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 except for G140L, λ=1105 Å)
= A (default and only allowed value for G140L, λ=1105 Å)
= B

This is the same as in TIME-TAG Mode; see Section 13.9.

EXTENDED
= NO (default), YES

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

FP-POS
= ALL, 1, 2, 3 (pre-Cycle 19 default), 4

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.

.

 

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.

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. 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.