Co-pointing & Collimation
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. Until these effects are better understood and modeled, the best approach to observing with LBC will be to locally correct the collimation, pointing, and co-pointing just before taking your science data. 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.
Properly pointed and co-pointed, the telescope pointing control system (PCS) will accurately generate the tracking polynomials that drive the telescope mount to follow the field. However, 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. 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 this trailing if it happens slowly enough. You are thus most sensitive to this positional drifting in the pointing and co-pointing when the parallactic angle changes the fastest (meridian crossings, especially at high elevation) as well as when the telescope is out of thermal equilibrium (just after opening of the dome, as well as changes in the weather).
Adjusting the optics to correct low order aberrations by analyzing the pupil shapes of multiple stars. See LBC Active Optics. Correcting the pointing and co-pointing immediately before and very near to your science observations will minimize the guiding corrections that would be necessary and produce better images.
Dohybrid or Dofpia
The collimation script takes the yourCoPoint.ob preset. Collimation needs to be done before the pointing correction as the collimation process for LBC is not done in a pointing-free manner in order to preserve range of motion of the primary mirrors. Use the IDL procedure dohybrid (or dofpia) to iterate on the collimation using extra-focal pupil images obtained by driving the primary mirrors -0.8mm down (away from LBC). Note that dohybrid (or dofpia) assumes that your collimation field contains only uncrowded stars and the background is reasonably uniform. Fields containing any significant nebulosity or a large galaxy are unsuitable. The script takes an exposures with both cameras and reads out only the top ~5 arcmin of chip 2, finds relatively isolated pupil images of sufficient signal-to-noise ratio and derives corrections to the wavefront. After waiting for the corrections to be applied, dohybrid will check the corrections versus convergence criteria and iterate as necessary. Details of running dohybrid can be found here.
Verifying that each primary mirror is pointing at the same target, using a bright, easily identifiable source.
Once collimated, re-send the yourCoPoint.ob and allow it to complete. An iraf script is available in the LBTtools/Observe package called “lbcrangebal” to do the optimal pointing and co-pointing corrections while maximizing the range available to the optics (hence the name “lbcrangebal“). This script takes as input the LBCB and LBCR filename time-stamps for the images taken in the co-point OB. It will read from the telescope control software two current pointing model parameters (IE and CA), and from the image headers the pixel coordinates of the mechanical rotator axis’ intersection with the telescope focal plane for both cameras. Tip-tilt requests to the two primary mirrors and an offset to the pointing model are calculated in a manner that maximizes the available range of the primary mirror while simultaneously correcting any common offsets from the pointing model with an offset to the mount pointing model. Details of running are available in Observing Procedure –> Co-pointing.