Spectroscopy

A new 40-arcsec wide, extra-long slit was designed and fabricated in collaboration with Raine Karjalainen (ING) and Ernst de Mooij (Queens, Belfast). The slit is used for accurate, low-resolution spectral measurements of planet transits.

The blue VPH grism #18 was recommissioned after it was sent back to, and re-aligned by Andrea Bianco and Alessio Zanutta (Merate, Italy) to account for the original too red transmission response. For VPH grisms the throughput as a function of wavelength is determined by accurate alignment of the internal components (prisms and VPH grating), with respect to themselves and with respect to the holding barrel. Re-alignment at the 1-2 degree level was necessary to get the transmission peak around 4400 Å, with a 24% peak total-system transmission efficiency

Standard exposure time for calibration observations with the new VPH grisms (nos. 18-20) were determined, and added to the system table to be used in calibration observing scripts.

There is some issue with flat fielding ALFOSC spectroscopic data in the blue (<4500 Å), where the light in the flat field is dominated by scattered red photons from the halogen lamp which is bright in the red, but faint in the blue. A different way of constructing a flat field using broad-band imaging flat field was developed. This should be of interest to all users, and instructions are provided on the ALFOSC web page, see:

http://www.not.iac.es/instruments/alfosc/flatfieldingInTheBlue.html

The 1.0 and 1.8 arcsec slits were re-clued; the ultra-thin slit plates had come loose on one side due to day-to-day use.