Single Segment Guiding

Single segment guiding is a way of obtaining slit spectroscopy without using either on-slit guiding or using a second star that lies in the same guider field of view as the science target.

In order to accomplish this, we use a unique character of a segmented primary mirror; the ability to take one of the segments out of the stacked image, thus creating a “satellite” image of the science target. This satellite image can then be used as a guide star, with some modifications to normal procedure. This seems to be most relevant to HIRES, where the bright science targets, the small guider field of view, and the ever present slit (!) can combine to prevent good observations of some targets. Observers have been known to skip observing some of their targets that disappear almost entirely down the slit, leaving nothing to guide on.

There may be an additional benefit. Even if enough light spills out of the slit to allow on-slit guiding, off-slit guiding on a satellite image may produce higher quality data. Tests still need to be designed and carried out to determine how much benefit, if any, this provides.

Procedure

  1. Kick out a segment.
    The OA should do this on the ACS CLI, the window that allows ACS commands to be entered. The idea is to move one actuator enough to provide the satellite image. In our tests we chose an inner segment, with the idea that the inner segments are already partially vignetted, so light loss in the main image would be minimized. The choice is really fairly arbitrary, although we think that an inner segment (1 through 6) is more appropriate, since they are already vignetted.

    On the ACS CLI, type:

    tilt segment 1 3 0

    The parameters are: segment number, segment tilt in arcsec in the X direction (elevation), and segment tilt in arcsec in the Y direction (azimuth). Note that the segment tilt is half the distance on sky that the segment’s image will move. So in the example above we tilt segment 1 by 3 arcsec in the azimuth direction, moving it’s image 6 arcsec on the sky. (In many cases the slit will be vertical, so a tilt in azimuth will move the segment image perpendicular to the slit.) Note that the tilt segment command is absolute, so typing tilt segment 1 3 0 a second time will not move the segment farther from the main image.

    Also note that when a segment is tilted the ACS sensor gains will change, leading to some degradation in control. It is expected that this will be small. However, some restraint in how far you throw the segment should be considered.

    Finally, note that the ACS will take about 30 seconds to settle. You may seem the stack fall apart momentarily, so beware of performing this step during an exposure.

  2. Set differential guiding parameters
    The two parameters which define differential guiding mode will be different for single-segment guiding than for normal guiding. This is because the segment will not follow the stars, but will follow the pupil, which rotates at a different rate, The two parameters are (sEL, sPARANG), which define the guide behavior as a function of change in elevation and change in parallactic angle, respectively. The values of (sEL, sPARANG) also depend on the rotator mode and focal stations, as follows:

    Table 1. Differential guiding parameters vs. rotator mode and instrument for single-segment guiding.

    Rotator Mode
    Instrument
    Differential guiding parameters
    sEL
    sPARANG
    VERTICAL
    all
    Turn differential guiding off
    POSITION ANGLE
    all
    0
    –1
    STATIONARY
    NIRC, LWS, ESI, LRIS
    (any Cassegrain instrument)
    Turn differential guiding off
    DEIMOS, HIRES, NIRC-2, NIRSPEC, OSIRIS
    (any Nasmyth or bent Cassegrain position)
    –1
    0


    You change (sEL, sPARANG) on the xguide GUI. Turning differential guiding off is equivalent to setting both parameters to 0.

  3. Center the star’s primary image and start guiding on the satellite image.

Visual Cues

While watching the stars and segment image on the guider, the main thing you are concerned about is that the star’s primary image stay fixed in the middle of the slit. If this happens, you are guiding properly. There are, however, some useful cues that can indicate problems and their solution: