[Aavso-photometry] When to submit"FainterThan"versusactualnumbers.

[Jeff Hopkins]

Hello Arne,

Thanks for popping in here.

I understand that CCD and single channel photometry are different. 
Each has advantages and disadvantages.

My point is that you can have a standard of deviation for CCD 
photometry if you group three readings. If there are no errors with 
CCD the three reading should be very close if not exactly the same. 
The idea implies that the program star would not actually change 
greater than the measurements error during the the time of the three 
measurements. What bothers me is it seems some CCDers are giving a 
standard deviation for a magnitude that is not a real standard 
deviation (determined by the spread of data) of the program star.

What are the sources of error for a CCD type magnitude determination?

I wonder just how valid using a surrogate star is. So what if it 
changes over the set of exposures. It may well do just that due to 
changing sky conditions. The results will show a large standard 
deviation, but how does that have any meaning on the accuracy of an 
individual program star's magnitude determination? Since as you say 
all stars are measured at the same instant (or close to the same), 
variation of the surrogate star or comparison star from image to 
image should not matter, only the actual difference between the 
comparison star and program star. A standard deviation of the 
surrogate star would provide an indication on sky quality, but have 
no value indicating the quality of the program star's magnitude 
determination.

In reality photon arrival and scintillation of the light (which both 
follow Poisson statistics) from the comparison and program stars is 
unpredictable and will vary from image to image and star to star. I 
have spent hundreds of nights observing this and see where one star 
has a fairly constant set of counts while another may be constant for 
some sets and jump around a lot (not only decrease, but sometimes 
increase significantly) for others. In other words, there seems to be 
no predictable correlation of this scintillation. The best that can 
be done is to try to smooth it by averaging data. This is where the 
grouping of the three sets of measurements should help. In my 
observing I have reduced instrumentation error to near zero. Can CCD 
instrumentation error be assumed to be zero? Since I am watching 
counts real-time I can easily spot a drift of the star and other 
possible errors. The only significant errors are the changing sky and 
the scintillation of the star light. Large sky changes can be fixed 
by waiting for the sky to settle (e.g., cloud or contrail to pass) or 
packing it up for the night. Perhaps the long period CCD exposures 
sufficiently reduce the scintillation to a non-significant level.

You state "For a single frame, you can use Poisson error as a lower 
limit to the true error."  How do you do that?

Thanks for your comments on this.

Jeff


At 10:16 -0700 1/28/05, Arne Henden wrote:
>The problem here is that the variable star, well, varies.  So it is
>difficult to determine a standard deviation for its magnitude during a
>time series.  That is why people are using a known constant star of
>roughly equivalent magnitude as a surrogate.
>   The CCD photometric procedure is just different than PEP.  For PEP,
>you often use a set of measures, such as CVCVCVCK (C=comp, V=variable,
>K=check) and consider it a single datapoint.  This is because there
>are a number of PEP error sources, such as how well you center each
>star in the diaphragm, how much the sky transparency changes during
>the set, etc.  For CCD, all stars are measured simultaneously, and
>centering errors are nonexistent.  You gain in data quality; you lose
>in not having that standard deviation error estimate.  For a single
>frame, you can use Poisson error as a lower limit to the true error.
>For multiple frames, such as a time series, you can use the surrogate
>star standard deviation from its mean magnitude as a good estimate
>of the true error in the variable.
>   I think we are talking apples and oranges here.
>Arne
>
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-- 
Jeff Hopkins
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