GHRS Instrument Handbook
Most objects observed with the GHRS are point sources (stars), and the majority of the remainder can be observed by first centering on a nearby point source and then offsetting to the object of interest. Point sources with accurate coordinates are very, very easy to acquire with the GHRS: just specify ACQ with BRIGHT=RETURN to have the instrument automatically center on the brightest object found within the LSA.
Target acquisitions always take place with the LSA because the SSA is too small to enable a field to be mapped effectively. Additional ACCUMs may be specified after the first so as to obtain spectra at several wavelengths.
A blind pointing with HST is likely to place the object of interest within 1 arcsec of the center of the Large Science Aperture if guide stars are used, and within about 10 arcsec if the pointing is done on gyros. That 1 arcsec accuracy is limited in part by the quality of coordinates provided by users and partly by errors in the positions of the FGSs relative to the GHRS apertures (see the FOS Instrument Handbook for a discussion of pointing errors). Using J2000 coordinates tied to the GSC reference frame can help to reduce the possibility of a failed acquisition. And don't forget to include proper motions if appropriate and to check the equinox and epoch of proper motions (see the Phase II Proposal Instructions).
After the initial pointing, a GHRS onboard target acquisition begins with a spiral search centered on the field of view. The motions are made by the telescope, and at each point of the search either a single flux measurement (with 8 science diodes) or a map of the LSA is made. The default is a 3 3 pattern (SEARCH-SIZE=3) of maps in a square 4.6 arcsec on a side. Other options available are a 5 5 pattern (SEARCH-SIZE=5), 7.7 arcsec on a side, or a single integration (SEARCH-SIZE=1) that is 1.74 arcsec square (this latter option can be useful for obtaining a MAP after an object is centered). The telescope motions are made in the x and y coordinate system of the GHRS with a step-size of 1.53 arcsec. The motions are not in the U2, U3 system of the telescope.
For stars with good coordinates, the default (3 3) acquisition strategy will suffice (we are unaware of any acquisition failures in the recent past because of a too-small search area). In a few cases the 5 5 pattern guards against minor coordinate uncertainties (the time needed increases in proportion to [SEARCH-SIZE]2, but the STEP-TIME for an acquisition is often so low that the total time involved is small).
You should use ONBOARD ACQ whenever:
- The object is a point source, or
- The object can be reached by offsetting from a nearby object that meets the above description, or
- An extended object is small enough that LOCATE=EXTENDED will work (see Section on page 45).
Also, the object to be centered should be the brightest object within the area searched (with some allowance for uncertainty in positioning as well). In other words, you should ensure that your object is the brightest one that HST will find within a box whose total size is about 6 6 arcsec (this size differs from the 7.7 arcsec square box mentioned above to allow for uncertainty in position). Note that the flux is measured in the ultraviolet (see Section on page 97).
The items you must specify for the ONBOARD ACQ are:
- The mirror to use. N2 and N1 will work for most targets. Mirror N2 provides a flat reflectivity over a broad range of ultraviolet wavelengths (see Section on page 97). Mirror A2 has a similar spectrum response but reflects much less light than N2, in order to acquire bright objects. You may specify mirror N2 for an acquisition to observe with Side 1; this may be desirable, for example, when observing cool stars. However, doing this will cost the time needed to activate D1 after shutting down D2 since both detectors may not be fully on at the same time.
- In almost all cases you should specify BRIGHT=RETURN, which is a feature that automatically centers the brightest object found. If BRIGHT=RETURN is specified, any FAINT limit given is ignored. Explicit BRIGHT and FAINT flux limits may also be used instead, but there is rarely a reason to do so because BRIGHT=RETURN is so robust (see Section on page 101 for more information).
- The size of the spiral search pattern to execute (SEARCH-SIZE). The default is a 3 3 grid (which covers about 4.6 arcsec square), but you may also request a 5 5 search over a square 7.7 arcsec on a side.
