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Instrument

Overview

SPIFFI is a near-infrared integral field spectrograph for the VLT. It allows observers to obtain simultaneously spectra of 2048 pixels in a 32x64 pixel field-of-view. In conjunction with the adaptive optics system MACAO it is possible to perform spectroscopy with variable slit widths sampling the diffraction limit of an 8m-class telescope. SPIFFI covers the near-infrared wavelength range from 1.1µm to 2.45µm with a moderate spectral resolving power ranging from R=1500 to R=6000.

Characteristics

2048x2048 HAWAI 2RG array increases both spatial and spectral coverage by a factor of 8 over the preceding instrument 3D
The field of view is sliced into 32 slices
With a scale changer in the pre-optics the slit width is selectable between 0.25", 0.1" and 0.025"
Corresponding field-of-view sizes are 8"×8", 3.2"×3.2" and 0.8"×0.8"
Each one of the 32 slitlets is imaged onto 64 detector pixels.
Four gratings on a wheel provide spectral resolving powers of R=2000, 3000, 4000, 1500 in J, H, K, and H+K band, respectively, in the largest pixel scale.
The high spectral resolving power allows one to "suppress" the OH sky-lines to achieve deeper integrations.
A sky spider facilitates the simultaneous observation of blank sky in one corner of the field of view, allowing long integrations without nodding the telescope

Image Slicer

The heart of SPIFFI is its image slicer, which converts the two-dimensional field of view into a one-dimensional slit. This slit is fed into a long-slit spectrograph to disperse the light of each pixel in the two-dimensional field of view at the same time. During the data reduction the two-dimensional spatial information together with the spectral information is used to create a data cube. The basic concept of image slicing is shown here:

The SPIFFI image slicer consists of two sets of plane mirrors. The first set of mirrors, the "Small Slicer" labeled B in the cartoon below, consists of a stack of 32 mirrors which slice the image into 32 slitlets. The second set of mirrors, the "Big Slicer" or A in the cartoon, rearranges the fan of 32 slitlets to form a pseudo long-slit.

Spectrograph

After the SPIFFI field of view is sliced into a one-dimensional slit, a long-slit spectrograph disperses the light onto a HAWAII detector.

In theory:

Shown here is an overview perspective of SPIFFI.

H : Sky spider	F : Filter wheel	V : Pre-optics
B : Image slicer	K1-3 : Collimator	G : Grating wheel
C : Camera	D : Detector	L : Enclosure
S : Enforcing structure

In Reality:

Detector

The detector used in SPIFFI is a Rockwell HAWAII Focal Plane Array with 2048² detector elements.

Control Electronics

The control electronics of SPIFFI is based on the ESO control scheme developed for VLT instruments. An HP workstation runs the graphical user interface for controlling the instrument and the detector and for displaying the data. A VME-bus-based real-time computer actually controls motorized functions and reads sensor and status signals. A SUN workstation in conjunction with an ESO IRACE controller collects the data from the detector.

Sensitivities

Assumptions for the computation of the limiting magnitudes:

S/N of 10 per spectral channel in 1 hour integration time on source (6×600sec on source, 6x600sec on sky)
Encircled energy

250 mas scale: 50 % in 0.65'' diameter.
100 mas scale: 50 % in 0.20'' diameter.
25 mas scale: 25 % in 0.10'' diameter.

Not using OH avoidance	250 mas scale	100 mas scale	25 mas scale
J	17.6	18.1	16.7
H	17.2	18.2	17.0
K	16.6	17.7	16.8
H+K	17.4	18.6	17.9

Using OH avoidance	250 mas scale	100 mas scale	25 mas scale
J	17.8	18.2	16.7
H	18.0	18.5	17.0
K	17.3	18.2	16.9
H+K	18.4	19.2	18.0