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 table: E(D) = E(0) + (Ds/2*Dlws)^2 * (E(inf) - E(0)) for Ds<= 2*Dlws and 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 change.) 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: ftp.estec.esa.nl The programmes can be found in the subdirectory: pub/iso/LWS_ITE 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 `helpdesk@iso.estec.esa.nl'.