4.7 Serendipitous Spectrum Recognition

Using the algorithm described in the previous section, the NEWSIPS software searches for low-dispersion spectral flux in both apertures, regardless of the information recorded on the original observing script as to which apertures were exposed. If flux is detected in an aperture that was not originally documented as having been exposed, the FITS keyword SERENDAP will appear in the output files for that image indicating that a serendipitous exposure was found and in which aperture.

When a serendipitous spectrum is detected with the NEWSIPS software, the aperture-dependent Core Data Items (CDIs) for the primary spectrum will be copied to the CDIs for the serendipitous spectrum. The coordinates for the serendipitous exposure will be calculated from the primary coordinates using the following algorithms:

$$RA(s) = RA(t) + \Delta RA~~~~{\rm and}~~~~DEC(s) = DEC(t) + \Delta DEC$$

where RA(s) and DEC(s) are the coordinates of the serendipitous pointing, and RA(t) and DEC(t) are the coordinates of the target. $\Delta RA$ and $\Delta DEC$ are computed in units of arcseconds as follows:

$$\Delta RA = -0.2680 (FESX(s) - FESX(t)) \cos \Psi + 0.2617 (FESY(s) - FESY(t)) \sin \Psi$$

$$\Delta DEC = -0.2680 (FESX(s) - FESX(t)) \sin \Psi - 0.2617 (FESY(s) - FESY(t)) \cos \Psi$$

where $\Psi$ = spacecraft roll angle + 28.31 deg, FESX(s) and FESY(s) are the FES x and y coordinates of the serendipitous aperture, and FESX(t) and FESY(t) are the FES x and y coordinates of the target aperture. The FES coordinates of the apertures are given in Table 4.7.

 

 

Aperture FES x, y

SWSA 243, -89

SWLA 94, -90

LWSA 47, 65

LWLA -106, 52

The target name for the serendipitous exposure will be NEAR XXX. The HISTORY portion of the label will reflect that the main object is in the other aperture and the APERTURE keyword will be changed to BOTH. In the case that the serendipitous exposure is in the small aperture and the large-aperture spectrum is trailed, the serendipitous exposure is considered a trailed overshoot. The large-aperture coordinates and object name are copied exactly to the appropriate entries for the small aperture, as well as the aperture-dependent CDIs. In this instance, the coordinates are not recalculated and the object name is not preceded by ``NEAR''.

The NEWSIPS system also searches for unexpected spectral data in images classified as sky background. These images are usually acquired simultaneously with a pointed spectral exposure in another camera. If an image has an object class of 07 (sky background) and spectral data are detected with the algorithm described above, processing is terminated and the IUE staff attempts to identify the pointed observation obtained in another camera contemporaneously with the image. Assuming the associated image can be identified, the coordinates will be derived from those of the main object and the object class changed to that of the main object. The target name is ``NEAR XXX''. If the associated image cannot be identified, then the object class is changed to ``UNKNOWN''. A COMMENT line in the FITS header will say, ``SERENDIPITY WITH CAMXXXXX''. If no spectral flux is detected in the image, processing continues as for a large-aperture extended source.