The NASA Marshall Space Flight Center (MSFC) has developed a science camera suitable for sub-orbital missions for observations in the UV, EUV and soft X-ray. Six cameras were built and tested for the Chromospheric Lyman-Alpha Spectro-Polarimeter (CLASP), a joint National Astronomical Observatory of Japan (NAOJ) and MSFC sounding rocket mission. The CLASP camera design includes a frame-transfer e2v CCD57-10 512x512 detector, dual channel analog readout electronics and an internally mounted cold block. At the flight operating temperature of -20 C, the CLASP cameras achieved the low-noise performance requirements (less than or equal to 25 e- read noise and greater than or equal to 10 e-/sec/pix dark current), in addition to maintaining a stable gain of approximately equal to 2.0 e-/DN. The e2v CCD57-10 detectors were coated with Lumogen-E to improve quantum efficiency (QE) at the Lyman- wavelength. A vacuum ultra-violet (VUV) monochromator and a NIST calibrated photodiode were employed to measure the QE of each camera. Four flight-like cameras were tested in a high-vacuum chamber, which was configured to operate several tests intended to verify the QE, gain, read noise, dark current and residual non-linearity of the CCD. We present and discuss the QE measurements performed on the CLASP cameras. We also discuss the high-vacuum system outfitted for testing of UV and EUV science cameras at MSFC

Beabout, B.

Beabout, D.

Champey, Patrick R.

Cirtain, J.

Hyde, D.

Kobayashi, Ken

Robertson, B.

Stewart, M.

Winebarger, A.

