VARIABLE STARS : Part 6
All brightness estimates should be recorded into an observation book, with the date and all other relevant information. It is advised that sometimes it is worthwhile to keep observations in a loose-leaf folder, with all information about the stars in the observation programme. This has an advantage as all the data can be compared, between each successive observation. It is also useful to determine the next time the star needs to be observed.
Often the changes in brightness are small, so it is not required to attempt the all your listed stars on a daily basis.
Recording the calculated date of the variable star observation is the most important. Variable observers are recommended to use the Julian Date, typically expressed to either 3 or 4 decimal places, depending on the nature of the variable. Mira variables and other red variables, only require two places because of their very long periods. Eclipsing binaries and other fast changing types will require three or more accuracy due to the rapid changes.
Number of Times an Observation Should be Made
Irregular variables should be observed once a night, if at all possible. For the long period variables, observations can be lengthened to about once a week, as the variations are typically not too large. Most of the cataclysmic types and the eruptive variables could be observed very frequently, with several estimates during the observing night.
When any variable undergoes rapid changes, such a fall in brightness, the frequency of observation should be increased. As with all variables, the increase in the number observations can produce problems with bias. Observers should also be aware that the early morning estimates are of the greatest value, even into the twilight hours. Usually, the frequency of observations become poor during this time, especially when related to the southern hemisphere. The aim of the collectors of the data is to get an even set of observations of a particular star, over a day, or the entire year, so the important changes in the light curve maybe captured.
Observations of Comparison Stars
Having identified the variable’s star field and having an rough estimate of the star brightness, it is worthwhile checking the comparison stars. It is sometimes recommended not to use comparison stars too far from the variable in question, especially if there are more suitable closer ones. Double Stars and faint stars should be avoided, except when the variable is faint or close to another bright star. Stars should also be selected so they appear along the line of sight, preferably as close to the horizontal as possible. Reflecting telescope an be regarded as an advantage, particularly if it has a rotatable tube on the telescope mount. Refractors or catodioptric telescopes have the advantage to parallel the stars by rotating the star diagonal. This places the stars in the most suitable position but complicated the orientation of the star chart.
Estimates of Colour
The colour of the variables should sometimes also be noted. Many stars display different spectral characteristics during the time of the fluctuations. The best method is to record colour using the Hagen Colour Index (HCI). The scale is as follows;
The HAGAN COLOUR INDEX (HCI)
Errors in Variable Star Observations
A summary of the most significant sources of error include
1. Identification of the Wrong Variable Star or Wrong
12. Uncomfortable Posture or Fatigue
Differences in colour between the variable and the comparison star should also be estimated, if possible. This can contribute errors to the variable estimates made at the telescope. For example, if you place a white star against a red one, the red star will apparently seem brighter. This is the psychological effect commonly called the Purkinje Effect. In some instances this causes the star to appear different by as much as 0.5 magnitudes. Basic experiments have shown that the degree of influence varies significantly among different observers. Causes are similar to the problems sometimes found with dark adaption. The effect on the eyes with dark adaption is minimised using red coloured sources because the eye sensitive in not good at these wavelengths when compared to a white or other coloured source.
Methods to reduce the Purkinje effect, is by not to directly looking at the variable in question, but by giving the star fleeting glimpses during the estimation. This let the light to fall on the outer areas of the retina, which is not as sensitive to the effect. One problem with this solution, is that the reduced sensitivity will make stars on the edge of visibility, seem to disappear.
If too bright, the safest way to reduce the apparent magnitude of the variable is to use a small telescope or pair of binoculars, or by reducing the telescopic aperture with a field stop. Reducing the aperture also has the advantage of flattening the telescope field and reducing the coma near the edge of the field.
Another solution is to observationally assess the degree of the Purkinje effect using red and white star of similar magnitude. By adding several different sized aperture field stops, the stars can be reduced in brightness until one of the stars disappears from view. The observed magnitude difference can be then quantified, and in effect maybe corrected and assessed for future reference. More useful is that the extinction magnitude can be determined against standard comparison stars. This is useful for those that use extinction photometers, so that suitable corrections can be made.
The use of filters could also be an advantage, though some references warn against attempting this. The drawbacks normally stated include loss of transmitted light through the filter, and the disadvantage of having to reset the comparison stars against any particular standard filter.
Light Time Corrections
Predictions for the observed time of the variable star is based on the heliocentric position of the Earth. This changes the times of eclipse or occultation with eclipsing binary or the luminosity changes for short period intrinsic variable. As the Earth orbits the Sun a particular event can vary in its light time by a maximum of +8 minutes. This depends on the position of the Earth and the variable in the sky.
