1. First Assessments of EUVI Performance on STEREO SECCHI
J. R. Lemen, J.-P. Wuelser, N. Nitta, and M. Aschwanden
Lockheed Martin Solar & Astrophysics Laboratory
First Assessments of EUVI Performance on STEREO SECCHI
028.01
Abstract
The SECCHI investigation on the STEREO mission contains two Extreme Ultraviolet Imagers (EUVI), one on the ahead spacecraft and one on the behind spacecraft. EUVI views the solar disk using multilayer-coated normal-incidence optics that image onto 1.6 arcsec-per-pixel back-thinned CCD detectors. Four wavelength bandpasses are observed in series, 17.1 nm (Fe IX), 19.5 nm (Fe XII), 28.4 nm (Fe XV), and 30.4 nm (He II), covering the chromospheric and coronal plasma temperatures. Science operations began in January 2007 and both EUVIs are working very well. The fine pointing system effectively removes low frequency spacecraft pointing jitter, so the image resolution quality is very good, approaching 3.5 arcsec. We present early on-orbit assessments of the performance of both EUVIS and updated predictions for the temperature dependent instrument response functions, which are compared to early observations. The lunar transit observed by the EUVI on the behind spacecraft is used to assess the point spread function.
This work is supported by the NASA STEREO mission under NRL contract N C-2035.
Key EUVI Characteristics
Field of View
1.7 solar radii circular field of view
Spatial Scale
1.6 arcsec pixels; 2048 x 2048 pixels
Band pass
He II 30.4nm Fe IX 17.1nm
Fe XII 19.5nm Fe XV 28.4nm
Optical System
Normal incidence (Ritchey-Chrétien)
Back-illluminated CCD (e2v)
Effective Focal Length
1750 mm
Aperture is 98 mm at primary mirror
Fine Pointing System Assessment
The EUVI employs a fine pointing system (FPS) which steers the secondary mirror in response to signals from the SECCHI Guide Telescope
Early in the commissioning phase, the FPS dramatically improved the quality of EUVI images, demonstrating the system is working very well
More recently, the spacecraft jitter is substantially reduced following tuning of the attitude control system algorithm
Images below with and without FPS demonstrate that the spacecraft contribution to jitter is now minimal
15 Feb :00:17 UT
171 Å
Response Assessment
Early in the mission when the two spacecraft were close together, images taken nearly simultaneously were used to compare the responses of the two telescopes.
As the plots (right) demonstrate, there is very good agreement between the telescopes. The pre-launch calibration was used to predict the response of each of the four band passes to solar spectra, and these will be used in coming months to compare the relative responses of the band passes.
Performance Assements
The EUVI instruments have been operating for about six months.
The performance of the EUVIs on the Ahead and Behind spacecraft is assessed
In particular, the following performance characteristics are investigated:
Response function
Fine pointing system
Spatial Resolution (Behind)
Roll angle (Behind)
Effective Area Response Functions
Temperature Reponse Functions
Roll Alignment Assessment
The transit of the moon across the sun on February 25, 2007 afforded a unique opportunity to calibrate the following parameters of the SECCHI instrument suite:  1. The absolute roll orientation of the EUVI, Cor1, and Cor2 detectors on Behind,     with respect to the spacecraft reference frame, and relative to each other  2. The absolute roll orientation of the EUVI Ahead detector
The Behind EUVI roll was calibrated by performing a best-fit of the observed Sun-Moon offset to the offset predicted by the ephemeris, and by taking into account the roll orientation of the spacecraft reference frame as measured by the star tracker
The Ahead EUVI roll was calibrated by cross-correlating solar features in co-temporal EUVI Ahead and Behind images, again taking into account the spacecraft  roll angles based on the star tracker measurements
We found the following roll angles  - The angle of the Behind EUVI north direction, measured eastward from spacecraft    north is degrees  - The angle of the Ahead EUVI north direction, measured eastward from spacecraft    north is degrees - The above roll angles have been incorporated into SECCHI SolarSoft routines  (euvi_point) and will be incorporated into the Level 0.5 FITS image headers in  the near future
Resolution Assessment
Lunar transit images taken on 25 Feb 2007 were used to perform a preliminary evaluation of the EUVI point spread function
A sum of Gaussians was fit to the lunar limb to place an upper limit on the resolution
Ray trace analysis (left) indicates that the geometric spot size is less than one pixel (=1.6 arcsec)
Mirror surface roughness, entrance filter diffraction, and charge spreading in the CCD will contribute to the resolution spot size
Fits to the core results in a 1σ diameter of 3.6 arcsec
The wing of the point spread function is expected to be mostly caused by entrance filter diffraction, but the analysis is not yet complete
Profiles over lunar limb
Lunar Transit
The Moon was observed to transit the Sun from the Behind spacecraft on 25 Feb 2007
The images below show the transit at different times (time goes left to right, top to bottom)
The Moon formed a natural occulter that was used to assess the spatial resolution of the EUVI
Preliminary result for core of PSF:
3.6 arcsec 1σ diameter
Conclusions
Preliminary assessments are that both EUVI telescopes are performing well and as expected from pre-launch design and calibration. In particular:
The effective area response functions of the two telescopes are very similar
The fine pointing system is working well, although the spacecraft induced jitter is much better than pre-flight predictions indicated
The a preliminary estimate to the point spread function is consistent with pre-launch predictions. Measured: ~3.6 arcsec 1σ
The roll angle of the Behind spacecraft was measured during the 25 Feb 2007 lunar transit. This will enable, by cross calibration, the establishment of the Ahead EUVI roll angle
Ray trace results for two field angles and different focus values between -150 µm and +150µm
Solar
Limb
Solar
Limb
May 2007