NASA Technical Reports Server

VUV Testing of Science Cameras at MSFC: QE measurement
of the CLASP Flight Cameras
Champey, P.a,b Kobayashi, K.b Winebarger, A.b Cirtain, Jb Hyde, D.b Robertson, B.b
Beabout, B.b Beabout, D.b and Stewart, M.a
aThe University of Alabama in Huntsville, Huntsville, AL, United States
bNASA Marshall Space Flight Center, Huntsville, AL, United States
ABSTRACT
The NASA Marshall Space Flight Center (MSFC) has developed a science camera suitable for sub-orbital missions
for observations in the UV, EUV and soft X-ray. Six cameras were built and tested for the Chromospheric Lyman-
Alpha Spectro-Polarimeter (CLASP), a joint MSFC, National Astronomical Observatory of Japan (NAOJ),
Instituto de Astrofisica de Canarias (IAC) and Institut D’Astrophysique Spatiale (IAS) sounding rocket mission.
The CLASP camera design includes a frame-transfer e2v CCD57-10 512×512 detector, dual channel analog
readout and an internally mounted cold block. At the flight CCD temperature of -20C, the CLASP cameras
exceeded the low-noise performance requirements ( ≤ 25 e− read noise and ≤ 10 e−/sec/pixel dark current), in
addition to maintaining a stable gain of ≈ 2.0 e−/DN. The e2v CCD57-10 detectors were coated with Lumogen-E
to improve quantum efficiency (QE) at the Lyman-α wavelength. A vacuum ultra-violet (VUV) monochromator
and a NIST calibrated photodiode were employed to measure the QE of each camera. Three flight cameras
and one engineering camera were tested in a high-vacuum chamber, which was configured to operate several
tests intended to verify the QE, gain, read noise and dark current of the CCD. We present and discuss the QE
measurements performed on the CLASP cameras. We also discuss the high-vacuum system outfitted for testing
of UV, EUV and X-ray science cameras at MSFC.
Keywords: Characterization, Camera, CCD, VUV, Sounding Rocket
1. INTRODUCTION
The Chromospheric Lyman-Alpha Spectro-Polarimeter (CLASP) is a sounding rocket instrument that is nearing
the scheduled launch date of September 3, 2015. NASA MSFC, NAOJ, IAC and IAS collaborated in the
development of the CLASP instrument.
The purpose of CLASP is to measure the linear polarization profiles caused by scattering processes and the
Hanle effect in the Ly-α line. The magnetic field information will be obtained from the measured Stokes Q/I
and U/I profiles themselves and through detailed radiative transfer modeling of the observed Ly-α intensity
and polarization using the most advanced magnetohydrodynamic models of the solar atmosphere.1,2 This will
provide, for the first time, a diagnostic tool for magnetic field measurements in the upper chromosphere and
transition region.
The CLASP instrument consists of a Cassegrain telescope optimized for reflecting the Ly-α line (121.6 nm),
a slit jaw imager and the spectro-polarimeter. The spectro-polarimeter produces two spectra simultaneously
(corresponding to two orthogonal polarization states). It consists of a slit, polarization modulation unit (PMU),
diffraction grating, two reimaging mirrors, two polarization analyzers, and two cameras. The PMU uses a
rotating 1/2 waveplate, which allows for measurement of both Stokes Q and U with fixed polarization analyzers.
The rotation of the waveplate sends simultaneous trigger pulses to the cameras, which initiates frame transfer,
effectively ending an exposure and beginning the next. The trigger pulses will be sent to the cameras every
300 milliseconds, and a total of 16 pulses will be sent for every rotation of the waveplate. The polarization
Further author information: (Send correspondence to Patrick Champey)
Patrick Champey: E-mail: patrick.r.champey@nasa.gov
https://ntrs.nasa.gov/search.jsp?R=20150018357 2019-08-31T06:27:19+00:00Z
produced by the Hanle effect in Ly-α is expected to be on the order of 0.1%.1 Therefore the two cameras must
be synchronized to a high accuracy and optimized for precision and stability.3,4
Strict science requirements imposed on the CLASP cameras required a complicated design and a demanding
development process. For this reason, we took advantage of the resources and facilities at MSFC and The
University of Alabama in Huntsville to develop the cameras in-house. Below we discuss a series of tests performed
on the CLASP flight cameras, which characterize key parameters of the camera’s performance.
2. CAMERA PERFORMANCE
The science goals for CLASP called for cameras with stable, low-noise performance to accurately measure sensitive
polarization signatures. The flight cameras were designed to operate with a gain of 2.0 e−/DN, dark current
≤ 10 e−/pix/sec, and read noise of ≤ 25 e−. Meeting the low dark current requirement is achieved by cooling
the detectors with liquid nitrogen (LN2) to 253 K ( −20◦C). The CCD is thermally strapped to an internally
mounted copper cold-block, which acts as a thermal reservoir. During the CLASP flight, the cold-block will be
cooled to roughly −40◦C and a heater will be used to maintain steady CCD temperature at −20◦C.
Figure 1: A typical dark frame has an offset between the left and right half. The first 280 columns are read out
by the left tap and the remaining 280 columns are read out by the right tap.
The e2v CCD57-10 detectors operate in frame transfer mode. This allows for continuous exposure without the
use of a shutter. The CCD57-10s are a 512×512 detector with several additional rows of inactive pixels, producing
528×560 frames. Each exposure is read out by two channels at the lowest readout rate, which optimizes for low-
noise performance needed to meet science goals. Reading the CCD with two independent channels produces
subtle differences in background intensity and the gain. This can be seen in a typical dark frame (Figure 1). The
offset between the two halves of the detector requires measurements to be made independently for both sides.
The Solar Instrumentation lab at MSFC hosts a 245 cm L × 60 cm D cylindrical vacuum chamber in a 10k
clean room. The chamber is outfitted with an optical bench, a cryogenic flow system, vacuum translation stages,
optical and thermal monitoring systems, and several UV, EUV and X-ray sources that can be interfaced to the
chamber. The system is capable of achieving vacuum < 1.0× 10−6torr, via a 3-stage system of roughing, turbo-
molecular and cryogenic pumps. For the CLASP camera testing program, an Acton VM-502 monochromator
and Hamamatsu deuterium lamp were used to generate the monochromatic Ly-α beam. A NIST calibrated Opto
Diode AXUV100G photodiode and Kiethley 6485 Picoammeter were employed to measure the irradiance of the
beam, which provided a standard of measurement to calibrate the camera at the Ly-α line.
Characterization of each camera was performed in vacuum, with the CCD cooled to the flight operating
temperature of −20◦C. Each test cycle included several data points for the measurable characteristics listed
