Jump to top page

Reports from Geisei Observatory <February 2, 2009>

C/2007 N3 (Lulin) and visual magnitude estimates

    C/2007 N3 (Lulin) is glowing in the predawn sky and getting closer to the earth. Currently its magnitude is from 7th to 8th and hasn't brightened as much as initially predicted. The coma with a solid nucleus is a beautiful green. I haven't see a bright comet like this for some time.

C/2007 N3 (Lulin)
16-minute exposure at 4:34, February 2, 2009, J.S.T.
70cm f/7 reflector, ISO 800 negative color film
Photographed by Tsutomu Seki

    In photographs this comet appears to be at the 8th magnitude, almost the same brightness as a nearby bright star. It is not easy to visually estimate the magnitude of a comet because the fuzzy image of a comet has to be compared with a pinpoint image of a star. It is often advised to compare the extrafocal image of a comparison star with the image of the comet. The telescope must be defocused to the extent that the out-of-focus star image either inside or outside the focus equals the diameter of the comet. However, many observers point out that this tends to underestimate the magnitude of the comet because the comet's brightness decreases as the comet itself is put out of focus. At a Comet Conference held in Tokyo in the 1970s, Mr Koichiro Tomita of the National Astronomical Observatory said that, contrary to this popular notion, we could overestimate, not underestimate, the comet's brightness. As the images of the star and the comet are being gradually defocused, the star image loses its brightness at a much faster rate than the fuzzy image of the comet does. Mr. Kiichiro Furukawa of the same observatory said that the values obtained by this method would differ depending on whether the inside focus image or outside focus image is used.
    I think this is a very important issue worth serious consideration. I tested this myself by comparing the defocused images using Comet Lulin. I estimated the comet to be at 7th magnitude, roughly one magnitude brighter than the nearby star. I am not sure which value is correct, but it may not be desirable to defocus the image too much. The method I have been advocating for many years involves defocusing the image to the extent that the star image becomes a little larger than a pinpoint, avoiding too much defocusing. It may not be easy, but less problematic. After this is achieved, you make a magnitude estimate by the fractional method choosing a star brighter than the comet and one fainter. Or use the step method and follow the procedure for a magnitude estimate for variable stars. I prefer the step method and my one step is 0.1 magnitude.
    In case of a comet bright enough for binoculars, you may be able to obtain a more accurate estimate because the amount of defocusing is small. The method I occasionally use is to choose a number of stars (more than 5, if possible; the more the better) close to the comet in a wide field and average out the estimates. It will be more accurate than a poorly applied fractional method using two stars with a large magnitude difference.
    There will also be a significant difference in results depending on whether you observe under an excellent dark sky or near a large bright city. In the urban areas you tend to underestimate the brightness of a fuzzy object like a comet. I experimented with a comet in 1965 and found that the difference was as large as one magnitude.
    I believe that for the physical study of comet brightness (i.e., the relationship between the heliocentric distance and brightness) it is better to use the results obtained only by the same person using the same observing site and same telescope. The comet's standard magnitude and the coefficient of log R are often worked out from the empirical formula. Because the comet occasionally displays unusual magnitude changes over a short period of time, it is important to collect a wide range of data. However, in case of a new comet with a near parabolic orbit, 10 is used as the coefficient of log R in the magnitude formula assuming that the comet's magnitude is inversely proportional to the 4th power of its distance from the sun; and for a periodic comet which frequently returns, 15 is used for the coefficient. These formulae have been commonly used without major problems. Personally, I prefer these simple formulae to more complex and cumbersome formulae.
    Problems relating to comet magnitudes are awaiting future research for solutions.
Copyright (C) 2009 Tsutomu Seki.