• Overview
• Mechanical Layout
• Optical Layout
• The Detector
• MIR-Filter
• Fabry-Pérot Interferometer
• Readout Electronics
• Sensitivity
• Observation Technique
• MANIAC at the 2.2m Telescope at La Silla
• MANIACs Future
• Acknowledgement

Overview

MANIAC (Mid- And Near-Infrared Array Camera) is an imaging camera for astronomical observations in the mid- and near-infrared wavelength ranges. It has been developed at the Max-Planck-Institute for extraterrestrial Physics (MPE) in Garching/Germany, in collaboration with the University of New South Wales (UNSW) in Sydney/Australia, and the University in Jena/Germany. MANIAC is designed for simultaneous observations in the near-infrared (NIR) from 1 to 5 µm and the mid-infrared (MIR) from 8 to 28 µm. This allows measurement of accurate relative positions of the mid and near-infrared images of an object. However, at present, only the mid-infrared channel is installed. Nevertheless, MANIAC is a fully operational instrument at the mid-infrared wavelength regime and has been used for numerous observations. In future phases, a cooled Fabry-Pérot Interferometer, a bigger detector array for the mid-infrared channel, and the near-infrared channel will be installed.

In the mid-infrared wavelength range, thermal radiation from the atmosphere, the telescope, and the instrument reduce the sensitivity significantly (the Planck curve for a Black Body at room temperature peaks at about 10 µm). To avoid a contribution to the thermal background emission from MANIAC, the instrument has to be cooled. Furthermore, the detector has to be cooled down below 12 K for optimum operation. Therefore, the whole MANIAC instrument is cooled down to 4.2 K using liquid helium. Due to high helium costs, the present cooling system of MANIAC will be exchanged by a closed cycle cooling system in the near future.

MANIAC is operated using a PC. All the electronics and the PC sit in a standard 19'' rack, which is mounted next to MANIAC on the telescope. The PC is connected to the display and the keyboard in the telescope control room via a 60 m long video and keyboard extension cable.

Mechanical Layout

The tanks for liquid nitrogen and liquid helium have a volume of 20 litres. As a result, the hold time of the filled helium tank with MANIAC in operation is about 35 h. Due to space considerations, the optical plate in MANIAC is mounted vertical on the surface of the liquid helium tank and the mid and near-infrared channel are set up on the opposite side of the optical plate. Figure 1 represents a section through MANIAC. This figure also shows the mid-infrared channel mounted on the optical plate.

Figure 1: Mechanical construction of the MANIAC cryostat (not to scale)

MANIAC has two filterwheels, with the stepper motors that drive the filterwheels are mounted outside the dewar. Figure 2 shows an overall view of the MANIAC instrument. The box mounted to the front of MANIAC is the analog readout electronic box. On the left hand side, outside the dewar, the two stepper motors for the filterwheels can be seen. Inside the dewar, the vertical optical plate with baffles, filterwheels, and the gear of the filterwheels are visible. Figure 3 shows a closer view of the mid-infrared channel without the baffles. The total weight of the instrument is roughly 100 kg.

Figure 2: Overall view of the MANIAC cryostat.	Figure 3: View of the MIR optics of MANIAC

Optical Layout

MANIAC is designed for a telescope with a focal ratio of f/35. The focal plane of the telescope is inside the instrument and is used as a cold field stop. The beam enters MANIAC horizontally, as can be seen in figure 4.

Figure 4: Schematic of the optical layout of the MANIAC mid-infrared channel

The entrance window of MANIAC is made out of KRS-5. All the optical components of MANIAC are purely reflective. Therefore, the optical design of MANIAC is intrinsically achromatic over the whole wavelength range. The first mirror (M1; off-axis paraboloid) creates an image of the telescope secondary mirror at the position of the second mirror (M2; spherical), which thus acts as the aperture stop of the system. A third mirror (M3; spherical) is used to fold the light to the detector. One filterwheel sits in front M2 and the second filterwheel is placed just in front of the detector. Figure 5 shows a photo of the actual optical plate with the mirrors mounted on it.

