Dear ISO Observer:
During the course of routine observing with LWS, some observing strategies
have been identified which facilitate optimal reduction of the data. These
suggestions reflect the current LWS consortium practice and encompass
all LWS observing modes; you may wish to consider incorporating them in
an update to your observing program. Updates will affect any observations
in your program which have not yet been scheduled. Instructions on making
updates can be found at the end of this note.
A brief summary of the recommendations is in the table below.
A detailed discussion follows.
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Recommendation Applicable to Procedure/Comments
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1) Make enough All LWS Observations Fast scan = "Y" and
independent scans (<10^4 Jys) Tint >= 2.4s ==> 6 scans.
to de-glitch the data For >~10^4 Jy see point
3).
2) Measure fringing All Obs. except Do a sufficiently wide
so data can be point sources and deep LWS01 range scan
de-fringed on axis (w/in within or in addition to
~5") the primary observation.
3) Avoid excessive ESSENTIAL for Sources Tint <- 0.7s. Forces
non-linearity >~10^4 Jy Forces 1/4s integration
ramps.
4) Use sufficient scan F-P Obs NSPEL=5. 2 more elements
width to cover will be added automatically
wings of FP spectral NOTE that this means all
elements (+/- 5 res. LWS04 observing times will
elements) increase.
5) Re-consider the Mapping Sample assuming a
spacing of well- SMALLER beam with
sampled maps. beam-width ~80"
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In detail:
1) Use at least 6 scans.
Previously, fast scanning was recommended to minimise the low-frequency
end of 1/f noise. It is now even more important to avoid correlated
contamination os successive spectral samples after a particle
hit. The recommendation to use 6 (rather than the previous 4) scans
is to ensure that sufficient uncontaminated samples are available at
each spectral point to allow median filtering. For 1/2s
integration ramps, used for all but very bright sources (see point 3),
this implies Tint per sample must be at least 2.4s.
2) Characterise fringing with a grating range scan.
Some examples of severe fringing are available on the WWW:
http://isowww.estec.esa.nl/images/fringes.gif
LWS spectra of off-axis point sources or of extended sources show
a modulation of the spectrum referred to as fringing. In wave-number space,
these fringes are cosinusoidal with a fixed phase and a constant
period of 3.6 cm^-1, independent of the detector channel. It is
therefore supposed that they arise from within the instrument and are
caused by the interference of two beams arriving at the detectors
along paths which have a fixed difference in length. In wavelength space,
the interval between successive peaks varies from about 1 micron at
the shortest wavelength to about 10 microns at the longest; at the
shortest wavelengths this is comparable to the grating resolution
element, complicating detection of lines.
If the interpretation of the origin of the fringes is correct, their
effects are present in both grating and Fabry-Perot spectra of
extended or off-axis sources. Note that different parts of a source
may have different degrees of fringing. For example, the spectrum
arising from a point source will not itself be fringed, even if it
lies within a field of extended emission. The resultant overall
spectrum, however, will exhibit fringes.
If you are CERTAIN that your source is point-like, well isolated and
its IR position is known to within 5", you may not need to take action.
In most cases, however, you will want to de-fringe your data.
Information about fringe removal can be found on the LWS Home page
(http://ast.star.rl.ac.uk/isouk/iso/lws/lws.html) and the subject will
be discussed in the Addendum to the LWS Observer's manual, which will be
available around the end of July from the ISO WWW Site
(http://isowww.estec.esa.nl/) as part of the material for the
Supplementary call for proposals.
In brief, the current procedure makes use of the constancy of the
period of the fringe to identify and isolate it. It requires at least
three complete fringes on each detector. Given present understanding,
IT IS IMPOSSIBLE TO DE-FRINGE spectra without this information.
The case of a point-like line source with strong extended continuum
emission is particularly difficult, since the amplitude of the line will
be unfringed, but the line will be superimposed on a fringed continuum.
In this case there is no way of determining the true strength of the
lines unless a spectrum of the surrounding continuum is also taken.
In detail, it is recommended:
A) For LWS01 observations, make sure that your range is sufficient
to cover at least three fringes per detector used. (Refer to
the figure on the WWW or in the Addendum, when it is available).
The observation must also have sufficient S/N on the fringe (SNc)
to get the desired S/N on the de-fringed line (SNl). If Nss is
the number of spectral samples you have in your LWS01 spectrum
(1,2,4 or 8 times the number of resolution elements; 65, for
a full grating scan. See section 6.3 in the LWS Observer's
Manual) then, at a minimum:
SNc=SNl/sqrt(Nss)
B) For LWS02,03,04 and narrow-range LWS01 consider an additional,
preferably concatenated, LWS01 scan meeting the above criteria.
The observer should calculate the S/N required as detailed
in A).
You may wish to bear in mind that a full-range minimum scan (43 to
170 microns, fast scan, 1 sample per resolution element, 3 scans)
takes about 8 minutes, and that the time does not decrease much if
you decrease the spectral range.
C) Note that if you are doing an off-source measurement or map you
will need to characterise the fringes in ALL POINTINGs (e.g., for a
2x2 raster you would need to repeat the raster with the minimum LWS01
scan.) For the case of a point source embedded in extended
emission, it is necessary to to do an off-source scan to remove
the fringed spectrum of the extended emission.
3. For bright sources, use 1/4s ramps.
For sources ~10^4 Jy at 120 microns, 1/4s ramps are necessary to prevent
the detector integration ramps from becoming too non-linear to process
successfully. Unfortunately, you must use Tint < 0.7s to get 1/4s ramps,
so you can only get 5 scans per AOT; to get more you must concatenate
2 or more successive AOTs or observe the same line repeatedly in one
AOT. (In most cases 5 scans should be sufficient to deglitch the data.)
For sources much brighter than 10^4 Jy, you should consider Fabry-Perot
observations since even 1/4s ramps may become excessively nonlinear.
4. Use a wide scan for Fabry-Perot observations.
The Airy profile of a Fabry-Perot has broad wings and sufficient width
around the line should be used to get down to the continuum. The maximum
number of spectral elements to each side of the line as specified in
PGA is 5, but the data suggest that 7 elements on either side is preferred,
especially for resolved lines. Up-link calibration tables are therefore
being modified to ADD 2 SPECTRAL ELEMENTS TO NSPEL. This will
substantially increase observing time; if you wish, you may compensate
for this by reducing NSPEL by 2, but use of NSPEL=5 is strongly
recommended.
5. Reduce step size for well-sampled maps
The LWS beam width is not yet well known, however the data suggest
that it varies non-monotonically with wavelength between 58" and 82".
It is probable that 80" is a good working figure but it could be less
than this at some wavelengths. Observers who are critically dependent
on full sampling are advised to reconsider their step sizes.
To make an update to your program:
The procedure is the same as it was for releasing the proposals post-launch.
You should contact the ISO helpdesk at helpdesk@iso.vilspa.esa.es (or IPAC
at iso@ipac.caltech.edu if you have been doing your PGA work there) and
request a visit to PDEC (or IPAC), a computer account to make remote changes
or give explicit instructions for changes to made on your behalf. When the
changes are completed you will be sent a PH report for confirmation -- this
is necessary to verify that the correct copy of your program will be uplinked
to ISO!
Most of these changes will unfortunately increase the time needed for an
observation. We remind you that the Supplemental Call for Observations will
be issued soon, and that you may propose to complete or follow-up on your
existing observing programmes.
sincerely,
ISO Helpdesk