VILSPA, 30 August 1996


Dear ISO Observer,

Evaluation of a representative sample of faint source measurements has shown
that the sensitivity limits which can be achieved in practice for ISOPHOT
detectors P3, C100, and C200 are significantly higher than pre-flight
expectations.

In order to make best use of ISO time, observers with ISOPHOT measurements
of faint sources are requested to review their programme. For the time being
all observations have been suspended which include ISOPHOT P3, C100 and C200
measurements of targets that are fainter than 50 mJy. These observations have
effectively been blocked from scheduling. A possible delay in the execution
of these observations is not expected to harm any programme since no skybins
will disappear in the coming 9 months of ISO operations.

This email is sent to all observers to alert them of the present situation.
Observers who have affected proposals containing faint observations will
receive Proposal Handling (PH) reports within the coming days. They should
re-examine the feasibility of their observations given the updated 
information.

* If the observations are worth carrying out as planned, the proposal should
be re-released:
==> send an e-mail to helpdesk explicitly RE-RELEASING the proposal.

* If the programme needs reworking, arrangements should be made to carry out
changes:
==> send a CHANGE REQUEST to helpdesk
Observers are encouraged to make changes by submitting a change request to
SOC/IPAC but for significant rework a remote login account can be arranged.

Attached you find information (1) on the current ISOPHOT sensitivity limits,
(2) where to find recommendations on the most suitable observing strategy 
for detecting faint sources, and (3) on the criteria which were used to 
suspend an observation.

Sincerely,
ISO Helpdesk

=============================================================================

1.	ISOPHOT sensitivity limits

The next table is an update of the table presented in the Helpdesk note sent
to all ISOPHOT observers on 18 April 1996. In this note the in-flight
sensitivity limits were derived from detector and photon noise analysis
and not from actual faint source observations. The limits in the table assume
a S/N=10, 128 sec on-source integration time and a differential measurement
for background subtraction. We have added in the last column the observed
10 sigma in-flight sensitivity limits based on a set of observations of 
faint sources.

------------------------------------------------------------------
det   filter   aper   Iback   Flim         Flim         Flim
               ture           pre-flight   in-flight    in-flight
                                                        observed
                ["]   [MJy/sr]  [mJy]         [mJy]       [mJy]
------------------------------------------------------------------
P1    11.5     23      15       17             63  	
P2    25       52      25       48            121
P3    60       99       8       34            164    
C100 100        -       2       15             26          150
C200 160        -       2       17             60          200
-------------------------------------------------------------------

In the table Iback is the background brightness level, Flim the sensitivity
limit. The data in the table shows that for detectors P3, C100, and C200
the observed sensitivity limits are factors 5-10 times higher than the
pre-flight predictions. Please note that the values of Flim given in the
last column of the table now include flat-field uncertainties, background
subtraction, and sky confusion, which are in addition to the already
noticed glitches and responsivity drifts.

Consequently, current instrument performance parameters have been changed 
to reflect the in-flight sensitivity limits from observed faint sources 
including all uncertainties. These have been published in the
"Addendum to ISOPHOT Observer's Manual". The document can be found in the
WWW on:

"http://www.iso.vilspa.esa.es/manuals/PHT_add/supcall11.html"

or can be retrieved as LATEX or postscript files from the ftp server:

"ftp.estec.esa.nl/pub/supp_call/PHT_add".

2. Recommendations on faint source observations

Recommendations and caveats to decide on the best observing strategy suited
to your programme can be found on the WWW under:

"http://www.iso.vilspa.esa.es/manuals/PHT_doc/PHT_doc.html"

or can be retrieved as LATEX or postscript files from the ftp server, query
the /pub/iso/supp_call/PHT_doc directory in:

"ftp.estec.esa.nl"

3.	Criteria for suspending faint source observations

The discrepancy between pre-flight predictions and in-flight observations
listed in the table above makes it necessary to revisit all faint source
observations with detectors P3, C100, and C200.

We have determined all ISOPHOT measurements in the mission database which
comply with either the following criteria:

case 1:		- Detector = P3, C100 or C200
		- POINT_OR_EXTENDED_SOURCE = P(oint)
		- EXPECTED_SOURCE_FLUX <= 0.05 Jy

case 2:		- AOT = PHT32
		- if detector = C100 then
			MINIMUM_BRIGHTNESS <= 0.27 MJy/sr
		- if detector = C200 then
			MINIMUM_BRIGHTNESS <= 1.00 MJy/sr

If these cases were met for any ISOPHOT measurement then we have set the flag
"Fixed time or periodical observation?" = Y and included the text:
"observation(s) on hold - contains PHT faint source measurement(s)".
In case the AOT is part of a concatenated chain we have entered the fixed 
time condition in the first AOT of the chain. Proposals for which all 
observations have already been executed or proposals for which all 
observations are fixed time observations have not been blocked.