[Aavso-photometry] When to submit "Fainter Than" versus actualnumbers.

[Jeff Hopkins]

Large errors usually indicate a problem. It could be an equipment 
problem, observing problem or it could  indicate a bad sky condition. 
One other possibility would be doing photometry beyond the capability 
of the equipment.

As for 0.002 m errors being a vast understatement of the error, I 
take case with that, however, it maybe true for CCD photometry. Doing 
single channel photon counting I routinely produce SDs of three sets 
of measurements in the area of 0.001 and sometimes even less. During 
the 1982-1984 Epsilon Aurigae Campaign, a fellow observer in Sweden 
observed the system several times at almost the exact UT as I did. In 
some of those cases our results were within 0.001 of each other. 
While I report data to three places I calculate magnitudes in the 
data reduction to 4 places and sometimes over many days, trends can 
be seen even in that range. This requires doing photometry correctly, 
e.g., observing close to the zenith as well as paying close attention 
to what is going on and knowing your equipment. Sky conditions can 
change and cause errors. Sometimes thin contrails that cannot be seen 
drift across a star. Recognizing that and not including that data can 
improve your accuracy. This requires experience and as mentioned 
above, knowing your equipment.

Single channel photometry has its own set of advantages and 
disadvantages, e.g., invisible dew on the corrector plate or star 
drift in the diaphragm. Even in dry Arizona I always use a heated dew 
shield. One way to watch the errors is to see if a significant drop 
in counts happens between readings. This almost always points to a 
problem and not an actual drop in the star's light. This is perhaps 
another advantage of photon counting. You see the result immediately 
and after observing a star for a bit can anticipate approximately 
what the counts should be. An increase in counts for a given air mass 
usually indicates the extinction has gone down, while a decrease may 
indicate the opposite. This is fine. A steady set of three readings 
(with less than 1% variation in the counts) usually indicates a very 
good night and if everything else is proper, the standard deviations 
will be very low. If the reading are erratic (> 10% variation in 
counts) either there is an equipment/observer problem or the sky just 
is not photometric.

Jeff

At 07:56 -0700 1/25/05, Michael Koppelman wrote:
>Aaron already addressed this but there is no problem with huge error 
>bars. Researchers can choose to exclude your point(s) or not. I'd 
>rather have a point with huge but well-determined error bars than no 
>point at all. I am still a bit bothered when I see people report 
>errors like 0.002. That is always a vast understatement of the 
>error. I never take a single image of any field so I can always get 
>a better idea of the error by the scatter in the comp stars. IMHO, 
>this is the only way to go.
>
>Cheers,
>Michael Koppelman

-- 
Jeff Hopkins
HPO SOFT
http://www.hposoft.com/Astro/astro.html

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