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

[Jeff Hopkins]

Having a comparison star close to the same spectral type is more 
important than having them close in brightness. Different color stars 
can have a wide range of different magnitudes in different bands. 
While it is true you could use a red comparison star to compare to a 
blue program star, when performing the transformation using the color 
coefficients for your system, you are most likely to get higher 
errors than if you use similar spectral type stars. You will be hard 
pressed to find two stars with exactly the same spectrum. Look at the 
UBVRI values of stars and then the (U - B), (B - V) and (I - R) 
values for those stars. They vary all over the place. The best you 
can do is try to get stars that are close to the same U-B, B-V or I-R.

Don't confuse precision to accuracy. They are two different topics. 
Precision is how many places you measure or calculate to. Accuracy is 
how valid the data is. As I have mentioned before, you can use a yard 
stick to measure something and say it's 1.5324". But you probably 
would not be able to accurately state a precision of more that 1.5". 
In fact measuring it with a more accurate measuring device may show 
it is really 1.5143".

When stating a V magnitude of 16.247 and a standard deviation of 0.05 
you are really saying the data is only in the range 16.20 to 16.30. 
While your precision indicated 3 places the standard deviation 
indicates the accuracy to less than two places.

Jeff

At 12:43 -0700 1/27/05, Geir Klingenberg wrote:
>Hi
>
>This is the way I do it too.
>
>I choose a star that is close in magnitude to the target and calculate
>the standard deviation of the differential magnitude between this
>check star and the comp star(s). But Gary also mention that they
>should be close in color. Why does color matter when it comes to
>precision? That is; wouldn't a blue and a red star that gives the same
>instrumental magnitude have the same precision?
>
>Geir Klingenberg

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

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