9.5 Science Exposure Overheads

Science-exposure overheads are dominated by the time required to move OSM1 and OSM2 and to read out the on-board memory buffer at the end of each exposure. While the Phase II overheads computed by APT may be less than the values presented below, it is important to plan Phase I proposals using the conservative overheads given below to ensure adequate time for each exposure.

The full overhead calculation for science exposures depends upon a number of factors including generic exposure set-ups (which depend on the detector and observing mode), whether an aperture change is required, whether a grating change is required, whether the central wavelength setting for the grating is changed, and the directional sense of any required motion to implement an FP-POS change. Table 9.5 lists these additional overheads.

When moving to a new grating, you may specify any combination of central wavelength and FP-POS setting with no additional overhead penalty. The FP-POS sequence 1,2,3,4 is more efficient than 3,4,1,2, because no backward motion is required.

New policies in effect for G160M observations are detailed in Section 9.5.1. In particular, G160M observations involve increased WAVECAL overheads, unless the G160M observations require the use of LP4. Figure 9.1 can be used to determine if G160M observations satisfy the requirements for an LP4 request. 

Table 9.5: Science Exposure Overhead Times.


Exposure set-up





Grating changesee Table 9.3see Table 9.4

Central wavelength change



FP-POS forward2



FP-POS backward2



PSA BOA Change



WCA BOA Change



SEGMENT reconfiguration

300 (off); 315 (on)


Memory readout3





G160M Wavecal Exposures4

82 + 242 x NFP-POS


1 This depends on the cenwaves involved and may vary by up to ±6 s. For G130M, allow up to 105 s if one of the cenwaves is 1222 and up to 132 s if one of the cenwaves is 1055 or 1096 and the other is 1291 or greater.
2 "Forward" refers to the preferred direction of motion of OSM1 or OSM2 (toward larger FP-POS values) and "backward" to the opposite direction.
3 ACCUM mode readout overheads can be hidden within subsequent exposures under certain circumstances, but the rules are complex. Use these values as safe upper limits for proposals.

4 NFP-POS = number of FP-POS exposures. 242 s of overhead time is required for each FP-POS exposure,  with an additional 82 s of overhead time for the first FP-POS exposure. This overhead is not applicable to G160M observations that may be observed at LP4. To determine if G160M observations satisfy the requirements for an LP4 request, consult Section 9.5.1.

To estimate the overhead for an exposure, round the desired exposure time up to the next whole second and add the generic exposure setup overhead from Table 9.5. If a grating change has occurred from the previous exposure, add the appropriate values from Table 9.3 and/or Table 9.4. If a central wavelength change is made, add the appropriate value from Table 9.5. If an FP-POS movement is made, add the appropriate value for motion in the preferred direction (toward larger FP-POS) or non-preferred direction. Note that all dispersed-light target-acquisition exposures are obtained with FP-POS=3. For all FUV observations except the G140L 800 Å and 1105 Å settings and those impacted by the COS 2025 policy, both detector segments are powered on by default. To turn one of them off, set SEGMENT to A or B and add the associated overhead. For G160M observations add on the associated overhead for WAVECAL exposures per FP-POS used.  Lastly, add the appropriate detector memory readout overhead.

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 ~1180Å (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 TAGFLASHes 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, and their associated WAVECALs, should be used instead. For more information, consult with your contact scientist.

Overheads associated with new settings introduced in Cycle 26 have not been tabulated, but they are expected to be similar to those of the other cenwaves in their respective gratings.

9.5.1 Policies when observing with G160M

The default lifetime position for G160M observations is now LP6. Wavelength calibration exposures at LP6 incur higher overheads than at other LPs due to the use of SPLIT wavecals (Section 5.7.6). In order to help minimize the impact of these overheads, a number of policies are in effect for G160M observations: 

  • The use of multiple FP-POS positions for each CENWAVE setting of the COS FUV detector is required in order to improve the limiting S/N and minimize the effects of flat-field artifacts (Section 5.8.2). While the use of all four FP-POS is required for FUV gratings (unless justified in the Phase I proposal), G160M users at LP6 are permitted to use less than 4 FP-POS based on the following S/N limits:
    • Users whose spectra require S/N > 25 are required to use 4 FP-POS
    • Users whose spectra require S/N 20-25 are required to use at least 3 FP-POS
    • Users whose spectra require S/N < 20 are permitted to use 2 FP-POS

The savings in overhead from using fewer than four FP-POS must be balanced against the potential reduction in wavelength coverage. Continuous coverage of the broadest possible wavelength range may be obtained by using either all four FP-POS or by using three FP-POS: 1, 4, and either 2 or 3. If there is a well-justified reason to use only two FP-POS, 1 and 4 are recommended. At LP6, this maximizes the wavelength coverage when two FP-POS are in use, leaving a gap of only 0.3 Å on Segment B. The 0.3 Å gap is expected to lie within the 1 Å ranges tabulated below, but its precise location cannot be predicted in advance due to mechanical uncertainty. If fewer than four FP-POS are desired, a justification must be included in the Phase I proposal.

G160M cenwaveGap range (Å)
15331439.0 - 1440.0
15771483.0 - 1484.0
15891494.9 - 1495.9
16001506.6 - 1507.6
16111518.3 - 1519.3
16231531.0 - 1532.0
  • If the total exposure time per target is less than one orbit, and there is a well-justified need for more than two FP-POS, users may request to use LP4 for their G160M observations. This is to mitigate the increase in overheads associated with wavelength calibration exposures at LP6, which are required for each FP-POS exposure. LP4 will be in 'available mode' for G160M to accommodate observations of this type and as such needs to be requested by the user during the Phase I process. Figure 9.1 provides a flow-chart to guide users in determining when the use of LP4 for G160M observations can be requested.

Figure 9.1: Users can request the use of LP4 for G160M observations by assessing the exposure time and S/N requirements per target.

  • When calculating science exposure overheads for G160M observations, additional overheads should be included to account for SPLIT wavecal exposures (Section 5.7.6).  G160M observations that satisfy the criteria for an LP4 request do not require the additional overheads. Science exposure overheads for all FUV gratings should be calculated using Table 9.5, or APT to derive a complete and accurate determination of overhead times.