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Space Telescope Imaging Spectrograph
STAN - July 1997

-------------+--------------------------------------------+----------------
             |               ___ _____ __  ___            |
STScI        |             ,' _//_  _// /,' _/            | STAN - STIS
Analysis     |            _\ `.  / / / /_\ `.             | Number 4
News         |           /___,' /_/ /_//___,'             | July 1997
             |                                            |
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CONTENTS:
 
       - STIS NEWS 
         + SMOV Nearing End         
         + Higher NUV-MAMA Dark Rate and the Impact on Cycle 7 Proposals
         + ETC Incorporates New Dark Rates
         + Cycle-7 Calibration Plan
         + STIS Acquisition Accuracy
         + What's New in the STIS WWW Pages
         + RPS2 Updates
         + Pipeline Calibration News
         + SMOV Progress

       - NEW STIS DOCUMENTATION 
         + STIS ISRs (Instrument Science Reports)...
         + STIS FAQs (Frequently Asked Questions)...
         + STIS Advisories...
 
===========================================================================

SMOV Nearing End
----------------

Since the May issue of the STIS STAN, a great deal of new and updated 
information has become available regarding the status of the STIS which will
be discussed in this issue, including new values for the dark count rates, 
ETC updates, improvements to the cycle-7 calibration plan, updates to the 
STIS calibration pipeline, and details of the lastest results from SMOV.  

Orbital verification of STIS is now nearing its end. The sensitivities, 
spectral resolution, and PSF of the MAMA modes have now all been checked 
out (although not extensively calibrated), and are in general close to 
pre-flight predictions. Various aspects of the commissioning are discussed 
briefly below. More detailed reports will be posted on the STIS WWW page 
over the coming months. Please bear with us as we try to put the information 
into an easily digestible form.
 

Higher NUV-MAMA Dark Rate and the Impact on Cycle 7 Proposals
-------------------------------------------------------------
 
  On July 15 all STIS GOs and GTOs using the NUV-MAMA received email with 
the latest results regarding the NUV-MAMA dark count as measured on-orbit.
This memo (posted on the STIS Advisory web page) indicated the dark count 
has stabilized at a much higher rate than anticipated, ranging from 5-11 
times that specified in the STIS Instrument Handbook.  The mean NUV-MAMA 
dark rate is now ~1x10^(-3) counts/sec/pixel.  Roughly 1/4 to 1/3 of all 
STIS NUV-MAMA proposals may be affected by the increased instrumental 
background.  Therefore, users were asked to review their programs to 
determine if changes were needed by running the revised ETC which has 
incorporated the increased background rate.  Minor changes which do not 
affect the pointing or orbit structure, such as a change of grating or 
exposure time, will be handled as part of the Phase B (science optimization)
review.  More significant changes which impact upon the scheduling of the 
observations such as visit structure, special requirements, orientations or 
target changes will require resubmission of the proposal and should be 
coordinated with your PC and CS.  By now, all STIS NUV-MAMA users should 
have responded to the mailing with indications of what impacts were 
determined to exist, if any.  Resubmitted proposals should be received by 
August 15 in order to assist in completion of a realistic Long Range Plan.


ETC Incorporates New Dark Rates
-------------------------------

With the stabilization of the dark count rates for the FUV-MAMA and 
NUV-MAMA detectors, updates to the Exposure Time Calculator were required 
to properly reflect accurate count rates for planned exposures.  As noted 
above, the NUV-MAMA mean dark rate is now higher by nearly a factor of 10 
at 1.0x10^(-3) counts/sec/pixel.  For the FUV-MAMA, the new dark count 
rate has been determined to be lower than the values quoted in the STIS 
Handbook, i.e. 7.0x10^(-6) counts/sec/pixel.  The ETC has now been updated 
to include the new values in both the spectroscopic and imaging ETCs.


