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COS G130M Segment=B Observations with the 1055 and 1096 CENWAVEs

Contents

  1. Overview
    Overview of the problem, the solution, and the scientific impacts.
  2. General APT Guidelines for non-CVZ targets
    General tips for creating Phase II submissions in APT.
  3. Creating a WAVECAL in APT
    Detailed instructions on how to create a WAVECAL in APT.
  4. Single CENWAVE examples (non-CVZ targets)
    Examples of the orbit layouts of programs that use a single CENWAVE.
  5. Mixed CENWAVE examples (non-CVZ targets)
    A discussion of the differences in programs that use both G130M/1055 and G130M/1096.
  6. Single CENWAVE CVZ programs
    A brief discussion of the impact on single CENWAVE CVZ programs.
  7. Mixed CENWAVE CVZ programs
    A brief discussion of the impact on CVZ programs that use both G130M/1055 and G130M/1096.

1. Overview

Due to the 100x difference in sensitivity between the COS FUVA and FUVB segments when observing with the G130M/1055 and 1096 CENWAVEs, it is expected that many observers will need to turn off FUVA when observing bright targets. (We refer to these SEGMENT=B observations here as either C1055B or C1096B). Only those observers using these two configurations are affected by this issue.

Under these conditions the zero point of the wavelength solution cannot be determined because the MgF2 window on the PtNe lamps (WAVECAL) blocks light below ~ 1180 (all WAVECAL light falls on FUVA). This results in a degradation of the resolution when FP-POS are combined by CalCOS and decreases the archival value of the COS data. In these cases, normal TAGFLASHs are not available and WAVECAL exposures with FUVA turned ON must be inserted into the observing sequence adjacent to each CENWAVE/FP-POS setting used. As a result, in these cases FP-POS=ALL should not be used. Individual FP-POS science exposures should be used instead.

To reduce the impact on the time-on-target, observers may need to decrease the number of FP-POSs observed with each CENWAVE setting. G130M/1055/1096 spectra are much broader in the cross-dispersion direction and do not require as many FP-POSs as the “standard” CENWAVEs to mitigate fixed-pattern noise. Observing strategies for single CENWAVE one-orbit visits have been defined that result in, at most, a 14% reduction of exposure time (7% drop in S/N). For all visits greater than one-orbit, the impact is less than 2% of the total exposure time.

Inclusion of WAVECALs with FUVA turned on only significantly impacts one-orbit visits that observe with both C1055B and C1096B. Due to the relative ratio of throughputs and BOP considerations, it is unlikely that G130M/1055/1096 observations that require FUVA to be turned off would have exposures in the same visit with other CENWAVEs. These exposures will likely also require an NUV TA.

This webpage is designed to provide the basic examples of various length programs and instructions to properly insert the WAVECALs into the Phase II submissions. We will first detail some general APT guidelines, and how to insert WAVECALs using APT. We will then examine non-CVZ programs that use either C1055B or C1096B, then we will consider programs which observe with both C1055B and C1096B. Finally, we will consider both kinds of programs when the target is a CVZ object. Please contact your Contact Scientist (CS) if you have any questions about this webpage.

2. General APT Guidelines for non-CVZ targets

Below are some general guidelines for creating the most efficient observing sequence for your Phase II submission when observing with C1055B or C1096B. These guidelines apply to non-CVZ orbits only. You will create your Phase II in the same way as other programs. Not using FP-POS=ALL will save you observing time as you will need to create each FP-POS independently. You will need to manually insert a WAVECAL exposure for each science exposure in your program. For HST Cycle 21 Phase II submissions, you must use APT Version 21.2 or higher. In APT 21.2 you will see a WARNING when a C1055B or C1096B science exposure is present. Once you insert the appropriate WAVECAL(s), the warning message will still appear in APT, but you can submit and execute your proposal once the WAVECAL(s) have been added. The bullets below are general guidelines for minimizing overhead. Specific examples are given in sections 4)-7).