- Whether or not to record a map of the field at the search points so that you can confirm the telescope's pointing after the fact. The LOCATE phase of an acquisition precisely centers the object in the LSA. The residual pointing uncertainty after the LOCATE (or, equivalently, a PEAKUP), is two deflection steps, which is 0.054 arcsec. A MAP is therefore unnecessary and not an effective use of spacecraft time. At most, a MAP=END-POINT should suffice. (See Section on page 101 for a discussion of MAPs.) Note that such a MAP occurs after the return to the brightest point in the field but before the object is centered in the LSA by an ACQ/PEAKUP. To determine the position of an object in the LSA before spectroscopic observations are begun, we recommend obtaining an IMAGE as a separate exposure line. The MAP=ALL-POINTS option may not be used with an ONBOARD ACQ.
- Any offset to apply once the object is centered (if appropriate).
After the initial acquisition, a peakup helps to precisely center the object in the aperture. Specifying an ACQ/PEAKUP before starting LSA observations is redundant (a peakup is done as part of the LOCATE phase of an ACQ) and unnecessary. However, an ACQ/PEAKUP before starting SSA observations is vital for achieving the best throughput with the small aperture. In general, you should use the same STEP-TIME for a PEAKUP that you used for the ACQ, or perhaps a slightly longer STEP-TIME if the total number of counts is low (see 48).
There are situations in which an object can be observed satisfactorily with Side 1 but for which the count rates for acquisition mirrors N1 or A1 are extremely low. One possibility is to increase the exposure time for the acquisition, but the maximum permitted STEP-TIME is 12.75 seconds. A better option may be to acquire with mirror N2. Because both detectors, D1 and D2, may not be active in the GHRS at the same time, and there is a 40-minute overhead involved in making one primary and the other secondary; e.g., to go from Side 2 to Side 1. Whether or not that is a "cost" or not to your program depends on specific details. It is often the case that an acquisition takes place over the first orbit, followed by science observations in later orbits. In that case, much of the 40 minutes can take place during the part of the orbit when the target is inaccessible. But for CVZ viewing or single-orbit visits the cost can be real.
Given the centering algorithm for the GHRS, which we will now describe, you can usually predict the results of an onboard target acquisition. Stepping, in both the x and y directions, is done in 0.027 arcsec steps, and on a point source the centering is expected to be good to within two steps. If the target is extended enough that the fluxes in the areas which are compared do not change significantly when a step is made, the centering accuracy will be degraded. An example is the case in which there is more than one source of light within the LSA.
Consider, for instance, two stars which are separated by 1.0 arcsec and for which the second star is 1 magnitude fainter than the primary star. Exact results will depend on the position angle between the two stars. The x balancing algorithm begins by placing the brightest source on the fourth of the eight diodes that are used during an acquisition (the LSA is imaged onto eight diodes), and moving until the flux on diodes 4 and 5 is balanced. The second star would not affect this balance at all unless its light fell on one of the same diodes as the primary star. In that case it would affect centering by a fraction of a diode.
In the y direction the results are different. If the second star is "above" or "below" the primary, it will "pull" the centering in that direction. In the case described, an extra source of light 40% as bright as the primary would be present in the upper or lower half of the LSA. The flux-balancing algorithm would divide the primary image 70-30, rather than 50-50, with the image displaced towards the half of the LSA which did not contain the second star. In this case the centering error should be less than 0.1 arcsec. If the LSA acquisition were to be followed by a slew to the SSA, PEAKUP, and an observation, the primary object should be successfully centered and observed. More complicated images, or sources more similar in brightness may not be suitable for onboard acquisition. (Note that balancing in the y direction is done before the x direction is balanced.)
Sometimes a star may be so faint that geocoronal Lyman-a can interfere with an acquisition. Some guidance for when this may be a problem is provided in Section on page 99. If it is, we recommend that you specify SHADOW as a Special Requirement on the acquisition line on your Phase II form. Doing so constrains the scheduling of your proposal, so SHADOW or LOW-SKY should only be requested when they are necessary.
