  We study the impact of photometric signal to noise on the accuracy of derived
structural parameters of unresolved star clusters using MCMC model fitting
techniques. Star cluster images were simulated as a smooth surface brightness
distribution following a King profile convolved with a point spread function.
The simulation grid was constructed by varying the levels of sky background and
adjusting the cluster's flux to a specified signal to noise. Poisson noise was
introduced to a set of cluster images with the same input parameters at each
node of the grid. Model fitting was performed using emcee algorithm. The
presented posterior distributions of the parameters illustrate their
uncertainty and degeneracies as a function of signal to noise. By defining the
photometric aperture containing 80% of the cluster's flux, we find that in all
realistic sky background level conditions a signal to noise ratio of $\sim$50
is necessary to constrain the cluster's half-light radius to an accuracy better
than $\sim$20%. The presented technique can be applied to synthetic images
simulating various observations of extragalactic star clusters.
