ISO SWS Frequently Asked Questions (FAQ)

Last update: 12-Jun-95

1) How do I run the SWS Observation Time Estimator (SOTE) at IPAC?

Click here for a usage summary and template files which you can cut and paste.

2) The TDT's I am getting from the PH software are very high compared to those output by the SWS Observation Time Estimator (SOTE). What's the deal?

Yes. Unfortunately, the overheads have increased significantly since Phase 1. The major new overheads include High Precision Pointing (HPP), extra telemetry uplink time ("uplink jitter"), and signal-to-noise ratios measured during the testing phase which for the grating detectors are ~70% to 90% of those assumed by SOTE. All SWS AOT's now have HPP built in at the level of the AOT-to-OCT logic. The HPP time overhead amounts to 98 sec. (Note that the AOT-to-OCT logic no longer imposes an extra 117 sec overhead for each SWS aperture, as it did during April and early May.)

A summary of the new overheads is as follows: 

		    Item      Item Cumulative    Total Cumulative 
                   ---------  ---------------    ----------------
Target acquisition:
	      Slew:  180 sec
  Pointing request:   19 sec
	 Telemetry:    6 sec 
  Total:                              205 sec              205 sec

HPP:   
               HPP:   98 sec           98 sec              303 sec 

Uplink jitter:      6*(#ICSs)          11 sec minimum      TBD sec

3) What is the recipe for computing the new SWS AOT TDTs?

As of 3-Apr-95, the PH is returning TDTs in line with the new realities described above. You can use the following recipes to predict what the PH will return for SWS AOT TDTs using the output of SOTE. However, note that as of PGA v4.0, the PGA itself computes correct OTT's for all SWS AOT's, so this procedure should no longer be necessary for most observers:

4) For AOT SWS01, should the flux I input into the PGA be the source flux, or the total (source plus background) flux?

This parameter is used to set the detector gains, to avoid saturation and to assure optimal signal-to-noise will be achieved. Thus, input the total (line, plus source continuum, plus Galactic background (FIR cirrus), plus Zodical emission (if significant) flux. The Galactic and Zodi emission can be easily estimated using ISKY or IBIS.

5) Do I need a background/reference observation?

At SWS wavelengths, unless your target is sitting along the line of sight to a Galactic cloud with significant dust emission, the background will be dominated by the zodiacal light. The typical zodical emission in an SWS aperture is ~0.3 Jy. Of course it depends in detail on the exact location of your target, and on the direction of viewing from the satellite at the time of observation (the solar elongation, etc.). To be safe you should use IRSKY (or IBIS for the zodiacal backgrounds) to measure the expected background flux density in the SWS aperture(s) you are using. If the background is a significant fraction of the expected flux density in your target, and if you want SPECTROPHOTOMETRY (absolute calibration of the continuum and lines) and/or accurate line EQUIVALENT WIDTHS, you should take an offset/background/reference integration and subtract it from the target spectrum. On the other hand, if all you're interested in is LINE FLUXES AND LINE RATIOs, and the noise in the background is not too high, you can probably get away without a background measurement. This is because the continuum around the line(s) of interest will also be sampled by the spectrograph, so regardless of the exact flux and shape of the infrared background which is added to your source's underlying continuum, you can fit the continuum surrounding the line(s) and subtract it, leaving only the emission or absorption lines. A detailed analysis requires a proper propagation of errors (noise) in each specific case. BE CAREFUL!

6) Will there be a new version of SOTE?

NO! The SWS Instrument Dedicated Team has abandoned SOTE for computation of FINAL TDTs. Version 6.0 (20 June 1994) was the last version of SOTE. It should be regarded as a tool to approximate the On Target Time (OTT), and to do "what if" experiments to figure out what S/N you will need to ask for to trigger the next step up (or down) in integration time, since this is quantized by the detector reset intervals of 1, 2, 4, or 8 sec. The recipes explained in item (3) above can be used to estimate the new overheads and TDTs, based on SOTE's computed OTTs (and TDT for SWS07). However, as of mid April 1995, the Proposal Handler (PH) software returns real TDTs by implimenting the new AOT-to-OCT logic for SWS. Likewise, PGA Version 4.0, released 24-April-95, now contains the current AOT-to-OCT logic and computes OTT correctly for all SWS AOTs.

