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T-ReCS Example Observations
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.
Optional Acquisition Step
In most cases, imaging with T-ReCS does not require an acquisition step. Very occasionally, (ie. extended objects that fill the field or require precise placement on the detecotr) one may wish to acquire. 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.
The following OT image shows an imaging observation with an initial acquisition step.
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.
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.
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.
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.
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.
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.
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:
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.