UIST IFU
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Spectroscopy: Target Acquisition
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Imaging acquisition of IFU sources
IFU targets are acquired in essentially the same way as
long-slit spectroscopy sources. Remember to use short exposures
and a few coadds rather than one long exposure to avoid latency
affects. This issue is particularly important with the IFU, since
almost all of the array is used for the spectroscopy.
By looking at a raw IFU image, it is very difficult to figure out
whether a source is well centred within the IFU 3.3"x6.0"
field-of-view. For example, consider the following (hope you've had
your coffee)...
| Before Centering | After Centering |
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| Raw Spectral Image (above) | Raw Spectral Image (above) |
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| Scrunched Spectral Image (above) | Scrunched Spectral Image (above) |
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| White Light Image (above) | White Light Image (above) |
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| Above we show the effect of moving
the telescope "up 1.5 arcsec" and "left 1.5 arcsec". In the
left-hand images, the target is off-center, though this is only
apparent in the reduced data (center and bottom); the "scrunched"
spectral image (where the IFU slices have been wavelength-calibrated and
re-ordered so that the
slices -- from top to bottom -- span the source region -- from right to
left) and the "white light image" (where the IFU data cube has been
collapsed along its dispersion axis).
The main target is
clearly:
1) Too far to the left in the IFU field, so it is only
covered by the left hand "columns" in the white-light image, and it
only appears in the lower IFU slices in the "scrunched"
spectral image.
It is also:
2) Too low in the IFU field, so the
spectra in the scrunched image are low in each of the
individual slices in the "scrunched" spectral image.
Note that a second, fainter star appears (at left) in the IFU field
after shifting the telescope. This secondary source is evident in the
scrunched and white-light image. The above data were obtained with a
position angle of 68 degrees. [The axes in the white-light images are
in arcseconds.] |
If you can't see the star in the white light image in the Gaia
display, try zooming in with the Z button, or changing the
autoscaling (under the "Auto Cut" pull-down menu). Alternatively, you
could try your favourite display tool, after converting the
file to fits with the convert
package.
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