1. On a nice clear evening, wait until the Sun is about 8 degrees below horizon. 2. Find an aperture which gives you a count rate of about 1,000,000 cps at a blank piece of sky. Use a filter, if useful. ALWAYS start with the smallest aperture when trying to approach this count rate. 3. Start recording data. Switch back and forth between this aperture and a second one with about half the other's diameter; about 20 seconds of integration in one aperture are a guide. Continue switching until sky counts drop below about 3000 cps in the larger aperture. 4. Take a number of dark integrations, then quit this run. 5. Check that there is no star in the larger aperture. 6. To determine the deadtime, first subtract the mean dark count from the data. Separate the curves for the different apertures. 7. Determine the aperture area ratio from the count rate ratio in the last (flat) part of the curves. Multiply the curve with the smaller count rates by this ratio. 8. Bring the steep parts of the curves to agreement by using trial deadtimes, choose the best value. Small adjustments to the aperture area ratio may be necessary. 9. Obviously, you can also do this in morning twilight in reverse order. ADVANTAGES: - More accurate - Incorrect measurements are easily recognized DISADVANTAGES: - Might take away precious observing time - Requires clear skies