Figure 5: Photo of the MANIAC mirror system.	Figure 6: MANIAC detector: Rockwell International S/N MF12

^a Signal value with 10% deviation from ideal curve.
^b Detective quantum efficiency: (real quantum efficiency)/(excess factor)

Table 1: Parameter of the MANIAC mid-infrared detector Parameter Size (128 × 128) pixel Pixel Size 75 µm × 75 µm Good Pixel 99.94% Number of Signal Outputs 4 Operation Temperature 12 K Active Layer Thickness 15 µm Active Layer Doping Density 1 × 1018 cm-3 Well Capacitya 8.5 × 106 e- Read Noise 590 e- Det. Quantum Efficiencyb 0.21 Dark Current 5 e-

MIR-Filter

The filters for the mid-infrared wavelength range used in MANIAC are made out of Germanium with the appropriate coatings for the individual band-passes. In addition to the broad-band and narrow-band filters, a Circular Variable Filter (CVF) is used in MANIAC. The filters are installed in two filterwheels. The specifications of the filters are given in Table 2.

The filter specifications are for 4.2 K

Table 2: MANIAC Filters center Tmax Filter (µm) (µm) (%) N-band 10.5 5.0 80 Q'-band 17.6 1.8 60 Q-band 20.0 5.4 45 Sweet Spot 8.025 0.15 45 Continuum 10.36 0.124 60 [S IV] 10.53 0.124 60 Continuum 10.68 0.124 60 Continuum 12.56 0.124 54 [Ne II] 12.82 0.124 54 Continuum 12.95 0.124 62 Continuum 18.43 0.124 40 [S III] 18.73 0.124 40 Continuum 19.00 0.124 40 CVF 7.7 to 14.0 R 60 70

In addition to the filters on the filterwheels a cooled (4.2 K) Fabry-Pérot interferometer for the N-band will be installed soon.

Fabry-Pérot Interferometer

Read-Out Electronics

The principle of the MANIAC read-out electronics is shown in figure 7. The analog box, which is mounted outside the dewar, reads the four detector outputs and amplifies them. It also generates the detector biases and the clock signals for the array. The digital box reads the preamplified detector outputs from the analog box, digitizes, and co-adds them at up to 400 frames per second. It also also communicates with the PC over two standard serial IO boards. Two read-out modes are available for the detector: the single read mode (SR) and the uncorrelated double sampling mode (UDS). In UDS, the detector is reset twice at the beginning of each integration. Since the UDS mode reduces the 1/f noise, this mode is normaly used for the observations. However, the read-out in UDS mode is slower than in SR mode. To avoid saturation of the detector in N and Q band, the faster SR mode has therefore to be used for broadband observations.

Figure 7: Schematic of the MANIAC readout electronics.

Sensitivity

The sensitivity of MANIAC in the N-band at the 2.2 m telescope is typically 3 - 5 mJy per square arcsec for a 1 detection within 1 h (on source) for an extended source (Lehmann, T., Böker, T., & Krabbe, A. 1997, ESO Msngr, 88, 9). For a point source the sensitivity is roughly 2 mJy for 1 within 1 h (on source) in N-band.

Observing Technique

Because of the high background radiation of the atmosphere and the telescope, the chop-nod-technique has to be used for the observations. Chopping means to look alternately at the object of interest and the empty sky. This is usualy done by wobbling the secondary mirror of the telescope at frequencies of 1 to 10 Hz. By chopping between the object and the empty sky and subtracting the sky-image from the object-image, the background radiation of the atmosphere is eliminated. However, because of the chopping, the beam hits slightly different parts of the main mirror of the telescope. Due to temperature gradients (and imperfections), the main mirror of telescopes is never perfectly uniform and after the chopping there is some ''structure'' left in the image. By nodding with the telescope one can get rid of this ''structure''. Nodding is carried out by moving the whole telescope so that the sky-beam will be the object-beam and the former object-beam will be a sky-beam on the opposite side (with respect of the object) of the former sky-beam. Nodding is done on the order of a few minutes.

 

MANIAC at the 2.2 m Telescope at La Silla So far, observations with MANIAC have been carried out at the ESO/MPG 2.2 m telescope at La Silla/Chile. The field of view of MANIAC at this telescope is 44.2'' × 44.2'' and the pixel size is 0.345''. MANIAC is diffraction limited at the 2.2 m telescope. The diameter of the diffraction disk is roughly 1.5''.

Figure 8: MANIAC mounted on the ESO/MPG 2.2 m telescope at La Silla.