The SPIFFI Instrument
SPIFFI - the acronym stands for SPectrometer for Infrared Faint Field Imaging - is a very efficient infrared integral field spectrograph. It consists of pre-slit optics to adjust the image scale to the requirement of adaptive optics and seeing limited observations. Furthermore an advanced reflective image slicer and an efficient spectrograph equipped with a large Rockwell Hawaii infrared array detector. Here a brief summary of the instrument characteristics:
- Rockwell Hawaii 2RD 2kx2k detector
- With a scale changer in the pre-optics the spatial pixel scale is selectable from 0.25", 0.1" and 0.025". This allows diffraction and seeing limited observations with well sampled PSFs.
- Corresponding field-of-view sizes are 8" x 8", 3.2" x 3.2" and 0.8" x 0.8"
- Four gratings (J, H, K, H+K) on a wheel provide a spectral resolving power of R=1500 to 4000
- 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. (Based on the commissioning results we do not recommend this sky subtraction mode).
The SINFONI instrument is operated with a 2kx2k Hawaii 2RG detector which provides a spectral sampling of about 2 pixels per spectral resolution element and image slice.
The science grade detector is operated in none destructive mode with permanent (none desctructive) reads of the exposure level of the detector. The detector noise level depends on the number of reads and accordingly on the integration time (DIT). The lowest noise level (less then 10e-) on the detector (RON and photon noise) is reached for DIT values between 15 and 500 seconds, with a minimum of 7e- for DITs arround 100 seconds. At longer exposure times the photon noise from the dark current, the thermal background and from the remnance effect is larger then the RON. The gain factor was found to be 2.42 +- 0.05 e-/adu. The detector is linear within 1% up to the saturation level which occurs at >45000adu. We recommend to keep the exposure level below 10000adu for science exposures. As expected: The detector response has improved by large factors in the J and H bands in respect to the engineering grade detector with which the public science verification data sets were obtained. The overall instrument response including telescope, MACAO, SPIFFI and detector is now between 20% and 28% over the full spectral range.
As for any other similar detector: The detector may never be saturated at night time to avoid strong remnance effects which typically would not recover until the end of the night. There should be no target brighter the 5th (J,H,K) or 6th (H+K) magnitude in the 8 arcseconds wide field of view of SINFONI. Please contact us if this is a problem for you project.
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 third, spectral information is used to create a data cube. The basic concept of image slicing (only 6 slices shown to simplify the figure) is shown here:
In SPIFFI the image-slicer consists of two sets of plane mirrors. The first set of mirrors (B), called "Small Slicer", consists of a stack of 32 mirrors which slice the image into 32 slitlets. The second set of mirrors (A), called "Big Slicer", rearranges the fan of 32 slitlets to form a pseudo long-slit. A picture illustrating the SPIFFI image-slicer is shown here:
Design overview of the instrument
| perspective view of the design || picture of the integrated spectrograph |