Contents

Focus maintenance system

Auto-focus

Calibrating autofocus "fine focus offsets" for the instruments

NB: the latter includes a standard protocol for measuring the offsets.

Focus Maintenance System

All structures change shape and dimensions with temperature and attitude shifts, and this is true of UKIRT. Finite Element Analyses carried out at the UKATC (then ROE) showed that elastic length changes of up to 1.0 mm occur between the zenith and an altitude of 30°; a length change of 0.5 mm occurs when the telescope steelwork changes temperature by 5°C, as can readily occur in the first half of a night. Such shifts are hundreds of times larger than the tolerances allowed by the performance goals.

UKIRT is accordingly equipped with a simple model which corrects for these effects using the attitude information from the Telescope Control System and temperature information from sensors attached to the truss steelwork. Position corrections are sent to the hexapod secondary mirror positioner whenever the expected length change exceeds 1 micron for more than 10 seconds.

Auto-focus

The autofocus system complements the focus maintenance system and allows instrument focus settings to be checked during observing far more rapidly and accurately that is possible with a standard "through-focus" run with an imager or spectrometer.

How autofocus works

The longitudinal position of the image formed by the front lens on its own depends on the overall focus of the telescope. Consequently the diameter of the converging pencil in any plane between the lens and its image is a measure of the telescope focus. The four sub-images are formed from sub-pupils of the converging cone from the single lens, so the radial separation of the four images is also a measure of the overall telescope focus setting. In auto-focus mode the four images are sensed by a 24x24 array of pixels, which are again binned up 3x3, now into 8x8 superpixels forming four (rather than one) 4x4 guiding arrays.

The larger readout area slows the read process somewhat: while in normal guide mode the read rate can be 100 Hz or more, in autofocus mode the image positions are sensed at 60 Hz. The position references are the four centrepoints of the central quartets of pixels, and for each readout of the array the displacement of the centroids of the sub-images from the reference positions is computed. The average radial component measures the current defocus. The X-Y component averaged over the four sub-images measures image movement, just as in normal fast guiding, so that tip-tilt correction is still available, with a closed-loop bandwidth of ~6 Hz.

Averaging periods in autofocus

The 60 Hz measures are averaged over a specified interval, during which the RMS of the focus fluctuations ("Z_rms") is also determined. Because the 4x4 sensor is non-linear at large excursions, such as might accompany a change of instrument (see below), when autofocus mode is initiated the 60-Hz focus corrections are averaged, and then applied, over consecutive periods of 2, 4 8, 16 and 32 seconds, first to facilitate convergence and then to allow seeing-induced fluctuations to be averaged out. (The 32s averaging is then repeated indefinitely.)

Back to Acquisition & Guiding, or to "Other guider modes".

Calibrating autofocus for the instruments

1. Autoguider fine focus

   Each scientific instrument has a slightly different optimum telescope focus setting. As we have seen the guider in autofocus mode can only correct the telescope focus to bring the images into coincidence with the reference points on the CCD. The single and quadruple lenses are both carried in the same lens wheel, which can be moved towards and away from the CCD. This adjustment is called "Fine Focus" on the Bottom-End Control screens and is a measure of the lens wheel position (in mm) relative to an arbitrary zero.

   The lens wheel can be translated. This adjustment is called "Fine Focus". A movement of the lens wheel towards or away from the guider CCD will change the radial spacing of the images on the CCD. Thus, if the "Fine Focus" setting is changed while the Fast Guider is in autofocus mode, the result will be a change in the Z position of the secondary, so as to bring the four images back to their reference positions. The change in secondary position then causes a change in the focus at the instrument.

   A series of measurements with a science instrument at different autofocus Fine Focus settings can then be used to determine the optimum setting of the Shack Hartmann position (Fine Focus), i.e. that which gives the best working focus for the instrument. This measurement is made for all the facility instruments, with and without relevant accessories (FPs, etc.) according to a standard protocol. Current defualt estimates for the optimum fine-focus settings are given here.

   NB: The IRPOL polariser module, being above the dichroic, is in both the instrument and autguider beams and its focus shift will be the same in both beams (modulo the difference in refractive index of its material in the visible and in the IR). Differences between the optimum autofocus fine focus settings with and without IRPOL should therefore be small. This is not the case for, e.g., the Fabry-Perot etalons, however, as these effect only the focus of the IR beam, so that a new focus determination is needed.

   
   

2. Different Fine Focus for autofocus and normal guiding modes

   The lens combinations used in "normal guide" and "autofocus" guider modes have different effective focal lengths: the "autofocus" lenses have their combined focal plane nearer to the lens wheel than that of the "normal guide" lens. Consequently different Fine Focus settings are used for autoguiding and for normal fast guiding. The settings for "normal guide" have no effect on the telescope focus and are selected simply to give the best Fast Guider performance.

Back to Acquisition & Guiding, or to "Other guider modes".