Diffuse Light in Dense Clusters of Galaxies. I. R-Band Observations of Abell 2029

We have developed an observing technique that uses a CCD detector, multiple overlapping exposures, and a tesselating algorithm that allows us to do reliable differential photometry over a large (about 0.6^deg^) field of view to a precision of 2 x 10^-4^ of the night-sky level, limited only by statistical noise. We have measured the point-spread function of the telescope-detector combination out to 20' from the nominal pointing position. We have found that it is necessary to correct the data for the contamination from the extended halos of stars brighter than m_R_ = 15.5 in order to measure diffuse light at levels below μ ~ 26, or about 1% of the average night-sky level. We present R-band observations of the very dense cluster of galaxies Abell 2029. We detect an elliptical component of diffuse light-extended halo of the cD galaxy-with constant eccentricity of 0.9 that follows a de Vaucouleurs profile out to a distance of 425h^1^ kpc [measured as d = (r_min_ r_max_)^1/2^]. The integrated luminosity of the cD galaxy with this halo is 5 x 10^11^h^-2^ L_sun_ (R band). The ratio of the diffuse light to the total cluster light in the elliptical annulus between d = 250h^-1^ kpc and d = 425h^-1^ kpc is 0.10 +/- 0.005, where the uncertainty comes mainly from the estimate of the contribution of the galaxies. We place an upper limit of 5% to the ratio of diffuse light to total cluster light in the elliptical annulus between d = 425h^-1^ kpc and d = 850h^-1^ kpc.