7.6 MAMA Spectral and Spatial Offsetting

7.6.1 FUV First-Order Spatial Offset to Avoid Repeller Wire

For the FUV-MAMA, the repeller wire produces a small shadow on the detector (with a depth of ~10%) which is apparent on FUV-MAMA flat-field images (see Section 7.4.1). To avoid first-order mode spectra falling on the repeller wire shadow, all data taken with the G140L and G140M gratings are projected to fall 3 arcseconds or ~120 low-resolution pixels below the repeller wire (to AXIS2=392) from the detector center (see also Section 11.1.2). This offsetting is done using the Mode Select Mechanism to tilt the grating. For these modes the projected field of view is therefore asymmetric with respect to the specified target coordinates. Note that in Cycle 7, prior to March 15, 1999, the offset from the detector center was done in the opposite direction (to AXIS2=632). However, as this caused G140L and G140M point source spectra to fall onto the region of the FUV-MAMA with the highest dark current (see FUV-MAMA Dark Current), this was changed to the current setting. Observers who wish to match the fields of view of Cycle 7 observations will need to take this difference into account.

7.6.2 Monthly Spatial and Spectral Offsetting of MAMA Modes

Beginning in January 1998 the projections of the spectra on the detector for all  NUV- and FUV-MAMA spectroscopic modes were shifted slightly each month. This procedure was instituted in order to minimize uneven charge depletion in the microchannel plates that would increase the non-uniformity of the flat fields. These monthly charge-offsetting shifts, recorded in header keywords MOFFSET1 and MOFFSET2, can shift the spectrum by up to ±15 low-resolution pixels in AXIS1 (dispersion) and up to ±40 low-resolution pixels in AXIS2 (cross-dispersion). Observers are advised to select settings that keep wavelength ranges and targets of interest away from the extreme ends of the long slits, lest they be inadvertently shifted off the edge of the detector.

Since August 2002, however, this monthly offsetting has been disabled for the MAMA echelle modes, though it continues to be done for 1st order MAMA spectroscopic modes. It was realized that moving the echelle spectrum in the AXIS1 direction shifts the echelle blaze function by a different amount than it shifts the wavelength scale. This misalignment of the blaze function with the wavelength scale makes proper flux calibration of the extracted spectrum significantly more difficult.  In addition, the larger shifts in the spatial direction could change the set of echelle orders included on the detector.

STIS flux calibration programs routinely disable the usual monthly offsets with use of the MSMOFF mode. Because the MAMA detectors have not shown any evidence of uneven charge depletion (see ISR 2021-02), beginning in Cycle 30 this previously restricted capability was made available-but-unsupported for those GO programs requiring very high photometric accuracy or reproducibility. Scientific justification for the use of the MSMOFF mode is required in the Phase I for it to be approved (contact the Help Desk for help implementing this). See Section A.3 for more information.