FLOYDS manual and pipeline

Stefano Valenti12, David Sand3, D. Andrew Howell\(^{1,2}\)

  1. Las Cumbres Observatory Global Telesope Network

  2. University of California, Santa Barbara

  3. Texas Tech University


The Las Cumbres Observatory Global Telescope Network (LCOGT) has recently deployed two new, robotic spectrographs on the Faulkes telescopes, FTN and FTS. This document details the characteristics of the spectrograph, the data reduction pipeline we have developed, and the data products that the user can expect.

Overview of the FLOYDS instruments

FLOYDS are a pair of nearly identical, low dispersion, robotic spectrographs deployed at the 2m Faulkes Telescopes, North and South (FTN and FTS). The instruments were designed with supernova classification and monitoring in mind, with a large wavelength coverage (\(\sim\)320 to 1000 nm) and a resolution (R\(\sim\)300 to 600, depending on wavelength) well-matched to the broad features of these events.

The design uses a low dispersion grating (235 l/mm) and a cross-dispersed prism in concert to work in first and second order simultaneously. A folded all-reflecting camera focuses first and second order light onto the CCD. This allows a \(\sim\)320 to 1000+ nm wavelength coverage in a single exposure. A 30“ slit length allows both orders to fit on the chip with no order overlap (Figure \ref{fig:floydsformat}). Four such slits are available at each spectrograph, with widths that bracket the median seeing at each site (0.9”, 1.2“, 1.6” at FTN; 1.2“, 1.6” and 2.0“ at FTS) and a 6” slit width for spectrophotometry.

\label{fig:floydsformat} A raw FLOYDS spectrum of SN2013ek (Valenti et al., 2013). The top trace is second order light, with a wavelength coverage of \(\sim\)320-570 nm, while the bottom trace is first order light, spanning \(\sim\)550-1000+ nm.

Since the FLOYDS spectrographs are mounted on the 2m telescopes’ Cassegrain rotator stages, the slit may be oriented along any desired position angle on the sky (note that this is not necessarily a good idea, though, given the very broad wavelength coverage of FLOYDS; we generally recommend orienting the slit along the parallactic angle). An Andor Newton 940 CCD controller package, with an e2V 42-10 CCD (13.5 \(\mu\)m pixels and a 2048\(\times\)512 format), is used with a broadband ultraviolet-enhanced coating. The CCD package is thermo-electrically cooled to -70 C, with negligible dark current. Spectra do fringe above \(\sim\)700 nm, but this can be corrected when a flat field is taken at the same position as a science frame.

Wavelength calibration is accomplished with a Mercury Argon (HgAr) lamp, and flat fielding with a combination of a tungsten-halogen and high-powered xenon lamp (whose light are combined with a dichroic). The calibration unit housing these lamps sits in a cabinet near the telescope with a fiber connection to FLOYDS. A deployable arm in the FLOYDS instrument can direct light from the calibration unit into the spectrograph, and accompanying optics delivers light with an f/10 beam, mimicking the actual Faulkes telescopes. A typical calibration arc is shown in Fig. \ref{fig:arc}, and a representative flat is shown in Fig. \ref{fig:flat}