  1. All SEGMENT=B science exposures should have a SEGMENT=BOTH WAVECAL at the same CENWAVE and FP-POS either immediately before, or immediately after the exposure.
    • Whenever possible, try to schedule the WAVECAL during the occultation.
  2. For maximum efficiency, the first orbit should contain a single science exposure following the target acquisition (TA), and all other orbits should not contain more than two science exposures.
  3. For all orbits which contain a single science exposure (FP-POS), the WAVECAL should be inserted after the exposure in the occultation.
  4. For each orbit containing two science exposures:
    • If the orbit is the first in the visit, then the WAVECAL for the first science exposure should be placed AFTER the target acquisition, but before the exposure.
    • If the orbit is not the first in the visit, the WAVECAL for the first science exposure should be placed in the previous occultation.
    • In either case, the WAVECAL for the 2nd science exposure should be placed after the 2nd exposure, in the occultation.
  5. For each orbit containing three exposures:
    • If the orbit is the first in the visit, then the WAVECAL for the first science exposure should be placed AFTER the first science exposure and paired with the WAVECAL for the second science exposure.
    • If the orbit is not the first in the visit, the WAVECAL for the first science exposure should be placed in the previous occultation, and the WAVECAL for the 2nd science exposure should be placed after the second science exposure.
    • In either case, the WAVECAL for the last science exposure should be placed after the last exposure, in the occultation.
  6. For each orbit containing four exposures:
    • If the orbit is the first in the visit, then the WAVECAL for the first science exposure should be placed AFTER the target acquisition, but before the first science exposure.
    • If the orbit is not the first in the visit, the WAVECAL for the first science exposure should be placed in the previous occultation.
    • In either case, the WAVECAL for the 2nd science exposure should be placed after the 2nd exposure and paired with the WAVECAL for the 3rd science exposure.
    • The WAVECAL for the last science exposure should always be placed after the last exposure, in the occultation.
  7. Orbits which contain more than four science exposures should be handled in similar fashion, but are increasing inefficient.

3. Creating a WAVECAL in APT

To create a WAVECAL in APT, the following steps are necessary:

  1. Create a new exposure
  2. Set the configuration to COS/FUV
  3. Select Target = WAVE
  4. Select Mode = TIME-TAG
  5. Select Spectral Element = G130M
  6. Select Aperture = WCA
  7. Select Wavelength = 1055 or 1096
  8. As an required “optional parameter”, set FP-POS to the FP-POS used in the exposure corresponding to this WAVECAL.

Note that APT will automatically select the appropriate exposure and buffer times for the WAVECALs. Once created, these WAVECALs can be duplicated and moved just like other COS exposures in APT. Remember when duplicating to change the CENWAVE and FP-POS as appropriate for your exposures.

As previously noted, in APT 21.2 you will see a WARNING when a C1055B or C1096B science exposure is present. The warning message will still appear in APT after you insert the appropriate WAVECALs, but you can submit and execute your proposal once the WAVECAL(s) have been added.

4. Single CENWAVE Examples (non-CVZ targets)

Targets which require that FUVA must be turned off are, most likely, too bright to acquire with FUV spectroscopic TA. The TA for these observations will therefore either be in NUV imaging or NUV spectroscopic mode. NUV imaging TAs are likely to be the most popular as they are often the fastest TA strategy. These NUV ACQ/IMAGEs are most likely to be BOA+MIRRORB observations with exposure times between 7 and 90 seconds (to achieve a recommended signal-to-noise of 60). The following examples assume an NUV ACQ/IMAGE TA of 30 second duration without an ACQ/SEARCH (the target coordinates are known to better than 0.4 arcseconds.

  1. Single CENWAVE One-Orbit Visits

    Single orbit visits with one CENWAVE, will need to be changed from using all FP-POSs to just two. This is not ideal, but using 3 FP-POS does not leave sufficient observing time. The configuration in APT is shown below for the modified orbit layout. In the worst case (C1055B), this results in a 14% reduction is observing time ( ~ -7% Signal-to-Noise).

    Single CENWAVE One-Orbit Visit With WAVECALs (2 FP-POSs)
  2. Single CENWAVE Two-Orbit Visits

    Single CENWAVE two-orbit visits, can be changed from using all FP-POSs to just three, or left with all 4 FP-POSs. Using 3 FP-POS increases the observing time by a few percent, while using 4 FP-POSs decreasing the observing time by ~10%. The configurations in APT are shown below for the modified orbit layouts.

    1. Single CENWAVE Two-Orbit Visit With WAVECALs (3 FP-POSs)
    2. Single CENWAVE Two-Orbit Visit With WAVECALs (4 FP-POSs)
  3. Single CENWAVE Three-Orbit Visits

    Single CENWAVE three-orbit visits, can either use 3 or 4 FP-POSs at the observers discretion. The 4 FP-POS configuration in APT is shown below for the modified orbit layouts. There is no impact on the observing time for 4 FP-POS, while the use of 3 FP-POS will increase the observing time by +1%.

