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EFOSC2 Documentation

1 Additional info to Manual

1.1 Imaging polarimetry

1.1.1 exposure/cycle loops

• if you ask for N exposures, the template does N cycles where all HWP angles are repeated; so the N exposures of angle, say, 22.5, are separated by those of the other three angles. In algorhythm:
  • for every exposure
    • for every angle
      • take frame

1.1.2 Doing fast polarimetry

Figure 1: A window to reduce read-out time for polarimetry; the source must be centered at pixel (427, 157); not that the RTD does not have the full FITS header, so this px will correspond to another position in the RTD

If you have a single point-source, you can reduce the read-out time by taking a CCD window of dimensions 500x280 (and starting at 1,1). See Fig. 1; the read-out time of this sub-frame is 3.07 in vfastL mode (not offered), 3.28 sec in fastL (aka normal). When including all overheads (transfer to ws, etc.) the total time is 12sec for the fastL/normal mode. So basically, this is the fastest time that can be offered for linear polarimetry.

For comparison, reading out the full frame in vfastLR mode (aka fast) takes 9.71sec, and to this overheads must be added.

1.1.3 Danger, you're close to the Moon!

Figure 2: Comparison of a two-beam image without (left) and with (right) Moon

When observing close to the Moon, the scattered and polarized light from the sky increases, inducing the effect shown in Fig. 2. The background of the ordinary and extraordinary beams are very different, but it's not an instrumental problem.

1.2 Grisms 7 and 14

Figure 3: Response functions of grisms #7 (black) and #14 (red), on November 28, 2006. For each grism two standard stars have been used to define the function (Hiltner 600 and LTT 2415.

Figure 4: ADU/sec vs. wavelength for the two spectrophotometric standards Hiltner 600 (top spectrum) and LTT 2415. For each standard the two spectra have been taken with grisms #7 and #14.

To clarify the difference between grisms #7 and #14, on 2006/11/28 two standard stars were observed, with both grisms, and in photometric conditions. The response functions are shown in Fig. 3, and allow to understand the differences between the two grisms. The conclusion is that grism 14 is slightly more efficient near the maximum ($2.6\%$ more than grism 7), but grism 7 extends a bit more to the red (ca $100$ Å). A similar conclusion can be reached by looking at Fig. 4, where the instrumental spectra of the two standard stars are shown.

Send comments to : ls-spectro Last modified: Sun Dec  3 03:05:03 CLST 2006