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["G.Comello" <comello@astro.rug.nl>]

Subject : [vsnet-chat 1217]  From the "A Guide to Dwarf Novae Observing" -
          "How to observe faint stars" 

What Dr. Kato means with "mental substraction of the sky background" is unclear
to me. I doubt if there are observers with "CCD-eyes".
To the contrary, I would warn against trying to see "too much", which is 
encouraged by the aforementioned aricle of Dr. Kato.
Looking long enough with high powers the observer may see all kinds of faint 
objects, close to or at the limiting-magnitude. In my opinion it is impossible
to detect a few photons of a faint star in the avalanche of backgroundphotons.
This is the realm of imagination which is demonstrated for instance by the
canals on Mars which were only seen with relatively small telescopes! Also I do 
not give much value to estimates at the limits of a telescope, particularly in
light-polluted areas : the archives of VSNET itself clearly demonstrate that
observers make mistakes with some regularity.
  
In an article in Sky and Telescope, November 1989, pp.522-525 : "Your telescope
's Limiting Magnitude", Bradley E. Scheafer deals with the results of an exp-
eriment to determine the limiting-magnitude of telescopes under different
circumstances. A formula is used in an algorithm to predict this limiting-
magnitude. With a 4-inch telescope 13.5 can be seen, under excellent conditions
14 is at the limit. This is in good agreement with an earlier, smallscale 
investigation among Dutch observers by the late Henk Feijth of the Dutch 
Variable Star Section, which was done between 1973 and 1982.
After almost 40 years of observing I can confirm the results of both 
experiments. It is possible to achieve better results under special conditions. 
Under very good conditions I could see stars down to 16 on the Observatory of 
Roque de los Muchachos on La Palma, at an altitude of 2400-m, with a 20.3-cm 
Celestron. This was in 1986 during an expedition to observe comet Halley.
The limit of the naked eye was 7.3 and the seeing bettter than 0.5".

Therefore, my advise is to be careful, and leave the variables close or at the
limiting-magnitude to the larger telescopes!
 

                                                  Georg Comello

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