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Long-slit Spectroscopy

FLAMINGOS-2 has a number of permanent long slits available for long-slit spectroscopy (see the MOS wheel schematic for the locations of these long slits within the wheel). The widths of the long slits are 1, 2, 3, 4, 6, and 8 pixels. They are all ~4.4' (263", 1460 pixels) in length. These long slits can be used with either the moderate or higher resolution grisms. The f/16 and f/32 pixel scales in spectroscopy mode are the same as those in imaging mode.

The available slits are shown in the following table. The slit widths are given in pixels and arcsec projected on the sky. The spectral resolution is lower in a wider slit (see the grism table), even for point sources since under most atmospheric conditions the seeing wings of pointlike objects fill the wider slits.

 

Slit width
(pixels)
Slit width
(arcsec)
1 0.18
2 0.36
3 0.54
4 0.72
6 1.08
8 1.44

Offset strategies


The standard strategy for point like or compact objects involves the use of ABBA offsets along the slit, represented in the Observing Tool sequences as "Q" offsets with amplitudes of 10 to 60 arcseconds depending on the requested conditions and target angular size. In the Observing Tool the use of the Flamingos-2 Iterator or the Repeat iterator before the ABBA offsets, allows to keep the sequence after each iteration (e.g. ABBAABBAABBA).


However, different approaches are possible: Flamingos-2 does not provide frame coadds, therefore when observing bright objects the exposures can be repeated a few times in each offset node, in order to save overhead time (i.e. AAAABBBBAAAABBBB).  After a couple of minutes of short spectral exposures in the same node, the telescope is then moved to next node. Other strategies involve using ABABABABAB offsets because the sky contribution is modeled and uniform time sampling is needed at each node.


Any offset strategy must take into account that the sky emission lines can vary significatively in scales of 2 to 5 minutes. 

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