Dear LWS observer,
 
This note is intended to give the observers using the LWS Fabry Perots
some additional information on the use of the FP AOTs L03 and L04.
The line scanning AOT (L04) has been commissioned and is available for
use. The range scanning AOT (L03) is in the process of being released, but 
with detailed caveats on when its use is suitable. 
During recent detailed analysis of the grating calibration observations, 
anomalies in its behavior were discovered, which impact in particular the
accuracy of the fluxes for the L03 AOT.
 
Observers using or intending to use the range scanning mode (L03) should 
review their observations in the view of the recommendations below and contact
the ISO helpdesk if they need assistance in modifying their programme.
 
 
The problem and consequences
----------------------------
 
The origin of the problem lies in the accuracy with which the grating 
can be set to the correct position, coupled with an uncertainty of the
spectral profile of the grating over a sufficiently wide range of
wavelengths. The effect of this on the FP AOTs is as follows:
 
- L04:
For each line an attempt is made to set the grating so that its response
peaks at the wavelength of the line. After the grating has been set, the FP
is scanned over the line. An uncertainty in the setting of the grating
will lead to an uncertainty in the peak strength of the line as well as some
distortion of the line (since it is assumed in the processing that the 
grating was actually put in the correct position). If the true grating
position can be reconstructed accurately, it should be possible to 
correct for both these effects.
 
- L03:
The original intention of this AOT was to reconstruct a range of the continuum
at high resolution as well as the lines within this range. The grating is set
at a number of standard positions (0.25 grating spectral elements apart for
FPL and 0.5 grating spectral elements apart for FPS; to reduce the distance 
between these standard settings much below this would require an inordinate 
amount of time in overheads). The FPs are than scanned over a wavelength 
range larger than the distance between the grating settings to provide
overlap between the scans. If we then are to reproduce the continuum and
match it between adjacent grating positions, we must accurately know
(a) the wavelength setting of the grating and (b) the grating profile 
out to beyond its half-power point. We do not yet know (b) with sufficient
accuracy, and, in the light of the discussion above, (a) is currently
impossible (but may be possible later).
 
More information on the anomalies with the grating setting will
be provided in a later note and on the World Wide Web.
 
 
Advice on the use of L03
------------------------

Circumstances in which to use L03
---------------------------------
 
L03 can be confidently used in the following circumstances:
 
a) you believe your source consists of lines which are no more than about
5 FP resolution elements wide (so that the whole line is well within a single
FP scan), sitting on a continuum which is smooth on the scale of a grating 
resolution element, and
b) you want merely to identify spectral lines, and assign a reasonably accurate
(about 20% relative) flux to them.
 
If you want in addition to have a measure of the continuum, youo should also
make an L01 scan of the same spectral region. Note that features in the 
continuum that are unresolved by the grating but resolved by the FP will 
distort the estimate of the line strength.
 
 
Circumstances in which the use of L03 is not recommended.
---------------------------------------------------------
 
We do not recommend the use of L03 if:
 
a) you want to obtain a high resolution measure of the continuum as well
as of the lines, or
b) you believe your spectrum consists of lines which are more than 5 FP
spectral elements wide, or
c) you believe that your continuum has significant structure on the scale of 
a grating resolution element, or
d) you are interested in brod "features" in the spectrum.
 
 
If there is no other way to make your observation, then L03 may be used, but
there will be great difficulty in interpreting the resulting data. On a best 
efforts basis, the LWS team is happy to help you with this.
 
 
 
Best regards,
 
ISO helpdesk and LWS Instrument team.