Introduction

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

Mode = TIME-TAG Config = COS/FUV

In TIME-TAG Mode, the COS FUV detector produces an event stream with a time resolution of 32 milliseconds. 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-millisecond period which contains photon events, a 32-bit time-of-day word is written to the data memory. The TIME-TAG is the recommended data-taking mode unless the target’s brightness leads to a total count rate in excess of that supported by this mode.

Aperture or FOV

The following apertures are allowed:

• PSA Primary Science Aperture
• BOA Bright Object Aperture (but not permitted for grating/wavelength combinations that use lifetime position LP5 or LP6; those need to request use of LP3 or LP4)
  
• WCA Wavelength Calibration Aperture; required for Target=WAVE
• FCA Flat-field Calibration Aperture; required for Target=DEUTERIUM

Enter a spectral element from Table 11.5: COS Spectral Elements and Central Wavelengths for the COS/FUV 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. Table 11.6: Additional configurations offered in AVAILABLE mode lists the AVAILABLE G130M and G160M central wavelengths that are not offered in SUPPORTED mode, (after following the link to the table you will need to click on expand to see the table).

Optional Parameters

SEGMENT

= BOTH (default except for these cases: G140L/1105, G140L/800 and G130M/1300, 1309, 1318, 1327 at LIFETIME-POS=LP4 and LP5)
= A (default and only allowed value for these cases: G140L/1105, G140L/800 and G130M/1300, 1309, 1318, 1327 at LIFETIME-POS=LP4 and LP5)
= B (not permitted for G140L/1230)

Specifies which segment of the FUV detector to use for an observation. A value of BOTH will activate both segments. If A is selected, only segment A of the detector will be activated for photon detection, and the spectrum will contain data from only that half of the detector. If B is selected, only segment B of the detector will be activated and used to generate data. The use of only a single segment may be warranted with sources that are too bright to observe safely over the entire detector, or when a source has a much higher expected count rate in one segment than the other and the important science is in the segment with the low count rate. Observations at LP5 with G130M, λ=1300, 1309, 1318, 1327 Å will use SEGMENT=A only, in order to protect the FUV detector SEGMENT=B from illumination by geocoronal Lyman-alpha emission. See the COS Instrument Handbook for more information.

If you specify grating G140L with the 800 or 1105 wavelength setting (or with the 1230 Angstrom wavelength setting when FP_POS is set to 4 or ALL), SEGMENT defaults to A.

For exposures with SEGMENT = B only and the central wavelength set to either 1055 or 1096, the wavelength solution is compromised because the wavelength calibration lamp produces no counts at the  short wavelengths seen by those settings. The best solution involves inserting a wavecal with segment A turned on with the same setting and FP-POS. A warning will be triggered whenever 1055/FUVB or 1096/FUVB settings are used (both ACCUM and TIME-TAG mode). The Phase II program can be submitted with this warning but the appropriate course of action must be discussed with the Contact Scientist assigned to the program before the observations are executed.

BUFFER-TIME
= 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 one.

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 and stim pulses (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. The COS Exposure Time Calculator (ETC) should be used to estimate a suitable value for BUFFER-TIME. See the COS Instrument Handbook for more information.

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. 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 strongly recommend using all four FP-POS positions (ALL). There are two exceptions: For G130M/1291 when SEGMENT = B or BOTH, only FP-POS = 3, 4 are allowed. And when LIFETIME-POS=LP6 is specified (or is the default) it is permitted to use fewer than 4 FP-POS based on the S/N of the observations (see Chapter 9 of the COS Instrument Handbook). Please see Table 11.5: COS Spectral Elements and Central Wavelengths for more information.

For FP-POS=All the specified exposure time will not be split for external targets (i.e., it is the exposure time for each FP-POS exposure).

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 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 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; restricted for G130M, λ=1055 or 1096 Å and SEGMENT=B), 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 Optics 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.

FLASH will automatically be set to NO for exposures having central wavelengths of either 1055 or 1096 Angstroms and SEGMENT set to B. Under these circumstances the wavelength calibration lamp produces no counts at the short wavelengths seen by these settings. The appropriate course of action must be discussed with your Contact Scientist before your observations can be executed under these conditions. (Restricted) FLASH can be set to YES.

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 11.8: COS WAVECAL Exposure Parameters.

When aperture BOA is selected, FLASH may not be specified and defaults to NO.

When aperture PSA is selected and LIFETIME-POS=LP6 is specified (or is the default), FLASH may not be specified and defaults to NO.

Number of Iterations

Number_Of_Iterations must be 1 in this Mode.

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 × 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/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 × 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.

Aperture or FOV

The following apertures are allowed:

PSA Primary Science Aperture

BOA Bright Object Aperture  (but not permitted for grating/wavelength combinations that use lifetime position LP5 or LP6; those need to request use of LP3 or LP4)

Spectral Element

Enter a spectral element from Table 11.5: COS Spectral Elements and Central Wavelengths for the COS/FUV 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. Table 11.6: Additional configurations offered in AVAILABLE mode lists the AVAILABLE G130M and G160M central wavelengths that are not offered in SUPPORTED mode.

Optional Parameters

SEGMENT

= BOTH (default except for these cases: G140L/1105, G140L/800 and G130M/1300, 1309, 1318, 1327 at LIFETIME-POS=LP4 and LP5)
= A (default and only allowed value for these cases: G140L/1105, G140L/800 and G130M/1300, 1309, 1318, 1327 at LIFETIME-POS=LP4 and LP5)
= B (not permitted for G140L/1230)

This is the same as in TIME-TAG Mode. See 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 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 strongly recommend using all four FP-POS positions (ALL), except for G130M/1291 when SEGMENT = B or BOTH, where only FP-POS = 3, 4 are allowed. And when LIFETIME-POS=LP6 is specified (or is the default) it is permitted to use fewer than 4 FP-POS based on the S/N of the observations (see Chapter 9 of the COS Instrument Handbook). Table 11.1: Supported settings for FUV ACCUM and TIME- TAG exposures for more information.

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.

Number of Iterations

Enter the number of times this exposure 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.

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.

Related Links

11.3.2 COS NUV Observing Modes

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