13.10 Mode = ACCUM
Config = COS/FUV

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.

13.10.1 Aperture or FOV

The following apertures are allowed:

PSA Primary Science Aperture

BOA Bright Object Aperture

13.10.2 Spectral Element

Enter a spectral element from Table 13.3 for the COS/FUV configuration.

13.10.3 Wavelength

Enter the value of the central wavelength in Angstroms. Table 13.3 gives the allowed values of the central wavelength for each grating.

13.10.4 Optional Parameters

SEGMENT
= BOTH (default)
= A (default for G140L, =1105 Å), B

EXTENDED
= NO (default), YES

Indicates whether the target is an extended source. This parameter is used only for data reduction.

FP-POS
= AUTO, 1, 2, 3 (default), 4

Indicates 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. FP-POS=AUTO indicates that the specified exposure time will be divided evenly among four subexposures, and each subexposure will be obtained at a different predetermined offset from the specified central wavelength.

The values FP-POS=1, 2, 3, 4 will result in the exposure being taken at an offset from the specified central wavelength determined by the value. The default value, or if FP-POS is not specified on the exposure, 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.

13.10.5 Number of Iterations

Enter the number of times this exposure should be iterated, and the exposure time (Time_Per_Exposure) per iteration. If FP-POS=AUTO is specified with Number_Of_Iterations>1, the exposures will be obtained in the order Number_Of_Iterations of exposures at each FP-POS position between moves of the OSM.

13.10.6 Time Per Exposure

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. If the exposure specifies FP-POS=AUTO, then the valid range is 0.4 to 26000.0 seconds. Values much larger than 3000 seconds (12000 seconds if FP-POS=AUTO) are normally appropriate only for visits with the CVZ special requirement. If FP-POS=AUTO, the Time_per_Exposure is divided equally among the FP-POS copies.

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.