above, and typically took 8–10 hours to complete. The experiment and test procedure were configured to
perform these tests in succession without having to break vacuum, or change the system.
Figure 2: Left : Photodiode and Fe55 source mounted to a translation stage in front of the camera. The Fe55
source is used to measure the gain of the camera. Right : The translation stage can be seen mounted to the optics
bench. A light baﬄe blocked the wings of the diverging beam, only passing the central Ly-α portion. The baﬄe
was also used to test linearity. A variable intensity LED array mounted outside of a vacuum window above the
chamber illuminated the baﬄe, creating a uniformly diffuse field.
2.1 DARK CURRENT, READ NOISE & GAIN
Dark current was measured by taking a series of dark images at different exposure times. The average dark
intensity was plotted against exposure time. The slope of the line fitted to the data is the dark current rate
[e−/pix/sec]. Dark current measurements were performed periodically throughout each testing cycle to monitor
the stability of the cameras over a long period of operation. We calculated read noise by fitting a gaussian
function to a histogram of pixel intensities after dark subtraction was performed. The width of the gaussian
fitted to the residual pixel intensity distribution is taken as the read noise.5 Results are listed in Table 1.
The camera gain is determined by the detector response to Fe55 photons. Fe55 decays into Mn, producing
Mn Kα1, Kα2, Kβ1, and Kβ2 lines. These lines have well known energies and emission probabilities. We generate
a histogram of the intensity of detected Fe55 photons and fit a quadruple gaussian function to the histogram. By
determining the relative amplitudes of the 4 Mn Kα and Kβ lines and the difference in intensity of the histogram
peaks, the camera gain is determined.5–8
2.2 QUANTUM EFFICIENCY
We define the quantum efficiency of the detector as the percentage of photons that generate a signal on the
detector. Taking the ratio of the number of photons detected to the number of photons incident on the detector
yields the QE. e2v delivers CCDs with anti-reflective coating optimized for visible, which means high absorption
in UV. In an attempt to increase the QE at the Ly-α wavelength, the CLASP e2v CCD57-10 CCDs were coated
with Acton’s Lumogen-E. This thin film phosphor coating absorbs UV photons then re-emits photons in the
visible (λ ≈ 530 nm), a process known as fluorescence.
The quantum efficiency at Ly-α of each of the 4 cameras was measured in high-vacuum (< 1.0× 10−6torr).
The cameras were mounted to an optical bench in the chamber, while the NIST calibrated photodiode was
mounted to a translation stage positioned in front of the camera. The camera and photodiode were exposed to
the monochromatic Ly-α beam in alternating sets, over a period of a one hour. The photodiode would translate
into the beam, record several hundred samples, then move out of the beam. This was followed by a set of camera
exposures, then dark exposures for both the camera and photodiode. Multiple data points were necessary to
average out any instability of the deuterium lamp. Each subset of photodiode measurements and CCD exposures
were taken over a period of ≈ 1 minute: the full QE test took ≈ 1 hour to complete.
Beginning with raw FITS images, standard image reduction techniques were performed to remove dark
current, fixed pattern noise and correct for gain. The average intensity over the full frame was calculated for
each subset of exposures. The photodiode data was also corrected for dark current and the average number of
incident photons was calculated for each subset using the NIST calibration data. Computing the ratio of the
incident photons on the camera to number of photons incident on the photodiode yielded the QE. The measured
QE for each of the four cameras is reported in Table 1.
Characteristic Requirement SN 001 SN 004 SN 005 SN 006
Gain [e−/DN]
(Left/Right)
2.0 ± 0.5 1.97/2.04 1.92/2.04 1.94/2.03 1.88/1.90
Dark Current [e−/sec/pix]
(Left/Right)
≤ 10.0 0.3/0.2 0.4/0.3 0.3/0.1 0.5/0.4
Read Noise [e−]
(Left/Right)
≤ 25.0 5.4/5.6 5.3/5.6 5.5/5.6 5.6/5.4
Quantum Efficiency [%] 30 14.2 ± 1.5 12.1 ± 1.5 20.0 ± 2.5 20.0 ± 1.9
Table 1: Summary of the performance of the three flight cameras (SN 004, SN 005, SN 006) and one engineering
model/flight spare (SN 001).
In an attempt to improve accuracy in the experiment, we repeated the QE test with SN001, but with an
aperture placed in the monochromatic beam. The justification for this approach was the beam uniformity was
worse than expected. We placed a 5 mm diameter aperture in the optical beam to mask the photodiode and
provide an equivalent area of exposure for both detectors. A correction factor of 0.8 was applied to all four
cameras, which reduced the QE by 20%.
Figure 3: Left : A 10 second Ly-α exposure. Right : A 10 second Ly-α exposure after a masking the CCD.
The quantum efficiency measurements reported in Table 1 include the correction factor from masking. Cam-
eras SN005 and SN006 were selected as the science cameras for their higher QE. SN004 was selected as the slit
jaw camera. SN001 had some damage to the Lumogen-E coating, which occurred during handling of the CCD.
SN001 was selected as the flight spare and was used for subsequent testing and troubleshooting.
3. CONCLUSION
The four flight cameras built for CLASP were tested in high-vacuum under flight-like conditions. The gain,
dark current, read noise and quantum efficiency were measured and verified against defined requirements. The
gain measured for all four cameras is within the 2.0 e−/DN requirement. Some differences in gain between the
readout channels were noted and a gain correction factor will be applied to science data. Measured dark current
and read noise in all four cameras exceeded requirements. Dark current rate is about ≈ 30 times better than the
requirement, while the readout noise is ≈ 4.5 times better than the requirement. The QE at Ly-α was measured
by comparing the average number of photons detected by the CCD to the number of incident photons detected
by a NIST calibrated photodiode. An aperture was installed in the monochromatic beam because the beam
uniformity was worse that expected. As a result, a correction factor of 0.8 was applied to the QE measured for
each camera.
ACKNOWLEDGMENTS
We acknowledge the Chromospheric Lyman-Alpha Spectro-Polarimeter grant funded by NASA’s Low Cost Access
to Space program under Research Opportunities in Space and Earth Sciences (ROSES). NASA MSFC led this
project; partners include the NAOJ, Lockheed Martin (LMSAL), IAC, IAS, Astronomical Institute of ASCR,
and the University of Oslo.
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VUV Testing of Science Cameras at MSFC: QE Measurement of the CLASP Flight Cameras

VUV Testing of Science Cameras at MSFC: QE Measurement of the CLASP Flight Cameras

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