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Space Telescope Imaging Spectrograph
STAN - August 1998

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STScI                        .  . __..___.                       Number 9
Analysis                     |__|(__   |                         AUGUST 1998
Newsletter                   |  |.__)  |   
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               G H R S     ( S | T | I | S )      F O  S  
                            \_/ \_/ \_/ \_/          
 
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CONTENTS:
 
       - STIS NEWS 
         + Second order contamination in G750M(10363) and G750L(8975) 
         + Shifts in G140L spectra in the cross-dispersion direction
      
       - SPECTROSCOPIC MODE PECULIARITIES (STIS ISR 98-24)

       - CYCLE 8 NEWS 

       - EXPOSURE TIME CALCULATOR UPDATES/FAQS

       - HDF-SOUTH UPDATE

       - STIS PIPELINE and SOFTWARE

       - STIS REFERENCE FILE UPDATES

       - STIS CALIBRATION NEWS

       - NEW STIS DOCUMENTATION 
         + STIS ISRs (Instrument Science Reports) and FAQs
         + Non-proprietary STIS Cycle 7 program listings posted

       - RECENT PREPRINTS
 
       - APPENDIX: HST Spectroscopy Contacts

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

                           STIS NEWS
                           --------- 
 
+ Second order contamination in G750M(10363) and G750L(8975)

We would like to draw your attention to recent calibration analyses which
have identified a problem with long wavelength settings using the G750M and 
G750L gratings.  As noted on the newly posted Foible under the Performance
Button on the STIS Home Page link entitled "Second Order Contamination in 
G750L and G750M at lambda>10300" (7/6/98), the settings for G750M 10363 and 
G750L 8975 are heavily contaminated.  Blocking filters begin transmitting 
light longward of ~5150A resulting in second order contamination longward 
of ~10300A, as illustrated in the posted figure.  While second order 
contamination for the G750L 8975A setting is also observed, almost the 
entire useful wavelength range for the G750L is covered by the 7751 prime 
setting.  Therefore, beginning in Cycle 8, the G750M 10363 and G750L 8975 
settings will no longer be supported for general use, and will be 
available-but-unsupported modes.
 
If your Cycle 7 program has not yet executed, you may wish to reconsider 
your choice of grating settings for the affected exposures.  Please contact 
your PC and CS to proceed with any necessary changes.  If your program has 
already completed, we want to be sure you are aware of the problem as you 
begin the analysis of your data.


+ Shifts in G140L spectra in the cross-dispersion direction

As noted in the revised STIS Instrument Handbook (June 1998), the FUV-MAMA
repeller wire produces a small shadow which is visible in detector flat field
images.  To avoid having first-order mode spectra fall on the shadow, the 
default Mode Select Mechanism (MSM) position for the G140L and G140M gratings 
was changed moving the spectrum image to a location 3 arcseconds, or ~120 
pixels (to AXIS2 = 632) above the repeller wire away from the detector center. 

Since this offset is achieved by a MSM motion, all apertures, including 
slitless observations, are affected.  Additionally, small monthly shifts in 
the projection of spectra on the detector have been introduced for all 
spectroscopic modes in order to minimize uneven charge depletion in the 
microchannel plates, thus avoiding an increase in the non-uniformity of flat 
fields.  Monthly shifts can approach +/- 30 pixels in AXIS1 (dispersion) and 
+/-80 pixels in AXIS2 (cross-dispersion).  For slitless observations, this 
means that the target will not be in the same location on the detector for 
the dispersed and undispersed modes. 

Although the change of default MSM position and the monthly offsets are 
relatively small, some portions of extended targets may fall near the end of 
a slit, or even out of the field of view, as a result of the change in 
default.  Another change of default position is planned during the Sept/Oct 
timeframe.  The current default position, while avoiding the repeller wire 
shadow, places the spectrum in a region of higher background counts.  The new 
move will place the spectrum at a location on the detector below the repeller 
wire in a region of very low background.  The final positioning (pixel offset) 
will be reported to users upon completion of the procedure.  Programs 
involving extended targets currently containing the special requirements 
POS TARGs or PATTERN=ALONG-SLIT, or those wishing to incorporate a POS TARGs 
to assure most or all of extended images are within the field of view may wish
to consider corrections to their programs at that time.  

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

         SPECTROSCOPIC MODE PECULIARITIES (STIS ISR 98-24)
         -------------------------------------------------

An Instrument Science Report (released 11 June 1998) describes a number of 
peculiarities in spectroscopic mode data which may affect the scientific 
goals of the observations, particularly for detection of faint features or
for objects located very close to a bright source.  STIS ISR 98-24, posted 
on the STIS Documentation web page, discusses three classes of spectroscopic 
images exhibiting problematic features.  These include images showing 
1) diffraction structures of the PSF reimaged at the various STIS detectors, 
2) 'railroad tracks', i.e. additional "spectra" displaced and parallel to the
primary spectrum and 3) ghost spectra which occur in some selected wavelength
settings.  Examples and scientific impacts are presented in the ISR.



