Project Leader: Alan Baldwin @alan-baldwin
As most readers will know, a visual observing project suitable for beginners and experienced people alike studying R Car and eta Car has begun. We have sufficient numbers, but more observers will generate a better study. Anyone who has not thought about participating, please do. We want you. As well as our own observations, we will add historical data into the AAVSO database to extend the lightcurves of the two stars backwards in time as well as forwards, and then examine the data for any insights into the objects.
- 1 Introduction
- 2 How to observe variable stars
- 3 Visual observing targets
- 4 Recording Observations
Our intent is to write up the work for publication somewhere, with all consistent contributors offered co-authorship. ‘Consistent’ here means anyone who’s observed the stars reasonably well through the year-long duration of the study. We don’t need to be perfect.
The AAVSO database, which today includes the historical VSS RASNZ data, contains observations of R Car going back to 1891. In passing, the same Scottish-South African, Alexander Roberts, was responsible for these early observations as well as those of R Cen elsewhere in this newsletter. A very fine observer! Entering ‘new’ historical data, we will extend the lightcurve backwards from 1891 for another 10 or 15 years. AAVSO and VSS observations of eta Car don’t go back anywhere near that far, so our input there will be very significant.
Our historical observations are those of John Tebbutt (1834-1916). He observed R and eta Car for 15 or so years in the late 19th century. Variable star work was only a small part of his contribution to astronomy. He published hundreds of articles, appearing everywhere from the Monthly Notices of the Royal Society to regular columns and essays in local Australian newspapers about things astronomical. He discovered two comets, a probable nova in Scorpius, did much double star work, and kept what are now very valuable weather observations for the western Sydney area. I don’t doubt he did much more. As for variables, he only observed R Car and eta Argus (as it then was) and made sporadic observations of T Cen.
As already stated elsewhere, 7 X 50 binoculars are a minimum requirement, and larger binoculars or a 90-100 mm telescope can be entirely sufficient. Correct VSS RASNZ charts will be provided, as well as any help necessary. For any comment or questions, please contact Alan Baldwin @alan-baldwin.
How to observe variable stars
Guidelines for visual observation
In this document you will find:
- Making and lodging a visual estimate of a variable star
- Two sources of error to avoid
- Comments on charts and atlases
- Stars to observe this season (winter-spring 2010)
1. Making and lodging an estimate
As an example, we want to estimate the Mira variable R Hya, and we have the correct VSS RASNZ charts (see below) made by Mati Morel. An excerpt of VSS chart 12B is pictured below.
- Locate the general area on an atlas, or use the AAVSO’s Variable Star Plotter to generate an ‘A’ scale finder chart, and then find the field in the sky.
- Find the field, not the target; the variable may or may not be visible. For example, using the chart below, when R Hya is faint we could look for the line joining the ‘91’ (9.1 mag) and SS Hya (a small amplitude variable unsuitable as a comparison star), with the two pairs of fainter stars incorporating the ‘98’ and ‘95’ aimed across it.
- If the variable is invisible, which is possible for a binocular observation of R Hya, log ‘fainter than’ the faintest star you can see, and move on.
- If visible, find two comparison stars that can define R Hya in brightness. For instance, it may be between ‘91’ and ‘86’ – the brighter of the two. Move into the center of the eyepiece the target and the comparison stars one after the other, and judge where between the ‘91’ and ‘86’ the target lays. Do this by dividing the ‘91’ and ‘86’ into steps 91, 90, 89 etc, and use your judgment.
- Write down your estimate, and the time. At some stage, either at the eyepiece later, the time will need to be in universal time (UT) or Julian Date (JD). Also you will need the chart identification number, and the comparison stars used to make the estimate.
- A data entry program such as WebObs from the AAVSO can automatically do your time conversions from local time, give you a desk top text file of all your observations for your own use and for any VSS projects, and lodge the observations directly to the AAVSO International Database.