Cycle-7 Calibration Plan
------------------------
 
This is a reminder that the cycle-7 calibration plans for STIS are
posted on the STIS web page.  Since the last STAN, the MAMA calibration
program has been firmed up and phase-2 proposals have been submitted,
so MAMA observers may want to take another look to see if the
calibrations will satisfy their needs. Comments or suggestions are
encouraged. Please send email to help@stsci.edu, with a clear title,
such as "Comments on STIS cycle-7 calibration plan," or talk to your
Contact Scientist.


STIS Acquisition Accuracy
-------------------------
  
As noted in last issue of the STIS STAN, the accuracy of the STIS 
acquisition flight software is better than specified in the Instrument 
Handbook.  These new accuracies may allow you to revise your acquisition 
strategy (e.g. remove ACQ/PEAKs for observations in the 0.2" slits), thus 
resulting in more time for science exposures.  If you wish to revise your 
programs, please feel free to do so during the science optimization review 
("Phase B" review, which occurs roughly 10 weeks prior to the earliest 
plan window in your program); there is no need to do so prior to that time.
 
The flight software changes for ACQ/PEAK (as well as updates to make the 
ACQ more robust) will be uplinked and verified in August, thus allowing 
for routine activation of science using CCD ACQ/PEAKs and confirmation of 
the acq/peak accuracies. 

 
What's New in the STIS WWW Pages
--------------------------------

During the past few weeks several new items have been posted to the STIS 
web pages including new ISRs, FAQs and other items listed below.  You may 
find these entries at the specified STIS web page link or in the "What's 
New" link on the STIS home page.  Also note that there is now a Search 
button at the bottom of the STIS home page to assist you in locating 
specific items in the growing repository of STIS related documentation.

   Spectral Format Plots of the STIS Instrument Modes
   STIS Cycle-7 Calibration Plan  [under "Calibration Resources"]
   STIS Parallels web page [off STIS Home Page]   

New on the STIS home page, is a link to some examples of the 
STIS Archival Pure Parallels.  "Several hundred images currently 
available in the STIS parallel archive have been examined to give a
flavour of the sort of data that has been taken and an indication of 
the longer term promise of this archive. The data range from low 
Galactic latitude star fields with bright (~16th mag) stars, to very
crowded star fields in the surroundings of the M31 dwarf companions 
to very sparse fields of faint galaxies."


RPS2 Updates
------------

RPS2 7.1.1 was released operationally on 05 June 1997.  If you are 
currently preparing a Phase II program for submission (late submission or 
Carryover) you should download the new version of RPS2 (7.1.1).  RPS2 
instructions note that you should also stop your current RPS2 servers 
using the command "stop-RPS2-servers" before using the new version. 

STIS related updates include...

   1) The direction that Patterns move a target has been corrected (it now 
      matches the proposal instructions). 
   2) The CR-Split exposure times are now accurate. 
   3) The frequency at which autowavecals are inserted has been updated. 
   4) Some fixes have been made to the data buffer dumping routines (this 
      includes a fix for WFPC parallels and NICMOS coordinated parallels
      with STIS primes). 

General RPS2 related fixes include...

   1) The "stay around" description generator (which caused some people 
      problems) has been turned off.
   2) The scheduling period for Cycle 7 has been increased to reflect the 
      new March 1999 end to Cycle 7.
   3) The special requirements MIN and MAX DUR (which did not work in some 
      situations) have been fixed.
 


Pipeline Calibration News
-------------------------

Version 2.0 of STSDAS and TABLES are required to read and analyze STIS 
data.  The release of version 2.0 is dependent on IRAF 2.11.  The latest 
information on the release schedule is available off the STScI home page 
by linking to STSDAS via the Science Resources and Software pages.  A beta 
release is currently planned for the week of July 28.

The STIS calibration pipeline (calstis) has been operating well over the 
past several months, with few serious procedural problems encountered in 
the production and archiving of SMOV, GTO and GO data.   However, the 
pipeline is still at an early stage of use.  Therefore, as the Cycle 
progresses, we do expect to identify bugs and needed improvements as science 
data is routinely processed through the pipeline.  We urge you to keep track 
of changes and improvements to the pipeline through the reading of the
STANS.

