Introduction

There is one detector on COS that can be used to obtain near-ultraviolet data, and that is the NUV.

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 time-of-day word is written to the data memory.

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
• FCA Flat-field Calibration Aperture; required for Target=DEUTERIUM

Spectral Element

See Table 11.5: COS Spectral Elements and Central Wavelengths.

Wavelength

Enter the value of the central wavelength in Angstroms. Table 11.5: COS Spectral Elements and Central Wavelengths 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.

Optional Parameters

= 80 or greater (integer seconds)

Specifies the estimated minimum time in which 2.35 × 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 unless the proposal is a calibration or engineering.

If the predicted total number of events from a TIME-TAG exposure exceeds the COS data buffer capacity of 4.7 × 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 × 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

Indicated whether the target is an extended source. This parameter is still in APT, but has been functionally replaced by the EXTENDED target parameter. Selecting this parameter will only trigger a warning message asking you to set the TARGET description parameter instead. This exposure-level parameter should not be selected for any Cycle 23 proposals and beyond. Please see the description in Target Extent.

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 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 and DEUTERIUM. Allowed values for exposures with Target=WAVE and DEUTERIUM are FP-POS=1, 2, 3 or 4.

FLASH
= YES (default), NO, SnnnnDmmm

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  unless this is a calibration or engineering proposal, 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 Table 11.8: WAVECAL Exposure Parameters.

Number of Iterations

Number_Of_Iterations must be 1 in this Mode.

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. Engineering and calibration proposals may specify a value for Time_Per_Exposure, subject to the restrictions defined above.

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: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 × 10⁷ counts, or about 2 GBits (close to 20% of the solid-state recorder capacity).

Special Requirements

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

Mode = ACCUM Config = COS/NUV

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/NUV configuration is 1024 × 1024 pixels. The COS data buffer can hold 9 such images. 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.

Aperture or FOV

The following apertures are allowed:

PSA Primary Science Aperture

BOA Bright Object Aperture

Spectral Element

Enter a spectral element from Table 11.5: COS Spectral Elements and Central Wavelengths for the COS/NUV configuration.

Wavelength

Enter the value of the central wavelength in Angstroms. Table 11.5: COS Spectral Elements and Central Wavelengths 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.

Optional Parameters

EXTENDED
= NO (default), YES

Indicated whether the target is an extended source. This parameter is still in APT, but has been functionally replaced by the EXTENDED target parameter. Selecting this parameter will only trigger a warning message asking you to set the TARGET description parameter instead. This exposure-level parameter should not be selected for any Cycle 26 proposals and beyond. Please see the description in Target Extent .

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 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.

Number of Iterations

Enter the number of times this exposure logsheet line should be iterated, and the exposure time (Time_Per_Exposure) per iteration.

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.

Related Links

11.3.1 COS FUV Observing Modes

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