FUSE Target Acquisition Summary

William P. Blair
FUSE Mission Planning Scientist
Sept. 13, 1999
Rev. 1.1

History: Version 1.0 May 14, 1999

Version 1.1 Sept. 13, 1999: Fixed (swapped) acq_case 2 and 3 discussion following Table 3.3.7-1 excerpt. (This was also wrong in the SCC-MP ICD, and will be until a new official version of that document is adopted.

1. Introduction and Purpose

Target acquisitions are one of the most demanding aspects of FUSE operations. This is complicated by the fact that most target acquisitions occur autonomously, so that failed acquisitions can have detrimental affects not only on a given observation, but potentially on those that follow in the schedule as well. We MUST take a conservative approach to target acquisitions to prevent major disruptions in scheduled activities.

From an operations standpoint, there are many aspects of target acquisitions that we do not know or understand in detail from our pre-mission perspective. This also pushes us into a conservative stance, from which we will move only as we learn more during In-Orbit Checkout (IOC) and the Science Verification (SV) period. Since the acquisition mode is tightly intertwined with the whole scheduling process (being set early in the process by the "ObsTyping" step), adjusting to the changing situation is a painful but necessary process that requires iteration of information in the Mission Planning Database (MPDB) and subsequent steps of the scheduling process.

In this document, I attempt to lay out the assumptions we are using pre-mission to determine the acquisition mode that will be used as a function of target and observation specifications. Much of what follows derives from other documents but is being pulled together here to provide "one-stop shopping" for people interested in these issues. I cross reference to other documents where the official records are kept.

2. Acquisition Cases

Acquisition cases are specified in the SCC to MP Interface Control Document (Rev. F), in section 3.3.7. An excerpt from this document, including the description of the different defined `acq_cases', are given below. The first four acq_cases are the one used most often for normal science observations. The nomenclature for these cases will be used heavily in what follows.

This activity provides information on the type of target acquisition to perform. An example is shown in Appendix A, section 5.9.

Briefly, the different acquisition types are as follows:

• 0 = No Acquisition: After slewing to a position, pointing is held with gyros and no additional target acquisition activities are performed.
  
  
• 1 = FUV Peak-up: The standard acq. mode in cases where channel alignment and/or careful centering of the target in the slit are required. (Assumes target is bright enough in FUV for this to work.)
  
  
• 2 = FES Target Acq plus FUV Peak-up: Target is sufficiently bright visually to be seen and centroided by the FES, and most of the positional uncertainty is removed by the FES Target Acq. Additionally. source is bright enough to permit a Peak-up (for optimum centering in the narrow slit, for instance).
  
  
• 3 = FES Target Acq: Target is sufficiently bright visually to be seen and centroided by the FES. Assumes no channel alignment is needed.
  
  
• 4 = FES Guide Star Acq: Target is too faint or diffuse to be centroided directly, but its position is well known with respect to the selected guide stars. Alternatively, target may be too bright visually to permit FES Target acq (i.e. it saturates the FES) but not so bright that it makes guide stars invisible to the FES. The FES acquires guide stars and assumes the target position is known. (This is basically a "blind offset" from within the FES FOV of the target, with no separate FLDT and no change in guide stars between the acquisition and FUV observation.)
  
  
• 5 = FES Offset from within FES FOV: Used in certain cases where a separate offset target close to the science target is provided for use in the acquisition process. Offset targets of this type are expected to be very close to the target position so that no change in guide stars is required between the offset star and science target. An FES Target Acq is done on the offset star, but then the "target" position is placed in the selected aperture. (The salient point here is that what is known accurately is the relative position of the offset star and the science target position. This is the case where the osi_ra and osi_dec in the TASA activity will be different from the target_ra and target_dec.
  
  
• 6 = Target to Slit: This refers to an acq mode whereby a GSAT table (i.e., an offset field within the FLDT of a target) is used to verify pointing position prior to putting the science target directly into an aperture (with no further refinement in position). GSIT guide stars are then acquired with the target star in the aperture. This may be the primary way we acquire visually bright stars that are too faint for FUV peak-up, although it would in principle work in other situations as well. Target position should be accurate in the guide star frame of reference, especially if placing target in MDRS.
  
  
• 7 = Bright Star Type 1: Placeholder Acq type whereby a visually bright target could be placed in HIRS by `bootstrapping' from the LWRS or MDRS aperture. (Not yet defined functionally.)
  
  
• 8 = Bright Star Type 2: A placeholder acq mode whereby the ND filter must be in place to attenuate a very bright target, and remains in place during the observation, with guiding done on the spillover light from the slit. (It is uncertain whether this mode will actually work.)
  
  
• 9 = Moving, Planet: Needed to acquire a moving target with accurately known position and trajectory.
  
  
• 10 = Moving, Comet: Needed to acquire a moving target with accurate trajectory, but absolute position at time of observation is uncertain.
  
