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)…
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.