For a routine spectroscopic observation the acquisition involves slewing to the target, acquiring the guide star, centering the target in the slit, and waiting for the primary mirror tip/tilt corrections to be derived. Aside from making sure that proper guide stars have been selected, the only step in this process that involves T-ReCS is getting the object into the slit.

For spectroscopy observations the acquisition steps are required for each observation: each wavelength range and each slit position angle. In each case the target has to be re-centered on the slit. For long spectral observations periodic re-centering may be needed as well. Each such acquisition takes of order 15 minutes overhead.

The image below shows how the main T-ReCS sequencer is set for a spectroscopic acquisition of a brighter target. It consists of four steps:

• imaging the slit to locate it on the detector;
• imaging the field to locate the target and move it to the slit position;
• carrying out a short image the field for the PI; and
• imaging the field through the slit for the PI.

The first step does not take long and is aborted once the slit position on the detector has been marked. No data is saved from this step. The second step is analogous to the imaging acquisition step, an exposure to allow the operator to offset the telescope as required to move the target to the proper position. This step also is aborted once the target is in position and no image is saved.

The next two steps are to take images that are saved for the PI, so the PI will know where the slit is positioned. First an image of the field is taken and saved. How long this needs to be depends on how bright any source in the field is. Then the slit is put in again and an image of the field through the slit is taken.

If the target of the spectroscopy observation is invisible in the N-band or the Q-band then spectroscopy is probably impractical. In principle we can peak up on a source in the K, L or M filters and proceed with spectroscopy from there. In practice due to the poor quantum efficiency of the T-ReCS detector at shorter wavelengths this is unlikely to work.

Fig. 1 (click for high-resolution version)

If one wishes to locate the slit on something that is best detected in a filter other than the N-band filter or the Q-band filter (whichever is appropriate for the spectroscopy) then the offsets caused by changing filter must be allowed for. This can be done but is more difficult than an N-band setup, and there is likely to be a larger overhead in setting up the observation. The filter to filter offsets can be measured using a star. For the moment, since we have no data about whether these shifts depend on the telescope elevation, we strongly suggest that such observations be done if this type of setup is needed. All such cases should be discussed with the contact scientist.