7) What is the minimum (maximum) line-to-continuum (continuum-to-line) ratio that I can expect the SWS to be able to measure?

Tests have shown that line-to-continuum ratios as small as ~1% (3-sigma?) can be measured with the SWS grating. See also question (8).

8) My source will have weak unresolved lines on a strong continuum. To detect the line at a given signal-to-noise ratio, what signal-to-noise ratio should I input into the SWS PGA (or SOTE) software for the total (line + continuum) flux density?

S/N(total)=S/N(line)*{1+[F_nu(continuum)*d_lambda/F(line)]},
where F_lam(continuum) is the expected flux density [W m^-2 um^-1] in the continuum, d_lambda [um] is the line width (use one resolution element), and F(line) is the expected total line flux [W m^-2] in the resolution element containing the line. For example, if you want S/N=10 in a line with total flux 6e-17 [W/m^2], and the continuum is 150 Jy (1.8e-14 W/m^2/um], the S/N required on the total flus is 1810!

See Sections 6.4 and 6.5 of the LWS Observers Manual for a good desciption of line-to-continuum and S/N issues.

9) Where can I find accurate wavelengths for common astrophyical emission lines observable with the SWS?

Here is a good start---
  • Atomic Lines
  • Molecular Lines

    • 10) What is the read-out-noise of the integrating amplifiers for the SWS detectors, and how does it limit the sensitivity of my observations?

    The minimum system noise (read-noise) for the grating detectors, measured at a signal power at which the photon noise is negligible, ranges 30--45 e-, or 0.32--1.00 uV/s at gain=16 and reset time = 2 sec (Table 5.1, p. 27, SWS Observer's Manual). The expected S/N ratio per resolution element for the SWS is expressed as: 
                       S * t_r * sqrt(n_int)
     S/N = ----------------------------------------,
           sqrt{N_r.o.n.^2 + (N_d^2 + N_s^2*S)*t_r}
    where S is the signal flux [uV/s] (use the curves in Figs. 5-3 - 5.8), t_r is the detector reset time in [s] and n_int is the number of integrations per resolution element (see Table 6.3, p. 50), and N_r.o.n., N_d, and N_s are the read-out noise [uV/s], dark noise [uV/s], and signal shot noise [sqrt(uV/s)] (see Table 5.2, p. 27).

    An observation is said to be "read-noise limited" when N_r.o.n. (sigma_r.o.n.) is greater than the other noise sources. (An observation is said to be "background limited" when sigma_b.g. >> sigma_r.o.n.) For example, the Si:As detector (12.0-29.5 um) has N_r.o.n.=1 [uV/s], N_d=0.41[uV/s] and N_s=0.14 [(uV/s)^0.5]. A read-noise limited observation for this detector would require 
           N_r.o.n. >> (N_d^2 + N_s^2*S)*t_r, or

              1 >> (0.17 + 0.02*S)*t_r.
    As an example, for an observation of a source with S_nu=0.5 Jy at 18.0 um, S=3.0[uV/s/Jy]*0.5[Jy]=1.5[uV/s], so we inquire whether 1 >> 0.2*t_r. Using t_r=1 or t_r=2 (e.g., the only possibilities for SWS01), we find that indeed the observation is read-noise limited. However, using SWS02 or SWS06 with t_r=8 (the maximum), the observation is not read-noise limited.

    The above recipe can of course be used to get an idea of the limitations of your SWS observations at other wavelengths and flux levels. The key data are in Table 5.2 (p. 27) [or Table 5.3, p. 39 for the FP] and Table 6.3 (p. 50).

    11) What is the expected precision of the SWS calibrations?

    References

    ISO SWS Observer's Manual
    ISO LWS Observer's Manual
    ISO PGA Reference Manual
    Various memos and communications from ESTEC and the SWS Instrument Dedicated Team.

    Disclaimer

    These are personal notes of J. Mazzarella which are in no way complete or intended to be a substitute for the original SWS document set. They are made available in case they may be of use to others.

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