Visual Magnitude-Estimation Methods for Comets

Note that there are a few different acceptable methods for properly obtaining a total visual magnitude estimate of a comet, which is a much more difficult process than obtaining a visual magnitude estimate of a variable star (for example), due to the extended size of the comet's coma and to the combination of diffuse coma plus central condensation. All of the acceptable methods compare a comet's brightness with that of comparison stars from acceptable catalogues of V (or visual) magnitudes, or from atlases/charts that have such data inscribed next to star images. [It is not acceptable to produce magnitude estimates of comets based on comparison with "deep-sky" objects (nebulae, galaxies, star clusters) or on "observer experience".]

The three most commonly used extrafocal methods for determining the brightness of a comet are known as the VSS, VBM, and Modified-Out methods:

• The VSS method, also known as the "In-Out method", is good for diffuse comets that do not have a strong central condensation; for this, you compare the memorized in-focus image of the comet to a de-focussed imaged of a comparison star (which is de-focussed to the same coma size as the in-focus comet). [This is often called the "Sidgwick" method, but predates Sidgwick by half a century via Vsekhsvyatskij, Steavenson, and others, and is now termed more historically-correct by the ICQ as the "Vsekhsvyatskij-Steavenson-Sidgwick" or "VSS" method.]

• The standard, easy-to-use, extrafocal VBM, or "Out-Out", method is only correct when the comet shows little coma (thus, mainly when comets are at small solar elongations and of mag 4 or brighter). With this method, which is sort of an "equal-out" extrafocal procedure (in that the comet and star are defocussed by the same amount), the observer slightly de-focusses both the comet and comparison star(s) the same amount (which makes this the easiest of the methods to use), until comet and star appear about the same size. [This has been called the "Bobrovnikoff" method, but predates Bobrovnikoff by many years via Van Biesbroeck and others, and is now termed more historically-correct by the ICQ as the "Van Biesbroeck-Bobrovnikoff-Meisel" or "VBM" method.]
  • However, the standard VBM method produces errant results for diffuse comets or comets with large coma sizes --- which is how comets usually appear! --- by giving magnitudes that are too faint, and should generally not be used.

• A "Modified Out" method was developed by Steve O'Meara and Charles Morris in the late 1970s that essentially combines the above "In-Out" and simple "Out-Out" methods into a single, better (though somewhat more difficult to use) extrafocal procedure --- in which the comet is defocused somewhat to smear out the nuclear condensation (and overall brightness gradient), making the defocused comet image as uniform as possible in surface brightness; the comparison stars are then (further) defocused to the same size as this defocused comet image, and one must make several observations of the memorized comet and star images for a good magnitude estimate. (In this method, the comet and star are usually defocussed by unequal amounts, yielding however a "more equally correct" surface brightness for the images of comet and star.)

• The "Extrafocal-Extinction" method of Max Beyer, in which both comet and stars are defocussed excessively to see which disappears first, is not recommended for use.

Note that a detailed history and explanation of these methods was published in the October 1996 ICQ (Green 1996, ICQ 18, 186) and also in the 1997 ICQ Guide to Observing Comets, p. 63.

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