Pointing & Co-pointing
[under construction…]
Why pointing and co-pointing matter
On traditional (one-eyed) telescopes one does not have to worry about co-pointing at all, and the pointing and collimation are well understood such that one can often “just” observe. Our asymmetric telescope design and extremely fast optics make the LBT more sensitive to thermal effects and relatively rapid drifts in collimation. The telescope metrology system (TMS) should maintain collimation as the telescope slews from one object to another, however it is still necessary to correct pointing and copointing after each slew.
With the LBCs’ large ~25 arcmin x 27 arcmin field of view, one may wonder why pointing and copointing matter. However, both the telescope tracking and LBC rotator tracking are based upon the assumption that the coordinates at the LBCs’ centers of rotation are accurate. Properly pointed and co-pointed, the telescope pointing control system (PCS) will accurately generate (1) the tracking polynomials that drive the telescope mount to follow the field, and also (2) rotator trajectories which it will send to the LBCs to drive their rotators. Even unguided, the telescope and LBC rotators will move so as to perfectly maintain the position of the field on the detector…
But that is not the case. Our sensitivity to thermal changes means our pointing and co-pointing can drift off this just-corrected position, by up to several tens of arcseconds over the course of an hour or less. This would result in differential field rotation, where the sources in your science images follow a circular path with a radius equal to this drift off the position the mount is tracking, and its effect will be greatest when the parallactic angle changes the fastest (meridian crossings at high elevation). The rate the sources cover this circle is proportional to the change in parallactic angle over the exposure, so the sources will trail over an arc of length r*Theta (radius in arcsec times the change in parallactic angle in radians).
Guiding can correct for drift in the mount (though only during an exposure – see LBC tech chip guiding) but not for errors in the rotator trajectory. It will correct the trailing on the right side of image, closest to the tech chip #1, while at the same time worsening the trailing on the left side of the image. Figure 1 illustrates this effect on an 173 second LBCB image obtained at 85.6 deg elevation.
Figure 1: Rotational trailing on a field at elevation 85.5 deg. This is most easily seen in the whisker plot on the left, courtesy of C. Veillet, though it can be discerned by zooming in on various regions of the image itself. The first place to look is the lower left corner, since it is furthest from the guide chip, and then a look at the upper left corner confirms that the elongation is due to rotational trailing. Near the rotator center, the images appear slightly elongated up-down.
Correcting pointing and co-pointing
Pointing and co-pointing should be corrected near the target after each slew and, for a field that is being tracking from low to very high elevation, before the elevation gets too high (e.g. just before the field reaches ~80 deg). This is achieved by preparing a “co-point” OB that provides a suitable coordinate reference source near your science target as well as multiple stars usable for collimation. See Co-pointing OBs for details.
The pointing and co-pointing corrections are achieved by moving the mount (changing the pointing model terms IE and CA) and by tip/tilt corrections to each of the primary mirrors. The optics corrections are made in such a way that they preserve collimation and the mount and optics corrections are calculated to maximize the available range to the optics.