    1. Single CENWAVE Three-Orbit Visit With WAVECALs (3 FP-POSs)
    2. Single CENWAVE Three-Orbit Visit With WAVECALs (4 FP-POSs)
  4. Single CENWAVE Four-Orbit or Longer Visits

    Single CENWAVE Four-Orbit or longer visits are only impacted by the inclusion of WAVECALs if there are two exposures in the first orbit, or more than two in any subsequent orbits. For single CENWAVE observations, these conditions are very unlikely, so there should be no impact on the observing time for visits lasting four orbits or more.

    Single CENWAVE Four-Orbit Visit With WAVECALs (4 FP-POSs)

    This table applies to C1055B observations with a 30s NUV ACQ/IMAGING TA. The WAVECAL times for C1096B exposures are 90 seconds shorter than for C1055B, so the C1096B exposures sequences are more efficient that the ones given here.

    C1055B CVZ Orbital Schemes
    # of C1055B w/ NUV TA non-CVZ OrbitsOriginal SchemeET_ORIG (s)New SchemeET_NEW (s)ET_NEW ET_ORIGS/N Loss or Gain
    1TA F1F2F3F4 (4FP)2,165TAW1F1F2W2 (2FP) 1,85986%-14%
    2,165TAF1W1W2F2F3W3 (3FP) 76235%-65%
    2TA F1F2F3F4 (2FP)5,325TAF1W1F2W2 (2FP) 5,564104%+4%
    5,325TA F1W1W2F2F3W3 (3FP) 5,453102%+2%
    5,325TA W1F1F2W2W3F3F4W4 (4FP) 4,75189%-11%
    3TA F1F2F3F4 (4FP)8,460TA F1W1F2W2F3W3 (3FP) 8,567101%+1%
    8,460TA F1W1W2F2F3W3F4W4 OR TA F1W1F2W2W3F3F4W4 (4FP) 8,460100%0%
    4TA F1F2F3F4 (4FP)11,595TA F1W1F2W2F3W3F4W4(3FP) 11,595100%0%
    5TA F1F2F3F4F5 (5FP)17,890TA F1W1F2W2F3W3F4W4F5W5(5FP) 17,890100%0%

    This table shows the observing options for mixed CENWAVE non-CVZ orbits that need to have WAVECALs inserted. The first column gives the number of non-CVZ orbits in the C1055B orbits simulated with 30s NUV ACQ/IMAGE TAs. The second column, ET_ORIG, shows the original scheme. The NUV TA is shown in GREEN, the C1055B observations are in BLUE. In this notation, each FP-POS is denoted by F#. At the end of the scheme the number of total FP-POSs is reported in the format of 2x(# of FP-POS per CENWAVE). The #'s given do not necessarily correspond to same number FP-POS, but it is most efficient to observe FP-POSs of the same CENWAVE in the preferred, increasing FP-POS number, direction. The third column gives the original estimated exposure time (in seconds) on the original observing scheme. The fourth column gives the new observing scheme with the WAVECALs inserted. In these schemes, the WAVECALs are indicated by W#. Wavecal exposures taken during the occultation are underlined. The fifth column, ET_NEW, gives the exposure time using the new scheme. The sixth column gives the percentage of original exposure time represented by the new scheme. The final column expresses this loss of exposure time as the loss, or gain, in signal-to-noise (assuming only Poisson Statistics).

5. Mixed CENWAVE examples (non-CVZ targets)

The efficiency impacts for mixed C1055B+C1096B visits follow the same general rules as for single CENWAVE exposures. However, these mixed cases will likely have more exposures per orbit and therefore will be more impacted by the inclusion of the WAVECALs.

The recommendations for achieving maximum observing efficiency when including WAVECALs for the mixed CENWAVE cases are :

  1. Visits of 4 orbits or more should see little or no impact. The biggest impact comes from visits which have two science exposures in the first orbit. In this case a WAVECAL is needed before the first science exposure (after the TA). For maximum observing efficiency, no orbit should contain more than two science exposures.
  2. Visits of 3 orbits should use 3 FP-POSs for each CENWAVE. The estimated impact in these cases is ~3% of the observing time.
  3. Visits of 2 orbits should switch to 2 FP-POS per CENWAVE instead of 4. Use of 3 FP-POS per CENWAVE is an option, but results in a ~40% reduction in observing time.
  4. Visits of 1 orbit will always be severely impacted. Reducing the number of FP-POS per CENWAVE to 2 from 4 results in only ~34% of the original observing time. Observers in this category will need to discuss the situation with their CS.

The table below gives the details for simulated typical mixed-CENWAVE non-CVZ exposure sequences. These estimates assume a 30-second NUV ACQ/IMAGE for the TA. For many of the # of orbits in a Visit possibilities, the exact combination to use depends on the brightness of the target and the science needs of the observations.

Please contact your CS for the latest information if you have mixed C1055B+C1096B visits.

