This post is meant to compare the S/N achieved with on-slit and off-slit nodding for alpha Tau on the second night of commissioning. The on-source nodding doesn't seem to be getting the anticipated gains in S/N.
On UT 10 Apr 2014, we observed alpha Tau in high-resolution mode at central waveno = 1548cm-1. The slit length was about 10.5" and the FWHM of the source in the spatial direction was about 3.7" (recall that this is worse than night 1 because we adjusted focus to optimize for spectral focus at the expense of spatial focus for alpha CMa).
We did a sequence of on-slit nods and off-slit nods.
on-slit
atau.sci.40053.fits 8 nods
atau.sci.40054.fits 4 nods
off-slit
atau.sci.40055.fits 4 nods
atau.sci.40056.fits 4 nods
The OTPAT was N6 S12 N5 D0, for a total of 27s integration time per side.
On-slit nodded observations had 12 nods * 2 sides * 27s = 648s on-source time
Off-slit nodded observations had 8 nods * 1 side * 27s= 216s on-source time
Here's the spectra, blue for on-slit, red for offslit.
And here's the S/N versus wavenumber.
The flux density of the on-slit nodded spectrum is about 70% that of the off-source spectrum. This could be because some light is spilling off the end of the slit, or being cancelled in region between the pos-neg beam.
Anyway, S/N_onslit would then be S/N_offslit * 0.70 * SQRT(648/216) = factor of 1.2.
Instead, its only a factor of 1.03.
We also have some similar on-slit/off-slit data from night 1 with alpha Boo. I'll add it to this post soon.