by Gary Poyner
Birmingham, Aug 1998 (updated 02/8/09)
You do not necessarily have to have any optical instrument to get started in variable star observing. The naked-eye is perfectly adequate for observing variable stars that are brighter than about magnitude 4. However, as there are only a few such stars, most observers will, sooner or later, want to get themselves equipped with either binoculars or a telescope.
Hand-held binoculars can be used to observe stars in the magnitude range 4 to 8. If they are mounted on a tripod then you should be able to reach down to magnitude 9 or below. There are several thousand suitable variable stars within this range - far more than a single observer can hope to cover.
To go much fainter than magnitude 9 you will need a telescope of some sort. The effective magnitude limits for various popular sizes of telescope are as follows. It should be noted that these values are only very approximate. An observer who lives in a dark rural site will reach fainter limits with the same aperture than one living in an urban area. The quality of eyepiece, condition of the mirror (if used) and experience of the observer are all factors to be considered. To be honest, tables of this nature are worthless, except to give a very basic idea of what to expect once some experience has been gained.
Aperture (mm): 60 150 200 300 400
Brightest (mag.): 6.5 8.5 9.0 10.0 10.0
Faintest (mag.): 11.0 13.0 14.0 15.0 16.5
If a variable star appears uncomfortably bright then you should either use a smaller instrument or else place a 'stop' (a cardboard ring) over the front of the telescope to reduce the effective aperture (be sure to make a note of the reduced aperture when recording your observation).
You will need a chart for each variable star that you plan to observe. The BAAVSS publishes charts for all of the stars on its various programmes, as do other major variable star organisations. Each chart shows the positions and the magnitudes of a series of standard comparison stars (the 'sequence') against which you make your magnitude estimates. The chart may also include a 'finder chart' (usually 9 degrees field) to help you locate the field of the variable star in relation to bright naked-eye stars. If it doesn't then you might have to refer to a star atlas such as Sky Atlas 2000, Uranometria or one of the many star chart programs available to the PC to plan your own way of locating the variable before you go outside. Computer generated star charts are excellent for producing finders for those variable stars which are difficult to locate, although the official chart should always be used for making estimates. The chart will also give the celestial co-ordinates of the variable which will be useful if your telescope is equipped with setting circles or is a GOTO.
As well as your instrument (if any) and your charts you will also need the following items...
A notebook to record your observations in. This should be reasonably sturdy, bound (not loose-leaf) and preferably A4 or A5 size to allow easy photocopying. Some observers use tape recorders, which are particularly useful if you are observing a fast eclipse.
A pen, preferably black or blue (again, to allow photocopying), with permanent ink (not water-soluble). On cold winter evenings, it's a good idea to have a pencil at the ready, just in case the pen freezes (which they always do!)
A loose-leaf ring-binder or some other means of holding your charts so they don't blow away.
A watch or clock to allow observation times to be recorded to the nearest minute.
A dim red light source to allow you to read and write without losing your dark-adaption. A small torch with the glass painted over with red acrylic paint is one possibility. Be sure to have spare batteries to hand.
A convenient surface to keep all of these items on, and for you to lean on when writing. Using the ground will not do your back any good!
Warm clothing. Cold is the astronomer's greatest enemy. You cannot make reliable observations when you are shivering and longing to get back into the warm indoors. A warm observer is a better observer!
Armed with a Variable Star chart and telescope or binoculars, the next step is to actually locate the Variable, or rather find the field in which the variable lies as it may be at a fainter level than your instrument will reveal. This may at first seem a daunting task, but a little patience and common sense will reap rewards. The binocular observer should be able to locate the Variable Star field without too much trouble, providing the field stars are within the magnitude limit of the instrument being used. The telescopic user should firstly orientate the chart with the view as seen through the eyepiece. Most Variable Star charts are drawn inverted (North to the bottom, and West to the left ). To orientate the chart with the eyepiece view, simply place a star near to the Variable Star field in the centre of the eyepiece and note which way it drifts across the field of view (if your scope is driven, switch off the drive). Whichever way the star appears to drift is West! The chart can then be orientated correctly. If an equatorial mount is used, it should be possible to 'drop' onto the field easily. However if an alt-az mount is used (dobsonian for example) then first locate a bright star near the Variable (one is usually included on the 9 or 3 degree chart), and if the chart has been correctly orientated it will be a simple process to 'star hop' to the Variable. Check very carefully that the correct field has been located, as it's surprising how similar star fields can appear in a telescopic field. Check to see if the Variable is visible. Once you are confident that you have located the correct field, move the telescope and repeat the process. If after 20 minutes or so you find that you are unable to find the field, take a break and return later to try again. Locating a Variable Star field for the first time can be a most frustrating experience, both for first timers and experienced observers alike. However once the field has been found for the first time, it will be a simple job to return to it on future clear nights. Confident that your in the right area, you are now ready to estimate it's brightness. Of course if you own a GOTO telescope, the telescope should take you directly to the field. However you should still spend some time in identifying the variable correctly.