  
• 11 = Unknown Star Acq: Slew to position and acquire (and guide on) unknown stars, but do not proceed further in acquisition process. (Added in Rev. F. Mostly needed for IOC and testing.)
  
  
• 12 = GS Re-Acq: There are differences in the scripting between the GS Acq (acq_case = 4) and a reacquisition after occultation or SAA. As of Rev. F, a GS Re-Acq case is defined separately for use in scripting.

A note about offset stars: There are two types of offset stars. The kind used in aqc_case=5 above is expected to be a star "near" the target in the FES FOV whose position is known accurately with respect to the target. The coordinates of such a star would be reported in the "osi_ra" and osi_dec" fields of the TASA activity (see section 3.3.6). The other kind of offset star is used in situations where an FUV peak-up is needed but the source itself is too faint or diffuse; these offset stars may actually be a degree or two away from the real target and will be reported as a separate "targets" (i.e. with their own separate field star/guide star information). The situation in acq_case 6 is slightly different still: a separate SET of guide stars, from within the same FLDT of the science target, are provided as a "GSAT" table (see section 3.3.17). This allows an offset FOV to be acquired first, prior to putting the science target directly into an aperture, and then acquiring the GSIT stars for guiding.

(End excerpt from SCC to MP ICD.)

3. Deciding on the Acquisition Type

What acq_case to apply to a given observation depends on a number of things, most of which are supplied by the user in their Phase 2 inputs. In addition. certain assumptions are made on our part as to how the instrument operates (or will operate!). Pertinent target/observation information includes the V magnitude, the F(950,1050,1150) reference fluxes, the expected count rate, and the desired aperture. Operational assumptions include FES performance parameters (e.g. how faint we can go for FES Target Acq, how bright we can go before saturation affects the FES centroiding, what the scattered light performance will be, etc.) and channel alignment and stability assumptions.

An important step in observation scheduling is a process we call "Observation Typing" (or ObsTyping for short), and one of the most important aspects of ObsTyping is selection of the acq_case. Here are premission assumptions used by the ObsTyping Tool (OTT) for selecting PKUPs and acquisition types. (Again, many of these assumptions are very conservative!)

- PKUP requires source type PC (point continuum type).
  (Note: We assume no PKUPs on emission-line sources!)
- HIRS requires PKUP; LWRS does not.
- MDRS requires PKUP when source Vmag is too bright for FES centroiding,
  otherwise no PKUP. If source too faint for PKUP, suggest LWRS OR manually
  verify target coordinate in HST-GSC frame and use FES GS acq.
- PKUP requires all 3 ref. fluxes > Fmin  (Fmin=1E-13 pre-mission) or else 
           requires a UV-bright offset star with all 3 ref. fluxes > Fmin.
- V ranges for PC acquisitions:
    V <= 1 requires separate offset star for acq.
    1 <=V < 8 too brt for FES Targ Acq, but FES GS Acq OK (with accurate coordinate
               wrt GSC).
    8 <= V <15 will use FES Targ Acq.
    V >= 15 requires FES GS Acq (with accurate coordinate wrt GSC).

From inspecting the above, it is clear that automatic selection of acq_case is not always possible. By taking the conservative approach, we can be assured that anything the OTT decides will be safe and accurate; anything that the OTT cannot decide uniquely gets kicked out for manual processing and decision making. For instance, judging PKUPs based only on the three reference fluxes is an over-simplification of reality. We ASSUME the weakest of the three reference fluxes is appropriate across a given detector segment; and thus disallow some sources that are probably bright enough for PKUP. THIS IS OK!! We MUST take a conservative route in the automated part of the process! Anything that gets ObsTyped for a PKUP acquisition had better darn well have sufficient flux. The targets that seem to require PKUP but are too faint SHOULD get kicked out for manual processing, where a case-by-case judgement can be made on the acquisition type to use (and in particular, whether a PKUP acq should be allowed for a given target). In most cases, the decision to go ahead and try a PKUP in a situation such as this will be left to the user to specify, and any failure due to such a decision will result in time charged to your program. Left to our own devices, FUSE MP will normally take the conservative approach.

Questions have arisen about performing PKUPs on other types of sources, such as slightly extended sources or emission-line objects. For now, these will only be allowed with manual analysis and interaction (i.e., when we manually adjust the step size for an extended source or when we judge the emission line fluxes to be sufficiently accurate and strong to allow PKUP to work).

