Telescope pointing and multiple sources

In this section we will discuss how to simulate the source position in the sky, relatively to the telescope pointing. In a real observation, the telescope is pointed to a certain location in the sky, where the target is expected to be, and other sources can be in the field.

Note

This pointing section has nothing to do with pointing stability simulations or with a pointing direction changing with time. Here we are only simulating the telescope ideal pointing.

The position of the target from the sky to the focal plane is handled in PopulateFocalPlane by the ComputeSourcesPointingOffset. The offset resolution is in integer multiple of the subpixel size.

Telescope pointing

In exosim the telescope pointing direction can be set in the main configuration .xml file (see General settings). Assuming we want to observe HD209458 we point the telescope to the target coordinates by adding

<root>
    <pointing>
        <ra> 22h03m10.8s </ra>
        <dec> +18d53m04s </dec>
    </pointing>
</root>

Then, we need to add the coordinates also to the star in the source description (see Sources):

<source> HD 209458
     <source_type> planck </source_type>

     <R unit="R_sun"> 1.17967 </R>
     <M unit="M_sun"> 1.1753 </M>
     <T unit="K"> 6086 </T>
     <D unit="pc"> 47.4567 </D>
     <z unit=""> 0.0 </z>

     <ra> 22h03m10.8s </ra>
     <dec> +18d53m04s </dec>
 </source>

Another important information is the channel plane scale. In fact, exosim needs to estimate the angle of view of each pixel to estimate the star position in the focal plane. Assuming the instrument has two channel: a photometer and a spectrometer, we can add this information under the detector section as

<channel> Photometer
    <type> photometer </type>
    <detector>
        <plate_scale unit="arcsec/micron"> 0.01 </plate_scale>
    <detector>
</channel>

<channel> Spectrometer
    <type> spectrometer </type>
    <detector>
        <plate_scale>
            <spatial unit="arcsec/micron"> 0.01 </spatial>
            <spectral unit="arcsec/micron"> 0.05 </spectral>
        </plate_scale>
    <detector>
</channel>

In this example, the spectrometer has different plate scales in the two detector directions.

Because we are pointing directly to the target, the star will be at the center of the focal plane:

Pointing offset

If we want to simulate an offset of the source on the focal plane, we can mode the telescope pointing. In this example we simply changed the pointing in the main configuration .xml file:

<root>
    <pointing>
        <ra> 22h03m11s </ra>
        <dec> +18d53m06s </dec>
    </pointing>
</root>

The result will be a different location of the target on the focal plane

Multiple sources in the field

Another useful case is the simulation of multiple sources on the focal plane. In this example we add two other targets. To keep things simple, we add two HD209458 stars to the field: HD209458 1 and HD209458 2. We change the star distances a little, to differentiate them on the focal plane: HD209458 1 is set at 55 pc and HD209458 2 is set at 35 pc instead of 47 pc as the original star location. Also the star locations on the sky are little changed to generate the offsets:

<source> HD 209458 1
    <source_type> phoenix </source_type>
    <path>/usr/local/project_data/sed </path>

    <R unit="R_sun"> 1.17967 </R>
    <M unit="M_sun"> 1.1753 </M>
    <T unit="K"> 6086 </T>
    <D unit="pc"> 55 </D>
    <z unit=""> 0.0 </z>

    <ra> 22h03m10.68s </ra>
    <dec> +18d53m03s </dec>

</source>


<source> HD 209458 2
    <source_type> phoenix </source_type>
    <path>/usr/local/project_data/sed </path>

    <R unit="R_sun"> 1.17967 </R>
    <M unit="M_sun"> 1.1753 </M>
    <T unit="K"> 6086 </T>
    <D unit="pc"> 35 </D>
    <z unit=""> 0.0 </z>

    <ra> 22h03m10.9s </ra>
    <dec> +18d53m04.7s </dec>
</source>

The results will be like:

For the following, it is important to separate the target source, which is the source that we expect to have an astronomical signal associated (see Astronomical signals), from the others. This can be done by adding the source_target attribute to the source description:

<source> HD 209458
     <source_target>True</source_target>
 </source>

Then the target source will be treated differently from the others. In the focal plane data product, the target source will be stored under the focal_plane group. The other sources are considered background sources and will be stored under the bkg_focal_plane group.