LWS Data Frequently Asked Questions

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).

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.

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/lws/lia/lia.html)

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

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.

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.

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.

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.

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 non-flat spectra or just for certain cases?
A. For all non-flat spectra.

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.

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.

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).

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).

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.

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.

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.

Q. I can clearly see broad wings in the [CII]158um line profile. Is there any instrumental effect that might cause this ?
A. It is possible that this result from detector memory effects. The LWS detectors, especially the long wavelength ones, correctly track decreasing fluxes, but tend to be slower to react to increasing fluxes. Since a line is scanned in both directions by the grating, the leading edge of the line will tend to exhibit a non-gaussian wing. If then you average both scan directions together, you will have broad wings on both sides. This effect is of course stronger on strong lines (on a relative faint continuum), and it is illustrated in the figures below.


In the left panel the two colors represent the two grating scan direction where is evident that the leading edge of the line
is broader (blue is scanned from right to left, green from left to right). A straight average would yield the red profile in the right panel; if instead, we zap the leading edge in both scan direction and then average, the white line is obtained.