Q. I tried running ISAP on an LWS Fabry-Perot observation and I get
detector  buttons for only detectors 6,7,8,9.

A. The problem has nothing to do with ISAP, IDL or the computer; it has
to  do with the fact that the observation is an L03/L04 AOT (FP). If a
specific  wavelength range or line was required with LWS which falls on
one of the ten  detectors, the instrument will also give you for free
the wavelength ranges that  are simoultaneously falling on the other
nine detectors. With the FP it often  happens that these extra bits of
spectra can not be calibrated. In this case the  Pipeline leaves those
bits (and hence those detectors) out of the product file  (the LSAN
file).

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Q. I would like to know some things about the origin of the uncertainty
in the  integrated line flux as given by ISAP.

A. The integrated line flux uncertainty is propagated from the
uncertainties in  2 free parameters of the fit: height and width. Those
two uncertainties come  from the diagonal element of the regression
matrix multiplied by the rms from  the baseline fit (if available from
10 or more baseline points) or the residual  of the gaussian fit itself.
Bevington has a discription of this calculation.

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Q. How do you get pointing information out (ie. is there a program
written, or  do I need to do it the hard way) ?

A. the INSPECT_RASTER routine is suited for this purpose. Check out the 
documentation for this routine in the LIA www page 
(http://www.ipac.caltech.edu/iso/lia/lia.html)

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Q. I'd like to know the aperture for the various LWS observations, do I
just go  back to the manuals or can I get it from the header ?

A. The LWS aperture is in principle fixed. In the paper by Swinyard et
al. in  the A&A special issue an aperture of 80" is quoted. Infact, it
is found that the aperture (the beam-size actually) varies with
wavelength. Check out this www  page for the most recent estimates of
the LWS aperture for the 10 detectors: 
http://isowww.estec.esa.nl/instr/LWS/note/beam_mem.html

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Q. How do I adjust L04 (FP) data to account for fringing ?

A. It is not possible to correct L04 data for fringes; you could do
something  if you also have a full grating scan for that source (L01
AOT). In that case you defringe the L01 observation and use the fitted
fringe to correct the L04 spectrum.

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Q. Where are the LWS resolutions listed?  I've looked in the handouts
from the LWS IPAC workshop, but can't seem to find them.

A. what it should be listed are the spectral element sizes. The
original  estimates are 0.29um for the first 5 detectors and 0.6um for
the last 5 detectors. 

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Q. I would like to know whether ISAP is available for an NT machine.

A. some people are successfully using ISAP on a PC but only under Linux.
We have  no reports of people using it under windows. 

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Q. How believable is the continuum level in our L04 data for weak/strong
lines?  Although it is not generally recommended to use FP spectra to 
determine continuum fluxes and shapes, are there cases where the 
continuum can be determined from FP spectra with high s/n?

A. The transparency of the FP is so low that the signal from the source
(unless  very bright) is comparable to the dark current; hence a small
uncertainty in the  determination of the dark current is enormously
amplified in the calibrated  spectrum. On a source like Orion BN
(100,000 Jy at 100um) the estimated  continuum was within 30% of the
values observed from KAO. On a fainter source,  the continuum
determination has essentially an unpredictable uncertainty.

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Q. What cases justify the need for grating adjustments in L04 data? The
documentation in fp_proc suggest that the only criteria for L04  data is
to flatten out the continuum in the spectra.  So should a  grating
adjustment be made for all un-flat spectra or just for certain cases?  

A. For all un-flat spectra.

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Q. Are there some general guidelines about the maximum safe grating 
position adjustment one can make in FP_PROC ? 

A. No guidelines; within the shown range, we do not know what the exact
position  of the grating was during the observation.

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Q. In some of our lines observed with the LWS FP (AOT L04), we are
noticing some  sag (a bowl shaped depression) in the middle of our
spectra. What can we  attribute this to ?  Adjusting the grating
position with FP_PROC doesn't seem to  help. Section 7.4 of the LWS IDUM
manual mentions that non-linear detector  behavior can cause 'sags' in
the shape of the spectra.  Can this also be causing  this depression in
the FP data?  And how can we correct for this?