Another way to acquire very faint targets reliably is to use the Faint Object Spectrograph. This can be especially useful for acquiring extragalactic objects to observe with grating G140L because the acquisition mirrors for Side 1 of the GHRS reflect only far-ultraviolet light and because the maximum permissible integration time per dwell point is only 12.75 seconds. FOS-assisted acquisitions for the GHRS have been tested and have been found reliable once the object has been acquired with the FOS blue side. It is possible in principle to acquire with the FOS red side and then move the target to the GHRS (the relative positions are precisely known), but the overall motion is about 2.5 arcmin. The same guide stars must be used for both the FOS and GHRS portions of your program, but it is unlikely that guide stars will exist that can move 2.5 arcmin without leaving the FGS field of view (the "pickle"). Only in very special situations will it be possible to find guide stars that stay within the fields-of-view of the FGSs from beginning to end of this operation, and such guide stars are especially scarce at high Galactic latitude.
See Example 2 on 57.
There are three classes of extended sources we can consider:
- Objects larger than the LSA that have roughly uniform surface brightness.
- Objects smaller than the LSA with roughly uniform surface brightness.
- Objects with significant structure, some of which is on scales smaller than the LSA.
The first class might be typified by Jupiter, and such objects are impossible to acquire directly with the GHRS because there is no clear photometric "center" to align on. In such cases it is necessary to offset from a smaller object which can be centered.
The second class of objects includes the Galilean satellites of Jupiter, and it is these for which the LOCATE=EXTENDED acquisition option was written. In a normal LOCATE, the object to be observed is moved in the x direction until the signal seen by the center two diodes (of the eight onto which the LSA is imaged) is balanced. LOCATE=EXTENDED in ACQuisition mode balances the four left diodes against the four right-hand ones to roughly center an object. In ACQ/PEAKUP mode, the EXTENDED option allows you to specify that the balancing be done over the central four, six, or eight diodes (specified as EXTENDED=2, 3, or 4).
The third class of objects can be the most problematic, especially if the target is an extragalactic one at high latitude. In such cases there may be no nearby star from which you could offset, but the source itself often contains point-like sources that can be centered on; in these cases an early acquisition or a pre-existing image is invaluable. The problem is then one of predicting acquisition count rates; that is treated in Section on page 85. You may also wish to consider an acquisition with the FOS, as described in the previous section.
Even if an ONBOARD ACQuisition will not work for your target, it may still be possible to acquire a nearby offset star and to then offset to your target. Such an offset will happen automatically if the coordinates given for an acquisition exposure are different from those given for the science exposure. You would normally use two or three lines in the Phase II proposal to achieve this: acquisition of a offset star, offset, peakup on the target (if desired), and a science observation. The first line would request an onboard acquisition of the offset star. It should specify the Special Requirement ONBOARD ACQ FOR <line 2>. Line 2 would then specify the observation of the science target. A PEAKUP in the SSA could also be added if appropriate, but whether it was done on the offset star or the science target would depend on the brightness of the target and any crowding in the field being observed.
You must, of course, include the offset star as one of the objects on your target list. It should be designated xxx-OFFSET, where xxx is the name of the target object. If desired, you may give the position of your target by using RA-OFF, DEC-OFF, or XI-OFF, ETA-OFF and FROM relative to the offset star. See the Phase II Proposal Instructions for details and notes on proper units. In the Phase II proposal, the Target Name for line 1 is xxx-OFFSET, and in the example above, the Target Name for lines 2 and 3 is xxx.
To make a successful offset, the relative positions of the offset star and target must be very well known - about as well as 1/4 the size of the aperture. (e.g., rms errors of 0.05 arcsec for the SSA.) One way of obtaining such positions is by requesting an early acquisition WFPC2 image, and measuring relative positions from it (at least 2 months prior to the science observation). The offset positioning accuracy of the HST is expected to be very good (of the order of 0.03 to 0.05 arcsec for a 30 arcsec offset), and the accuracy of the placement will be primarily determined by the accuracy of your positions. An offset of more than 30 arcsec may require the telescope to acquire new guide stars, which would worsen the accuracy of the positioning.
Very bright objects may need to be acquired with one of the attenuated mirrors, A1 or A2. Such acquisitions will take substantially longer than if N1 or N2 were used because of internal calibrations that are performed to determine the location of the aperture on the photocathode (the DEFCAL, or deflection calibration). In some cases you may wish to use N1 or N2 even if the object is bright as long as your object is not a threat to the GHRS. Very high count rates lead to a non-linear detector response, but that may not matter as long as the search algorithm can accurately determine where the center of the stellar image is.