The BAAVSS employs two main methods for estimating magnitudes: the Fractional method and the Pogson step method.
With the fractional method you compare the variable against two comparison stars. You look alternately at each star and try to judge the ratio of the brightness differences between them. This sounds more difficult than it really is. As an example, suppose the variable (usually denoted V) appears to be midway in brightness between comparison stars A and B. You would then record your estimate as A(1)V(1)B. However, if the A-V difference appears to be twice as big as the V-B difference then you would record A(2)V(1)B. The brighter star should always be written first, and if possible the difference between the magnitude of comparison stars should not exceed 0.6 mag. - although this is not always possible. It should be noted that the larger this difference, the less accurate the reduced magnitude. If the variable appears to be equal in brightness to one of the comparisons then you would record, for example, V=B. It is possible to make extrapolated fractional estimates, such as V(1)A(2)B and A(4)B(1)V, if the variable is brighter or fainter than all of the listed comparison stars. However, extrapolated fractional estimates tend to be rather unreliable, so you should always try to use one comparison star that is brighter than the variable and one that is fainter if you can.
In order to derive the magnitude from a written fractional estimate, you form a weighted mean of the magnitudes of the comparison stars used. For example, assume the following comparison star magnitudes:
A = 7.10 - B = 7.55 - C = 7.83
The estimate of A(1)v(2)B would then be worked out thus..
Difference between A&B = 0.45
Divided by the fraction used (3) = 0.15
Multiplied by the step from A (1), and added to A = 7.25, rounded up = 7.3, which is the estimated magnitude of the variable.
Alternatively you can multiply by the step from B and subtract from the value of B, which equals the same.
Extrapolated fractional estimates should first be converted to equivalent interpolated ones involving negative differences before the weighted mean is taken: thus V(1)A(2)B becomes A(-1)V(3)B which gives (3x7.10 +(-1)x7.55)/(-1+3) = 6.875. These sorts of calculations are best carried out with a pocket calculator after you have finished observing, rather than in your head at the telescope.
With the Pogson step method you compare the variable against just one comparison star at a time and you try to judge the brightness difference in units of tenths of a magnitude. This does require some practice but it is not all that difficult. Suppose the variable appears to be two tenths of a magnitude fainter than star A, then you would record the estimate as A-2. If it was one tenth brighter than B then you would record B+1. If it was equal to B then it would be =B.
To derive the magnitude from a written Pogson estimate you must remember that the magnitude increases as the brightness decreases (the Pogson step unit is really minus one tenth of a magnitude). Following our observation, we derive a Pogson step estimate of A-1,B+2. Assuming the above magnitudes for comparison stars, then B+2 gives 7.55 + 2x = 7.35 and A-1 gives 7.10 -1x = 7.20. The average is then taken between the two reduced estimates and rounded off to the nearest tenth e.g. 7.20; 7.35 difference = 0.15 (rounded off = 0.2), averaged = 0.1, then added to A (or subtracted from B) gives the reduced magnitude of 7.3
If the variable star is too faint to be seen then you can still make a useful observation by recording the faintest comparison star visible. For example, if the variable was unseen and the faintest comparison visible was star C then you would record <C, meaning 'fainter than C'. Such a negative estimate provides an upper bound on the brightness of the star at the time of the observation. In some circumstances this information can be very useful. The derived magnitude from a negative estimate is simply the magnitude of the comparison star prefixed by '<' . For example, <C gives <7.8. It is important to note here that a negative observation of (for example) <105 for a star with a maximum magnitude of 12.0 is a waste of the observers time. Some preparation time should be spent when compiling an observing programme. Magnitude ranges of each variable should be taken into account, considering the limiting magnitude of the instrument being used to make the observation (see below).
