Context : The nuclear star cluster of the Galaxy is an important template for understanding its extragalactic counterparts , which can currently not be resolved into individual stars .
Important drawbacks of observations of the Galactic center are , however , the presence of strong and spatially highly variable interstellar extinction and extreme crowding of the sources , which makes the use of adaptive optics techniques necessary .
Both points pose serious obstacles to precise photometry that is needed for analyzing the stellar population .

Aims : The aims of this work are to provide accurate photometry in multiple near-infrared broadband filters , to determine the power-law index of the extinction-law toward the central parsec of the Galaxy , to provide measurements of the absolute extinction toward the Galactic center , and finally to measure the spatial variability of extinction on arcsecond scales .

Methods : We use observations of the central parsec of the Milky Way that were obtained with the near-infrared camera and adaptive optics system NAOS/CONICA at the ESO VLT unit telescope 4 .
The photometric method takes into account anisoplanatic effects and limits the corresponding systematic uncertainties to \lesssim 2 \% .
Absolute values for the extinction in the H , Ks , and L ^ { \prime } -bands as well as of the power-law indices of the H to Ks and Ks to L ^ { \prime } extinction-laws are measured based on the well-known properties of red clump stars .
Extinction maps are derived based on H - Ks and Ks - L ^ { \prime } colors .

Results : We present Ks -band photometry for \sim 7700 stars , and additionally photometry for stars detected in the H and/or L ^ { \prime } -bands .
From a number of recently published values we compute a mean distance of the Galactic center of R _ { 0 } = 8.03 \pm 0.15 kpc , which has an uncertainty of just 2 \% .
Based on this R _ { 0 } and on the RC method , we derive absolute mean extinction values toward the central parsec of the Galaxy of A _ { H } = 4.48 \pm 0.13 mag , A _ { Ks } = 2.54 \pm 0.12 mag , and A _ { L ^ { \prime } } = 1.27 \pm 0.18 mag .
We estimate values of the power-law indices of the extinction-law of \alpha _ { H - Ks } = 2.21 \pm 0.24 and \alpha _ { Ks - L ^ { \prime } } = 1.34 \pm 0.29 .
A Ks -band extinction map for the Galactic center is computed based on this extinction law and on stellar H - Ks colors .
Both its statistical and systematic uncertainties are estimated to be < 10 \% .
Extinction in this map derived from stellar color excesses is found to vary on arcsecond scales , with a mean value of A _ { Ks } = 2.74 \pm 0.30 mag .
Mean extinction values in a circular region with 0.5 " radius centered on Sagittarius A* are A _ { H,SgrA* } = 4.35 \pm 0.12 , A _ { Ks,SgrA* } = 2.46 \pm 0.03 , and A _ { L ^ { \prime } ,SgrA* } = 1.23 \pm 0.08 .

Conclusions :