OSIS Active Guiding

Introduction

The guiding system in OSIS computes the instantaneous centroid of a given guide star and corrects for its random motion at frequencies up to ~20 Hz. The OSIS active guiding is rarely used in Infra-Red observations, as its impact on the image quality is much lower than in visible.

Principle

The OSIS active guiding can be used in three different ways, one being the normal mode of operation, the others alternatives which could be used if needed.

• The normal way of using active guiding with OSIS is to correct the slow drifts in telescope tracking with the Telescope Control System (TCS) and then correct, to first order, the fast random motions due to atmospheric turbulence with an independant tip-tilt mirror in front of the instrument. The electronics associated with the active mirror directs the low frequency (<0.3 Hz) error signals to the TCS for correction.
• The tip-tilt mirror only mode allows compensation of both low and high frequency motions with the tip-tilt mirror alone. It can be used only for short exposures. In the case of a noticeable drift, the mirror would progressively deviate from its mid-point position and would rapidly reach the limit of its movement (the maximum amplitude is a few arcsec). It is useful in the event of failures with TCS.
• The "no TCS-OSIS link" mode leaves the slow corrections to be computed independantly with the Cassegrain bonnette guiding system (acting on another guide star),the OSIS active guiding taking care of the rapid motions, as well as an eventual drift between the CAss guiding ond OSIS. This latter drift means that the OSIS guiding should start with the guiding star close to the center of the guiding. The result of this combination is as good as the normal mode, but should be considered only as a back-up mode in case of problems with the communications between OSIS and TCS.

Tip/tilt mirror and APD

Figure 3 shows the guide probe window displayed when the "GUIDE-PROBE" form is activated. Since the guide probe location in the focal plane versus position on the detector is well known. When you enter the detector coordinates of a star, measured on a previous exposure, into the guide probe window, the probe will move to that position when you click on the button Move Probe to Guide Star. To maintain this mapping accuracy, the procedure CALIB must be run after each mounting of a detector. This is done by the observatory staff during the set-up.

FIGURE 3: The guider window

Figure 4 shows the available domain for the guide probe motion, compared to the science field with the (now-defunct) CCD, STIS2 (2048 x 2048 21 µm pixels). Figure 5 shows a typical science image, again with STIS2, showing the vignetting caused by the guide probe.

The accessible field is about 3' x 4.5'. The scale for the probe motion is: 1 step = 7.5 µm in the focal plane. The field of view for the probe is 3", divided in 4 quadrants. The "garage" position reads X = 4000; Y = 6200 for the probe coordinates. When moving the guide probe to a star, allow some time for the display of the star position in the window to update or look at the oscilloscope which gives a real time display.

The present guide probe is quite large, and there is no way with to avoid a shadowing effect of the guide probe on the field. For some programs doing multi-slit spectroscopy, locating the guide probe in the science field may be tolerated. On the other hand, it is better to minimize the occultation for direct imaging programs (see Figure 5). This can be achieved with the X coordinate of the guide probe close to the maximum value of 4000. We recommend that observers choose guide-stars for each of their fields in advance. Sometimes, a slight decentering and/or bonnette rotation may also be needed in order to minimize occultation. The size of the obscuration region may be seen in the following schematic:

                     A     B     C
no obscuration      68"   55"   81"
full obscuration    51"   42"   63"

             +--------------+
            /               |
 ----------+                |
    ^                       |  ^
    B          X            |  A
    v                       |  v
 ----------+                |
            \               |
             +--------------+

             |<----- C ---->|

FIGURE 4: Guide probe domain

FIGURE 5: Science image with Guide Probe

Performance

The OSIS guiding system was tested on various stars after the change of detectors (from photomultipliers to avalanche photo-diodes). In the field of H1413+117 (a typical high galactic latitude field) only 3 stars were bright enough for guiding attempts. Below are listed their R magnitudes and measured total fluxes (in counts/s above the sky level of 2800).

Guiding on brighter stars is more practical and allows a range of integration frequencies up to 100 Hz. The gain in image resolution, versus Cass bonnette guiding, is of the order of 0.1"- 0.2" (typically, 0.5" - 0.6" versus 0.7") but we do not yet have a large enough body of statistics to study the dependance of image improvement on guide-star brightness.

We observed image improvement relative to the Cass bonnette guiding for the first 2 stars with a guiding integration time 0.1 s and still for star 40 with an integration time of 0.2 s. However, this corresponds to the practical limiting magnitude; the S/N ratio in each quadrant was only ~2.3 with this integration time. A practical rule for choosing a suitable guide star is to have at least 1000 counts/s above the sky background, which corresponds roughly to R = 18.0 and V = 18.5.

The degree of image improvement also depends on the turbulence regime when you observe. We can, however, recommend the following values for the integration time of the tip-tilt mirror command:

• 100 ms integration can be used if the net flux (above sky) is at least 1000 counts/s;
• 30 ms integration can be used above 3000 counts/s;
• 10 ms can be used above 10000 counts/s.

This assumes a dark sky (sky background around 3000 counts/s). During grey time, adjust the integration time in order to have at least S/N = 2 for each quadrant. The best images obtained to date have image qualities a little below 0.4", which is similar to HR Cam performance. Long exposures (~30 min) show resolutions similar to the short (30 s) exposures, and demonstates the good reliability of the stabilizing system. Another point, important for the multi-slit spectroscopy mode, is that the image position is very stable on the CCD. For a given field, the centroid of a star moves by less than 0.09" in 2 hr. The flexures between the guide probe and the detector are thus negligible.