On 2007 October 29 the outbursting comet 17P/Holmes passed within 0.79″ of a background star .
We recorded the event using optical , narrowband photometry and detect a 3 % to 4 % dip in stellar brightness bracketing the time of closest approach to the comet nucleus .
The detected dimming implies an optical depth \tau \approx 0.04 at 1.5″ from the nucleus and an optical depth towards the nucleus center \tau _ { n } < 13.3 .
At the time of our observations , the coma was optically thick only within \rho \lesssim 0.01 \arcsec from the nucleus .
By combining the measured extinction and the scattered light from the coma we estimate a dust red geometric albedo p _ { d } = 0.006 \pm 0.002 at \alpha = 16 \arcdeg phase angle .
Our measurements place the most stringent constraints on the extinction optical depth of any cometary coma .