  CDG-1 is a tight grouping of four likely globular clusters in the Perseus
cluster, and a candidate dark galaxy with little or no diffuse light. Here we
provide new constraints on the luminosity of any underlying stellar emission,
using HST/UVIS F200LP imaging. No diffuse emission is detected, with a
2$\sigma$ upper limit of F200LP>28.1 mag/arcsec$^2$ on the 5'' scale of CDG-1.
This surface brightness limit corresponds to a 2$\sigma$ lower limit of >0.5
for the fraction of the total luminosity that is in the form of globular
clusters. The most likely alternative, although improbable, is that CDG-1 is a
chance grouping of four globular clusters in the halo of the Perseus galaxy
IC312.
