SOHO Intercalibration Joint Observing Program
06
SOHO/SERTS CROSS-CALIBRATION
(SERTS/CDS/EIT/CELIAS)
Author: R.J. Thomas
(Updated: 1997 November 06)
OBJECTIVES:
CDS radiometric and wavelength cross-calibration using the SERTS rocket.
EIT and CELIAS/SEM radiometric cross-calibration.
Determination of EIT 304A channel spectral composition.
CONDITIONS NECESSARY TO RUN:
Flight of the SERTS rocket experiment and operations of CDS to assure co-temporal observations of overlapping fields of view on the sun at appropriate wavelengths.
DESCRIPTION OF THE SERTS EXPERIMENT:
The GSFC Solar EUV Rocket Telescope and Spectrograph (SERTS) is scheduled for flight on 1997 November 18, with a principal objective of providing radiometric and wavelength calibrations for several experiments on the Solar and Heliospheric Observatory (SOHO) satellite mission. SERTS provides imaged EUV spectra and spectroheliograms of selected areas on the sun. For the '97 flight, it will be configured to observe the wavelength interval 298-355A with an intensified CCD detector and multilayer-coated grating. It will simultaneously record high resolution spectra over a spatial area of 7x360arcsec, and spectroheliograms of two regions 3x8.5arcmin in size. An end-to-end radiometric calibration of the rocket instrument was carried out at Rutherford-Appleton Lab in the same facility used to characterize the CDS experiment on SOHO, and using the same EUV light source recently re-calibrated by PTB against the BESSY synchrotron. Pre- and post-flight wavelength calibrations will be done at GSFC using well known lines of He II and Ne II within the SERTS bandpass.
The payload will also include a full-sun EUV monitor provided by the University of Southern California. This device was recently calibrated at the NIST SURF-II synchrotron, and is equivalent to the middle channel of the CELIAS/SEM instrument now flying aboard SOHO. Its measurements will provide a direct update to the radiometric calibration of CELIAS, and a check on the atmospheric extinction models used for correcting the primary SERTS data.
Operating Details:
Detailed observing plans are being developed. Following are some preliminary considerations for various instruments involved in this project.
SERTS: Launch is scheduled for 1935UT on 1997 Nov 18, with a window of 15 min. This allows SERTS observations within 1-hour of local solar noon at WSMR, while YOHKOH is also in sunlight. Observations of Target 1 occur from T+102sec to T+165sec. The rocket then slews to the second pointing position, and observations of Target 2 occur from T+170sec to T+500sec. The baseline objective for Target 2 is an active region near disk center (if available), with Target 1 positioned so that this active region falls within one of the SERTS lobes. The slit will most likely be aligned to the solar rotation axis with a pointing accuracy of +-10arcsec in X,Y and +-5deg in roll.
CDS: Sixteen 25-pixel windows will be observed with the NIS, using the 4x240arcsec slit and 30s exposures. The number of observed CDS windows is limited by the telemetry rate and need to have measurements co-temporal with the brief SERTS flight. To assure FOV overlap, 30 raster steps of 4-arcsec each will be made covering a total area of 120x240arcsec centered on Target 2. The observed lines include: He II 304, Mg VIII 315, Si VIII 316, Si VIII 320, Fe XV 327, Cr XIII 328, Al X 332, Fe XIV 334, Fe XVI 335, Fe XII 338, Mg VIII 339, Fe XI 341, Si IX 342, Si IX 345, Si X 347, Si IX 350, Fe XII 352, Fe XI 353, and Fe XIV 354. (He II 304 is seen by NIS2 in second order.) This full sequence takes approximately 500sec; it will be run at least 3 times: before, during, and after the SERTS flight. Before and after this sequence, another one will be run using the same raster pattern, slit, and exposure time, but reading out the full NIS1 spectrum for context. Spatial co-registration will be done by matching intensity distributions in He II 304 as measured by the two instruments.
EIT: Full disk images will be used before the SERTS flight for targeting, and after the flight for context information. During the flight, emphasis will be on the target regions and in particular on Target 2. The most directly relevant data are images at 304A, both for co-registration and for radiometric cross-calibration. Since SERTS clearly resolves the strong Si XI 303A and He II 304A lines blended in EIT for all of the solar features observed, SERTS measurements will permit better understanding of the spectral composition of these images. EIT data at other wavelengths would be also be desirable, especially to compare plasma parameters derived from EIT image-ratio techniques with those from the spectroscopic diagnostics available from SERTS observations.
MDI: Magnetograms will be made of the SERTS target fields during the flight. The high-resolution mode will be used if selected SERTS targets overlap the MDI-HR field of view.
CELIAS: Measurements will be made of the integrated 304A solar flux during the flight with the Solar Extreme Ultraviolet Monitor. These will be compared to SERTS observations of 17% of the solar disk scaled to the full sun through the EIT images. In addition, readings from the central SEM channel will be directly compared to the equivalent calibrated EUV monitor carried by the SERTS rocket.
We invite ALL observers to participate with collaborative observations. Spectroscopic data obtained during the SERTS flight will be provided for collaborative work upon request.
November 18, 1997 -- SERTS SUCCESSFULLY LAUNCHED
The SERTS sounding rocket was launched successfully today at 19:35 UT. All indications at this point are that the mission was successful. The instrument appeared to be well pointed, and a full set of data was obtained. The payload has been recovered safely.
November 18, 1997 (T - 2 hours) STATUS
The primary pointing target has been finalized as N21.2E18.4 (-286,+315arcsec), which will be observed for the first 70 sec in the SERTS lobe and then for the remaining 330 sec with the narrow spectrograph slit. This position is near the center of active region NOAA-8108 between its two main spot groups, so that observations will concentrate on the tops of the active region loop system. The 'T - 3 hour' checkout has been performed, and all is ready for launch at 1935UT.
