In ACCUM Mode, the detector coordinates of each photon event are used to reference a 16-bit word in a memory array, which is incremented as each event occurs. An
ACCUM mode image in the COS/FUV configuration is 16,384 x 128 pixels in each of the two detector segments. The COS data buffer can hold two such images, assuming both segments are used.
ACCUM should be used only when absolutely necessary, such as for high count-rate targets.
TIME-TAG is the recommended data-taking mode unless the target’s brightness leads to a total count rate in excess of that possible with this mode.
Enter the value of the central wavelength in Angstroms. Table 13.3 gives the allowed values of the central wavelength for each grating.
SEGMENT= BOTH (default except for G140L
, λ=1105
Å)
= A (default and only allowed value for G140L,
λ=1105
Å)
= B
This optional parameter is required for external and
WAVE targets. It specifies whether to take the exposure at a single offset, at multiple offsets, or at no offset from the nominal central wavelength. Obtaining exposures at small wavelength offsets from the specified central wavelength aids in the correction of the fixed-pattern defects of the detector. For the best data quality, we strongly recommend using all four
FP-POS positions (value
ALL)
FP-POS=
All indicates that
the specified exposure time will be the exposure time for each
FP-POS position.
The values
FP-POS=1, 2 or
4 will result in the exposure being taken at an offset from the specified central wavelength.
FP-POS=3 will result in the exposure being obtained at the nominal central wavelength (i.e., at a zero offset). The exposure will be for the specified exposure duration. See the
COS Instrument Handbook for specifics of the offsets and for guidance on how to use this capability.
Time_Per_Exposure must be an integral multiple of 0.1 seconds (if it is not, its value will be rounded down to the next lower integral multiple of 0.1 sec, or set to 0.1 seconds if a smaller value is specified), and may range from 0.1 to 6500 seconds. Values much larger than 3000 seconds are normally appropriate only for visits with the
CVZ special requirement.
Note: ACCUM Mode exposures longer than 900 seconds using the
G130M or
G160M gratings may blur the FUV spectra by 1-2 pixels (~1/6 to 1/3 of a resolution element) due to wavelength dependent deviations from the mean Doppler correction.