RADIOMETRIC INTER-CALIBRATION of SOHO

SUMER and its Radiometric Calibration Status
K. Wilhelm
Max-Planck-Institut für Aeronomie
D-37191 Katlenburg-Lindau, Germany

The SUMER instrument and especially its radiometric calibration is described, from laboratory calibration to in-orbit monitoring and performance.

CDS and its Radiometric Calibration Status
J. Lang
Rutherford Appleton Laboratory
Chilton, Didcot, Oxfordshire OX11 0QX, UK

The CDS calibration status is shown, with main focus on the NIS and pre-holiday period. Some remarks on the post-holiday period are given.

The Radiometric Calibration Status of the EUV Channels of UVCS
L. Gardner
Harvard-Smithsonian Center for Astrophysics
Cambridge, MA, USA

UVCS was calibrated in 1995 before launch using secondary detector standards traceable to the (US) National Institute of Standards and Technology. We will present those results, some new laboratory results based on replicas of the flight gratings, and comparison to on-orbit cross calibrations with other instruments through solar (e.g., SUMER) and stellar (e.g., IUE) observations. We will also discuss the stability of the UVCS calibration from beginning of mission to date.

SUMER Calibration Using Stars
P. Lemaire
Institut d'Astrophysique Spatiale, Universite de Paris XI
91405 Orsay cedex, France

Before the launch of SOHO, a set of stars near the ecliptic plane has been selected to check and to update the SUMER calibration. Since the launch, only the two brightest stars have been kept to make regular observations: alpha and rho Leo, seen near the Sun each year in August. The data were collected using several instrument modes of operation. The results obtained with the two detectors are discussed and compared.

LASCO and its Calibration Status
R. Howard/D. Wang
Naval Research Laboratory
4555 Overlook Ave SW, Washington DC 20375-5000, USA

The LASCO radiometric calibration status will be presented. The radiometric calibration has been performed using pre-flight calibration and stars. Several relative calibration sources are included within the LASCO instrument. Ground based eclipse observations provide a calibration check. The long-term relative response of the instrument will also be presented.

EIT Calibration Overview
F. Clette/J.-F. Hochedez
Observatoire Royal de Belgique
Avenue Circulaire, 3, B-1180 Bruxelles, Belgium

After a five-year effort, the analysis of EIT's pre-flight and in-flight calibrations is finally yielding firm results. In this introductory overview, we will summarize what we learned "internally" from EIT itself. This includes the interpretation of the pre-flight calibrations, the original flat-field components (CCD, grid), the in-flight PSF and straylight determination and NRL's calibration rocket program. Specific topics like the statistics of exposure time fluctuations, the effect of pixels binning, the filter factors or the plate-scale determination will also be introduced. This will lead to a discussion of past and future in-flight diagnostic techniques: which ones are feasible, productive or promising?

CELIAS/SEM Calibration and In-flight Monitoring
D. McMullin/D. Judge
USC Space Sciences Center
Los Angeles, California, USA

The CELIAS/SEM was calibrated pre-flight at the National Institute of Standards and Technology (NIST) Synchrotron Ultraviolet Radiation Facility (SURF-II). Since the pre-flight calibration, the stability of the CELIAS/SEM has been monitored periodically by rocket underflights. A CELIAS/SEM clone instrument has been calibrated at NIST SURF-III, and has flown 3 times during the SOHO mission. A comparison of the rocket and SOHO SEM instrument results will be presented.

What can VIRGO radiometry bring to the assessment of degradation of UV-Instruments onboard SOHO?
I. Rüedi/C. Fröhlich
PMOD/WRC
CH-7260 Davos, Switzerland

Recent analysis of the long-term behaviour of the VIRGO radiometers shows that more sophisticated methods are needed to reliably account for the degradation. The most important results are: (i) Long-term changes of optical properties are better described by a hyperbolic function than by an exponential, (ii) The time-dependence of the dose has to be taken into account explicitly.

CDS - SUMER Intercomparison
A. Pauluhn
INTEC HTA Bern & Institut für Astronomie ETHZ
CH-8092 Zürich, Switzerland

The results of the in-flight intercomparison of the CDS and SUMER instruments (JOP Intercal_01) are presented. The data set for this study consists of more than four years of almost monthly simultaneous observations of quiet Sun areas with SUMER and CDS. We investigate and compare the absolute radiance measurements of both instruments as well as their stability before and after SOHO's summer holiday.

