For a routine imaging observation the acquisition involves slewing to the target, acquiring the guide star, finding the target in the T-ReCS field of view, 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 to the center of the field view.

In some cases, especially in observing standard stars which are point sources to T-ReCS, the astronomer carrying out the observations may chose to not bother moving the target to the array center. PIs can also skip the acquisition step if they so wish. This will save a small amount of setup time provided the object is found in the T-ReCS field of view. On the other hand it will cause a loss of time should the astronomer find that the target is poorly positioned.

To do imaging acquisition it is necessary to have an acquisition step defined. This must be a separate acquisition observation.

It is possible to skip the acquisition step for simple imaging observations if the target is a point source and if one does not care where the target ends up on the detector. Our experience from randomly selected standard stars and from queue observations without acquisition steps taken is that an object with a good position given in the OT file will appear somewhere in the central part of the T-ReCS field of view unless it has a significant proper motion, usually within a radius of 3 arc-seconds of the center of the field. This depends somewhat upon possible errors in the guide star position and the raw pointing accuracy of the telescope. If this is sufficient for the science program to be done, no acquisition step need be defined.

The Acquisition Step

In the OT the acquisition step is a single observation that is done with the following parameters:

• dataMode: "Discard All"
• obsMode: either "Chop" for brighter targets or "Chop-Nod" for fainter ones
• timeOnSource: set to some fairly big value such as 600 seconds--note that the observation will be aborted long before this time is reached
• filter: in most cases, should be same as the filter for the first step after acquisition
• timePerSaveset: when "Chop" mode is being used, set to a small value such as 1 second, otherwise take a value such as 10 seconds
• nodDwell: optionally this can set to a shorter value than the default, for example 10 seconds, if acquisition is to be done in "Chop-Nod" mode.

In the above the division between a "bright" source where "Chop" mode is used and a faint source where "Chop-Nod" mode is used is roughly 0.2 Jy in the N-band if using either the N filter or the Si filters, while the value would be higher when using the narrow-band filters. In the Q-band the brightness has to be of order 2 Jy because the sensitivity is about a factor of 10 worse than for the N filter. If observations have to be made at Qb the threshold is higher still by a factor of 2 or 3. These are conservative values. We can try on targets a bit fainter than this.

The following OT image shows an imaging observation with an initial acquisition step.

Fig. 1 (click for high-resolution version)

Other Acquisition Options

It is possible to peak up on a K-band, L-band, or M-band target and then take images in the N or Q windows. Due to the intrinsically poor detector sensitivity at wavelengths below 6 microns this is not normally going to do much good. For blind pointing one should pick an astrometric target as close to the source field as possible and carry out an astrometric observation to center this target, followed by slewing to the field. No T-ReCS acquisition step would be needed in that case, but the astrometric acquisition step would be charged to the program. Astrometric "HOTSPOT" standards must have accurately known positions and be bright enough to be easily detected by T-ReCS in (say) the N-band filter. For more information see the MIR Astrometry page.