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