12.11 Mode = TIME-TAG
Config = COS/NUV

In TIME-TAG mode, the COS detectors produce an event stream with a time resolution of 32 millisec. The X and Y pixel coordinates of each photon event are stored in a 32-bit word in data buffer memory. At the start of an exposure, and after every subsequent 32 millisec interval which contains photons events, a 32-bit tine-of-day word is written to the data memory.

12.11.1 Aperture or FOV

Allowed apertures for this mode are:

•	PSA: Primary Science Aperture

•	BOA: Bright Object Aperture

•	WCA Wavelength Calibration Aperture; required for Target=WAVE

12.11.2 Spectral Element

See Table 12.3: COS Spectral Elements and Central Wavelengths.

12.11.3 Wavelength

Enter the value of the central wavelength in Angstroms. Table 12.3 gives the allowed values of the central wavelength for each grating.

Note: No wavelength should be specified if MIRRORA or MIRRORB is the Spectral Element.

12.11.4 Optional Parameters

BUFFER-TIME
= 80 or greater (integer seconds)

Specifies the estimated minimum time in which 2.35 x 106 photon events (half of the COS data buffer capacity) will be accumulated during a TIME-TAG exposure. BUFFER-TIME is a required parameter if the target is not WAVE. If the target is WAVE, then BUFFER-TIME may not be specified .

If the predicted total number of events from a TIME-TAG exposure exceeds the COS data buffer capacity of 4.7 x 106 photon events, data must be transferred to the HST onboard science recorder during the exposure. Transfers of data from the COS buffer during an exposure will be made in 9-MByte blocks (half the buffer capacity). The value of BUFFER-TIME should be the half-buffer capacity (2.35 x 106 counts) divided by the estimated maximum average count rate in photons per second.

Note that BUFFER-TIME should include expected counts from the detector dark current (see the COS Instrument Handbook) as well as the detected photon events, factoring in the instrument quantum efficiency. A conservative value of BUFFER-TIME is recommended (err slightly on the low side) to avoid data loss.

The absolute minimum BUFFER-TIME of 80 seconds corresponds to a maximum average count rate of ~30,000 counts/sec over the entire detector, which is the maximum rate at which the flight software is capable of processing counts.

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 (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.

Note: FP-POS is not allowed to be specified when the spectral element is MIRRORA or MIRRORB.

Note for internal targets: FP-POS is not allowed for internal targets except Target=WAVE. Allowed values for exposures with Target=WAVE are FP-POS=1, 2, 3 or 4.

FLASH
= YES (default), NO

Indicates whether or not to "flash" the wavelength calibration lamp during exposures. These flashes are needed to compensate for the effect of post-move drift of the Optic Select Mechanisms. The default behavior will be that the wavecal lamp is turned on briefly at the start of an externally targeted exposure, and at intervals later in the exposure. The grating-dependent "flash" durations and the time-since-move-dependent flash intervals will be defined by the STScI. Specifying the value NO will disable automatic flashing for the current exposure.

Note: FLASH is not allowed to be specified (and defaults to NO) when the spectral element is MIRRORA or MIRRORB, or the aperture is BOA.

When flashing is enabled, the exposure time must be at least as long as a single flash. Flash durations, as functions of grating and central wavelength, are given in Table 12.5.

12.11.5 Number of Iterations

Number_Of_Iterations must be 1 in this Mode.

12.11.6 Time Per Exposure

Enter the total time of data collection as 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.

For Target=WAVE, enter DEF for Time_Per_Exposure.

If BUFFER-TIME < 110 seconds, photon events may be generated faster than data can be transferred out of the buffer during the exposure. In this case, Time_Per_Exposure should be less than or equal to 2 * BUFFER-TIME so that the exposure can complete before data transfer is necessary. A BUFFER-TIME of 110 seconds corresponds to an average count rate of ~21,000 counts/sec.

Note that TIME-TAG exposures have the potential to rapidly use up the HST onboard storage capacity. Caution is advised on any exposure with an exposure time greater than 25 * BUFFER-TIME, which corresponds to ~6 x 107 counts, or about 2 GBits (close to 20% of the solid-state recorder capacity).

12.11.7 Special Requirements

The special requirement RT ANALYSIS is not permitted on TIME-TAG exposures.