Acquisition of Targets

Caution: The precision achieved by fine offsetting has yet to be established definitively.

The Gemini spectrographs often are designed with the option of very narrow slits to take advantage of the high quality images delivered by the telescope. The positioning of targets in such narrow slits, typically to one-tenth of the slit width, requires care. The basic procedures outlined below will provide the best slit centering possible given current telescope offsetting performance.

Targets Visible in the Instrument's Imaging Mode

Blind and Offset Acquisitions

When centering on the actual target is not possible, a reference star at an accurately known reference position is used instead. The telescope is slewed to the science target first, and guiding is started. The telescope is then 'offset' to the reference object, which is used for fine-tuning the slit centering. The offsets are then removed to return the telescope to the position of the science target. 

The accuracy of such offsets decreases with distance to the reference star, so it is important to choose the nearest reference star possible. The current offset accuracy is approximately 0.2 arcsec for a reference star 50 arcsec from the science target.

The Phase II Science Program must contain clear instructions on what the offsets are to the reference object. There are currently two ways to do this. The preferred way of specifying offsets to the reference object is to include accurate coordinates for the reference object stored as a "User" target (i.e. Tag=User in the Target component).  The base position is the science target. It is essential that the the coordinates of the reference star be accurate and be in the same reference frame as the coordinates of the science target so that the telescope control system will determine accurate offsets between the two positions. Both coordinates must be specified in J2000. 

Alternatively, the user may supply relative offsets in arcseconds in RA and Dec. These offsets should be specified in a note in the OT program. The note must be unambiguous about the directions (i.e., N, E) from the science target to the reference object. Please name the note so that it is clear that the note contains acquisition offsets.

With either method it is important that the relative offsets between the reference star and the faint target be very accurate.  These offsets are best determined by measuring the positions of both objects on the same image with good relative astronometry.  Then the coordinates of both objects, but at least the target, need to be put on the J2000 coordinate system.  If the science target, reference star, or the guide star have proper motions then these should be entered in the Proper Motion tab.

No matter which method is used to specify reference object offsets, the wavefront sensor used for guiding must remain active during the offsets to the reference star. It is important to check that the guide star is accessible for both the science target and the reference star. 

Offsetting to a reference object may also be helpful for brighter targets when the science target identification might be confused, as in crowded or complex fields. Additional information about complicated setups should be included in a Note in the OT file. This image shows an example of how the reference ("user") target is defined in the OT.

For mid-infrared observations using Michelle and T-ReCS, one additional step is needed to ensure that the object is properly aligned on the slit or to allow accurate absolute astrometry. Since reference objects bright enough to be visible at 10 to 20 microns are uncommon, the observer should add an observation of a mid-IR astrometric star (the "hotspot" star, a mid-IR standard or other star brighter than ~4 mag at 10 microns with reliable coordinates) prior to the acquisition sequence. This star should be as close as practical to the science target, and certainly within a few degrees. The hotspot star is placed at the centre of the Acquisition Camera, and then (known) offsets applied to move it to the Michelle or T-ReCS "hotspot" (i.e., to a location at or close to the instrument array centre but off-centre on the Acquisition Camera). An image is then taken with the mid-IR instrument to record the location of the star on the array. Next, the telescope is slewed to the User 1 star which is then moved onto the instrument hotspot. Once this star is accurately positioned on the hotspot (as judged by its position on the Acquisition Camera), the telescope is offset to the coordinates of the science target, which should then appear at the same position on the array as the bright 10 micron star. The procedure is described in detail on the mid-IR astrometry web page.

Multi-object Spectroscopy Acquisition

See the GMOS target acquisition information.

Last update August 31, 2006; Bob Blum and Rachel Mason