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

                          CYCLE 8 NEWS
                          ------------

A number of new features, or removal of some others, will be evident as you 
prepare your Phase II submissions later this year.  We note a few of these 
here in order for you to be aware of issues which may affect the developement 
of your Phase I proposal.

+ G750L (8975) and G750M (10363) no longer supported in C8
  (see accompanying article related to second order contamination)

+ Local rate check image (MAMA) eliminated:
  Elimination of the local rate check image for the MAMAs will recover 
  between 2-4 minutes of overhead allowing more time for science observations.

+ 0.2X0.2 aperture with G750L now available-but-unsupported.
 
+ Jenkins slit (0.1X0.03) now supported with echelle gratings
  The smallest slit of STIS provides the highest resolution capable with 
  STIS and will be supported slit in C8.  

+ FPSPLIT slits now supported
  FPSPLIT slits will allow the observer to obtain S/N ratios >300:1 with the
  echelles gratings.

+ Smaller subarrays (16X16) for CCD ACQ/PEAKs that use the MIRROR

+ Automated correction of hot pixles on acquisitions (ACQs)

+ External shutter to be closed for GO wavecals:
  Saturation of GO wavecals when observing bright targets will no longer
  be a problem.  (The external shutter stayed open until now).

+ MAMA wavecals now allowed in occultation:
  This allows for the ability to take MAMA wavecal exposures during 
  occultations.  It will save up to 5 minutes for on-orbit science time



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

                 EXPOSURE TIME CALCULATOR UPDATES/FAQS
                 -------------------------------------

The Exposure Time Calculator was updated on August 12, 1998 to support the 
Cycle 8 Phase I proposal period.  This update includes new echelle through-
puts created from on-orbit calibration data.  Unlike the previous set of 
echelle throughputs released May 1998, these echelle throughputs compensate 
for the increased flux in the regions where the orders overlap.  In the case 
of the Echelle modes, there is some extra noise because of the scattered 
light which runs across the orders. The updated version of the ETC takes 
this extra noise into account (to a first order approximation) in the 
signal-to-noise calculations.

Comparisons of ETC output and calibration data were made for the updated 
ETC.  For the Imaging ETC, CCD simulations are typically within 5% of the 
measured results.  Most of the discrepancy is probably due to a small 
mismatch between the ETC assumptions and the measured PSF, difference between 
the number of pixels the ETC integrates over and the real number of pixels 
under the PSF.  The MAMA mode estimates are within 5% of the real 
measurements, except when using the F25CIII filter, where the observed/ETC 
ratio is 0.75.  This discrepancy is due to the PSF for the F25CIII filter, 
which has much wider wings than the other filters.  This will be resolved in 
the future by allowing the ETC PSF assumptions to vary with the filter used.
For the spectroscopic ETC, the count rate predicitions are typically
accurate to within 10% for point sources (for all gratings).  Observations 
in a few wavelength regions (e.g., the E230M grating with central wavelength 
2707 A) yield throughputs up to 25% higher than the ETC simulations. The 
cause of this is the fact that the ETC assumes a fixed PSF for each detector, 
whereas the PSF changes with wavelength.  For the wavelength regions in which 
the PSF is better than the ETC assumption, the measured intensities will be 
higher than the ETC simulations for a given extraction height.  This is NOT a 
concern for (local) bright object protection issues for MAMA observations - 
the ETC does calculate the highest flux per pixel correctly to within 5%.

Users should also be aware of the fact that, since the ETC does not have 
information of the intrinsic line width, it may not always give correct 
answers for 'narrow-emission line extended sources'.  Users should continue 
to use the instructions provided in the help file for adjusting the countrate 
to account for narrow emission lines in the case of extended sources.

A new extensive set of ETC FAQs are being generated covering common questions 
from Cycle 7 and questions regarding the latest ETC updates, which will also
be released by August 20.  The new FAQs can be reached from the STIS 
instrument webpage under FAQs.  Note also that the help file will contain a 
dated list showing when major file updates have occurred so that users can 
understand why the same input parameters may not yield the same results as 
they did several months ago.



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

                         HDF-SOUTH UPDATE
                         ----------------


The Hubble Deep Field South observations are scheduled to begin September
28 and end October 10.  Data release is slated for November 23.  STIS will 
be observing a field centered on a magnitude (R)=17 QSO adjacent to the main 
WFPC2 field.  The STIS observations emphasize deep spectroscopy with the 
medium resolution NUV echelle, and deep imaging with the CCD clear aperture.  
However, other modes are being used as well.  Details can be found on the 
HDF-S web page (off the STScI Home page under the 'Staff science' link).

