The Payload!

OGRESS is a soft x-ray spectrograph designed to observe large diffuse sources. These types of objects are too large to observe effectively with the current suite of orbiting telescopes.

Here's a SolidWorks model of the full payload which I made during the design process.

The full payload, buttoned up and ready to go to the field.

OGRESS consists of two identical spectrographs, which have three main components. Wire-grid focusers (to focus the light), off-plane grating arrays (to diffract the light), and GEM detectors (to collect the light).

The passive focusers consist of 24 wire-grid plates. One of these is shown below.

When you stack the plates together, light can only travel down the focuser if its coming from the direction we want (in other words, it must be coming to a focus). Light coming from different angles is vignetted by the wires. This produces a line focus at the focal-plane of the telescope.

Notice from the picture below (showing the main payload components from two different angles), that light is only focused in one dimension, resulting in long lines at the focal plane.

A view down the front end of the payload. My face is reflected in the star tracker alignment flat.

Here's a Ray Trace of a single focuser. On the left, you can see rays being vignetted inside the focuser. In the middle, the light hits the grating array and is split into three components. This trace shows light at ~20.7 Angstroms (simulating highly ionized Oxygen) in 0 and +/-1st orders.

Once the converging beam has been sculpted, the light is diffracted by an array of 67 gratings with a 4.4 degree graze angle. Off-plane gratings are oriented such that the incoming light is quasi-parallel with the grating grooves, and diffracts in an arc, as shown below.

The 67 gratings in each array are held under tension in a titanium flexture mount, shown below. This prevents the gratings from bending and hitting each other during launch vibrations.

After the light is diffracted into a spectrum, it must be collected and detected. This is done with Gaseous Electron Multiplier (GEM) detectors. GEMs were initially developed at CERN as particle detectors. More recently, researchers have begun using GEMs as x-ray detectors in laboratory settings. Proving that GEMs can be reliably flown in space (on a sounding rocket) can enable their use in higher-budget orbital missions. A schematic of a GEM is shown below. X-rays detected by a GEM pass through a thin window into a gas-filled chamber and ionize one or more gas molecules. Due to an applied voltage, photo-electrons then drift toward a series of perforated copper plates. The plates are held at sequentially higher voltages causing the electrons to accelerate into more gas molecules and produce an electron cascade. The results in an electric pulse hitting the detector anode, which is then amplified and digitized in the electronics section.

A picture of one of our GEM detectors. The thin window, seen on top, is attached to a grid structure to support it against the internal gas pressure.

A close-up SolidWorks diagram of the electronics section. This is where all the electronic and gas system components are located. The gas system contains the reserve gas for the detectors, as well as proportional valves to maintain the pressure within the detector. The electronics system has high and low-voltage power supplies, pressure gauges, amplifiers, and Time to Digital converters. The purpose of the electronics system is to convert our raw detector data into the format preferred by WFF so that it can be telemetered to the ground.