Modification to LWS Sensitivity Figures

Prepared by the ISO Science Operations Team and the LWS Principal Investigator,
based on inputs from the LWS Consortium.

10th April 1995

Version 1.0

Following the initial analysis of the LWS calibration data, which provided the
figures for the LWS User's Manual [LWS/PEC/2039.01] and the exposure time
estimator, further analyses have been made providing new figures for the
sensitivity of the LWS. Whilst it is believed that the new figures are a 
correct estimate of the instrument's performance on the ground, it must be 
emphasised that only after the in-orbit Performance Verification Phase will 
the actual in-orbit LWS sensitivity be known.

1. Revised Sensitivity Figures.

(a) Efficiency of the ISO telescope and matching to the LWS.

It has been found neither the obscuration of the telescope nor diffraction
effects were included in the published sensitivity figures. Table 1 gives the
fraction of the flux incident at the entrance of the telescope which
enters the LWS, as a function of wavelength, both for very extended sources
(governed only by the obscuration of the telescope) and for point sources; for
the latter, the diffraction effect becomes more significant at the long
wavelengths. Observers should be able to decide themselves as to whether this
observation is properly described as point or extended at their intended
wavelength of observation.

Our suggestion is that the efficiency for an extended source can be estimated
using simple interpolation between point- and extended-source figures in the

       E(D) = E(0) + (Ds/2*Dlws)^2 * (E(inf) - E(0))     for  Ds<= 2*Dlws
       E(D) = E(inf)                                     for  Ds > 2*Dlws

where E(0) is the efficiency for a point source, E(inf) is the efficiency for 
a source of infinite extent, Ds is the source size and Dlws is the LWS beam
size (FWHM ~ 96 arcsec). This formula assigns infinite extent to a source 
larger than 2*Dlws.

              Table 1.  Telescope efficiencies.

        | Wavelength |             Efficiency            | 
        |  (micron)  |-----------------------------------|
        |            | Extended source  |  Point source  |
        |   45       |     0.87         |     0.87       |
        |   60       |     0.87         |     0.86       |
        |   80       |     0.87         |     0.79       |
        |  100       |     0.87         |     0.68       |
        |  120       |     0.87         |     0.65       |
        |  140       |     0.87         |     0.65       |
        |  160       |     0.87         |     0.62       |
        |  200       |     0.87         |     0.57       |

Observers must correct their calculated fluxes according to the above
information and must enter the *corrected* values into PGA.

[the time estimator takes into acount the new detector sensitivities]

Observers who have already entered phase 2 data into PGA at PDEC
will have to update their *inputs* to PGA to account for the above
efficiency changes.  (The SOC software system will be updated as of 
24 April 1995 to reflect the revised NEP values.)

Observers should evaluate the impact on their programmes prior to a
possible re-visit to PDEC, via the following sequence of events:
  (a) Observer consults the PH user report that will be emailed to
      them from PDEC.
  (b) Observer calculates corrected fluxes (as in 1a), and sends a list 
      of modified fluxes (in a format that will be specified when the
      PH report is sent to the observer) back to ISO HELPDESK.
  (c) PDEC staff will - as quickly as workload permits - update the
      fluxes in the Observer's PGAMDB(s).  They will then re-run
      the PH/Logic system to calculate updated times and e-mail an
      updated full PH User report to the Observer.
  (d) Observer then decides, in consultation with his/her
      support astronomer as needed, the next steps for their programme.
  (e) Depending on the size of the changes , the observer can make an
      appointment for a re-visit to PDEC or can ask the PDEC staff
      to enter the changes into the system.
  (f) If the PDEC staff have entered the changes into PGA, the observer
      will be sent a full PH user report and will be asked to confirm
      that the programme has been correctly entered according to his/her
      wishes in the light of the current sensitivity figures.
All these actions have to be finished by the time PDEC closes, i.e. 
mid July 1995.

3. Actions for Observers still to visit PDEC

Prior to their visit to PDEC, Observers should prepare corrected
fluxes as described in section 1a above, in order to be able to input
these into PGA on arrival.  Thereafter, Observers must adjust their
programmes based on the outputs given by the SOC software systems.  As
always, PDEC staff will be available for consultation.

4.  Question for all Open Time LWS Observers

The Chairman of the Observing Time Allocation Committee (OTAC) has been
informed of the change in LWS sensitivity.  The OTAC has recommended
specific times for Open Time programmes; thus, with the reduction in
sensitivity, it may be that fewer observations can be done in the
recommended time. (However, some observations will be dominated by
overheads, and will therefore not be significantly affected by this

Therefore, the OTAC requests that, after evaluation of the impact of the
sensitivity changes, the PI's of affected proposals send an explicit 
statement to the ISO HELPDESK confirming that they can still meet those 
scientific aims of their proposal for which the OTAC recommended time.  
Should an observer be unable to make this confirmation, he/she should 
explain why not.

These inputs should be marked "Attention ISO Project Scientist"
as they will be reviewed by him and, as necessary, OTAC.

5.  Further Information

The new version of the LWS time estimator (version 3.5), including the new,
updated sensitivity tables can be retrieved as of 25 April 1995) by anonymous 
ftp from:
The programmes can be found in the subdirectory:  


Observers should first read the file README.1st in this directory and 
then retrieve either the VAX-VMS version from subdirectory LTE or the 
PC version from subdirectory LTE_PC.

Should you require any further clarification on this note, please
send your queries in the usual way to the ISO HELPDESK, i.e. send an
e-mail to `'.