In particular, this past month, through careful checking of calibrated data 
sets by external GOs and the STIS pipeline group, several problems have been 
uncovered that have recently been corrected, or for which fixes will be in 
place soon:

   - calstis was multiplying by instead of dividing by the 
     flat field reference file data. This was corrected in
     calstis version 1.3, which was installed into the 
     operational pipeline on July 16. OBSERVATIONS CALIBRATED
     PRIOR TO THIS DATA SHOULD BE RECALIBRATED WITH CALSTIS
     VERSION 1.3.

   - The reference files used for photometric calibration
     (the *_pht.fits files) did not contain the OTA through-
     put.  New reference files correcting this problem were
     delivered on July 24. DATA CALIBRATED PRIOR TO THIS DATE
     SHOULD BE RECALIBRATED USING THE NEW REFERENCE FILES,
     WHICH ARE AVAILABLE THROUGH THE ARCHIVE.

   - STIS PATTERN observations were being incorrectly associated
     into a single data file.  A fix for this problem will be 
     in place in a few weeks.  A temporary workaround was put 
     in place on July 24.  If you have pattern data taken prior 
     to this date, check with your Contact Scientist.

New calibration reference files have been delivered recently to the pipeline 
and are also accessible from the archive:

   - In-flight CCD darks have now been delivered for the period 
     4/25/97 through 7/20/97.  Weekly darks will be routinely 
     delivered to the pipeline within at least 4 weeks of their 
     having been taken.
   - One previously delivered superbias reference file and previously 
     delivered in-flight darks for the period prior to May 31 contained 
     corrupted data in the DQ (data quality) image extension.  
     These have been corrected and redelivered.
   - Corrected versions of the photometric throughput tables
     (*_pht.fits files) were delivered on July 24 to include 
     the OTA throughput.

A new Data Handbook covering STIS and NICMOS is scheduled for release on 
November 1.  Observers who get STIS data before this date should contact 
help@stsci.edu to obtain an early "preview" version of the STIS Data 
Handbook chapters.  Comments on the early version are welcome, and should 
be sent to help@stsci.edu.


SMOV Progress
-------------

Sensitivities - A brief summary of the comparison to the pre-flight
Exposure-time calculator (ETC) sensitivities is given below for each
of the first-order spectroscopic modes. The medium-resolution (M) modes
have been measured at only three wavelength settings so far.

G140L	- Ranges from 0.8 to 1.2 times the ETC prediction.
G140M	- Ranges from 1.1 to 1.4 times the ETC prediction.
G230L	- Ranges from 0.8 to 1.0 times the ETC prediction.
G230LB	- Ranges from 1.0 to 1.6 times the ETC prediction.
G230M	- Ranges from 0.6 to 1.1 times the ETC prediction. 
G230MB	- Ranges from 1.2 to 1.3 times the ETC prediction.
G430L	- Ranges from 1.1 to 1.8 times the ETC prediction.
G430M	- Ranges from 1.2 to 1.7 times the ETC prediction.
G750L	- Ranges from 0.8 to 1.4 times the ETC prediction.
G750M	- Ranges from 0.9 to 1.2 times the ETC prediction.

Some of the most discrepant values may be due to uncertainties in the ground
calibration of the order-sorting filters which cut in at the short wavelength 
ends of the M gratings, in particular.  Contamination, if present, has 
affected the sensitivities by less than 3% over the duration of SMOV.

The places where these sensitivities may make a significant difference are:
        G230M - The sensitivity is lowest at the shortest central wavelength 
                setting (1687 Angstroms).
        G140M - The higher sensitivity is at the shortest central wavelength 
                setting (1173 Angstroms), and could in principle put objects 
                over the bright object limit.
        G430M - The highest sensitivity is at the shortest central wavelength 
                setting and could cause objects to saturate. 
        G430L - The highest sensitivity is at the shortest wavelengths and 
                could cause objects to saturate.
The time to rework exposure times is when you finalize your proposals in the
phase-B phase, starting 10 weeks before the observations are scheduled.

