SOHO Intercalibration Joint Observing Program 06
SOHO/SERTS CROSS-CALIBRATION
Author: R.J. Thomas
(SERTS/CDS/SUMER)
(Updated: 11-November-1996.)
OBJECTIVES:
CDS and possibly SUMER radiometric cross-calibration using the SERTS rocket.
CDS wavelength cross-calibration using the SERTS rocket.
EIT and CELIAS SEM radiometric cross-calibration using the SERTS rocket.
Determination of EIT 304A channel spectral composition using SERTS rocket.
CONDITIONS NECESSARY TO RUN
Flight of the SERTS rocket experiment, and operations of CDS and possibly SUMER to assure co-temporal observations of overlapping fields of view on the sun at appropriate wavelengths.
DESCRIPTION OF THE SERTS EXPERIMENT
The Solar EUV Rocket Telescope and Spectrograph (SERTS) is scheduled for flight on 1996 November 13, 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 '96 flight, it will be configured to observe the wavelength interval 296-352A with an intensified CCD detector and multilayer-coated grating. It will simultaneously record high resolution spectra over an area of 5x276arcsec, and spectroheliograms of two regions 5x8arcmin in size. An end-to-end radiometric calibration of the rocket instrument will be 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.
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 1830UT on 1996 Nov 13, with a window of 30 min. Observations of Target 1 occur from T+99sec to TBD. Rocket then slews to the second position, and observations of Target 2 occur from TBD to T+530sec. Baseline for Target 1 is a quiet region at solar coordinates X=0, Y=+390arcsec (defined by center of narrow slit), with Target 2 at disk center X=0, Y=0. If a suitable solar active region is available, it may be targeted instead. In either case, the slit will be aligned to the solar rotation axis and pointing will be to an accuracy of +-10arcsec in X,Y and +-5deg in roll.
CDS: Ten lines will be observed with 20-pixel windows from the NIS, using the 4x240arcsec slit and 30s exposures. The number of observed CDS lines is limited by the telemetry rate and need to have measurements co-temporal with the brief SERTS flight. To assure FOV overlap, 15 raster steps of 4-arcsec each will be made covering a total area of 60x240arcsec centered on Target 2. The selected lines are: He II 304, Mg VIII 315, Si VIII 316, Si VIII 320, Fe XVI 335, Mg VIII 339, Si IX 345, Si X 347, Si IX 350, and Fe XII 352. (He II 304 is seen by NIS2 in second order.) This full sequence takes 536sec; 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.
SUMER: Since its spectral range does not overlap that of SERTS, no lines are observed in common. On the other hand, there are emission line pairs with one in each spectral bandpass which have known, invariant intensity ratios so that the measurement of one directly gives the value of the other. Unfortunately, all such candidates identified so far are either affected by blends or else too weak to be useful for cross-calibration. If any suitable line pairs of this type can be found, SUMER will be set to observe the UV component(s). Otherwise, it might observe He I 584A to investigate the question of helium line emission in conjunction with simultaneous He II 304A measurements obtained by SERTS and CDS. SUMER observations would be made in a raster pattern covering a width of 60arcsec, similar to that described for CDS. The 4x300arcsec slit would provide an optimum match to SERTS but may not be allowed due to count-rate limitations, especially if the final target is an active region.
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, which the baselined targets do.
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.
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 22, 1996 STATUS
The instrument and crew are back at Goddard Space Flight Center, where we are having a wonderful time looking over our new data set, and marvelling about the high sensitivity of the intensified CCD-detector, especially when compared to prior experiences with film. Quiet sun images at He II 304A were saturated in 3 sec, much shorter than even our most optimistic predictions! Since a wide range of exposure times down to 1 sec were planned and taken, we do have well exposed data for all lines and images at both pointing positions during the flight. In particular, solid measurements (more than 20-sigma) were made of all ten EUV spectral lines selected for cross-calibration with the SOHO/CDS instrument. Although the telemetry signal was far weaker than expected, we already have decoded 12 of the 19 data frames taken, and hope to retrieve the rest with some further effort.
