F110W and F160W photometric bands contains \sim 2400 stars with a formal photometric error \lower 2.58 pt \hbox { $ \sim$ } \kern - 7.75 pt \raise 2.795 pt \hbox { $ < $ } 0.1 mag down to m _ { F 110 W } \approx 23.5 and m _ { F 160 W } \approx 22.5 .
The fiducial photometry has a completeness factor higher than 50 % down to m _ { F 110 W } \approx 21.5 and m _ { F 160 W } \approx 20.0 .
We describe the data processing which is required to calibrate the instrumental peculiarities of NICMOS .
Two different packages ( DAOPHOT and StarFinder ) for PSF-fitting photometry are used to strengthen the photometric results in the crowded stellar field of NGC 1569 .

The resulting CMD is discussed in terms of the major evolutionary properties of the resolved stellar populations .
For a distance modulus of ( m - M ) _ { 0 } = 26.71 and a reddening of E ( B - V ) = 0.56 , our CMD samples stars down to \sim 0.8 M _ { \odot } , corresponding to look-back times of more than 15 Gyr ( i.e. , an entire Hubble time ) .
This is clear indication of star-formation activity in NGC 1569 spanning an entire Hubble time .
The metallicity of the reddest red giant branch ( RGB ) stars is in better agreement with Z = 0.004 as measured in HII regions , than with Z = 0.0004 as expected from the stellar ages .
The presence of — yet undetected — very metal-poor stars embedded in the stellar distribution around m _ { F 110 W } = 22.75 and m _ { F 110 W } – m _ { F 160 W } = 1.15 is , however , not ruled out .
The youngest stars ( \lower 2.58 pt \hbox { $ \sim$ } \kern - 7.75 pt \raise 2.795 pt \hbox { $ < $ } 50 Myr ) are preferentially found around the two central super star clusters , whereas the oldest population has a more uniform spatial distribution .
A star-formation rate per unit area of 1 M _ { \odot } yr ^ { -1 } kpc ^ { -2 } and a mass formed in stars of \sim 1.4 \times 10 ^ { 6 } M _ { \odot } in the last 50 Myr are derived from the CMD .
The near-infrared ( NIR ) CMD places strong constraints on the lower limit of the onset of star formation in NGC 1569 .

The exceptionally high crowding in the NICMOS images of NGC 1569 is a challenge for the photometric analysis .
As a result , optical and NIR images of NGC 1569 sample different populations and can not be cross-correlated .
Nevertheless , we demonstrate the consistency of the star-formation histories derived from the optical and NIR CMDs .