Target Acquisition

Field Centering

The physical centres of the various imaging arrays are not necessarily aligned precisely on the optical axis. Normal procedure is therefore to first place telescope's nominal pointing centre on the target (i.e., "blind pointing") and then issue a "Focal Plane" command which offsets the centre of the instrument field of view onto the requested coordinates (subject to the typical pointing error of around 10 arcseconds). While this is perfect for the imagers, more precise acquisition is required for the spectrometer.

Acquisition on the spectrometers may be achieved in two ways. A sky image is always first obtained using RATCam centred on your target position. That image is then used either to offset the telescope to fine tune the pointing and place your specified target position (+/- 2arcsec) on the fibre input array or you may request that the telescope simply centres up on the brightest star in the field. For more details see the FRODOspec instrument page.

Cardinal Pointing

Due to flexibility problems with the coolant pipes, we were forced to dispense with the practice of routing them through the cable wrap in the cassegrain axis rotator.

For most observations the rotator is therefore to one of the "cardinal" sky angles of 0, 90, 180 or 270 degrees. This means that images will be rotated to one of these angles, the choice being the one that will give the longest possible observing time before reaching a mechanical rotator limit. It is possible that the sky angle will change during a (very) long series of exposures - but not during an exposure!

Specific control of field rotation is available for cases where the science requirements of the programme demand it, but in general we recommend leaving the rotator on its default automated setting.

The combination of these two changes means that the overlap between repeated images of the same field with the LT is now maximized, which is important for programmes relying on secondary standards within the fields. As a side effect, it also means that a standard "North at the top" image orientation is now simple to achieve in an image viewer, using only 90 degree rotations and "flips".