Spacecraft & Instruments
The IRIS Spacecraft is comprised of a disk-shaped spacecraft bus and a 1.4-meter telescope that protrudes from the disk. The Spacecraft weighs 167 Kilograms and is 2.1 meters long and 3.7 meters across.
The spacecraft mass includes 90kg for the scientific payload and 77kg for the spacecraft bus. The bus is three-axis stabilized using momentum wheels built by Goodrich Aerospace, but IRIS is not equipped with a propulsion system and is not capable of adjusting its orbit. The reaction wheels provide fine-pointing with guidance from the Guide Telescope and quick slew times.
Two deployable solar arrays provide electrical power to the spacecraft. Total end of life power of the solar arrays is 294 Watts. The vehicle is equipped with batteries for power storage and avionics for power distribution as part of a 28-Volt main bus. Solar array EOM power provides a margin of 30% over critical vehicle limits.
The spacecraft and instrument elements use a large number of heritage components from the Solar Dynamics Observatory Program (SDO) to reduce risk and cost. Although SDO, is a much larger spacecraft, its payloads and IRIS are sharing a number of elements.
The spacecraft is equipped with a RAD750 flight computer. RAD750 is a single-card computer that is manufactured by BAE Systems in Manassas, Va. The processor can endure radiation doses that are a million times more extreme than what is considered fatal to humans. The RAD750 CPU itself can tolerate 200,000 to 1,000,000 rads. Also, RAD750 will not suffer more than one event requiring interventions from Earth over a 15-year period.
“The RAD750 card is designed to accommodate all those single event effects and survive them. The ultimate goal is one upset is allowed in 15 years. An upset means an intervention from Earth — one ‘blue screen of death’ in 15 years. We typically have contracts that (specify) that,” said Vic Scuderi BAE Business Manager. The RAD750 processors operate at up to 200 megahertz. It operates at temperatures of -55°C and 70°C with a power consumption of 10 Watts. RAD750 can tolerate 100,000rads.
The IRIS flight software is written in C++ and runs on the computer using the VxWorks operating system (known from the Mars Rovers). The computer system is largely based on SDO heritage to control IRIS autonomously with minimum spacecraft and ground interaction. A limited amount of onboard fault management is provided by the system to deal with any malfunctions should they arise. Flight software can be re-loaded into the computers, providing flexibility to teams that can make modifications and adjustments to the flight software during the mission.
The data handling system acquires data from the cameras of the instrument and compress it using configurable lossless and/or lossy compression algorithms to format the data into standard CCSDS packets that are then stored in the Solid State Memory of the spacecraft.A 1553 data bus is used to provide housekeeping data exchange between the systems of the spacecraft.
IRIS is equipped with an S-Band communications system to send telemetry and housekeeping data back to Earth and receive commands from Earth. An X-Band system is used for data downlink with a maximum data rate of 10Mbit/s and an average speed of 0.7Mbit/s. Data collected by the payload is stored in a 48 Gbit (6Gbyte) memory.
IRIS Instrument Architecture
The single instrument of IRIS is an Ultraviolet Telescope combined with a spectrograph and imager. The 20-centimeter telescope is feeding the stigmatic UV spectrograph and a slit-jaw imager that provide an unprecedented combination of 0.33 arcsec imaging with simultaneous high-resolution spectroscopy generating simultaneous intensity and velocity maps (spectroheliograms). Supported UV bands cover a range of chromospheric, transition region and coronal temperatures. The slit-jaw imager provides a new image every five seconds while the spectrograph generates a spectrum every second. 3D numerical modeling will allow scientists to couple both capabilities in order to trace how solar material at different temperatures courses through the chromosphere into the transition region and to the corona.
IRIS data will record observations of solar material at specific temperatures from 5,000K to 10,000,000K during solar flares in order to monitor material traveling on the surface of the sun, the photosphere, to the lower layers of the chromosphere and into the transition region to the corona.
The 1.4-meter UV Telescope largely builds on SDO’s Atmospheric Imaging Assembly (AIA) and is a 20-centimeter aperture, 240cm² geometric area classical Cassegrain with an effective focal length of 6.895 meters. Cassegrain telescopes use a combination of a primary concave mirror and a secondary convex mirror aligned on the optical axis with the primary mirror having a hole in the center to permit light to enter the imager/spectrograph.
