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How to observe variable stars?

What equipment do you need?

A pair of binoculars and/or a telescope

In fact you don't need a lot of equipment to observe variable stars. A simple pair of binoculars, like a 7 X 50, will already show you several tens of variable stars. Telescope owners also don't need to worry, since every type of telescope is suitable for variable star observations. It is however very important to have an easily controlable telescope. An unmanageable instrument will definitely cool down the enthousiasm. Every type and size of telesope has its own observing program : a list of variables that are well in reach and easily observable. With a small telesope, you won't be able to observe faint variables. On the other hand, a big telesope is not suitable for the observation of bright variable stars. Hence, always use the smallest instrument with which you can comfortably see the variable in order to make an estimation of its brightness.

The finder scope

The most difficult part of the observation of variable stars is looking for the variable on the night sky. You will need some perseverence! But a good finder scope will facilitate the job to a certain extent. This, of course, applies to other objects as well. It is worth it to replace a 6X24 "finderscope" by a 6X30, or better a 8X50.

Eyepieces

A variable star observer generally uses two eyepieces. The first yields a low magnification to localize the variable and to estimate the brightness of a bright variable. For the observation of fainter variables, a second eyepiece is used to push the magnification. Exactly which magnification you will use depends on the scope and your preference. There's only one rule : observe comfortably.

Star atlas

Before you start looking for the variable on the night sky, you need to find it on a map first. Very suitable atlases are for example the AAVSO atlas and the Uranometria. Of course you can also make your own (finder)charts with computer programs such as Megastar and Guide. Note however that you better should not report observations based on the magnitudes given by these programs. Use AAVSO charts in stead.

Finder charts

The members of the working group mainly use the charts edited by the AAVSO (American Association of Variable Star Observers). Only if no other charts are available or if the sequence on the existing AAVSO charts is not reliable, charts of other organisations are used. Members of the VVS receive charts for free on request. However, thanks to the WWW they can be downloaded from the AAVSO homepage charts.aavso.org. Do not make your own charts for observation purposes, by no means!
The AAVSO charts exist in different scales and limiting magnitudes. The scales are depicted by a letter in brackets, at the top right corner of the chart.
The table below gives an overview of the different scales.

Chart scale /mm FOV
a 5' 15°
b 1'
c 40"
d 20"
e 10" 30'
f 5" 15'
g 2.5" 7.5'

A- and ab-charts show north on top of the chart and west at the right side. These charts are suitable for naked eye, a pair of binoculars or a finder scope. On the other charts south is up, east is right. Hence, these charts are used at a telescope with inverted vision. Observers who use a diagonal can use reversed b- till g-charts (north is up, east is right).
In the header of the chart you will of course find the name of the variable. At the top left you can find the Harvard designation in a little frame. This designation is the approximate position of the star for Epoch 1900. The first two digits represent the right ascention in hours, the next two in minutes. These four digits are followed by a + or - sign and two digits to represent the declination. When several variables are located near this position, the designation is followed by a letter.
You will also find the exact position, the mean maximum and minimum brightness, the type of variable, spectral class and the date the chart is produced. On the map itself, the variable can be found as an little open circle. This circle is often highlighted by four lines or - as on older charts - with dot in the middle. Next to the stars, you find numbers. Such a number is the brightness of a comparison star. However, decimal points are omitted. For example, a star with a number 95 is of magnitude 9.5.

Observing Technique

Looking for the variable

The best method for localizing the variable is the "starhopping" method. This means that you "hop" from one star to another. First, try to locate the variable in an atlas (e.g. AAVSO atlas) and compare the AAVSO-chart with the atlas. On most AAVSO-charts there are stars that are brighter than 8th magnitude. Now, try to locate this bright star in the atlas. If everything is going well so far, try to find the easiest way the locate that star, starting from a star visible with the naked eye. Aim the finderscope on the latter star and compare the field in the finderscope with that in the atlas. Try to make shapes like triangles, squares,... with brighter stars. Rotate the atlas according to the orientation of the field in the finderscope. Move the telescope in the direction of the bright star near the variable. Always compare the field in the finderscope with the atlas. Don't forget to make shapes, and be sure that you can always find the shapes in the atlas. This way you prevent of getting lost. In case the field in the finderscope doesn't correspond to the atlas, return to the previous step. If this doesnt help, you better start all over again. Especially in the beginning, finding the variable isn't an easy task. Once you have a perfect command of this technique, loacting the variable will be (more) easy. From the moment you found the brighter-than-magnitude-8-star, you put this star in the field of your telescope. In fact, you proceed as you've done before by comparing the field of your telescope with the AAVSO-chart. Always check that what you see corresponds to what's on the chart. This way, you can be sure of correctly locating the variable star.

Estimating the brightness of the variable

Once you find the variable, you can estimate its brightness. You do this with the help of two comparison stars : one that's a little bit fainter and one that's a little bit brighter than the variable. Defocusing helps to compare the brightness of the stars in consideration. Please note to keep the difference in brightness small enough. If the two stars differ too much in brightness, the estimate will become less accurate. Therefore, keep the difference less than one magnitude, unless there's no other possibility. Suppose you find two comparison stars, of magnitude 9.8 and 10.4 respectively. Now try to divide the difference in brightness into equal parts, in this care for example 6. Compare the variable with the two comparison stars. When the variable is as bright as one of the comparison stars, for example 9.8, then you estimate the variable at magnitude 9.8. More often, however, the brightness of the variable is somewhere in between. If you for example notice that the variable is a little bit fainter than the star of magnitude 9.8 and a lot brighter than 10.4, you could write down the estimation as 9.8-1-V-5-10.4, in which V is the brightness of the variable. Hence, your estimation is 9.9. If you find that the variable is 2 parts fainter than 9.8 and 4 parts brighter than 10.4, your estimation will be 10.0.
When you're making your estimations, please take into account the following simple rules :

  • Always observe in averted vision, i.e. look a little next to the variable and comparison stars.
  • Try to keep the imaginary line connecting the variable and comparison star parallel to the line connecting your eyes. You can eventually turn your head with regard to the eyepiece.
  • Defocus the stars so they become little discs. Comparing the brightness of discs is easier than of pinpoint stars. Beware however that the discs of the variable and the comparison stars don't overlap.
  • Try to compare the brightness of the compare star and variable by placing them alternately on the same spot of the retina.

More information on observing techniques can be found in the handbook of the working group.

How frequent can I make observations?

Our observing programme contains stars that slowly change in brightness in a more or less predictable manner as well as variables that show fast and unpredictable light changes. It is clear that "slow" variables need less frequent observations that the unpredictable one's. The table underneath gives the frequency of observations.

 

type
frequency
Mira (P>180 days)
7 days
Mira (P<180 days)
4 days
Semi-regulars
4 days
RV Tauri stars
4 days
Dwarfnovae
daily
Symbiotic stars
daily
R CrB stars
daily
Novae
daily
Supernovae
daily
Irregulars
daily

What do you need to record for each observation?

In order to make an observation report, you need to record at least :

  • date and time in UT, with a time resolution of 1 minute
  • name of the variable
  • magnitude of the variable
  • used comparison stars
  • used chart
  • remarks

 

Reporting observations

You can find instructions on how to report your observations by clicking on the next link : Reporting.

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