Multi-Anode Microchannel Plate Array Detectors (MAMA)

The Multi-Anode Microchannel Plate Array (MAMA) Dectectors were developed by Ball Aerospace and are both permanently sealed tubes with MgF2 entrance windows. Each MAMA detector incorporates a 1024x1024 element anode array, but the processing electronics centroid event positions to half the spacing of the anode array, providing improved image sampling and higher resolution (but with larger flat-field variations) in a 2048x2048 image format.

For UV modes,two types of MAMA detectors are employed on STIS. A photocathode optimizes each detector to its wavelength region. Each detector's photocathode provides maximum sensitivity in the wavelength region selected, while it rejects visible light not required for the observations.

The heart of each MAMA detector is a microchannel plate (MCP) ¡V a thin disk of glass approximately 1.5 mm thick and 4 cm in diameter that is honeycombed with small (12.5-micron) holes or pores. The front and back surfaces are metal coated. With a voltage applied across the plate, an electron entering any pore is accelerated by the electric field, and it eventually collides with the wall of the pore, giving up its kinetic energy to liberate two or more secondary electrons. The walls are treated to enhance the secondary electron production effect. The secondary electrons continue down the pore and collide with the wall to emit more electrons, and so the process continues, producing a cascade of a million electrons at the end of the pore.

The anode array is a complex fingerlike pattern. When electrons strike certain anodes, a signal is sent to the computer memory indicating the position and time of arrival of the photon.

The anode array has been designed so that only 132 circuits are required to be able to read out all 1024 x 1024 pixels. As the MAMA records the arrival of each photon, it can provide a time sequence. For instance, if an object is varying in time, like a pulsar, the data can be displayed to show if there is any periodicity. Similarly, to create an image, the data must be integrated in the computer memory before it is displayed. The MAMA data is recorded to a time resolution of 125 microseconds.

When used in the normal mode, each detector has 1024 x 1024 pixels, each 25 x 25 microns square. However, data received from the anode array can be interpolated to give a higher resolution, splitting each pixel into four 12.5 x 12.5 micron pixels. This is known as the high-resolution mode; however, data taken in this mode can be transformed to normal resolution if required. The high-resolution mode provides higher spatial resolution for looking at fine structural details of an object and ensures full sampling of the optical images and spectra.

Data Acquisition

The MAMAs take data in the high-resolution mode. For normal imaging and spectroscopy, the data is integrated in the onboard computer and stored in this format on the solid-state recorders for later downlink. The MAMAs also have a time-tag mode with a time resolution of 125 microseconds, where each photon is stored individually with its arrival time and location (x, y, t) .

Charge-Coupled Detector (CCD)

The STIS CCD was developed with GSFC and Ball input at Scientific Imaging Technologies (SITe). Fabricated using integrated circuit technology, the detector consists of light-sensitive picture elements (pixels) deposited onto a thin wafer of crystalline silicon. Each element is 21 x 21 microns. The elements are arranged 1024 to a row, in 1024 columns. The 1024 x 1024 format contains 1,048,576 pixels.

Each element acts as a small charge collector. As light falls on the pixel, it liberates electrons, which are stored in the pixel well. The number of electrons stored is then proportional to the instensity or brightness of the light received. When the exposure is completed, the charge is transferred across the array in a "bucket-brigade" like manner: all the charge in the image area is shifted by one row -- the row that reaches the edge of the array (the serial register) is then shifted one pixel at a time to the readout amplifier, where an electrical signal is generated that is proportional to the amount of charge in the pixel. The array of pixel intensities is then stored in onboard memory and later telemetered to the ground, where the image array is reconstructed as a two-dimensional picture.

The CCD is most sensitive to red light, but the STIS chip has been enhanced through what is known as a "backside treatment" to provide a usable sensitivity in the near-ultraviolet. The CCD is sensitive from approximately 200 nm to the near infrared at 1000 nm.

The CCD can make exposures ranging from 0.1 seconds to 60 minutes. In space, above Earth's protective atmosphere, radiation from cosmic rays is higher than at Earth's surface. CCDs are sensitive to cosmic rays, which can produce large numbers of electrons in the pixels. For this reason, two shorter exposures of up to 1 hour are made and comparison of the frames allows cosmic ray effects to be subtracted.

Target Acquision

Normally an object is acquired using the CCD camera with a 50x50-arcsec field. Two short exposures are taken to enable substraction of cosmic rays. Once identified, an object is positioned via small angle maneuvers to the center of the chosen science mode slit position. Two more exposures are made, the calibration lamp is flashed through the slit to confirm the exact slit position and a further peak up on the image is performed. Acquisition can take up to 20 minutes.