Mixed CENWAVE non-CVZ Orbital Schemes
# of C1096+C1055B w/ NUV TA non-CVZ OrbitsOriginal SchemeET_ORIG (s)New SchemeET_NEW (s) ET_NEW ET_ORIGS/N Loss
1TA F1F2F3F4 F1F2F3F4 (2x4FP)1,544TA W1F1F2W2 W1F1F2W2 (2x2FP)53034%-66%
2TA F1F2F3F4 F1F2F3F4 (2x4FP)4,836TA W1F1F2W2 W1F1F2W2 (2x2FP)4,80599%-1%
4,836TA F1W1W2F2F3W3 W1F1F2W2F3W3 (2x3FP)3,21967%-33%
3TA F1F2F3F4 F1F2F3F4 (2x4FP)7,845TA W1F1F2W2W3F3 F1W1W2F2F3W3 (2x3FP)7,72798%-2%
4TA F1F2F3F4 F1F2F3F4 (2x4FP)10,747TA W1F1F2W2W3F3F4W4 W1F1F2W2W3F3F4W4 (2x4FP)10,62999%+1%

This table shows the observing options for mixed CENWAVE non-CVZ orbits that need to have WAVECALs inserted. The first column gives the number of non-CVZ orbits in the C1055B + C1096B orbits simulated with 30s NUV ACQ/IMAGE TAs. The second column, ET_ORIG, shows the original scheme. The NUV TA is shown in GREEN, the C1055B observations are in BLUE, and the C1096 observations are in RED. In this notation, each FP-POS is denoted by F#. At the end of the scheme the number of total FP-POSs is reported in the format of 2x(# of FP-POS per CENWAVE). The #'s given do not necessarily correspond to same number FP-POS, but it is most efficient to observe FP-POSs of the same CENWAVE in the preferred, increasing FP-POS number, direction. The third column gives the original estimated exposure time (in seconds) on the original observing scheme. The fourth column gives the new observing scheme with the WAVECALs inserted. In these schemes, the WAVECALs are indicated by W#. Wavecal exposures taken during the occultation are underlined. The fifth column, ET_NEW, gives the exposure time using the new scheme. The sixth column gives the percentage of original exposure time represented by the new scheme. The final column expresses this loss of exposure time as the loss in Signal-to-noise (assuming only Poisson Statistics).

6. Single CENWAVE CVZ programs

CVZ orbits are more impacted by the addition of WAVECALs since there is no occultation in which to hide the WAVECALs. For a single-orbit visit, reducing the number of FP-POS from 4 to 2 results in a 20% loss of observing time, while reducing to 3 FP-POS results in a 35% loss in observing time. The longer the CVZ visit the smaller the impact of inserting the WAVECALs.

The table below gives the details for simulated typical single-CENWAVE CVZ exposure sequences. These estimates assume a 30-second NUV ACQ/IMAGE for the TA. For many possibilities, the number of FP-POS to use depends on the brightness of the target and the science needs of the observations.

This table applies to C1055B observations with a 30s NUV ACQ/IMAGING TA. The WAVECAL times for C1096B exposures are 90 seconds shorter than for C1055B, so the C1096B exposures sequences are more efficient that the ones given here.

C1055B CVZ Orbital Schemes
# of C1055B w/ NUV TA CVZ OrbitsOriginal SchemeET_ORIG (s)New SchemeET_NEW (s)ET_NEW ET_ORIGS/N Loss
1TA F1F2F3F4 (4FP)4,310TA F1W1W2F2 (2FP) 3,43080%-11%
4,310TAF1W1W2F2F3W3 (3FP) 2,78065%-20%
4,310TAF1W1W2F2F3W3W4F4 (4FP) 2,36055%-26%
2TA F1F2F3F4 (4FP)10,060TA F1W1W2F2 (2FP) 9,20091%-4%
10,060TA F1W1W2F2F3W3 (3FP) 8,60085%-8%
10,060TA F1W1W2F2F3W3W4F4 (4FP) 7,86578%-12%
3TA F1F2F3F4 (4FP)15,800TA F1W1W2F2 (2FP) 13,00082%-9%
15,800TA F1W1W2F2F3W3 (3FP) 14,30091%-5%
15,800TA F1W1W2F2F3W3W4F4 (4FP) 13,71087%-7%
4TA F1F2F3F4 (4FP)21,560TA F1W1W2F2F3W3(3FP) 19,50090%-5%
21,560TA F1W1W2F2F3W3W4F4 (4FP) 19,45090%-5%

