The nature of the star is an important factor to take into consideration when you attempt to put together an observing programme.
The slow variation of Mira stars, for example, need not be observed more than three times a month – perfect if you can only manage the odd night here and there to observe.
Eruptive and cataclysmic stars, however, need to be monitored on a nightly basis, which takes a little more dedication.
Bias is a major problem to both inexperienced and experienced observers alike, and it’s important not to over-observe a variable.
A star that varies over a period of 400 days will change in brightness very slowly indeed, so any change in its magnitude is not likely to be noticed on consecutive nights.
However, the memory of the observation made a night earlier will still be fresh in your mind and will be difficult to ignore.
Plan your observing sessions around the period of the stars you are observing.
Limiting magnitudes for various optical instruments
Aperture / Instrument / Limiting mag.
10×50 / Binoculars / 9.5
15cm / Reflector / 13.0
22cm / Reflector / 14.5
25cm / Reflector / 15.0
35cm / Schmidt-Cassegrain / 16.7
40cm / Reflector / 16.8
45cm / Reflector / 16.7
Choosing the correct star to observe with your chosen instrument is another important consideration.
A 6-inch (15cm) reflector will not show you a ‘dwarf nova’ in outburst if its magnitude range is +15.0 to +18.0.
12-inch (30cm) reflector is really too large an instrument for the RV Tauri-type star R Scuti, which varies between magnitude +4.2 and +8.6.
Choose your stars carefully and build your programme around the magnitude limits of your telescope or binoculars.
The table above gives limiting magnitudes for a variety of instruments used in light-polluted skies near Birmingham.
This should only be used as a very rough guide, as experience and quality of optics/eyepieces both play a major role on the limiting magnitude.
To inexperienced observers, the prospect of measuring the magnitude of a star to a few tenths of a magnitude may seem daunting, but providing a few procedures are followed accurately, it is straightforward.
The first thing that is required is an accurate variable-star chart.
Magnitudes taken from commercial planetarium software can sometimes lead to inconsistent results when the data is analysed and compared with official charts.
To make the observation, compare the brightness of the variable to two or more of the comparison stars on the chart – one fainter and one brighter.
Two methods are in common use, the Fractional and the Pogson Step method, the former being more suited to beginners.
The Pogson Step is recommended for those who have some experience in this type of observing, as it requires the eye to recognise differences of one tenth of a magnitude.
The fractional method simply requires the brightness of the variable to be expressed as a fraction of the difference between two comparison-star magnitudes.
There are several hurdles to overcome before a confident observation can be made.
Correctly identifying the variable itself is the most common problem faced by the newcomer.
The Purkinje Effect – the shift of the eye’s peak sensitivity towards the blue end of the spectrum at low light levels and its consequences on red stars – is another.
However, the satisfaction of developing a light curve over weeks or months more than makes up for all the preparation that has gone on before.
In our star-hopping guide to observing T Cephei we take a look at how to observe the Mira-type star T Cephei.
It has a range of magnitude from +5.2 to +11.3 over a period of 388 days, it’s circumpolar from the UK and is due at maximum brightness during early November 2005.