While photoionization codes have been carefully intercompared, a fundamental need for clean tests against real nebulae remains. NGC 2610 is a high-excitation planetary nebula which, even at HST resolution, is smooth and symmetric. Helium is He$^{++}$ throughout this nebula, which has a high electron temperature (20,000 K) resulting in strong UV lines. It is the best object we know of to test the performance of photoionization codes without the complication of low ionization knots or filaments. Its large angular diameter (40$\prime\prime$) allows spatial gradients to be observed. In 2001 and 2003, we obtained HST STIS long-slit observations to test against models. Observed lines cover wavelengths from 1240Å to 6563Å. Interstellar reddening is small.

We have constructed photoionization models of this nebula, and compare one with our observations. Most lines are in good agreement. The most discordant line is [Ne IV] $\lambda$2424, which is observed to be twice as strong as predicted. Collisional excitation of H$^0$ is the most important coolant, responsible for 30% of the total. Observations of the Balmer decrement in this nebula can put useful constraints on H$^0$ collision strengths.