Reporting Variable Star Observations
Having made observations they useless unless they are reported. Specific societies of the RASNZ (Royal Astronomical Society of New Zealand, Variable Star Section) and the AAVSO (American Association of Variable Star Observers) collect your observations and will actively help you improve them. Observations when combined with the other independent observers are of great benefit to the understanding to the particular classes of variables. More important is not to bias your observations by the influence of other observers. The observation submitted must be independent from their determinations.
All serious observers report their observations on a monthly basis to either the RASNZ if the observer is southern, the AAVSO if in the northern skies. Both these groups share and liaise observations, collecting the observations, compiling the information and passing on their comments. Details are also collected on the instrumentation, observing conditions and the magnifications that have been used.
When observations are made they should be left unaltered, commenting if the results are actual errors. Data should be recorded in black ink, and not in pencil, as this makes it easier for photocopying.
It is found that sometimes the adopted magnitudes may not appear to agree with the comparison stars in the star field, even from one chart to another. If the relative magnitudes are the same, this will not matter as much, as a least the comparison stars against that variable will be in the same order. (If a Minor Planet should cross the variable stars field, a magnitude estimate should be sent to the U.S. Naval Observatory for evaluation.)
Observational accuracy should always be kept in mind. Most observers can achieve an accuracy of 0.1 of a magnitude, and for the novice, this may take a month or two to achieve. Many find substantial improvements during this time, so that more arduous and difficult variables can be attempted.
When observations are taken seriously, the commitment to continued observation is very important. It is important to keep your skills at their optimum. Normal practice is to select a number of well known variable stars. These need to be observe them at regular intervals and constantly. These can be combined with a few specialised stars. Ultimately the value of these observations depends only the total number of observations and the number of years of good experience.
The Observing Programme
The program consists of keeping a continuous watch upon certain variable stars, and recording and reporting their change in magnitude. Some are long period variables of the Mira type, others are the Eruptive variables stars including novae and others are Eclipsing Binary stars. The observation of variables is largely in the hands of amateurs, as professionals do not have the time or telescope availability for continuous routine work. Observers in the Northern Hemisphere cover the northern and central regions of the celestial sphere., so in the Southern Hemisphere, who are few in numbers, concentrate on stars in the southern region which is invisible in the northern hemisphere.
Astronomical Research Ltd. of the Royal Astronomical Society of New Zealand collects observations from all over the Southern Hemisphere, processes them and sends the results to the AAVSO in America who coordinates the efforts of all variable star observers throughout the world.
Observations at eight or ten day intervals are usually sufficient for Mira type variables. Eruptive variables and novae must be regularly monitored and closely watched at outbursts, and any of such should be reported at maximum speed to Astronomical Research Ltd. Eclipsing binaries need continuous observation.
The simplest way of determining the magnitude of the variable is by companion stars. Maps are provided for the field around the star giving the brightness of a number of stars, the numbers written against these stars on the map being their magnitudes in tenths. The magnitude of the variable is obtained by estimation, by comparing it against two comparison stars, one brighter, one fainter than the variable and no greater than a magnitude difference. This method is particularly useful with fainter stars, around 9th and 12th magnitude.
With brighter stars the comparison may be made by defocussing. Find how many twists or half-turns of the focussing knob will extinguish the variable, and do the same with a couple of comparison stars, one brighter and one fainter The proportion out the differences to obtain the magnitude of the variable. But with still brighter stars the defocussing will cause the image to fill the field of view before extinguishing, so some other means must be used.
Observations and associated data should be recorded immediately they are made in a log book. Then at the end of the month (10pm on the first of the next month.) they should be written in a monthly report and posted Air Mail to Astronomical Research in New Zealand. They are desirous of receiving all reports within a week of the month’s end so they can enter them up in their ledgers.
Maps of starfields with comparison magnitudes are available from Astronomical Research Ltd. who have produced over 20 series with nearly a thousand star charts.
When recording observations quote the G.M.A.T., which is 22 hours less than Eastern Australian Standard Time, and this very often involves reducing the date to the day before. Also give the Julian Day Number, comparison star magnitudes, note the presence of Moon and age, approximate the altitude and general seeing conditions. Quote the six number designation of the star, the first four figures being IRA of the star in 1900, the last two the declination, underlined in the south. It is recommended to report the stars in order of their designation number. The month ends at 22 hours EAST on the first day of the next month. Writing out the report and post to;
(A Sample of a Variable Star Report is on Page xx.)
Last Update : 13th November 2012
Southern Astronomical Delights © (2012)
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