2. Two Sources of Error
Red stars such as R Hya can fool the eye into thinking they are brighter than they really are. This is called the Purkinje effect. Take only quick glances at the target and you can avoid this.
Position error can occur when where one star above another in the eyepiece will appear a different brightness because the light falls on a different part of the retina. Keep the comparison and target stars as much as possible in the same line as that between your eyes.
3. Comments on charts and atlases
Although a high degree of standardization is required to produce reliable estimates, there is plenty of room for personal preferences here. For gross finder charts, an easily available laminated Sky Atlas 2000 is good but not cheap. Being laminated means being dew proof, and can be written on and erased.
The AAVSO’s Variable Star Plotter is free and can be used to generate ‘A’ scale charts that cover large areas of sky with a cross marked where the target is. Then, you can generate smaller scale ‘B’, ‘C’, etc, charts until you get to where you are going.
The VSS charts produced by Mati Morel and others are in my opinion superior to the computer generated ones, and are freely available on a CD upon request. In reality, a combination of the two systems is the most flexible. The ‘high degree’ of standardization referred to earlier concerns the comparison sequences; the two systems are now synchronized in this regard.
For visual observing it is not suggested that you use charts displayed on a computer screen at the telescope. Use paper charts in plastic sleeves and a red light of some type.
4. Stars to Observe
This is a very personal choice that is governed by interests and the frequency of your observing sessions. Before I say anything, I strongly urge new observers to research this themselves.
A list of well observed stars that are good for beginners would include R Hya, R Cen, T Cen, S Car, R Car, RS Sco, and so on.
Here is a list of Long Period Variables compiled from the Bulletin 74. The selection criteria used are that these stars are over 11th magnitude this current season (the list will be updated seasonally), they are (mostly) easy to find southern objects, and are in ‘very urgent’ need of visual observation today. Some of these stars have many decades of visual observations on record, and there are scientific benefits to that continuing. LPVs need only be observed once a week or less, so with practice many can be observed in a night.
There is a multitude of classes of objects that visual observers can usefully target: Luminous Blue Variables, cataclysmic variables, R Coronae Borealis stars, Cepheid variables, and more. I urge you to find out where you own interests lie.
Notes on the list of stars:
- Mags are visual and from GCVS, so VERY approximate
- Predicted maxima are VERY approximate
Visual observing targets
These stars are ideal for beginners to experienced observers. Please see How to observe variable stars for information on techniques, charts, recording and submitting observations.
|Star||Approx. Range||Pred. Max. 2011|
|R Hor||6.0-13.0||22 Mar|
|R Ret||7.6-13.3||9 Mar|
|X Cen||8.0-13.4||02 May|
|RT Sgr||7.0-13.3||11 Mar|
|T Cen||5.5-9.0||14 Apr|
|RS Lib||7.5-12.0||21 Apr|
|R Sgr||7.3-12.5||17 May|
Logging Your Variable Star Observations
- Date and Time in Universal Time (UT)
- Julian Day – from a JD calculator. (Can be filled in later)
- Magnitude estimate Mag C1 ( x ) V ( y ) Mag C2
- Derived magnitude
- Sky clarity S1 to S3; Confidence of variable star estimate C1 to C3
- Comments on conditions – see below.
- Chart source (RASNZ or AAVSO) and number.
AAVSO Comment Codes
The AAVSO in its submission page WebObs allows for comments on each observation using a number of letter codes. The codes are defined in a lookup dialogue box on the submission page.
It is worth being aware of these classifications and the associated code letters when making your observations, although the actual comments should be written in the Workbook. The codes are listed here for your information
|B||Bright – Moon, twilight, light pollution, etc|
|U||Clouds, dust, smoke, haze, etc.|
|L||Low in sky, obscured view|
|V||Faint – Near observing limit|
|Z||Magnitude estimate uncertain|
|D||Unusual activity – Fading, flare etc|
|K||Non AAVSO chart|
|S||(Companion star) Sequence problem|