Examples of T-ReCS Observations

This page gives a basic discussion of T-ReCS observations as defined in the OT and should be read in conjunction with the T-ReCS OT details page. Example observations are available in an OT library file that can be fetched from the Observing Tool database.

Simple Imaging Observations

The following figures show how the T-ReCS component in the OT will look for (a) a simple imaging observation for a brighter target with the default position angles for chopping and for the detector array, and (b) an observation that needs to be taken with the specific position angle of 80 degrees. The way this last observation appears in the Position Editor is shown below.

Fig. 1 (click for high-resolution version)

Fig. 2 (click for high-resolution version)

The values that most commonly need to be set are the "Filter" and the "Total On-Source Time". The latter corresponds to the usual idea of the exposure time. For a point source or for a small extended target, say less than 5 arc-seconds radius, one does not need to change the "Chop Angle" or the "Chop Throw", unless the source is bright (say more than 10 Jy) in which case it is better to change the "Chop Angle" to 45 degrees.

If one wants to have the "negative" beams on the detector, one can change the "Chop Throw" to a value of, say, 10 arc-seconds. One can also change the "Chop Angle" to 45 degrees. Since the telescope is not guiding in the off-source chop position, these negative images will not be suitable for combining with the "positive" image. If all one is interested in is aperture photometry one may be able to use the negative images but this should still be approached with some caution.

The Position Window

The following image shows the T-ReCS position window with the PWFS2 field of view shown. Here a catalogue search has been done to find a guide star, and a particular guide star has been chosen. See the guiding options page for more details about WFS use with T-ReCS.

Fig. 3

The next image shows the Position Editor view zoomed into the source region. This corresponds to the imaging setup displayed in Fig. 2. The target, planetary nebula NGC 6302, has a bipolar axis just off of the east-west direction. The observation places the long axis of the T-ReCS detector along the central "waist" of the bipolar nebula, nearly in the north-south direction. The chopping is arranged to be along the bipolar axis, on the assumption that the mid-infrared emission peaks on the waist and is smaller along the lobes.

Fig. 4

Simple Spectroscopy Observations

When spectroscopy is carried out the setup is somewhat different than that for imaging. A single step spectroscopic observation in the N-band would have a T-ReCS main component set-up similar to the following:

Fig. 1 (click for high-resolution version)

Items to note are the choice of the Disperser, the Focal Plane Mask, the filter (almost always N-band for LowRes10 or HighRes10 observations, and almost always Q-band for LowRes20 observations, although in rare circumstances a different filter might be chosen), and the Chop Angle. For a point source one might wish to chop along the slit. It is also possible to chop less than 15 arc-seconds along the slit and thus have the two "negative" spectra in the frame, although it is not clear how to use these negative spectra since the observation is unguided in the off-source beam. For an extended source one would normally chop perpendicular to the slit so that one is not chopping the source spectrum onto itself.

The next image shows how the spectral set-up of an extended target would normally look in the Position Editor. The slit is positioned along the waist of NGC 6302, and chopping is done at right angles from the slit direction.