Omitting the ONBOARD ACQ FOR Special Requirement from an acquisition can make it seem as though the ACQ takes less time. What is really happening is that without this Special Requirement the LOCATE phase of the acquisition is not being done. That saves time but results in poor centering, and we recommend using LOCATE.
Use the ONBOARD ACQ FOR Special Requirement to indicate the relationships between exposure lines. The general idea is that an ONBOARD ACQ is either done to prepare for a series of science observations (probably ACCUMs) or for a peakup. If an acquisition passes control to a peakup, then it should say ONBOARD ACQ FOR 20, say, where 20 is the line number of the peakup. But if the acquisition or the peakup is the last line before the science starts, then it should say ONBOARD ACQ FOR 20-60, or whatever the range of line numbers is that it pertains to.
Step 1: Mode=ACQ
- Aperture should always be LSA ("2.0").
- MIRROR is usually N2 or N1 unless object is too bright (then use A2 or A1; see Section on page 85). It is permissible to acquire with one side (mirror N2, say) and observe with the other (grating G140L, perhaps), but with a cost in time.
- SEARCH-SIZE=3 is the default and adequate almost all the time. Values of 1 or 5 may also be used.
- BRIGHT=RETURN is the default for finding the target and should be used unless you are forced not to. Do not specify FAINT unless you must specify an explicit BRIGHT limit. (FAINT is ignored if BRIGHT=RETURN is used.)
- LOCATE: Default is YES for an ONBOARD ACQ and NO for an early acquisition or INT ACQ. We recommend these defaults. Note that LOCATE=EXTENDED is now available. With an ONBOARD ACQ, LOCATE=NO may be used only if MAP=END-POINT is specified.
- MAP: (See Section on page 101.) No image is generated by default for an ONBOARD ACQ. However, MAP=END-POINT will provide one if the target is in the LSA, but it will be made before the target is centered. If you wish to determine the actual position of the object in the LSA before spectroscopic observations are begun, you should obtain an IMAGE as a separate exposure line, and you should not specify a MAP at all. MAP=ALL-POINTS may not be used with an ONBOARD ACQ.
- The time per exposure can be calculated from:
- Special Requirement is ONBOARD ACQ FOR <exp list>.
- In general, use PEAKUP only for SSA observations; it is unnecessary for the LSA. However, formally the aperture can be specified as either the LSA ("2.0") or SSA ("0.25"); use the one that will be used for the science observations that immediately follow.
- Specify the MIRROR as for Mode=ACQ; i.e., N1, A1, N2, or A2 depending on target brightness.
- The time per exposure can be calculated from
- We urge you to be precise and explicit about the way in which you specify an ACQ/PEAKUP and the order in which observations are to be made. The defaults that apply to ACQ and ACQ/PEAKUP modes will usually accomplish what you wish, but the way to be sure is to specify the details. Confusion can arise particularly when a program mixes LSA and SSA observations. We recommend that you do an ACQ on the first line of your exposures, then on line 2 specify ACQ/PEAKUP if SSA observations follow and indicate the lines to which the PEAKUP applies (all of which should use the same aperture). Then specify another ACQ/PEAKUP before starting observations in the other aperture.
- Special Requirement is ONBOARD ACQ FOR <exp list>.
- Please note the STEP-TIME to be used as a comment.
There are other types of acquisitions - interactive and early - that have not been used by anyone for some time because strategies using ONBOARD ACQs work and are more efficient. Also, the ability to get MAPs is now little used. These parameters have been moved to the end of Chapter 7 (see Section on page 100) to simplify this chapter.
Onboard Strategies for Special Situations
Side 2 Acquisitions for Side 1 Science Observations
Acquiring Faint Targets with the GHRS or FOS
Acquiring Extended Sources with the GHRS
Acquisitions and RPS2
Acquisition Parameters -- A Summary
Other Acquisition Parameters