The RAIDS sensor suite includes eight instruments in three classes—photometers, spectrometers, and spectrographs— which together span the EUV to NIR to passband 55–874 nm. The photometers, spectrometers, and the FUV spectrograph include off-axis telescopes feeding an entrace slit, while the EUV spectrograph uses a mechanical-grid collimator to defined its field-of-view. The phometers have fixed 1.5 nm passbands at 630, 777.4, and 765 nm. The NIR (722-874 nm), NUV (295-399 nm), and MUV (190-317 nm) Ebert-Fastie spectrometers can operate at fixed selectable wavelengths or as scanning monochromators. The FUV (130-170 nm) and EUV (55-115 nm) spectrographs include array detectors and return multi-wavlength spectra continuously. Seven of the instruments are coaligned on the stepper-motor-driven scan head which points to or scans up to 16.5° across the limb, but the FUV spectrograph is mounted separately and performs fixed-angle vertical imaging of the limb. Thus the RAIDS sensor suite provides a highly flexible combination of spectral scanning, vertical scanning, fixed pointing, and selectable monochromatic photometry capable of addressing a wide variety of aeronomy problems.
A few key features of the sensors are:
- EUV Spectrograph
The EUV spectrographs is a f/5 imaging Wadsworth design using a wedge-and-strip two dimensional detector, although the spectrograph is not operated in imaging mode—the spatial dimension of the spectrum is collapsed to save bandwidth. The spectrograph uses a mechanical grid collimator to minimize reflective losses. The grating may be positioned to measure the 500-850 passband or the 770-1100 passbands at 1.2 nm spectral resolution. The 128-bin spectra are read-out every 0.5 seconds. The peak sensitivity is 0.5 counts/sec/Rayleigh at OII 83.4 nm, the emission feature of primary interest.
- FUV Spectrograph
The FUV spectrograph is an f/5 imaging Wadsworth design which uses a 0.125-m off-axis telescope to image the atmospheric limb in the 130-170 nm passband onto a sealed wedge-and-strip detector. This spectrograph is used in imaging mode and generates 64 spatial pixels by 256 sprectral pixel images. Each image is integrated for at least 64 seconds and read-out piecewise into telemetry as 128 bins every 0.5 seconds. This was originally the “premier” NRL sensor; however, for this ISS mission its limited 4.1° vertical field-of-view is not well suited to the low 375 km orbit altitude. Moreover, the sensor has suffered a significant degradation of its short wavelengt sensitivity below 145 nm which could not be corrected for this mission. Peak sensitivity near 150 nm is approximately 1.7 counts/sec/Rayleigh, and spectral resolution is 0.7 nm.
- MUV Spectrometer
The MUV spectrometer is an f/5 0.125-m Ebert-Fastie monochromator fed by a 0.25-m off-axis telescope and uses an EMR "F" CsTe photomultiplier tube detector. The spectrograph covers 195-317 nm at 1.0 nm resolution in a 25-sec scan. Sensitivity near 240 nm is ~7 counts/sec/Rayleigh.
- NUV Spectrometer
The NUV spectrometer is an f/5 0.125-m Ebert-Fastie monochromator fed by a 0.125-m off-axis telescope and uses an EMR photomultiplier tube detector. The spectrograph covers 295-399 nm at 0.7 nm resolution in a 25-sec scan. Peak ensitivity near 330 nm is ~7 counts/sec/Rayleigh.
- NIR Spectrometer
The NIR spectometer is an f/5 0.125 Ebert-Fastie monochomator fed by a 0.125 off-axis telescope. Its detector is not directly attached to the spectrometer, but fed by a fiber optic cable to the radiative cooled IR detector box which houses the Hamamatsu R943-07 series photomultiplier tube. The NIR spectrometer covers the passband 725-870 nm ar 0.84-nm spectral resolution during its 25-sec scan. This system uses a glass filter to restrict out-of-band light.
- 777 Photometer
The 777.4 nm photometer is tuned for an nighttime O++e recombination line of atomic oxygen. It uses a 0.25-m off-axis telescope and a Barr Associates 1.5-nm passband interference filter coupled to a Hamamatsu R943 photomultiplier tube via a fiber optic cable.
- 630 Photometer
The 630 nm photometer is tuned for a O emission feature present in the nightglow. A 0.25-m off axis telescope feeds an interference filter coupled to a Hamamatsu R943 PMT through a fiber optic cable.
- 765 Photometer
This sensor is an updated version of the original 589 nm sodium line photometer. A new 1.5-nm passband interference filter was installed, optimized for a portion of the O2 Atmospheric (0-0) band suitable for temperature sensing. This filter stage is fed by a 0.125-m off-axis telescope and connected to its detector via fiber optic cable. This sensor was checked to assess its off-axis light rejection, which was unchanged after extended storage.
|Sensor||Passband||Field-of-View (°)||Integration Time (sec)||Aperture Area (mm)||Telemetry Rate (bits/sec)||Sensitivity (counts/sec/Rayleigh)|
|EUV||55 - 111 nm @ 1.25 nm res||0.1 x 2.4||0.5||70 x 70||2560||0.2 - 0.5|
|FUV||130 - 170 nm @ 0.7nm res||4.1 x 0.1||64-90||21 x 25||4096||0.1 - 0.7|
|MUV||190 - 317 nm @ 0.95nm res||0.1 x 2.1||0.025||42 x 50||800||1 - 7|
|NUV||295 - 399 nm @ 0.7nm res||0.1 x 2.1||0.025||21 x 25||800||4 - 7|
|NIR||722 - 874 nm @ 0.84nm res||0.1 x 2.1||0.025||21 x 25||800||0.05 - 0.13|
|630 Phot||1.5 nm @ 630 nm||0.2 x 2.1||0.1||42 x 50||200||4.03|
|765 Phot||1.5 nm @ 765 nm||0.1 x 2.1||0.1||42 x 50||200||2.57|
|777 Phot||1.5 nm @ 777 nm||0.2 x 2.1||0.1||21 x 25||200||0.86|