A very powerful method for reducing the errors in visual magnitude estimates is to make multiple estimates using different sets of comparison stars and to take the mean of the derived magnitudes. For example, the multiple estimate A(2)V(1)C, V=B gives 7.59 and 7.55 which average to 7.57. Similarly, A-4, B+0, C+2 gives 7.50, 7.55 and 7.63 which average to 7.56. Multiple estimates are particularly useful in the Pogson step method because, by using comparison stars that are brighter and fainter than the variable, it can compensate for systematic errors in the Pogson step unit.
As you make your observations you should record them in an observing log book. As mentioned above, this book should be fairly sturdy and the record should be clear and permanent. Your observations will, believe it or not, become a potential source of valuable scientific information. In the years to come someone may want to refer back to a particular observation or set of observations that you made. Your observing log will be the original source. If you want people to take your observations seriously (as they surely should) then you have to go about recording them in a professional manner. Many observers now use spreadsheets on PC's to log their observations, copying the previous nights set of estimates up the following day, and including additional information which the spreadsheet can easily calculate for you (JD & UT decimal conversion for example). These observations can then be sent to your chosen VS organisation on a monthly basis for inclusion into the database. However the observations made and noted at the telescope should be recorded as accurately as possible, as these are of course the original source.
The accompanying example illustrates a possible layout for an observing log used at the telescope. One of your main aims in recording your observations should be to make everything as explicit, clear and as robust to errors as possible. Along with the date, it is a good idea to record the day of the week as well as a check. This is something that will not cause you much trouble at the time and it may become very useful if, at a later date, someone finds that you have made an error in the date. In the front of your log book you should record such things as details of the instruments that you use, the abbreviations you use for them, the longitudes and latitudes of any different observing sites that you have used, and so on. Note there are no magnitudes listed. These should be worked out and entered into the log when the observing session is over.
Note that the time is recorded in UT (or GMT). Some observers continue to record their observations in GMAT (Greenwich Mean Astronomical Time), where 0h starts at noon. This method is now out of date, and out of step with all major VS organisations worldwide. UT should be employed at all times!
Tuesday Feb 24th 1998
| Star | Time (UT) | Estimate | Class | Inst | Notes |
| ER UMa | 20.49 | L(3)v(1)M | 1 | R400x167 | H |
| DI UMa | 20.51 | <B | 1 | R400x167 | H |
| SY Cnc | 21.52 | D(1)v(3)E | 1 | R220x50 | H |
Wednesday Feb 25th 1998
| Star | Time (UT) | Estimate | Class | Inst | Notes |
| V1113 Cyg | 03.36 | <E | 1 | R400x290 | |
| V1016 Cyg | 04.59 | B(3)v(2)C | 1 | R220x50 | |
| CR Boo | 05.40 | 5+4 | 1 | R400x118 |
STANDARD COMMENTS:
The following single-letter abbreviations can be used to indicate the presence of other factors which may affect the observation:
C = Cloud (possible patchy obscuration in the field of the variable)
E = Extrafocal (the estimates made with stars out of focus)
H = Haze (or fog or mist evenly obscuring the field of the variable)
M = Moon (brightening the sky around the variable)
P = Photographic (include details of emulsion and filters)
T = Twilight (Sun brightening the sky around the variable)
L = Light Pollution (an ever increasing problem to observers)
If you have access to a PC, the observations can be copied from your logbook into a spreadsheet where additional information can be added and reduced magnitudes entered. Here is an example of my own spreadsheet set-up....
The spreadsheet is written to conform with the BAAVSS standard visual observation report format. It works out UT decimal and JD, and will automatically convert to AAVSO and VSNET format. The UT decimal and JD decimal are worked out from the Time UT & Date columns by entering simple formulae into the spreadsheet. The classification of the observation is noted under 'Cl'. There is a standard classification used in the BAAVSS which is as follows...
1: Very confident of the estimate made under ideal conditions and confident of an accuracy of 0.1 magnitudes.
