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| HST Spectroscopy                      STIS/GHRS/FOS |
| STScI Analysis Newsletter                  Feb 2001 |
|                                                     |
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How do I best determine an observation's actual 
orientation?


+-STIS------------------------------------------------+
  



How do I best determine an observation's actual 
orientation?

We have identified potential problems with the orienta-
tion keyword in the headers of data taken prior to 15 
September 1997. The ORIENTAT keyword can be in error 
by as much as 0.5 degrees. Please see the URL above for
instructions on how to check for this problem. Users 
who have tight ORIENT constraints that were determined 
using images that might be affected by this problem 
should get in touch with their contact scientist (CS) 
or contact the helpdesk at help@stsci.edu for aid in
correcting their Phase II proposals.

Note also that the slit angles for the 52X0.05, 52X0.1,
52X0.2, 52X0.5, and 52X2 apertures were also updated 
in July 2000.  These angles were changed by about 0.4 
degrees.  This can also have an effect on observations 
with tight ORIENT constraints.  These slit angles will 
be updated in the upcoming revision of the STIS Instru-
ment Handbook, but for reference, the values in Table 
11.2 on p. 235 of V4.1 should be

                        Offset
        Aperture        Angle
        --------        -----
        52X0.05         45.35
        52X0.1          45.35
        52X0.2          45.35
        52X0.5          45.35
        52X2            45.35


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