One sad event was that the payload landed in a hard-rock lava field beyond the target site, and its door smashed into the telescope's primary mirror, which broke. Fortunately, we had planned to replace it anyway on future flights with a new, state-of-the-art telescope that is almost ready now, so there should be no serious impact to our ongoing SERTS program. Overall, we can now say that the SERTS-96 flight was very successful indeed.
November 13, 1996 STATUS
SERTS has been launched!
At exactly 18:30 UT today, the SERTS rocket was launched at White Sands, New Mexico. Preliminary indications are that all systems performed well. The instrument has been recovered. More news later.
The SOHO observations in support of the SERTS flight also went well. Here are a few of the images that were taken:
CDS 4x4 arcminute context map taken prior to flight. (GIF, PostScript)
CDS images taken during the SERTS flight. (GIF, PostScript)
EIT 304A image taken during the SERTS flight. (GIF, PostScript)
In addition, here are Yohkoh SXT images taken just before and after the SERTS flight. (GIF)
November 12, 1996 (T - 1 day) STATUS
The rocket instrument and all of its support systems passed a final full-scale flight simulation this morning in preparation for launch tomorrow. The latest solar images show that our targeting positions for NOAA AR7994 are still valid. The observing plan has been reviewed by the SOHO experimenters and all is in readiness for their collaboration. Several final practice runs of the ground command system are being carried out to assure that operations will go as smoothly as possible during flight.
November 11, 1996 (T - 2 days) STATUS
SERTS is now on the launch rail with its two-stage rocket motors attached and ready to fly. Over the weekend, a small active region appeared that will be near the center of the solar disk at the time of our flight at 1830UT on November 13. Our preliminary observing plan is first to place the center of the SERTS field of view at heliocentric coordinates +85, +210 arcsec, which would put the active region (NOAA AR7994) in one of our spectroheliogram lobes. Then the pointing would be changed to +85, -180 arcsec, placing AR7994 just below the center of our high resolution spectrograph slit which is aligned with the solar rotation axis. The first position will be observed between T+102 and T+165 sec, while the second position will be viewed from T+170 to T+500 sec, where T is the time of launch. This plan will be refined over the next two days.
The GIF images in the previous two links were formed from an EIT image taken in the 195A wavelength band. The active region target is shown both at the position when the image was made (blue rectangle), and also rotated to the position at the time of the flight (trapezoid).
November 08, 1996 (T - 5 days) STATUS
Detailed analyses of prior test runs show that all systems are 'go' for launch on schedule next week. Today the payload is being moved to the launch rail where it will be mated to the Terrier - Black Brant rocket motors, and tested again there. Focus now shifts to selection of observing targets, as the solar disk that will be visible during flight rotates into view. The sun remains very quiet, with no sunspots or other significant activity, and forecasts indicate that this situation will probably continue through our flight period. Thus, our primary target will most likely be disk center, with a secondary target about 6 arcmin due north of center.
November 06, 1996 (T - 7 days) STATUS
The payload was buttoned up yesterday in its final flight configuration. At the Mission Readiness Review, the experiment team and all support groups indicated that there were no outstanding unresolved problems, so we are on schedule for flight at 1830UT on November 13. Today the full-up formal 'horizontal' flight simulation was run, with all systems working as planned for a nominal sequence. Additional testing was then carried out to check all backup command sequences, using both primary and redundant secondary ground-link command systems, each of which also proved to work well.
November 04, 1996 (T - 9 days) STATUS
Analysis of dark images taken after vibration show that the CCD-camera is still working well. The high-speed science data can now be correctly read through the entire link of on-board compression, commutation, and telemetry, then ground recording, decommutation, stripping, and decompression. The attitude control system is actually fine; test results turned out to be within normal ranges after all. Post-vibration optical alignments were all 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 that the spectrograph has no visible light leaks. The H-alpha video slit-jaw camera was put though its paces, and is working fine. Tests were also made at this time of the upgraded ground-link command system, which starts the experiment observing sequence, allows for in-flight pointing corrections, and can shut down the vac-ion pumps in an emergency. Practice was done on both the normal flight sequence, as well as contingency procedures to recover from a wide range of potential in-flight anomalies.
