GHRS Instrument Handbook
Target Acquisition Reference Information
We mention here for completeness several aspects of acquisitions that are now unused but which may be helpful in some circumstances. We are not aware of any usage of these in Cycles 4 or 5. There is nothing wrong with these features, but few or no GOs have found them necessary.
We first discuss interactive acquisitions (INT ACQ) in the hope of dissuading you from using them. An INT ACQ requires real-time contact between the ground and HST. Real-time contact is a limited and expensive resource that should only be used as a last resort. In almost all cases where an onboard acquisition will not work (because the object is in a crowded field, or is variable, or is a moving target), it is sufficient to use an early acquisition to get a WFPC2 or FOC image a few weeks in advance of the GHRS observation. The image can then be analyzed to pinpoint the source to be observed without requiring real-time contact. INT ACQ may be needed in a few instances where the object changes in its ultraviolet brightness unpredictably. We suggest that you consult with us before requesting INT ACQ.
If an interactive acquisition with the GHRS has been specified, a spiral search will be run after HST makes its initial pointing. A map of the LSA is made at each dwell point and each map is then downlinked to STScI in real time and may be viewed almost immediately in OPUS. After the spiral is complete, the telescope remains at its final dwell point, awaiting instructions. The final 9 (or 25) maps are assembled into a mosaic and displayed for the observer to identify the target, either from a cursor position or from calculation of a centroid. The motion needed to center the specified position in the LSA is computed and uplinked to HST. The recentering of the target usually takes place about an orbit after the spiral search, and must be scheduled for a specific time. If you have so requested in your proposal, an image of the LSA will be made after the recentering so that you may confirm the position of the target (but additional interaction at that time is not normally possible).
In an early acquisition an image of the field of interest is obtained several weeks (8 or more) in advance of the spectroscopic observation that the GHRS is to make. The image may be obtained with WFPC2, with the FOC (especially if an ultraviolet image is desired), or with the imaging capability of the GHRS itself. The GHRS is relatively slow at getting images, so if you wish to map an area much larger than about 2 2 arcsec we recommend that you consider WFPC2 or FOC. However, the GHRS has the capability of obtaining a monochromatic map (see below and "Image Mode" on page 49) in IMAGE mode, which can be useful in some situations. For an early acquisition, be sure to note the relationship of the image to the spectroscopic observations as a Special Requirement (use ON HOLD). Also, you should plan ahead so that the early acquisition image can be analyzed quickly and the positions measured sent back to STScI for incorporation into the telescope observing schedule.
Explicit BRIGHT and FAINT limits may be specified if you desire, although there is an increased risk of a failed acquisition unless you are confident of those fluxes. Also, a few very bright stars cannot be acquired with Side 2 if an explicit BRIGHT value is given but they can be acquired automatically with BRIGHT=RETURN (this is because different bit levels of counters apply in the two cases).
Details on computing BRIGHT and FAINT limits are given in "Predicting Target Acquisition Count Rates for Stars" on page 85. Please note that although we discourage the use of explicit BRIGHT and FAINT values unless they are unavoidable, you still need to estimate the target acquisition count rate in order to ensure that you choose the acquisition mirror correctly and that the STEP-TIME is determined properly.
Please note that using BRIGHT=RETURN and explicitly specifying BRIGHT and FAINT limits result in fundamentally different acquisition procedures. If BRIGHT and FAINT are specified, the acquisition stops as soon as those conditions are met and the point at which that happened is moved to the center of the LSA. With BRIGHT=RETURN, the entire spiral search region is sampled and the brightest object in it determined before any movement is made to center on the target. Both procedures require the same amount of telescope time because the schedule must allow for the entire region to be sampled.
The GHRS has the ability to make a MAP of the LSA by raster scanning one or both of its small focus diodes over the aperture. You may, for example, want a map to confirm the pointing at the time your spectrum was taken. The default for ONBOARD ACQuisitions is to make no map. If you ask for MAP=END-POINT, you will get a map after the spiral search has found your target, but before it has been centered (with LOCATE) in the LSA. If you want a map after the final centering, you can add a single line using IMAGE mode. An IMAGE may also be obtained of the SSA, which can be a useful means of determining what was observed in a crowded field. The MAP=ALL-POINTS option may not be used with an ONBOARD ACQuisition.
The time per exposure is calculated from
where (i.e., 1, 9, or 25), and is the number of dwell points mapped (=1 if MAP=END-POINT is chosen and = if MAP=ALL-POINTS. MAP=ALL-POINTS can only be used with INT ACQ or an early acquisition.).