A. Non-linearity in LWS is something that arises with very bright
objects in  grating mode. With the FP the flux incident on the detectors
is generally so low  that saturation (hence non-linearity) never arises.
The reason for the sag may  be due to an incorrect dark current
estimate. Reprocessing with FP_PROC is  advised. However, this is not
very important for the determination of a line  flux since this is not
influenced by an incorrect dark current subtraction. 

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Q. In defringing, the cookbook now says to only defringe LW detectors.
For a few of our sources, there are what certainly look like fringes in
some of the SW detectors (usually SW2 and 3, once SW5). In previous
versions of ISAP, trying to defringe these detectors failed, but in ISAP
1.6a, they cleaned up nicely for at least some sources.  Why don't  you
recommend defringing SW detectors? Since we get an improvement on the 
quality of some SW data, are you sure we shouldn't defringe?

A. We do not recommend defringing SW data because they are more
difficult to  recognize in the those detectors. This is because the
frequency of the fringes  (in wavelength space) increases with
decreasing wavelength and a series of bumps  in the spectrum can easily
be interpreted as a fringe and corrected accordingly. Only attempt
defringing if the the period of the fringe is of the order of 3.4 (in
wavenumber space).

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Q. I noticed that you no longer recommend shifting the data to align the
detectors (rather emphatically, too!). Is this likely to be the  final
recommendation? For most of our spectra, the offsets are less than 15%,
but a few have offsets greater than 30%, usually between SW1 and 2 or LW
4 and 5. (I will try LIA, but it didn't make much difference for the
source I tried it on already.)

A. Moving the detectors spectra to align them to one another is in
principle an  arbitrary operation. Unless you know the cause of the
misalignment (e.g.  incorrect DC subtraction), you do not know the type
of shift to apply (offset or  gain ?). Besides, what's a good detector
to be used as a reference one ? We cannot give a GENERAL recommendation in this respect. Besides, we realized that a dominant cause for the misalignment of the detectors is due to the fact that the transmission profiles we use to calibrate the data have been derived on a point-like source. If your source is extended, your spectrum will be incorrect to an extent that depends on the extension and the position of emitting areas in the beam (which, by the way is not yet fully characterized). 

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Q. The OI and CII line profiles with LWS show very broad wings which look like an underlying broad component. As far as I can see there is no physical reason for such a component, but I can't pin down any processing
artefact to be made responsible for it.

A. The problem has to do with the sampling of the spectrum. The LWS detectors are known to be slow in response to an increase in flux, in the sense that they rise slowly ; on the other hand they are very good when tracing a decreasing flux: i.e. you can trust the shape of the decreasing part of the line. This is a known problem for many of LWS spectra which are done with fast scan and oversampled of a factor 4 but I see it really becomes critical when no oversampling is present. If you plot the forward and backward scans separately, you will certainly see that in the two groups of scans only one line wing at a time is apparent. The easiest fix is to zap the wings and fit a line to what's left of the line and baseline; fixing the linewidth to the instrumental spectral element width might help a meaningful convergence of the fit.

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Q. I asked for 4 lines observed with LWS AOT 04, but when I open the
LSAN file in ISAP I can only find three lines. Where is the missing line
?

A. This is due to the LWS pipeline having problems with the FP data
processing. The LWS grating acts as an order sorter for the FP; the
degree of uncertainty in the positioning of the grating is sometimes
preventing the OLP from being able to determine the correct order of the
observed wavelength range. Since the number of combinations of
wavelength ranges and order is virtually limitless, only those
combination which guarantee the maximum sensitivity have been
calibrated. As a result, if the wrong order is picked-up for that
grating position, the data cannot be calibrated and are lost in the LSAN
file. 

The fix to this, is to reprocess the data using the LIA routine FP_PROC.
This routine applies a different algorithm with respect to the automatic
pipeline, for the determination of the FP order and the information is
never lost.

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Q. How it comes that when I average my LWS or SWS data, some points have a 0 standard deviation attached ?

A. average will yield 0 standard deviations whenever it has only one
point in a delta_lambda bin. This often happens in LWS data when you
have a small number of scans. It can also happen with a high number of
scans, when you zapped many points at a particular wavelength. In SWS it
is more common at the edge of each band, where the band coverage of the
detectors was less than perfect. 

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