Preliminary sensitivity estimates for several of the echelle modes
have also been determined and again appear in the nominal range
	
The bottom line is that pre-launch exposure time calculations should
be acceptable for nearly all STIS observations. 

Neither the ETC nor the pipeline calibration have yet been updated to 
reflect on-orbit sensitivities.  Updates are planned for later this summer 
and into the fall as more complete on orbit measurements are made and the 
methodology for deriving sensitivities for STIS is finalized.  We will
keep you informed of planned updates through the STAN.

Slit Throughputs -  The STIS image quality has been optimized at the slit 
plane to maximize throughput of the 0.1x0.09 arcsec aperture.  The point 
source throughput of this slit is excellent, at approximately 60% in the 
near UV, indicating nominal performance of the corrector mirror system.  
Users may plan with confidence using the slit throughput values indicated 
in the HB; actual transmissions may be slightly higher, especially in the 
FUV.  Also note a typo in Table 13.22: the throughput for the 0.2x0.2 
aperture for grating E140M at 1700 Angstroms should have been 55% (but the 
actual value, not directly measured as yet, is likely closer to 60%).  
Observers using the G430L and G750Lgrating modes should be aware of 
effective throughput losses for very narrow slits, due to the combination 
of diffraction at the slit and the Lyot stops in front of those gratings.  
While this effect is not yet well characterized, it is expected to lower 
the efficiency of the 52x0.05 slit by as much as 50% at wavelengths near 
1 um; shorter wavelengths and wider slits are affected to a much lesser 
degree.  Direct point source throughput measurement of most of the supported
slits is scheduled for next month and updates will be incorporated into 
the ETC, the calibration pipeline, and the WWW postings sometime thereafter.

CCD performance - The CCD continues to perform well. Monthly warming
and recooling of the CCD is managing to keep the hot pixels under control.
There are three CCD features, (which have been mentioned before) worth
noting for observers:
1) The number of hot pixels is significantly higher than for WFPC-2, 
   as expected from the larger physical area of the pixels. Weekly dark
   frames are taken and delivered to the pipeline; they can be used in post
   processing to remove hot pixels. GO's can also choose, when appropriate, 
   to dither their observations to allow hot pixel removal.
2) Short exposures taken in with GAIN=4 show large-scale pattern-noise
   (ripple) that is not removed by the standard bias images. This was seen in
   ground testing, and results in the higher read noise quoted for the
   GAIN=4 mode. The thing to note is that there is a large amount of
   coherence in the noise pattern, with the peak-to-peak ripple varying from
   near zero to ~ 1 DN. The coherence makes background determination 
   difficult and could limit the precision of photometry in short 
   observations taken at GAIN=4. This coherence is not present in GAIN=1 
   exposures.  
3) Spectroscopic observations longward of ~7000 Angstroms are affected by
   fringing. Several tests have been performed to see how well the
   fringing pattern can be removed with a contemporaneous flatfield image.
   Preliminary results look very promising, with peak-to-peak fringe
   amplitudes reduced from ~30% prior to flatfielding to <= 1% after 
   flatfielding. An instrument science report is in preparation describing
   the tests.  We encourage all GOs observing longward of 7000AA to
   insert contemporaneous fringe flats with their science observations
   (see Phase II CCD update for more details). These should be added
   during the Phase B review of your proposal just prior to scheduling on
   the telescope. They can occur during occultation and so should not
   affect your on orbit science time generally.

