Photometric standard stars in astronomy science are used to calibrate electronic photometers and imaging detectors, like a CCD camera or a digital single-lens reflex camera (DSLR), like a Canon EOS or Nikon digital camera. This article discusses a selection of white dwarfs as primary standard stars used for the photometry and calibration of flux with my observations with a Vixen VC200L astronomical telescope and Canon EOS DSLR cameras. 

Astronomers easily find several standard catalogues, like Landolt's catalog of photometric standard stars for the UBVRI spectral range: UV (U), blue (B), visual (V), red (R) and infrared (I). Here, V means the visual spectral range, in fact it is only an approximation of the spectral range of the human eye. The maximum transmission of the V-band is centered around green or yellowish green color. This depends on the transmission of color filters and the spectral response of the detector and is not identical to a human eye. The V values only provide an approximation of the visual appearance of the stellar brightness (illumination). On the other hand the V values are very accurate within the photometric system itself. Between several photometric systems of different observatories with different filter sets and detectors, however, there may be deviations between measured V magnitudes. Therefore, don't think about having found absolute precision with a V (visual) magnitude found in the literature. Every observatory has its own challenge to calibrate its own color system to those published by other authors.

Without further background knowledge there are certain issues with these catalogs of standard stars. First of all, most of the stars provided are located close to the celestial equator (declination around +/- 0°. ESO, the European Southern Observatory, naturally is more interested in the southern hemisphere. Recommendations provided as spectrophotometric standards include more stars having negative declination. The declination is a celestial coordinate and corresponds to the geographic latitude of the surface of the earth. Stars located close to or below the celestial equator, will not move far from the horizon or becomes invisible to an observer located at the northern hemisphere. The same star found at zenith will appeare more red if it is closer to horizon. Low declination thus means a certain reddening of the color of a star as a function of its location above horizon. This effect is well-known and will also cause the famous sky reddening at sunset. Therefore, an astronomer located at the northern hemisphere will not find these "European references" very useful. This is the reason why I provide my own selection of standard stars collected from different stellar catalogs here. Over years some standard stars provided cannot be used anymore. There are many reasons to discard a certain standard star later. For example amateur or professional astronomers found these stars variable after the catalogs had been published. One has to take care, which star to use. Never trust a publication record in the whole without further examination.

White dwarfs may be used as standard candles, as many of them will not show much variations of their flux. This is because white dwarfs reached a new thermal equlibrium after the star became a "white dwarf".  In other words, it is proven (until today), that these selected white dwarfs are not variable stars or eclipsing binaries. A further selection criteria was to have selected stars with positive declination above declination 0°. Higher declination makes the star more useful for comparison with stellar clusters found in the zenith from a site in the northern hemisphere (here in Germany for example). Thus photometric results are expected to improve with higher declination closer to DE=50°. A further criterion was to have at least one star for a certain oberving period. The stars are collected and compared from different sources. These are the original references and some online publications. References are given below. The columns in the table will mean:

Accuracy

Values in italic letters ¹) are computed values from photometric data obtained from Simbad database at CDS. This is my current approach to determine best known values for these stars. ESO compilation did not provide U, R and I for these stars. Further issues with accuracy and completeness are provided in the next section.

Remaining Issues

This compiled list still is incomplete. There is only one standard candle with declination around +50°. The table is also incomplete in terms of U, R and I magnitudes. ESO provides a compilation for standard white dwarfs with high declination, but U, R and I are not provided. B-V data are sufficient for a certain astronomical task, like the creation and interpretation of a color-magnitude diagram. However, the public data base records provided at ESO are not sufficient to calibrate a multi-color photometer like a modified DSLR, which spans a spectral range from the near UV to the near infra-red. There also remain some gaps for a whole year survey. Perhaps, one will need more than one star per night to have a standard candle close to zenith for the whole observing night and close to the object of interest. Thus, I will try to complete the records, with more references and stars to get a denser completeness over the northern sky hemisphere.

A calibration between those values provided and the instrumental parameters of the own telescope and camera setup depend heavily on the detector and color filter set used. Colors or difference color magnitudes derived with any CCD and even a Johnson filter system will not provide the same photometric values with different detectors using the same filter set. This is even true for a digital single lens reflex camera (DSLR). These cameras have color filters different from the Johnson filter system. Therefore, a more sophisticated calibration method ist needed.

However, the many amateur astronomers have a certain advantage over the professional institutes. Professional astronomical observatories use different color filters and detectors with any of their telescopes. This means calibration is much more difficult and tells a story by itself. The many amateurs use very similar equipment and detectors from mass production. This may sound very promising, as results with certain telescope setups seem much more comparable compared to the results obtained by the individual professional observatories. This may sound crazy to your ears, as you might not have expected that professionals create their own handicaps. Therefore, this article needs to be continued...

References

Landolt, A.U., 1983. UBVRI photometric standard stars around the celestial equator, Astronomical Journal, vol. 88, Mar. 1983, p. 439-460 (primary source) 

Landolt, A.U., 1992. UBVRI photometric standard stars in the magnitude range 11.5-16.0 around the celestial equator, Astronomical Journal, vol. 104, no. 1, July 1992, p. 340-371 (primary source)

Optical and UV Spectrophotometric Standard Stars, ESO online publication, visited on 22 August 2010

Website of the Centre de Données Astronomiques de Strassbourg