The current breakdown of STIS exposure times is listed below.  This is 
subject to change, as the Phase II proposal is still evolving.

        Config            Mode      Exp Time
                                     (sec)
        -------------     ------    -------
        STIS/CCD          MIRROR    203766 
        STIS/NUV-MAMA      G230L     17924 
        STIS/NUV-MAMA     MIRROR     21716 
        STIS/FUV-MAMA     MIRROR     24080 
        STIS/CCD           G430M     47592 
        STIS/FUV-MAMA      G140L     16560 
        STIS/NUV-MAMA      E230M    157080 


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

                    STIS PIPELINE AND SOFTWARE
                    --------------------------

A new version of CALSTIS (version 1.9) has been released and is now used 
in the pipeline (effective August 3, 1998).  This update is available 
for downloading from the STSDAS Advisories page (off the STScI Home page
'Reducing the data...' link to STSDAS).

Changes that have been made to CALSTIS in version 1.9 include:

Improved algorithm for extraction of echelle orders
---------------------------------------------------
When extracting the orders of an echelle observation, CALSTIS6
determined the location of each order individually, based on the
signal in that order.  If a given order had a weak signal, the
location algorithm failed, and the spectrum was extracted at the
nominal position (zero offset).  We have modified CALSTIS6 to enhance
the extraction of weak orders.  An average offset (compared with the
nominal position) for all orders is determined, with discrepant points
(more than 2 sigma) removed from the average.  If a given order is too
weak to locate the spectrum, then this average offset is used when
performing the extraction.  A message is printed in the trl file for
all orders where this global average was used, and a complete list of the
offsets used for each order is included in the resultant x1d table.

Correction to sky level determination for CCD filtered observations
-------------------------------------------------------------------
The CCD filters F28X50LP, F28X50OII, and F28X50OIII are significantly
smaller than the full detector size, especially in the second image
axis (i.e. the filters are 28" in height, while the detector is 52").
This can cause a problem with determining the sky level during cosmic
ray rejection, because the illuminated and non-illuminated regions are
comparable in area.  If the sky is improperly determined, it could
lead to one of the two pieces of a cr-split image being rejected as
mostly cosmic rays.  To resolve this potential problem (which has been
seen in one observation that we are aware of), CALSTIS now flags
regions that are outside the aperture, but in the illuminated portion
of the detector, with a value of 4 (bad detector pixel or beyond
aperture) in the DQ extension when doing DQICORR for CCD observations.
This will prevent the unilluminated portion of the detector from being
used in determining the sky value.  

Note that this flagging is not done, however, for apertures that cover
nearly the full detector in the second image axis, such as 50CCD or
long slits, or for apertures smaller than six arcseconds.  For
spectroscopic type observations, only regions above and below the
aperture are flagged, since a spectrum could extend beyond the
aperture to the left or right.

Correction to the calculation of the error array in 1-d spectra
---------------------------------------------------------------
Perviously CALSTIS6 (x1d) summed pixel values within the extraction
region to obtain the flux in the object.  However, when computing the
corresponding error value, a weighted average was used.  We have corrected
CALSTIS6 to compute the errors by adding in quadrature.

New error initialization value for MAMA data
--------------------------------------------
Previously, when CALSTIS1 initialized the error array (ERR extension)
for MAMA data, the minimum value it assigned was one.  However, if there
is no science data (flux=0), then an error of one is not meaningful.
We have therefore changed the minimum value for the error to zero.  In
other words, if the science data value is greater than zero, the error
array value should be set to the square root of the science array
value; otherwise, the error array value should be set to zero.
 
When computing statistics, a zero error should only be regarded as
a bad value if the science value is greater than zero.  If both the
science and error array values are less than or equal to zero, the
signal to noise should be set to zero.  Currently all pixels with
error less than or equal to zero are counted as bad.


A number of additional enhancements and minor problem fixes include:
--------------------------------------------------------------------

- the pipeline now performs the bias correction prior to the cosmic ray
rejection processing, to allow for proper bias correction in those cases
where the 2 (or more) exposures used for cosmic ray rejection have unequal
exposure times

- the pipeline now searches for bad voltages (indicative of a CCD reset),
and produces a warning message (in the trl file) if the voltages are
below the minimum allowed values


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

                     STIS REFERENCE FILE UPDATES
                     ---------------------------

+ Weekly DARKs continue

We are continuing with our deliveries of the weekly CCD darks.  The last
dark delivered is valid for the week of July 27 - Aug. 3.