November 03, 1996 STATUS
Yesterday, spin balance and bend tests were run successfully. The final observing sequence was thoroughly validated and burned into a ROM chip, which was then installed as part of the on-board instrument controller. Today, flight simulation tests again verified that the experiment can be started in both primary and backup modes, and that the new observing sequence is carried out as planned. The payload was then put through full-level vibrations in all three axes, with the vac-ion pumps operating. Afterwards, a second set of flight simulations showed that all experiment systems continue to function well according to housekeeping data, although there was some indication of a possible problem with the rocket's attitude control system. Dark images taken by the science CCD-camera during these tests are being processed and will be carefully analyzed to verify that the camera is still working properly.
November 01, 1996 STATUS
High-speed compressed data is now routinely being decoded properly, even for a special read-out mode that had not been checked before. However, some data drop-outs occasionally occur during telemetry decommutation, which is being investigated by the ground support group. Final trajectory predictions are available, and have been used to establish a detailed observing sequence. This sequence is being tested, and when validated, will be burned into the onboard instrument controller. The two redundant payload timers have been reprogrammed to match the new trajectory as well; they have been qualified and installed in the rocket telemetry section.
October 31, 1996 STATUS
An error was located in the software that passed telemetered data from the ground recorders to the experimenter's computer. This has been fixed, and the problem of properly decoding the high-speed compressed data seems to be solved. To make sure, the complete end-to-end system is being thoroughly validated.
October 30, 1996 STATUS
During testing, a problem was found in reading high-speed compressed data from the rocket telemetry system. It seems that the ground computer is not properly decompressing telemetered data using a system that works for direct readout of the detector. Trouble-shooting is underway, and the problem will be fixed or bypassed before the instrument goes through T&E and vibration.
October 29, 1996 STATUS
The scientific payload was fully assembled yesterday with all of its engineering support systems for a series of mechanical tests, such as weight and center of gravity. Ballast is now being added and the tests will be repeated. Once the final weight values are determined, the predicted trajectories (nominal and two-sigma low) will be recomputed, at which time we can finalize the spectrometer's exposure sequence. A recent upgrade to the rocket command system now allows the mid-course pointing maneuver to be controlled automatically by ground-computer, which should be a big improvement over the manual procedures of past flights; software is being developed to do this, and will be thoroughly checked-out over the next two weeks. Today, the SOHO satellite carried out a full practice run of observing sequences developed to coordinate with the SERTS launch (see plan below); early quick-look data from CDS, SUMER, EIT, and MDI all show good results so far.
October 28, 1996 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. Software were exercised that allow target selection from the latest SOHO/EIT and YOHKOH images. Using these, we picked disk center as the target position for the practice run of SOHO coordinated observations, which will be carried out tomorrow. Solar conditions remain very quiet, so that the target for our actual launch in 16 days will most likely also be quiet sun at disk center.
October 25, 1996 STATUS
The initial flight simulation tests run today 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-programed 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. Tests of the vac-ion pumps demonstrated that they can be turned off for at least 30 minutes without problem; this is important since they must be off for that length of time while the rocket is being armed on launch day.
October 24, 1996 STATUS
The SERTS instrument arrived at White Sands Missile Range yesterday, on schedule and in good shape. Tests show that the vac-ion pumps and intensified CCD detector, critical new components for this flight, are working well. Our launch date is now officially authorized for a 30 minute window starting Wednesday November 13 at 1830UT, with backup for the same times on Thursday November 14. A preliminary flight timeline has been established, and will be tested in upcoming timer runs. Electrical interface checks are underway, and pre-vibration optical alignment measurements are planned for later today. Tests will also be carried out to determine safe operating limits for the vac-ion pumps.
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