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USE OF THE IMAGE ROTATOR AND PARALLACTIC ANGLE CONSIDERATIONS
(JTR MARCH 2001)
CONTENTS
- IMAGE ROTATOR
- PARALLACTIC ANGLE CONSIDERATIONS
- IMAGE ROTATOR
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:
- 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
- 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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PARALLACTIC ANGLE CONSIDERATIONS
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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