November 17, 1997 (T - 1 days) STATUS
The rocket instrument and all of its support systems passed a final full-scale flight simulation this morning in preparation for launch tomorrow. Based on the latest solar images, our present observing plan is first to place the center of the SERTS slit at heliocentric coordinates -345, -132 arcsec, which would put the active region NOAA-8108 in one of our spectroheliogram lobes. After the first four exposures, taking 70 sec, the pointing will be changed to -345, +303 arcsec, placing the brightest EUV loops in NOAA-8108 onto the SERTS spectrograph slit, which is aligned with the solar rotation axis. This observing plan has been reviewed by the SOHO experimenters, and all is in readiness for their collaboration. Several final test runs of the ground command and telemetry systems are being carried out to clean up a few noise problems on housekeeping data that showed up during this morning's 'vertical' flight simulation. (These were found to be due to the land lines between the rail and blockhouse, so will not affect any flight data.)
November 15, 1997 (T - 3 days) STATUS
Liquid nitrogen lines were connected to the instrument on the launch rail, and procedures to cool the CCD detector were developed and practiced. The VLA collaborative observations have now been officially authorized. Modifications to the CDS observing windows were worked out that include a pair of Fe XIII density-sensitive lines (320.80A/348.18A), providing additional scientific value to the planned data set. Active region NOAA-8108 continues to be our baseline observing target, at a predicted position near N20E15.
November 14, 1997 (T - 4 days) STATUS
The Mission Readiness Review was held this morning; the experiment team and all support groups indicated that there were no outstanding unresolved problems, so we remain on schedule for flight at 1935UT on November 18. This afternoon, the payload was moved to the launch rail and mated to the Terrier - Black Brant rocket motors. Unless something else develops over the weekend, the solar target will be active region NOAA-8108, which is now predicted to be at N19E17 at launch. We just learned that Dr. Gopalswamy (Catholic University) may be able to obtain VLA observations at 20, 6, and 3.6 cm during our flight, which would greatly enhance the scientific value of our EUV data. We invite ALL observers to participate with collaborative observations. Spectroscopic data obtained during any SERTS flight will be provided for collaborative work upon request.
November 13, 1997 (T - 5 days) STATUS
Today the full-up formal 'horizontal' flight simulation was run, with all systems working as planned for a nominal sequence. Post-vibration optical alignments were measured and found to be well within tolerances. The instrument was moved to the optical room where sunlight is fed into it from a heliostat mirror. Detector images there verified that the spectrograph has no visible light leaks. The H-alpha video slit-jaw camera was exercised, and is working fine, although not at best focus. Measurements were made setting the monitor cursor on the center of the spectrograph slit, to allow for pointing corrections during flight if needed. The CDS studies to be run during our SERTS flight will be tested this Friday and Saturday on SOHO, when CDS will be switched over to its high-telemetry mode. At this point, we are considering the active region now on the north-east limb as our baseline flight target; final target selection will be made on Monday.
November 12, 1997 (T - 6 days) STATUS
Complete flight-simulation testing showed that all essential systems continue to function well after vibration. The instrument can still be started in both primary and backup modes, the vac-ion pumps are working beautifully, and the observing sequence runs as planned. In particular, dark images taken by the science CCD-camera during these tests demonstrate that it is in good health. The EUV monitor from USC also survived vibration in fine shape.
November 10, 1997 STATUS
On Saturday, the payload was spin-balanced, bend-tested, and had measurements made of its moment of inertia in roll. Today, the pitch and yaw moments of inertia were measured, and adjustments were made to the alignment of the rocket's igniter housing. The entire payload was vibrated in all three axes to the full levels required for launch certification. Also, dark images taken on Friday with the intensified-CCD detector were examined in detail to verify that all stages of data compression, telemetry, recording, stripping, decompression, and display are working properly.
November 7, 1997 STATUS
Various support sections have now been attached to our scientific payload, including command/telemetry, attitude control, and the all-important parachute. Additional timer tests were run to verify all functions in this configuration. In particular, since the payload was under vacuum for the first time here, we were able to demonstrate that the detector door and bypass-valve work properly. The EUV monitor supplied by USC was given its first test here, and seems to be working fine. The CDS observation study to be run during the SERTS flight was defined (see Operating Details: CDS, below), and is now being validated at RAL in England.
November 6, 1997 STATUS
Initial flight simulation tests were run yesterday and verified that the experiment can be started by ground command, as well as by backup signals from the internal timer, and that the instrument goes through its pre-programmed sequence of observations exactly as planned. Optical measurements showed that alignment of the telescope, spectrograph, and sun-sensor are all well within specs; these will be checked again after vibration testing scheduled for next week. The instrument is now in the heliostat to test the H-alpha slit-jaw camera, and to allow practice operating the ground-command pointing system.
November 4, 1997 STATUS
Due to range conflicts, the launch time is now set for a 15-minute window starting 1935UT on Nov 18. This launch window was chosen to allow SERTS observations while Yohkoh was in sunlight, and within 1 hour of local solar noon at WSMR.
November 3, 1997 STATUS
The SERTS payload has been successfully delivered to the launch site at White Sands Missile Range in New Mexico. Preparations are beginning for launch on November 18.
August 1, 1997 STATUS
Our launch date is now officially authorized for a window starting Tuesday November 18 at 1830UT, with backup for the same times on Wednesday November 19. A primary objective for this flight will be to provide radiometric and wavelength calibrations for several instruments aboard the SOHO satellite mission, as described below.
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