Comparison star sources
The brightness of planets
Sometimes it’s difficult to determine the magnitude of very bright comets since there are no adequate comparison stars (the brightest star, Sirius [brightness –1.5m] may be unavailable for comparison or may be too faint contrasted with a brighter comet). If that is the case, it is reasonable to use planets as comparison objects. The current brightness of planets can be looked up in corresponding electronic planetarium programs. In this case it’s important to remember that the brightness of Saturn must be considered together with its ring.
The Yale Bright Star Catalogue
Catalogue of the brightest stars. This contains information about 9110 stars brighter than magnitude 6.5 spread across the sky. Despite the acronym BSC (Bright Star Catalogue) the stars have index numbers starting with HR. The basis of the catalogue is the Harvard Revised Photometry Calatlogue which was established in 1908 at Harvard Observatory. The catalogue has a fixed number of stars (it is no longer updated) and no comments are added to the list. The current version (1991) is the fifth. Alongside the star coordinates (for epoch 1900 and 2000) are spectra and UBVRI photometry. Listings from this catalogue are automatically included in many of the electronic planetarium platforms (StarCalc and Carte du Ceil etc). These programmes allow you to choose suitable comparison stars along the paths taken by the brightest comets. This catalogue listing is very accurate as the errors in the magnitude values of these stars is no more than thousands or ten-thousands of a magnitude.
The Hipparchus star catalogue
The Hipparchus star catalogue was the result of a satellite observing programme (1989-1993) of measuring the precise values of stellar parallaxes. The program was named after one of the first compilers of a star catalogue,the ancient astronomer Hipparchus. The catalogue contains information on approximately 118,000 stars. Information about the exact brightness of these stars measured in a V filter (with errors as small as a few thousanths of a magnitude) is complete for stars as faint as eighth or ninth magnitude. There are some stars incuded that are as faint as twelfth v magnitude. Thus this catalogue is suitable for use in choosing comparison stars for moderately bright comets that can be seen in small telescopes and binoculars. The catalogue is easily imported into the free electronic planetarium programme Cartes du Ciel.
Tycho-2 catalogue
Improved version of the Tycho catalogue (significantly increased volume and accuracy of the information), complied on the basis of data obtained from the star identification device of the satellite HIPPARCOS. The astrometry contains information and data on the exact position and proper motions of the measured stars in the sky. Also included is photometric information (VT and BT) for 2.5 miliion stars in our galaxy. Also included are the coordinates of binary stars, which have separations greater then 0.8 arc sec. The catalogue contains 99% of all stars brighter than 11 magnitude and 95% of stars brighter than 11.5 magnitude. The faintest stars contained in the catalogue have magnitudes around 13.
Photometric errors for the brightest stars in the catalogue (brighter than 9m) is in the order of hundredths of a magnitude (0.01) and for the fainter ones around tenth of a magnitude (0.1-0.2)
Tycho-2 catalogue (named after the famous medieval astronomer Tycho Brahe) is now in common use due to the high accuracy of the data and the availability of bright stars. It is most often used by observers in the preparation of maps and charts for planning observations.
An important note is that the magnitudes of stars in the Tycho-2 catalogue are not shown in the V band, but in a band VT which is specific to Tycho-2. When preparing observations recalulate the magnitudes using the following formula based on VT and BT (blue filter specific to Tycho-2)
V = VT - 0.090 * (BT - VT)
Some planetarium programmes recalculate the magnitude of VT into V (eg Carte du Ceil), some do not (StarCalc). Remember this when preparing and planning and processing observations.
APASS catalogue
Photometric catalogue, based on the results of the photometric all-sky survey conducted by the American Association of Variable Star Observers (AAVSO). The latest version of the catalogue (published in 2013) covers 97% of the sky. It contains information about 50 million objects (predominantly in the range 10m to 17m).Stars are measured in five filters these being Johnson v and b as well as Sloan g', r' and i'. All these filters have been chosen to extend the traditional historical observations which link into the work that was related to the creation of the Tycho-2 catalogue. Further work is continuing to improve this catalogue and completion is expected by 2016.
Form the directory access APASS
*Right Ascension and Declination in the output directory APASS is given in degrees.
We strongly recommend visual comet observers to use this directory to choose comparision stars when observing comets fainter than magnitude 10m.
North Polar Sequence
This is a group of stars situated close to the north celestial pole, which have accurately measured photometric characteristics. It was created between 1906 and 1922 by many of the leading astronomical observatories of the day (including Mount Wilson, Greenwich and Harvard Observatories) The series contains information abou the visual brightness of 79 stars with magnitudes ranging from 2 to 20. Its use as a source of comparison stars in the visual observation of comets is often inconvenient because the stars are usually situated at considerable distances from the comet under study.
Currently there are two versions of the North Polar Sequence available.
North Polar Sequence Stars published in Henden and Kaitchuck (1982, Astronomical Photometry, NY: Van Nostrand Reinhold), p305. Download
North Polar Sequence , published AAVSO (ICQ 1,17,3,7) Map1 --- Map2 --- Map3
ASAS catalogue
The main purpose of the ASAS (The All Sky Automated Survey) is photometric monitoring of relatively bright stars distributed across the entire sky in order to identify and study different types of photometric variablity (especially of variable stars).The catalogue contains information about the brightness of around 15 million stars with magnitudes in the range of 8m -14m. The upper limit is determined by the presence of bright stars in crowded starfields. Errors associated with the photometry, usually about 0.05m, although in some cases it can reach 0.1m and above.The initial listing was complied from Southern Hemisphere observations and only includes stars that are south of +28 degrees declination. It is planned that a future listing ASAS-4 will include northern hemisphere stars. Access to the listings are given below. In it there is reference not only to the measured magnitude of the stars but to the photometric light curves of all the observations given in ASAS-3.Thumbnail images of parts of the frames are included as are links to other sites containing information about the objects.
From the Directory access ASAS
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