The telescope is attached to the spacecraft top panel via six struts. Its optics are mounted in a graphite-epoxy metering tube to provide alignment and thermal stability. The primary mirror is installed on the rear tube flange of the telescope via titanium flexures. The spectrograph is attached to the telescope by a stiff mounting base at the end of the tube.
Both, spectrograph and slit-jaw imager, are housed in an aluminum honeycomb case with graphite epoxy sheets for thermal stability during operations of the spectrograph. The case has multiple compartments to optically separate the different detectors that comprise the individual instruments for stray light reasons. The telescope is equipped with a front door based on the SDO design to protect it from contamination on the ground and during early orbit operations. The front door has a single use redundant wax actuator assembly as opening mechanism.
IRIS’s telescope has a 20-centimeter primary mirror and a secondary mirror magnification of 4.9. The secondary mirror is a tip-tilt mirror that provides image motion compensation and scans the solar image across the slit of the spectrograph. Secondary mirror scan range is +/-60 arcsec and it provides +/-5 arcsec jitter control. The slit-jaw imager and spectrograph magnify the image by another factor of 2.33 to achieve the 0.167 arcsec/pixel resolution. The telescope has a field of view of 170 by 170 arcsec at the 2800-angström wavelength. (1Å=one ten-billionth of a meter)
One big engineering challenge for Sun-observing missions is heat rejection and, in case of IRIS, visible-light rejection.
The spacecraft accommodates a number of features, taking a new approach to heat and light rejection. Due to the narrow bands IRIS is observing (1332-1406 Å in the far UV and 2785-2835 Å in the far UV), a simple but effective light rejection method can be applied.
The primary mirror that receives all radiation passing inside the telescope, is coated with a dielectric multilayer coating that reflects about 70% of the far-UV band radiation and 12% of near-UV radiation while allowing 94% of visible light and infrared radiation to pass through the mirror’s transparent ULE glass. Behind the mirror, a heat sink is mounted that is coupled with a radiator on the side of the telescope. A very small amount (<3W) of VIS/IR radiation is absorbed by the primary mirror causing its temperature to vary by <1.4°C.
An even smaller amount of light is reflected off the back surface of the mirror. This is sent back towards the entrance aperture in a diverging beam.
The secondary mirror uses the same UV coating to reflect the desired near- and far-UV radiation and allow VIS/IR radiation to pass to a heat sink behind the mirror. Of the 36 Watts of solar heat load incident on the primary mirror, 31W are absorbed in the primary heat sink, 3W are absorbed by the primary mirror and 2.2W are incident on the secondary mirror. Of those 2.2W, 1,9W are absorbed by the secondary heat sink, 0.2W are absorbed by the mirror and 142mW are allowed to pass into the spectrograph. There, a field stop blocks 140mW and 2mW of thermal radiation is allowed to pass through into the spectrograph.
The UV Spectrograph is a Czerny-Turner design with a pre-disperser at the slit and separate optical paths for the far- and near-UV radiation. In the common Czerny-Turner design, light is directed through an entrance slit (available light energy depends on light intensity of the source as well as the dimensions of the slit and acceptance angle of the system). The slit is placed at the effective focus of a collimator that directs collimated light (focused at infinity) to a diffraction grating or prism. Another mirror refocuses the dispersed light onto a detector. For IRIS, the design is somewhat more complex because of the number of optical paths used in the system:
Light from the telescope which is almost entirely UV radiation, enters the spectrograph through a small Magnesium Fluoride Prism with vapor-deposited aluminum slit jaws on the first surface to generate the 170 by 0.33 arcsec entrance slit of the spectrograph for both, near- and far-UV. The pre-disperser – the Magnesium Fluoride Prism – is strongly dispersive for wavelengths of 1400 to 2800Å – the near-UV band (2785-2835 Å) and the far-UV band (1332-1406 Å) are separated and directed onto separate halves of the collimator that has a UV-enhanced Aluminum coating. Another effect of the pre-disperser is that most of the incoming visible and infrared light is directed to the near-UV band so that the undesired long-wavelengths contribution in the far-band is reduced.
Now, there are two optical paths as the collimator passes the two UV bands through different diffraction gratings with 3600lines/mm and to two different camera & fold mirrors that direct the two bands to the detectors. The The fold mirrors and detectors are out of plane (above the plane for near-UV and below the plane for far-UV) to minimize aberrations.
The far-UV camera and fold mirrors are coated with a narrow band dielectric UV reflective coating that reflects 75% of the desired band and prevents any contamination from the 2800 Å band. The near-UV camera and fold mirrors are coated with the standard dielectric laser mirror coating to reduce contamination from VIS/IR. The grating is used in 1st order for near-UV and second order for far-UV.
