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P2PP: VISIR Information

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  • Imaging in burst-mode is offered in visitor-mode only. Sensitivities in the burst mode are similar to those in regular imaging (although with superior control of the PSF), but with significantly increased overheads in terms of observing time.

    • Table of Content

    Introduction

    This page contains information specific to the creation of Observations Blocks (OBs) for VISIR Service Mode programmes. For more general OB creation information and rules, see the P2PP page or the Service Mode Guidelines for Phase 2 preparation.

    Requirement Compliance Policy: Observing runs which do not adhere to procedures and policies presented in this document will not be scheduled for execution. If you feel you must violate one of these procedures or policies, you must submit a Phase 2 Waiver Request before submitting any Phase 2 material.

    Read the Manuals!

    It is absolutely essential that users read the following manuals: The manuals above are listed in a hierarchical way - the most general p2pp manual on the top and the more specific below. Similarly the instrument specific instructions on this VISIR service mode WEB page are supplementary to the more general Service Mode Guidelines page.

    Further useful information and tools for the OB preparation phase can be accessed from the User Support Department's Web pages (such as links to the Exposure Time Calculator, object visibility, catalogues, etc.).

    Observing Modes

    VISIR is offered in the following two basic instrument modes:
     
     
    IMA  imaging 
    SPEC  long-slit and cross-dispersed spectroscopy

     

    Imaging is available for two field of views (19.2'x19.2' and 32.3x32.3') for various broad and narrow band filters in the N- and Q- band. Long-slit spectroscopy is offered in low-, medium-, and high-resolution for specific central wavelength settings in the N- and Q- bands. Cross-dispersed spectroscopy is offered for specific central wavelength settings in the N- and Q-band.

    Refer to the VISIR instrument page which configurations, filters, scales, slits and central wavelengths are offered in detail.

    Classification Rules for Service Mode OBs

    General Policy:

    Quality Control of OBs executed in service mode will be mainly based on airmass and seeing constraints as well as sensitivity estimates obtained from standard star observations during the observing night.

    Optical seeing will be used to classify the seeing constraint requested in the observing proposal for a specific OB. The resulting image quality in the mid-infrared image is better than the optical seeing. As a rule of thumb, optical seeing between 0.6-0.8" will result in diffraction limited images in the N and Q band. A measured image quality of 0.3"(~10μm) and 0.6"(~20μm) will always satisfy the seeing constraint.

    Atmospheric classification like PHO/CLR/THN/THK are not necessarily related to the photometric stability and sensitivity in the mid-infrared. The main guideline to classify a specific OB will be therefore based on photometric observations of mid-infrared standard stars, frequently monitored during the night. The sensitivities guaranteed by the observatory are listed in the corresponding section of the VISIR instrument page.  The OB will be considered to be fully within constraints, regarding sensitivity, if an observation of a standard star at zenith in the same filter as the service OB results in a equal or better limiting flux as published ("grade A"). If the limiting flux obtained with standard stars exceeds less then 10% these published sensitivities in the same filter, the OB will be considered as "grade B", while the OB will be classified as "grade C" in all other cases. An analog classification scheme will be applied for spectroscopic observations.

    Moon constraints are usually irrelevant in the mid-infrared and are not taken into account for the classification of the OB. Telescope guiding and active optics can, however, under certain circumstances be compromised for moon distances <30deg.

    Guide stars with VISIR

    Sensivity in the mid-IR for a ground based observatory is strongly limited by the sky brightness. In addition, the VISIR field is small compared to other VLT instruments. Therefore images of a field can often appear empty in short to medium length exposures. However, objects may become visible in longer ones. Combining different exposures taken on different nights may be tricky if a proper astrometric alignment is not carried out. Since the overall astrometric accuracy of an image is actually limited by the accuracy on the coordinates of the guide star, it is strongly recommended that all OBs of a same field use the same guide star, in particular for faint objects. In addition, objects within (optically) dark molecular clouds may have few or no suitable guide stars, at least in the catalogues currently used by the Telescope Control System: the Guide Star and the USNO-A1 catalogues. Alternatively, some bright nebulae may saturate the region of the sky surveys used by the telescope operator to find guide stars. Considerable amount of telescope time will be saved if such cases are identified before an OB is executed. In these cases, examination of other catalogues, such as the USNO-B1.0, may provide suitable guide stars. Providing the coordinates of a guide star in the acquisition template of an OB is therefore strongly recommended in a number of situations: In all these cases, the use of the guidecam tool for VISIR is strongly recommended and the coordinates of a suitable guide star should be inserted in the acquisition templates.

    Finding Charts

     

    The  finding chart plugin to SkyCat provides a convenient way to prepare ESO-compliant finding charts.

     

    VISIR Pipeline