2: Less confident than 1, maybe cloud or stray light interfering. Tiredness. Accuracy of 0.2 magnitudes.
3: Observation made under extremely poor conditions; Variable just glimpsed a couple of times. Accuracy of 0.3 magnitudes or less.
A copy of this spreadsheet is available from the BAAVSS web pages
Visual variable star observations are notorious for the large errors they often show. Sometimes these can be in the order of a magnitude or more. However with a little care, it is possible to keep them to within a few tenths of a magnitude. An experienced observer can rightly claim to reach an accuracy of 0.1 magnitude under good conditions. The following points should help you to minimise the errors in your observations.
Always allow 10 to 15 minutes for your eyes to dark-adapt after coming out of a fully-lit room before starting to observe. The light-adapted eye has a quite different spectral response from the dark-adapted one.
If averted vision is used to see the variable, then the same method must be used when comparing it to a comparison star.
Always take care over identifying the variable and the comparison stars. Misidentification is one of the commonest and most pernicious errors.
Always take care over recording your observations. It can be very easy to mix up the names of comparison stars, especially when you are in a hurry.
Do not plot the light-curves of stars just before you go out to observe them, or refer to your previous observations of that star from your log book. Subconscious bias will tend to distort your observations to fit the previous trends. This is especially relevant to observers of eclipsing binary stars.
Try to avoid observing when you are very tired, ill, or in a great hurry.
Hold the instrument as steady as possible while you make your estimates.
Bring each star (variable and comparison) to the centre of the field of view of the instrument before estimating it. This will minimise various errors which depend upon the star's position in the field of view.
Try to be as consistent as possible (within reason) in your choice of comparison stars, instruments, and magnifications.
Purkinje effect. This is due to the colour sensitivity of the rods and cones in the human eye. When a red variable is estimated against a white comparison star in a small telescope, it will appear to be fainter than when observed with a larger telescope. The most accurate estimates are of stars which are of no more than four magnitudes above the limit of the instrument used. A smaller instrument (or stopping down the aperture) should be employed if the variable’s brightness is greater than this. Defocussing a very red variable and estimating it against a defocussed comparison is a useful aid in observing stars with a very red appearance.
Not all observations are of the same value. Some are more useful than others. The following suggestions will enable you to concentrate on making useful observations.
Avoid observing stars that are already grossly over-observed. You can get an idea of which stars are over-observed from the annual star totals published in the Section Circulars. At certain times of the year some non-circumpolar stars are only visible in the early morning and so tend to be poorly observed. Observations of these stars at these times are particularly valuable. If you have a large telescope then concentrate on observing stars which are relatively faint. There will be plenty of other observers to look after the brighter ones.
Use the rotation of the Earth to aid you in detecting faint stars. The eye is very susceptible to movement, and this can be used to your advantage when observing at the limit of your instrument. If you have a driven mount, switch the drive off. Place the area where you suspect the star to be at the Eastern side of the field of view and wait and watch patiently whilst the field drifts slowly across the eyepiece field. With practice the star can usually be seen as it approaches the centre of the field. The process should be repeated with the comparison stars to obtain a 'fairly' accurate estimate.
Observe each star no more frequently than is necessary to show its most rapid variations. The following list gives the recommended minimum interval between observations for various types of stars.
Type |
Interval (days) |
Type |
Interval (days) |
|
| AGN | 1 | RCB | 1 | |
| Eclipsing | See Below | RV | 2-5 | |
| GCAS | 5-10 | SDor | 5-10 | |
| In | 1 | SN | 1 | |
| L | 5-10 | SR | 5-10 | |
| M | 5-10 | UG | 1 (see below) | |
| N & NL | 1 | ZAnd | 1 |
For certain types of Dwarf Novae (UGSU) undergoing bright superoutbursts, observations can be made every few minutes when attempting to detect superhumps visually. Other types of Dwarf Novae show eclipses when in outburst. In these cases estimates should be made every minute or so once it has been established from an ephemeris when an eclipse is imminent. Occasionally a UG star will be seen in the early stages of an outburst. Observations can then be made every 30 - 60 minutes or so to capture the rise to maximum brightness. For eclipsing binary stars (type E) the recommended minimum interval can be as little as 30, 20 or even 10 minutes during eclipse, depending upon the rapidity of the fade and rise.