Two CCDs (charge-coupled devices) mounted on a single carrier are covering the far-UV bands of 1332-1358Å and 1380-1406Å and a single CCD covers the near-UV band from 2785-2835Å.
The telescope and spectrograph assembly provides a total effective area over the far-UV of 2.8cm² which represents a significant increase over previously flown missions. The near-UV area is 0.3cm² because the Mg II lines are much brighter than the far-UV lines.
The overall resolution of the telescope/spectrograph is 40mÅ for far-UV and 80mÅ for near-UV with spectral pixel sizes for 12.5 and 25mÅ. The cameras have an identical focal length of 1.257 meters.
Two fast shutters are used to allow a high repetition rate and optimized exposure of each CCD. Combined with the fast data read-out of the cameras, IRIS can obtain one spectrum each second.
The spectrograph is equipped with a 14-Watt heater to limit its thermal variation to 1.1°C throughout its orbit.
The second component of the optical assembly is the slit jaw imager that also has two optical paths. Light reflected by the aluminum slit jaws on the pre-disperser prism is directed onto a filter wheel that contains two different narrow-band near-UV filters and two mirrors with different narrow-band far-UV coatings. The two remaining positions of the wheels are holding a clear glass element and an aluminum mirror for ground testing purposes.
The position of the wheel determines the wavelength of the band that passes to the slit jaw imager. The two near-UV filters transmit a 15Å wide spectral window to the near-UV optics. One is centered around 2796, the other around 2816Å to cover the desired band. These optics consist of a spherical re-imaging mirror and two flat fold mirrors. Each mirror is coated with a reflective dielectric coating that is optimized for the respective UV-band. Between the second Near-UV fold mirror and the CCD is a Lyot/Solc birefringent filter that reduces the bandwidth of the near-UV radiation to 5Å. It consists of three quartz plates with thicknesses of 0.92, 3.68 and 0.92 millimeters and a wire grid polarizer on each side of the plate assembly.
The two mirrors on the filter wheel reflect far-UV radiation around 1335 and 1400Å, respectively, and direct the bands to an re-imaging filter that passes the light onto the two fold mirrors of this respective optical path.
Both optical paths image onto separate halves of the common slit-jaw imager CCD through a single shutter that controls the exposure time. The spatial resolution of slit jaw images is 0.167 arcsec/pixel. The effective area of the slit jaw imager is 0.7cm² for the 1400Å band and 0.005cm² for the 2800Å band.
All shutters and mechanisms within the system use brushless DC motors with high torque margins to provide a minimum lifetime of at least two years.
IRIS contains four identical CCDs, three in the spectrograph and a single detector in the slit jaw imager. The detectors are 1024 by 2048 pixels with 13 µm pixels. The detectors are custom versions of commercially available detectors using enhanced processes and thin-gate technologies. The transfer mask of the commercial unit is excluded in the flight model.
The detectors are radiatively cooled down to an operational temperature of -50°C to produce an insignificant dark current. CCD heaters are used to provide a uniform detector temperature during solstice orbits. The two cameras used on IRIS are two flight spare cameras left from the SDO mission that provide sufficient capabilities to IRIS. The cameras allow simultaneous operation of all CCDs with CCD read-out at 4Mpixels/s per CCD.
The Guide Telescope is located on the exterior of the main tube. It provides fine-pointing measurements that are used for the Image Stabilization System to suppress jitter frequencies above the attitude control system bandwidth and it provides an error signal for the ACS used for S/C fine-pointing.
The GT is a refractor telescope with a Barlow lens to increase the focal length of the optical system. The focal length of the telescope is 1.88 meters. It can be pointed anywhere within 18 arcmin of the mechanical axis of the telescope using two glass wedges each mounted in a hollow core motor to move the GT optical axis. The GT images the sun onto a limb sensor with four photodiodes.
The signal that is provided by the GT is digitized by the IRIS electronics system and routed to the spacecraft bus via a 1553 data link. The Image Stabilization System also receives the GT signal.
Image Stabilization System
The Image Stabilization System (ISS) senses jitter by using GT data and removes it in real time by controlling the active tip-tilt secondary mirror. The mirror is also used to scan the sun image across the spectrograph slit. The GT error signals are processed and sent to the IRIS secondary servo controller that controls PZT actuators that tilt the secondary mirror.