The third bullet above (for MDRS) is a slightly different twist on acquisitions from what we said in the FUSE Observer's Guide and in the Phase 2 Instructions, and so some explanation is in order. In Phase 2, we had told people that use or HIRS required an FUV PKUP (wither on the target or an offset star) to align the channels and centroid the target, and use of MDRS did not require FUV PKUP. The assumption for MDRS naively assumed that any positional error at the 1-2 arcsec level for MDRS would get removed by performing an FES Target Acq. What we have more recently come to realise is that there are a LOT of requested MDRS targets that are visually too bright (or too faint) for FES Target Acq! If such targets have enough FUV flux for a PKUP to work, we default to a PKUP in the MDRS. If the FUV fluxes are not sufficient for a PKUP, we MUST acquire with an FES Guide Star Acq, which means the target MUST have coordinates accurate at the 1-2 arcsec level relative to the HST Guide Star Catalog (from which the guide stars are selected). Note this is not an absolute coordinate accurate at that level, it is a RELATIVE coordinate accuracy that matters to us.

Most people with targets too faint for PKUP have paid attention to the instructions and provided coordinates in the HST GSC frame. If this was clearly stated in the Phase 2 proposal, these people are ready to roll! Interestingly, it is the people with visually BRIGHT (but FUV-faint) targets (1 < V < 8) that have a potential problem. The HST GSC simply lists initial Hipparcos coordinates for many such targets, which may or may NOT be accurate with respect to the HST GSC reference frame. Hence, we could find ourselves in the embarrassing situation where we can more confidently place a V=15 target in the MDRS than we can with a V=5 target!

For those of you in the latter category, the options are the following: 1) bail out of the MDRS and switch to the LWRS aperture (for which an FES Guide Star Acq will work with confidence), or 2) do the extra leg work (e.g. astrometry, careful proper motion analysis, etc.) and provide verified coordinates with respect to the HST GSC frame of reference.

4. More Information on FUV PKUPs

PKUPs will be used in one of two modes, either stand-alone or in conjunction with an FES-assisted Target Acq. Also, they will be used with either the HIRS (1.25 arcsec aperture width) or MDRS (4.0 arcsec) apertures. There are differing requirements for these different cases. In particular, in the FES-assisted case, the step sizes can be fewer and somewhat smaller. Here are the assumptions we are assuming pre-mission about step sizes and number of steps for the various cases. (Note: delta-X is the total X-motion in arcsec from the 1st to the nth position, where n=num_steps.) (This will be an Exploder "rule.")

1) Straight PKUP (acq_case=1), HIRS aperture
   num_steps=9, step_size=1.2 arcsec, delta-X=9.6 arcsec
   (Robust to coordinate errors of up to +/-4.8 arcsec.)

2) FES targ acq + PKUP (acq_case=2), HIRS aperture
   num_steps=5, step_size=0.8 arcsec, delta-X=3.2 arcsec

3) Straight PKUP (acq_case=1), MDRS aperture
   num_steps=7, step_size=3.5 arcsec, delta-X=21.0 arcsec
   (Robust to coordinate errors of up to +/-10.5 arcsec.)

4)* FES targ acq + PKUP (acq_case=2), MDRS aperture
   num_steps=5, step_size=2.5 arcsec, delta-X=10.0 arcsec

* Note: Case 4 will not happen very often, because we assume that a target 
which can be acquired with the FES can be placed into MDRS without a PKUP.

PKUP Dwell Times

The dwell times per step needed for FUV PKUP depend primarily on source brightness, as provided by the user as reference fluxes at 950, 1050, and 1150 Angstroms. We require a minimum of S/N=10, and so we take the conservative approach of ensuring we reach this exposure level even for the "faintest" of the three fluxes, and even in the "least sensitive" detector segment.

The following table summaries PKUP times required for S/N=10 for various flux levels using the on-line Count Rate Tool:

Dwell times for PKUPs (in seconds)

 Flux      SiC1    SiC2    LiF1    LiF2
--------  ------  ------  ------  ------
1.0E-14   743.27  760.30  170.29  197.56   
2.0E-14   224.29  229.03   58.83   67.20   
4.0E-14    75.31   76.74   22.84   25.70  
6.0E-14    42.02   42.76   13.76   15.38   
8.0E-14    28.44   28.92    9.77   10.88  
1.0E-13    21.28   21.63    7.56    8.39    
5.0E-13     3.31    3.36    1.34    1.48

From this, we set 1.0E-13 ergs/cm^2-s-Ang as the limiting flux allowed (at any of the three reference wavelengths) for PKUP in Cycle 1, to keep dwell times per step from getting too long. (Note: This is a change from the flux limit stated earlier, which was 4.0E-14. Until demonstrated otherwise, we are concerned about accuracy of PKUPs with dwell times larger than 30 seconds per step, let alone the concern for the total acquisition times that derive from such large dwell times.

5. Future Developments

New information will rapidly become available during the IOC/SV period. Much of this new information will affect the assumptions outlined above, and hence will affect the acquisition types specified from our pre-mission efforts. Some things may get clarified very early, such as the FES performance related assumptions, while others will take more time, analysis, and testing to resolve. You will be kept apprised of these changes via the FUSE electronic Newsletter and/or other electronic bulletins as needed.

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