[daip] GMRT and Channel numbering and possm !!!

[Eric Greisen]

Hans-Rainer Kloeckner wrote:
> 
> 
> Hi Eric,
> 
>     Sorry for bothering again, but I think that something funny is going 
> on with POSSM and possibly other task if using the 0.5 channel increment 
> for GMRT data and if the reference pixel is actually negative!
> 
> Could you please check that I'm not making a mistake here.
> 
> I have extracted 3 channels out a lager UV file and the header 
> information for the frequencies axis is:
> 
> 614000000.0                Frequency
>  -1.5                 at Pixel
> 125000.0            coord incr
>  3                    Pixels
> 
> so the calculate the NEW  reference frequency for the NEW reference 
> channel (integer)
> 
> new frequency                 =  614000000.0  + 125000.0 * 
> -1.5              =  613812500.0
> new reference channel  = 
> round(-1.5)                                                    = - 2
> 

This is wrong.

Freq at chan=-2  =   RefFreq + (-2 - OldRefChan) * Incr
                  =   614000000.0 + (-0.5) * 125000.0
                  =   613937500.0

Then at chan 1   =   613937500.0 + (1 - (-2)) * 125000.0
                  =   614312500.0


> 
> Now calculating the frequency of the first channel in the data would be.
> 
> freq at channel(1)            = 613812500.0 + 125000.0  * (1 + abs(-2)) = 
> 614187500.0

abs ??? no way

> 
> 
> But by plotting the spectrum with possm the first channel is by 
> 614312500.0 =     613812500.0 + 125000.0  * 4
> could you please let me know where my error lies.

You must take the formula I gave you literally not change it with abs 
values and ignoring channel number etc

Eric Greisen