Re-Calibration of SOHO by SERTS
R.J. Thomas
Laboratory for Astronomy and Solar Physics NASA/GSFC
Greenbelt, MD, 20771 USA

GSFC's Solar EUV Research Telescope and Spectrograph (SERTS) is a rocket instrument that obtains imaged high-resolution spectra of individual solar features to study the Sun's corona and upper transition region. Flights in 1997, 1999, and 2000 also provided radiometric and wavelength calibrations for several experiments on the SOHO satellite mission. For that purpose, end-to-end radiometric calibrations of SERTS were carried out after each flight in the same facility used to characterize the SOHO/CDS experiment at RAL, using the same EUV light source specially re-calibrated by PTB against the primary EUV radiation standard of BESSY-I. These measurements can establish the absolute SERTS response within 25% at 12 wavelengths covering its bandpass of 300-365A. Post-flight wavelength calibrations were done at GSFC using laboratory standard lines of He II and Ne II. Each SERTS payload also carried an EUV solar flux monitor kindly provided by USC; its readings were used to validate calculations of atmospheric EUV transmission over the rocket's trajectory, and to provide an updated calibration for SOHO/CELIAS. During each flight, SERTS and CDS observed the same solar locations, as demonstrated by subsequent data co-registration with simultaneous SOHO/EIT images, allowing the SERTS calibrations to be directly applied to both CDS and EIT. SERTS clearly resolves the strong Si XI and He II lines blended in EIT's 304A-channel, providing information on the spectral composition of those images as well. Cross-calibrations will be compared for periods before and after SOHO's 1998 loss of pointing control.
This work is supported by NASA RTOPs 370-18-37 & 344-17-38.

Comparison of CDS irradiance with SEM and EIT measurements
W.T. Thompson/J. Newmark
Emergent Information Technologies NASA/GSFC
Greenbelt, MD, 20771 USA
D. McMullin
USC Space Sciences Center
Los Angeles, California, USA

Observations of the solar irradiance have been made with the CDS Normal Incidence Spectrometer between 308-379 A and 513-633 A, on a semi-regular basis since March 1997. Also observed are the He II and Si XI lines at 304 A in second order. We have used these observations to estimate the signal that SEM should see on those dates. In order to fill in the wavelengths below 300 A, a DEM curve is generated from the CDS observations. Comparisons are also made with the EIT measurements for the same dates. Both pre-accident and post-recovery data are considered.

EIT In-flight Calibration and Inter-Calibration
J. Newmark
Emergent Information Technologies NASA/GSFC
Greenbelt, MD, 20771 USA

The EIT response has varied both temporally and spatially throughout the mission. Before intercalibrations can be attempted, these two components of degradation in EIT must be well modelled. We report here the latest results in this effort which allow us to compute well calibrated data, on a pixel by pixel basis, for the entire mission.
Useful intercalibrations consist of full disk irradiance measurements, full disk averaged spectral comparisons, and spatially resolved spectral comparisons. In order to obtain these comparisons we combine our calibrated image data with a differential emission measure technique we have developed. With our DEM maps we can calculate the intensity of any individual line, or the intensity through a well defined instrumental bandpass for each pixel in the EIT field of view. We use our DEM technique to approach each of the three intercalibration areas. First we compare our modeled SEM irradiance with their reported irradiance. Second, we compare our modeled disk irradiance spectra, covering 17-34 nm with the CDS results. Lastly, we compare our spatially resolved spectra with results from the SERTS 1997 flight.

Coronal polarized brightness measurements.
UVCS White Light Channel and LASCO inter-calibration.
M. Romoli
Dip. Astronomia e Scienza dello Spazio, Universita' di Firenze
Firenze, Italy

The electron density is one of the key parameters for the plasma diagnostics in the extended solar corona. One of the classical techniques to perform a direct measurement of the electron density integrated over the line of sight is to measure the K-corona polarized brightness. On board of SOHO there are two coronagraph polarimeters that are designed to perform such measurement: the White Light Channel (WLC) of UVCS and LASCO C2 and C3. These instruments have been carefully characterized in the past years in order to take into account all possible systematic errors, due mainly to stray light, spatial and time effects. The UVCS/WLC and C2 field-of-views overlap, as well as LASCO/C2 and C3 FOVs, thus, allowing a direct inter-calibration. Additional inter-calibrations have also been coordinated between the SOHO instruments and other spaceborne coronagraphs (Spartan 201) or ground based telescopes (Mauna Loa coronagraph and total solar eclipse observations). Both instruments' characterizations will be reviewed, together with the status of the inter-calibration.