MAMA performance - The MAMA detectors continue to perform well. The high
dark current seen in the NUV-MAMA has now been characterized. 
The dark count rate seen on orbit is temperature dependent and
over the course of a typical SAA free period is seen to range roughly
monotonically from 6.2 x 10^(-4) counts/sec/pixel (or 5 times the
handbook advertised value) to 1.4 x 10^(-3) counts/sec/pixel (or 11
times the handbook value), some times exhibiting a plateau on the high end
towards the end of the SAA free period.  A mean NUV-MAMA dark value seen is
~1x10^(-3) and the dark current used in the STIS ETC has now been updated
to this value. Likewise, we have updated the STIS ETC to reflect the on
orbit realized FUV-MAMA dark current of 7.0 X 10^(-6) (or 1/9th the
handbook value). 

While possible ways to reduce the NUV-MAMA dark current by cooling the
NUV MAMA for brief periods are being investigated, it is too soon to
know if they are feasible or practical. Cycle-7 observers must assume
for now that there is no control over the dark current and that it will
be in the range mentioned above.  An advisory was sent to all NUV-MAMA
Cycle 7 Users giving them an opportunity to alter their proposal to
adjust to the higher dark rates, while staying within their orbital
allocation and approved scientific aims and is posted under the
Advisory Button on the STIS WWW Page. If you have questions, contact
your CS.

Spectroscopic image quality - Spectral resolution has been measured
using the internal calibration lamp for all modes and is consistent
with pre-launch measurements: that is, at or in some cases
considerably better than the specification of 2.5 pixels FWHM.  

Point Spread Function - STIS focus has been optimized at the slit plane. 
The measured PSF in imaging mode is consistent with expectations. The 
prelaunch PSFs posted on the STIS web page should be usable for planning 
purposed. An on-orbit PSF library is under construction but will not 
likely be available until early next year; all calibration data is available 
from the archive however, so you can retrieve the data and construct your 
own in the interim.  Observers are reminded of the extended wings of the 
NUV-MAMA PSF (instrument handbook page 95) and the long-wavelength CCD PSF 
(CCD phase-II update).

Thermal Stability - Various tests have been carried out to monitor
the thermal stability of the STIS optical bench. The spectrograph
appears to be internally quite stable, drifts typically less than one
pixel in the wavelength zero point with time over an orbit. The default
wavecal frequency used in preparing phase-2 proposals should be
sufficient to track these drifts. Data has been taken to measure drifts in
position of the STIS slit relative to the fine guidance sensors, but
the analysis is still in progress.

Detector Resets - As mentioned in earlier STANs, opto-isolators in the
STIS detector electronics occasionally react to cosmic rays. Flight
software modifications have now been put in place to ensure that when
one of these hits occurs the MAMA high voltage is turned off, so the
effect should be relatively benign. At the moment the best guess is
that about one MAMA observation per month will be lost due to such a
reset.

 

===========================================================================
 
                           NEW STIS DOCUMENTATION
                           ----------------------
 
All new STIS related documentation noted here, along with previous
items, may be found on the "Documentation" link off the STIS home
page.  New STIS advisories may be found on the STIS home page Advisories
link.
 
 

STIS ISRs (Instrument Science Reports)...
-----------------------------------------

   97-12  STIS Target Acquisitions During SMOV 
   97-11  STIS Paper Products (Revision A)



STIS FAQs (Frequently Asked Questions)...
-----------------------------------------

Orbit Limit - Update:   
   What do I do if my observation requires more than 6 consecutive orbits? 

WAVECALs and slitless spectroscopy:      
   Do I need to take auto WAVECALs for my slitless spectroscopy? 

Spacecraft Drift:  
   If my observations need to be obtained on single guide stars, what 
   type of drift can I anticipate? 



STIS Advisories...
------------------

   1) Update on NUV-MAMA Dark Count Rate
       ...plus info on resubmission of proposals if needed.
 

 
===========================================================================
                           APPENDIX: STIS Contacts
                          -----------------------
 
Any questions about the scheduling of your observations should be
 
addressed to your Program coordinator. Post-Observation questions can
be addressed to your Contact Scientist. If you do not know who these
persons are, you can find the information on the WWW at
http://www.stsci.edu/public/propinfo.html.
 
Analysis, STSDAS or any other HST-related questions can also be
addressed to help@stsci.edu
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