This plot monitors the XSHOOTER response function. 
For this reason the response functions are
compared to 
<ul>
<li> a) a reference response functions acquired on 2015-10-25 and
<li> b) the master response function which is a clean and weighted average of
response functions with a certain validity range. 
</ul>

</font><p><font size="2">

On 2015-10-25, the first flux standard star was acquired with the effmon
template, meaning this observation was acquired under strict photometric
conditions. The calibration product 'response function' from that date
serves as a <b>reference</b> in the following. It is marked by a vertical red bar in
the history plot.</font><p><font size="2">

The <b>master</b> response function once generated as a weighted and clean mean of
response functions for the purpose to flux calibrate XSHOOTER science observations 
(internal data producs = IDPs) is a representative response function for a given
time range.


The comparison between the reference
response function  (from 2015-10-25, ARCFILE=XSHOO.2015-10-26T01:23...) and the current response function 
as well as the comparison between the master response and the current response
is parameterized via</font><p><font size="2"> <p>

</font><p><font size="2">
current(lambda) / reference(lambda) = a + b * (lambda - lambda_0 )

</font><p><font size="2">
current(lambda) / master(lambda) = a + b * ( lambda - lambda_0 )


</font><p><font size="2"> 
where lambda_0 is the center wavelength of the considered wavelength range, 
parameter a is the relative overall flux of the spectrum, corresponding to atmospheric variations and 
parameter b is a first order chromatic deviation (the slope between the reference(/master) and
the current). </font><p><font size="2"> This plot shows both parameters a and b, of which parameter b is the important one.
</font><p><font size="2"> On 2015-10-25, there is a=1 and b=0 by definition.

The response function is a flat-fielded efficiency, hence the response
function depends on the spectral shape of the flat field lamp, 
on long-term changes as well as on exchanges of the lamp. For this reason, dates of
lamp exchanges are marked by blue vertical bars. 

</font><p><font size="2">Notes:
</font><p><font size="2">
<ul>
<li> <b>UVB-arm</b>: the D2 lamp illuminated orders [305 - 360] nm and the QTH lamp
illuminated orders [370 - 500] nm are fit separately. The redest order of the QTH
lamp illuminated orders is not used for the fit due no known variations
of the dichroic. </li> 
<li> <b>VIS-arm</b>: The bluest orders are not used for the fit due to
known variations of the dichroic: [560 - 1020] nm.</li> 
<li> <b>NIR-arm</b>: The full range is used for the fit [1000 - 2460]
nm.</li> 
</ul>