The White Light Channel (WLC) on UVCS is a coronagraph polarimeter dedicated to the determination of the electron density in the extended solar corona, through the measurement of the broadband white light polarization brightness (pB).
At the beginning, the measurement of pB was based on the assumption that the stray light is not polarized and therefore contributes only to the accuracy of the pB measurement, but not to the absolute value of pB.
A discrepancy between pBs measured with WLC and pBs that could be found in literature had lead to a careful analysis of the data, of the calibration, and of the algorithm that computes the pB. The deep investigation that followed has brought up the existence of a stray light polarization.
The WLC goal is to measure pBs as little as half of a percent of the total measured signal. The coronal pB measurement is extracted from three consecutive exposures, called a pB cycle, under the assumption that the instrumental ``noise'', i.e., stray light and electrical noise, is not polarized. This measurement is extremely sensitive to any instrumental effect and instability that produce hardly noticeable variations in the UV measurements, both because the photon statistics is much less and because total brightness and not variations of it are measured.
The polarization of stray light has been characterized through several in flight tests. A model has been developed and a correction factor is given in order to remove instrumental stray light from coronal pBs.

SOHO Cleanliness
R. Thomas
ESA, ret.
now: Whiteoaks, Monkmead Lane, West Chiltington, UK

This paper reviews the advice given to the instrument teams about particulate and molecular cleanliness for their hardware and the practical measures taken at the assembly, test and launch preparation phases of production of the SOHO spacecraft. Special attention is paid to the thermal vacuum testing of both the service module and the complete spacecraft. The topic of molecular cleanliness modelling is further considered in the hope that this may be found useful for critical missions.

Discussion on realisation and monitoring of cleanliness:
Where was the cleanliness programme really effective?
U. Schühle
Max-Planck-Institut für Aeronomie
D-37191 Katlenburg-Lindau, Germany

The cleanliness programme within the SOHO project was a common effort of ESA and all SOHO experimenters with the primary goal to ensure stable performance of the spacecraft during the SOHO mission. Since each experiment on SOHO had its specific cleanliness requirements it was necessary to set up a cleanliness plan for each individual instrument. The performance of the instruments during the phases of scientific operation has proven in various ways whether this cleanliness effort was really effective, or how effective it was. We shall discuss and work out the experience gathered with SOHO in reducing performance degradation by cleanliness control, so as to communicate to future projects what lessons have been learned. Each participant of the discussion is welcomed to "tell his story" about the specific measures implemented for contamination control that were successful in the reduction of degradation. The special occasion of the SOHO loss-of-attitude phase may have given rise to degradation effects on spacecraft and/or instruments. These degradation mechanisms and their interpretation in regard to cleanliness shall be presented in this discussion.

The ISAS SOLAR-B mission - Science and Payload
J.L Culhane
Mullard Space Science Laboratory
Holmbury St. Mary, Dorking, Surrey RH5 6NT, UK

An introduction to the SOLAR-B mission will be given.

SOLAR-B EIS - The Laboratory Calibration
J. Lang
Rutherford Appleton Laboratory
Chilton, Didcot, Oxfordshire OX11 0QX, UK

The EIS instrument will be described, particularly the plans for the radiometric calibration.

STEREO
R. Howard/D. Moses
Naval Research Laboratory
4555 Overlook Ave SW, Washington DC 20375-5000, USA

The STEREO mission is designed to reveal the three dimensional nature of the Solar corona and interplanetary medium. The objectives are to reveal the physics related to the origin, evolution and interplanetary manifestations of Coronal Mass Ejections. Two identically instrumented spacecraft will be launched into an orbit about the Sun. One spacecraft will lead Earth and will separate from Earth at about 22 degrees per year. The other will lag Earth and also separate at 22 degrees per year. The mission objectives and instrument characteristics will be presented.

TIMED and EOS SORCE
T. Woods
LASP/U. Colorado
Boulder, Colorado, USA

I would like to present the instrument capabilities of our NASA solar irradiance instruments:

• UARS SOLSTICE (119-420 nm, 1991-present),
• rocket (1-122 nm, 1988-present, about once per year),
• TIMED SEE (1-190 nm, March 7, 2001 launch), and
• EOS SORCE (1-34 nm, 115-2000 nm, July 2002 launch).