Interval analysis

Dividing the observation period into intervals

If your observation period lasted longer than 3~hours, you should divide it into intervals of 1.5 to 2.5 hours' duration. The interval boundaries must coincide with any time labels reported in your notes (breaks included).

Intervals should not be shorter than 1 hour unless you had to stop your watch due to other reasons like clouds. If you observe a major-shower maximum, periods should also be shorter than 1 hour. There is no need to report intervals of exactly 1 hour duration. If circumstances change considerably and quickly (e.g. change of limiting magnitude by more than 0.5mag) take the time of such a change as an interval boundary. Typical period lengths for a major-shower peak are 15 minutes instead of 1 hour.

Next, on the back of the Visual Observing Form you will find three equal-sized tables, and thus you have space for three intervals per form. If you have more than three intervals to report, use an additional form. As an example we will use the case of Section 5.7. This observation lasted from 20h 12m - 22h 02m UT, and we record this in the first line of the interval analysis.

Limiting magnitude

The second table is for the limiting magnitudes. Fill in the times, field numbers ('Nr'), and numbers of stars ('N') seen during your watch. Then, using the conversion Table 3 in Part 1 of this description you must find the corresponding limiting magnitude (lm). As you can see, the lm obtained in field Nr. 1 in our example differs considerably from that in the other fields. Looking at the conversion table we find that the 10th star in field 1 is of magnitude 5.3 and the 11th one of magnitude 6.0. Although the limiting magnitude is 5.85 as determined using fields 7 and 15, only 10 stars in field 1 were seen which would correspond to lm = 5.3mag, since there is no star of 5.8mag and the one of 6.0mag cannot be seen. Field 1 should thus not be used in this case. Now, the reason for using three fields to determine the limiting magnitude becomes clear, as there are similar cases in some of the other fields. Whenever your limiting magnitude lies in a 'gap' wider than 0.3mag, you should ignore this field. When all the limiting magnitudes have been determined, you can calculate the weighted average (lm_avg). Do not compute the average number of stars: only magnitudes themselves can be averaged.

where t_i is the time for which lm_i is valid. In our example lm_1 = 5.85mag is valid from 2012-2036 (2036 is in between 2012 and the next lm check made at 2100 hours). This results in t_1 = 24 minutes. lm_2 = 6.1mag is valid from 2036-2115. With a break of 6 minutes (2050-2056) we find t_2 = 33 min.

        5.85 mag 24 min  +  6.1 mag 33 min  +  6.07 mag 47 min
lm   =  ------------------------------------------------------  =  6.03 mag
  avg                 24 min  + 33 min + 47 min

This procedure can be somewhat cumbersome, especially where the lm is very variable, but simple averaging is usually possible in most cases.

Cloud cover or field obstructions

Now we can fill in the third table. Here we need to compute the weighted average cloud cover. Multiply every percentage k by its corresponding time interval in minutes. These products are then totalled, and their sum is divided by the total observing time, also in minutes, and by 100 to give a percentage:

In our example t_total between 2012-2202 equals 110 minutes minus 6 minutes (break), which equals 104 minutes.

      10 %  9 min  +  20 %  4 min
k  =  ---------------------------  =  0.016
            100 %  104 min

k is the average part of the field covered by clouds during the entire watch. The final correction factor results from:

          1
F  =  ----------
        1 - k

In our example:

          1
F  =  ---------  =  1.02
      1 - 0.016

Note that only obstructions of the field of view are reported. The field has a diameter of roughly 100 degrees. Because of the reduced vision at the periphery of the field of view, clouds at the edge have a negligible effect on your meteor numbers; please take care that F is not overestimated.

Net observed time T_eff

Having filled in the "Breaks" table, the last but one line of the form starts with "Time for plotting". See the minor-shower observation page for details about plotting. If you were not plotting meteors leave this blank, as in our example. At the end of this line fill in the total of any breaks. The net observed time then naturally results from the time span start-end minus the total duration of the breaks. In our example:

20h 12m - 22h 02m --> 110min minus 6min, T_eff = 104min = 1.73h.

Give T_eff in hours and decimals of an hour, not minutes.

T [in h] = T [in min] / 60

Every interval of your observation should be processed in this way.