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To estimate the brightness of a variable we compare the brightness of the variable against that of a star (comparison) that does not vary in brightness. To help in this process the section produces star charts that show the position of the variable and its comparison stars, along with their respective magnitude.
On these star charts the magnitude of the comparison stars are often quoted to two decimal places. Our eyes cannot discern such small differences (0.01 mag) in brightness, so it is the usual practice to round any brightness estimate to the nearest 0.1 magnitude.
Examples: 3.35 to 3.44 is rounded to 3.4 and 3.45 to 3.54 is rounded to 3.5.
Fractional Method:
This is the beginners’ method of making magnitude estimates.
With this method, you select two stars from the comparison star charts.
The first star selected should be slightly brighter than the variable, and the second star slightly fainter. The observer then records the difference in brightness between the two stars.
The following example is for the star RZ Cassiopeiae. The selected comparison stars taken from the star charts are star B (mag. 6.8) and D (mag. 7.4). If RZ Cas appears midway between the two stars, then this are recorded as B(1)V(1)D and the magnitude of RZ is 7.1. The brighter star is always written first.
If the difference in brightness between B and RZ appears to be twice that of RZ and D, then this is written as B(2)V(1)D and the magnitude of RZ is recorded as 7.2. If the variable appears to be the same brightness as the comparison star B, then this can be recorded as =B and the magnitude of RZ would be 6.8.
The Step Method:
When an observer has gained a bit more experience, then it is recommended that the step method be employed in the making
of estimates. In the step method, you choose one or more comparison star that differ by no more than half (0.5) a magnitude from the variable. Steps of one tenth (0.1) of a magnitude are then used to make estimates of the variables brightness.
For example if Delta Cephei is estimated to be 1 step fainter than comparison star D (Alpha Lac) and 2 steps brighter than star E (Epsilon Cep), then this would be recorded as D-1,E+2.
The magnitude of the variable is then worked out from each of the step estimates. So Alpha Lac (mag.3.77) – 1 gives us a figure of 3.87 and Epsilon Cep (mag.4.19) +2 gives us 3.99, the average
of the two gives us 3.93, this is rounded to the nearest tenth (0.1) of a magnitude, thus the brightness of Delta Cep would be 3.9.
Please note that since fainter stars have numerically larger magnitude values, we have to add 0.1 to the magnitude of Alpha Lac, and subtract 0.2 from Epsilon Cep.
Also when using the step method, the two estimates tend not to agree exactly, this is quite normal.
Helpful hints and tips
1. Dark Adaptation
On average it takes around fifteen minutes to become fully dark-adapted. If possible try to avoid making any estimates during this period.
2. Bias
Always record what you see, not what you think you should be seeing. For example, when observing an eclipsing star the predicted time of mid-eclipse is only an approximate. If a change has taken place in the orbits of the stars then the eclipse could be occurring longer or earlier than expected. Also try and put any previous nights observations out of your mind when making estimates.
3. Red Stars
Stars that have a red colour like Mira’s and Semi-regulars, will appear to be brighter than they really are when looked at for long periods. Short glances will produce a more accurate estimate.
4. Altitude
Stars that are closer to the horizon will appear to be fainter, because of the greater depth of atmosphere that its light must travel through. If possible, always use comparison stars that are nearly at
the same altitude.
5. Binoculars/Telescopes
Always bring the variable and comparison star to the centre of the field of view. As a star near the edge of the field of view can gain in apparent brightness due to the effect of contrast.
If the comparison stars are a long way from the variable, then use a lower power eyepiece or a smaller instrument with a wider field of view. If a variable cannot be seen by direct vision, then it may be glimpsed by using averted vision. Always record that the variable was glimpsed with averted vision.
6. Comparison star charts and magnitudes
All of the comparison star charts on the SPA programme have north at the top.
On variable star charts it is the normal practice to omit the decimal points, thus 67 represent a star of magnitude 6.7 and 693 represent a star of magnitude 6.93.
Submitting Observations
The variable star section welcomes any observations that an observer may wish to submit by email or by post.
The section requires just four pieces of information for each observation.
1. Name of the star being observed
2. The date of the observation,
3. The time in U.T. and
4. The estimated brightness of the variable.
The following is an example for the star Delta Cep.
Delta Cep
Jan 02,2210 / 3.6
Jan 04,1950 / 4.0
Jan 09,2130 / 3.8
Observations can be submitted in the main email or in a text attachment using Windows Notepad. We can also accept observations submitted in Excel.
You can also submit your observations to us using our observing form HERE. Simply download, print it out, fill in your revelant data and send it to the director.