Re: Tech Note 97: Photometric properties of TOM1 data

[Arne Henden <aah@nofs.navy.mil>]

I must be missing something.  TN97 indicates the equation is

  calibrated mag  =   raw mag  +  a   +   b * (raw color)  +  k * airmass

You should be using "standard color" in here.  If the zeropoint for
each filter changes, then you get
        b * [(mag1 + z1) - (mag2 + z2)]
        b * [mag1-mag2] + b*[z1-z2]

This second term will influence your calibrated magnitudes unless
I don't understand what you mean by "raw color".

kv = 0.2 is too low for ground-level sites.  Values of 0.3 would be closer.

Why are you calculating the color term on each night?  Mean values
should be better since the color coefficient is *very* slowly changing
(like, years).

What are the values for the two color terms?

You should *not* use the Tycho2 transformations to determine color
terms.  The Ic values are nowhere near close enough to Landolt.
You should only use those Landolt standards that fall within scans
for that determination.  So set zeropoints perhaps with Tycho2,
but determine coefficients separately.

Arne

Stupendous Man wrote:
>   I've written up my work on photometric calibration of a subset of the 
> Mark IV data Tom has taken.  You can find my notes at
> 
>        http://stupendous.rit.edu/tass/technotes/tn0097.html
> 
>   I'd like to hear comments on this, so please don't hold back.
> 
>                                       Michael
> 
> 
> 
>