This table shows the observing options for mixed CENWAVE CVZ orbits that need to have WAVECALs inserted. The first column gives the number of CVZ orbits in the C1055B orbits simulated with 30s NUV ACQ/IMAGE TAs. The second column, ET_ORIG, shows the original scheme. The NUV TA is shown in GREEN, the C1055B observations are in BLUE. In this notation, each FP-POS is denoted by F#. At the end of the scheme the number of total FP-POSs is reported in the format of 2x(# of FP-POS per CENWAVE). The #'s given do not necessarily correspond to same number FP-POS, but it is most efficient to observe FP-POSs of the same CENWAVE in the preferred, increasing FP-POS number, direction. The third column gives the original estimated exposure time (in seconds) on the original observing scheme. The fourth column gives the new observing scheme with the WAVECALs inserted. In these schemes, the WAVECALs are indicated by W#. The fifth column, ET_NEW, gives the exposure time using the new scheme. The sixth column gives the percentage of original exposure time represented by the new scheme. The final column expresses this loss of exposure time as the loss in Signal-to-noise (assuming only Poisson Statistics).

7. Mixed CENWAVE CVZ programs

Mixed C1055B+C1096B CVZ Visits are the most severely impacted programs. Single-orbit mixed CVZ visits that drop from 4 FP-POS per CENWAVE down to 2, still lose ~43% of the observing time. As with single CENWAVE CVZ visits, the longer the CVZ visit, the less the impact on observing time.

The table below gives the details for simulated typical mixed-CENWAVE CVZ exposure sequences. These estimates assume a 30-second NUV ACQ/IMAGE for the TA. For many of the # of orbits in a Visit possibilities, the exact combination to use depends on the brightness of the target and the science needs of the observations.

Please contact your CS for the latest information if you have mixed C1055B+C1096B visits.

Mixed CENWAVE CVZ Orbital Schemes
# of C1096+C1055B w/ NUV TA CVZ OrbitsOriginal SchemeET_ORIG (s)New SchemeET_NEW (s)ET_NEW ET_ORIGS/N Loss
1TA F1F2F3F4 F1F2F3F4 (2x4FP)3,660TA F1W1W2F2 F1W1W2F2 (2x2FP)2,14058%-24%
3,660TAF1W1W2F2F3W3 W1F1F2W2W3F3 (2x3FP)1,20033%-43%
2TA F1F2F3F4 F1F2F3F4 (2x4FP)9,590TA F1W1W2F2 F1W1W2F2 (2x2FP)8,14085%-8%
9,590TA F1W1W2F2F3W3 W1F1F2W2W3F3 (2x3FP)6,95072%-15%
9,590TA F1W1W2F2F3W3W4F4 F1W1W2F2F3W3W4F4 (2x4FP)5,31055%-26%
3TA F1F2F3F4 F1F2F3F4 (2x4FP)15,310TA F1W1W2F2 F1W1W2F2 (2x2FP)13,74090%-5%
15,310TA F1W1W2F2F3W3 W1F1F2W2W3F3 (2x3FP)12,55082%-9%
15,310TA F1W1W2F2F3W3W4F4 F1W1W2F2F3W3W4F4 (2x4FP)11,14073%-15%
4TA F1F2F3F4 F1F2F3F4 (2x4FP)21,090TA F1W1W2F2F3W3 W1F1F2W2W3F3 (2x3FP)18,40087%-7%
21,090TA F1W1W2F2F3W3W4F4 F1W1W2F2F3W3W4F4 (2x4FP)16,89080%-11%

This table shows the observing options for mixed CENWAVE CVZ orbits that need to have WAVECALs inserted. The first column gives the number of CVZ orbits in the C1055B + C1096B orbits simulated with 30s NUV ACQ/IMAGE TAs. The second column, ET_ORIG, shows the original scheme. The NUV TA is shown in GREEN, the C1055B observations are in BLUE, and the C1096 observations are in RED. In this notation, each FP-POS is denoted by F#. At the end of the scheme the number of total FP-POSs is reported in the format of 2x(# of FP-POS per CENWAVE). The #'s given do not necessarily correspond to same number FP-POS, but it is most efficient to observe FP-POSs of the same CENWAVE in the preferred, increasing FP-POS number, direction. The third column gives the original estimated exposure time (in seconds) on the original observing scheme. The fourth column gives the new observing scheme with the WAVECALs inserted. In these schemes, the WAVECALs are indicated by W#. The fifth column, ET_NEW, gives the exposure time using the new scheme. The sixth column gives the percentage of original exposure time represented by the new scheme. The final column expresses this loss of exposure time as the loss in Signal-to-noise (assuming only Poisson Statistics).