Comet Track

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Magnitude Estimation Methods

Some contributors are now starting to specify the magnitude estimation method that is used (this is not required here, but is for observations submitted to The International Comet Quarterly). The goal of making a magnitude estimate is to obtain the total integrated brightness of the comet's head or coma. This is done by comparing defocused stars, of known brightness, to the comet. Specifically, the average surface brightness of the comet is compared with the surface brightness of defocused stars. Here is a quick summary of the different methods:
The Sidgwick or In-Out Method:

The in-focus comet is compared to the out-of-focus comparison stars. It is very important that the defocused stars must be the same size as the comet. This is the most popular method and works very well for diffuse comets. Strongly condensed objects, such as C/1995 O1 (Hale-Bopp), are more difficult to estimate using this method because it is very difficult to determine the comet's "average" surface brightness.

The Bobrovnikoff or Out-Out Method:

The comet and comparison stars are put out-of-focus together. Very easy to do. Works well for very strongly condensed (high DC) objects. Can result in a significant underestimate of brightness in very diffuse and/or large comets.
The Morris or Modified-Out Method*:

This method was developed to bridge the gap between the Sidgwick (works well for really diffuse comets) and Bobrovnikoff (best for strongly condensed comets) Methods. The comet is put slightly out-of-focus - just enough to "flatten" the brightness profile so that it is easier to determine the comet's average surface brightness. The average surface brightness of the comet is memorized as is its out-of-focus diameter. The comparison stars are then defocused to the comet's out-of-focus diameter (somewhat larger than its in-focus diameter). This method is considered more difficult than the other two methods by some observers. Note that when the comet is very condensed, this method "becomes" the Bobrovnikoff Method and when the comet is very diffuse, it becomes the Sidgwick Method. Thus, the other two methods are subsets of this method...

There are other methods, most notably the Beyer or "Way-Out" Method, but the ones given above are the methods recommended for making magnitude estimates today. Each method requires practice, particularly when comparison stars are not in the comet's field.

For the record, the author of this page uses the Sidgwick Method for comets with DC < 3 and the Bobrovnikoff Method for comets with DC = 8 or 9. The Morris Method is used for all other DC values. Obviously, exceptions occur if there is a star in the comet's coma...then the Sidgwick Method is the obvious choice. Other observers have different is not the intent to set a standard here.

- I used to call this method the Equal-Out method, because even though the comparison stars and comet are defocused, the defocused images are the same diameter (which isn't usually true with the Out-Out method). Daniel Green, Central Bureau of Astronomical Telegrams, pointed out that "equal-out" could be confusing...because the In-Out method has equal diameters and the Out-Out method is "defocused" equally. To avoid confusion, I have changed the name to "Modified-Out" method.

Daniel Green has written an article published in the October 1996 International Comet Quarterly on the history of comet magnitude estimate methodology.


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