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(JTR MARCH 2001)





    SpeX contains an internal K-mirror image rotator which makes it a simple matter to rotate the plane of the sky on the slit rather than manually rotating the instrument on the back of the telescope as is the case for NSFCAM and CSHELL. The two uses for this are to:

    1. align the slit on the object of interest; along the central meridian of a planet or with the position angle of a double star for example

    2. align the slit along the parallactic angle to minimse slit light losses due to atmospheric refraction for 0.8-2.5 micron spectroscopy

    The rotator is moved by clicking on the
    ROTATOR icon and entering the POSITION ANGLE (defined as positive east through north). It takes about 40 seconds to rotate 90 degrees. The position angle can be entered as 0 to 360 degrees or +180 to -180 dgrees. The optical and mechanical axes of the rotator are not perfectly aligned, so any object in the centre of the slit will need to be recentered a few arcsec following a large rotation.

    In the guider (Guidedog) Data Viewer (DV) display, north is up and east is left when the position angle is 0.0 degrees. See 'How to set up the IR guider' for instructions on how to move objects around in the guider/slit-viewer field. The field of view is 60x60 arcsec and the image scale 0.12 arcsec/pixel.

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    The parallactic angle is the position angle at which the slit is normal to the horizon ie. the angle at which atmospheric refraction is along the axis of the slit.

    For spectroscopy across the range 0.8-2.5 microns atmospheric refraction can be significant compared to the smallest slit widths used in SpeX. For example, at an air mass of 1.155 (distance from zenith of 30 degress), the differential atmospheric refraction across 0.8-2.5 microns is about 0.15 arcsec - half the width of the smallest slit. Therefore, to minimise light loss the slit should be aligned to the parallactic angle. In practice, the gain by placing the slit at the parallactic angle depends on the slit width, seeing, air mass, and the difference between the parallactic angle and actual slit position angle. To take an example:

    object positioned in slit at K 0.8 arcsec seeing (IRTF median@K circa 2000) 0.3 arcsec slit 2.0 airmasses 90 degree (max) difference between slit pa and parallactic angle

    = differential light loss at 0.8 micron is about 15%

    See plots for light loss estimates as a function of these parameters. (Plots to be included.)

    Observers need to judge for themselves whether the additional overhead required to set up for the parallactic angle warrants the reduced light loss. Ask your support astronomer for advice. Flat fielding and wavelength calibration is independent of position angle. In the range 2-5.5 microns atmospheric refraction is neglible for SpeX.

    At the RA and Dec of the object the parallactic angle is automatically calculated and read by the XUI. To set the slit to to the parallactic angle, in the
    ROTATOR icon click on UPDATE PARALLACTIC ANGLE SUMMARY and then SYNC TO PARALLACTIC ANGLE to set the rotator. Once the position angle is changed the objects needs to be recentered in the slit using the guider. Remember, when nodding along the slit, the nod direction needs to be reset when position angle is changed. See 'How to set up the IR guider' for details.

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