+ Spectral traces files updated

We have delivered updated spectral traces for most modes based on
observations of standard stars; the traces for the following
configurations are still based on ground-based measurements: G140M
(1218, 1387, 1400, 1540, 1640), G230M (1884, 2600, 2800, 2828), and
G430M (3305).  For the first order modes, a small rotation in the
spectral format between pre and post launch has been accounted for,
while for echelle modes, a small amount of curvature in the spectral
orders has been corrected.

+ Echelle Photometric throughputs updated

We have delivered updated throughput tables (pht) for the echelle
modes based on the revised spectral traces.

+ Upcoming Reference File updates

We plan to update the following reference files within the next 3 months:

Dispersion coefficient files (dsp) for E140H (1453), G230M (1713),
        and G750M (9806. 9851, and 10363) - impacts wavelength scale.

Imaging throughputs (pht) - impacts photometry for imaging modes.
 
CCD Spectral Flats (pfl) - impacts signal-to-noise achievable.

Cosmic Ray rejection table (crr) - impacts quality of cosmic ray rejection

Aperture Throughput table (apt) - impacts photometry

MAMA darks/flats (drk/pfl) - impacts signal-to-noise achievable.
 
Aperture description table (adp) - impacts the wavelength scale
        for slitless spectroscopy using full field apertures.



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

                       STIS CALIBRATION NEWS
                       ---------------------

The NUV-MAMA distortion coefficients listed in the Cycle 8 STIS 
Handbook appear to be incorrect.  Further measurements are expected
(from comparison to WFPC2 data) within the next month. 


============================================================================
  
                       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.
 
 
STIS ISRs (Instrument Science Reports)...
-----------------------------------------
 
  98-24: Spectroscopic Mode Peculiarities
         (C. Bowers and S. Baum, 14 Jul 1998)

  98-23: Plate Scales, Anamorphic Magnification and Dispersion:  CCD Modes
         (C. Bowers and S. Baum, 14 Jul 1998)

  98-22: Cosmic Ray Rejection in STIS CCD Images 
         (Dick Shaw and Phil Hodge, 19 Jun 1998)

  98-21: Scattered Light from the Earth Limb Measured with the STIS CCD
         (Dick Shaw, Merle Reinhart, and Jennifer Wilson, 17 Jun 1998)

  98-18: Absolute Flux Calibration For Prime STIS Echelle Modes With The
         0.2X0.2" Slit
         (R. Bohlin, 01 Jun 1998)


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

    Will I incur the full 'change of configuration' overhead for changes
      in sub-slits (0.2X0.06FPA - 0.2X0.06FPE, for example)?


    Does the ETC produce the same results as SYNPHOT (when SYNPHOT is run 
    locally)? 



Non-proprietary STIS Cycle 7 program listings posted
----------------------------------------------------

A complete list of STIS Cycle 7 programs containing non-proprietary data 
from SMOV2 to date has been posted on the STIS web site.  The list includes 
the proposal id, proposal type, and title along with more detailed tables 
for each program which include the target names and descriptions, instrument
configuration, opmode and spectral elements used.  Not all programs have 
completed execution of all visits, but those visits that have completed are
now available in the archive.

============================================================================
 
                         RECENT PREPRINTS
                         ----------------
 
We draw your attention to these papers, based on STIS, FOS and GHRS data, that
will appear in the next few months.  This list includes all preprints received
by the STScI Library not yet published in the journals.  Please remember to
include our Library in your preprint distribution list.

WEYMANN, R.J.; JANNUZI, B.T.; LU, L.; BAHCALL, J.N.;
BERGERON, J.; BOKSENBERG, A.; HARTIG, G.F.; KIRHAKOS, S.;
SARGENT, W.L.W.; SAVAGE, B.D.; SCHNEIDER, D.P.; TURNSHEK,
D.A.; WOLFE, A.M.  "The HST Quasar Absorption Line Key
Project. XIV. The Evolution of Ly-alpha Absorption Lines in
the Redshift Interval 0 to 1.5"  ApJ 10-10-98

BAPTISTA, R.; HORNE, K.; WADE, R.A.; HUBENY, I.; LONG,
K.S.; RUTTEN, R.G.M.  "HST Spatially-Resolved Spectra of
the Accretion Disc and Gas Stream of the Nova-Like Variable
UX Ursae Majoris"  MNRAS accepted
 
KOEKEMOER, A.M.; O'DEA, C.P.; BAUM, S.A.; SARAZIN, C.L.;
OWEN, F.N.; MEDLOW, M.J.  "Constraints on UV Absorption in
the Intracluster Medium of Abell 1030"  ApJ accepted
www.stsci.edu/science/preprints/prep1269/prep1269.html

============================================================================
 
                 APPENDIX: HST Spectroscopy 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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