A-MKID - Eine große inkohärente Sub-Millimeter-Kamera für inkohärente APEX-Empfänger

Large sub-millimeter direct detection mapping projects like the AtlasGal survey can only be performed with a good telescope at an excellent side and a large, sensitive camera working close to the background noise. The APEX telescope with its current instrumentation offers already the capability for such large surveys. But new technologies now make possible cameras with an even larger field-of-view, much larger pixel counts and therefore much higher mapping speeds.

A-MKID is a camera based on latest available technologies. It operates two frequency bands simulta­neously, in the atmospheric windows around 850µm and 350µm, and offers in total about 25000 pixels to the user.

The camera uses so-called microwave kinetic inductance detectors (MKIDs) as sensing elements. Their design allows for efficient frequency multiplexing and therefore high pixel counts for the overall camera system. At the same time the detectors show highest sensitivity and their production on large wavers with high yield has been demonstrated.

The camera design has been completed, integration of the cryostat is currently on-going and commis­sioning at the APEX is foreseen for autumn this year.

Specifications

A-MKID will be a wide field-of-view (squared 15 x 15 arcmin²), dual colour direct detection camera for APEX. Both colours will be operated simultaneously. The sky frequency detection bands are defined by the atmospheric transmission windows:

• LFA: 347 GHz ( 34 GHz width, from 330 to 364 GHz / HP width)
• HFA: 850 GHz (100 GHz width, from 800 to 900 GHz / HP width)

The hexagonal pixel arrangement together with the pixel spacing of 1 Fλ result in 3520 pixels in low-frequency array (LFA) and 21600 pixels in high-frequency array (HFA).

The detectors shall operate with sensitivities dominated by the sky background. Good system stability shall allow mapping of extended structures. The system design shall allow for the measurement of linear polarization.

The cryo-system will consist of two 4K pulse-tube closed cycle systems, one to cool the optics, one to reach the bath temperature for the He 10 sorption cooler. The so-called He 10 cooler will provide temperatures <300 mK, needed to operate the detectors.

Technology

MKIDs are superconducting pair breaking detectors. Their antenna determines the RF-band, while the resonator determines the KID readout frequency. Hence they offer detector and multiplexing filter in one structure.

To read out the MKIDs we create a tone-signal at the resonator frequency of each KID in the detector chip. After passing the detector, this tone is analysed in amplitude and phase. Changes to these values are a direct measurement of incoming photons. The tone-generation is realized by four 1.1 GHz wide DACs. These four signals are combined in an integrated IF-system to one monolithic 4 GHz wide band with up to 2000 tones for the detectors. After passing the detector chip, the signal passes again an IF circuit. The signal is then analysed by a fast ADC. A schematic of the readout principle is shown below.

To allow for mass production of in total 24 IF chains we designed highly integrated IF circuits. The up-converter board houses two separate computer controlled synthesizers, 4 RF filters, 2 amplifiers, two IQ-mixers and a computer controlled attenuator. The down converter board provides two separate down-converter circuits, each with a computer controlled synthesizer and attenuator, a low-pass filter and an amplifier.

The optics layout is mainly defined by the limited space available within the APEX Cassegrain cabin. We chose a fully reflective optics approach to avoid losses due to imperfect anti-reflection coatings and absorptions. In total the optics consists of four imaging mirrors (M3, M4, M5 und M6) and two “flat” mirrors (FM1 und FM2). The optics layout and mirror optimization was – within the limited development time - done in 2-D only. The third dimension was introduced by the flat folding mirrors. To make use of the additional degrees of freedom in the 3-D space, FM2 is used to compensate for aberration ("glasses" for the dewar-optics) and therefore has also a slightly structured surface. The overall magnification is ~4 and the size of the dewar-window of ~150 x 150 mm2. M5 and M6 are located inside the dewar. A Lyot-stop in between these both mirrors defines the telescope illumination. A commercial filter set defines the RF-bandpass of the both A-MKID sub-arrays.

Integration

In-house production

A-MKID was produced mostly in our in-house workshop (pictures of the cryostat and the mirror production, see below). Interaction with the workshop guaranties highest accuracy, e.g., for mirrors and provides full control over all manufacturing steps. The production is fast and reliable and allows for changes on short notice.

Detector assembly

• cooling is provided by a Chase Cryogenics He10 system;
• four temperatures (at 4K, 1K, 0.4K, 0.25K);
• three nested carbon-fiber hexapods are used to isolate the low temperature stages from each other;
• superconductive IF-lines keep the thermal loading as low as possible.

Cryostat Integration

The A-MKID consortium

In January 2011 a consortium between the Max-Planck-Institut für Radioastronomie, the SRON Netherlands Institute for Space Research and the Delft University of Technology, Faculty of Applied Sciences has committed to the development and operation of A-MKID as a PI instrument for APEX.