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The VIRGO instrument update

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VIRGO message # 93 on Wed Dec 6 03:57:25 GMT 1995

Mail from: cfrohlich@obsun.pmodwrc.ch

Subject: VIRGO Switch On and Cover Release

Original Date: Wed, 6 Dec 1995 04:53:53 --100

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Dear all

We just finished to celebrate the first scientific result of SOHO: 1425 Wm-2 (EGSE value with no guarantee) which was measured to prove that the cover mechanism was released properly (not for scientific reasons really). By the way the SOHO is already about 500 000 km from the Earth. All the instruments within VIRGO have been switched on and are in good shape. We will now wait for the daily pulse to synchronize VIRGO which will take place in about 20 hours. Then we will let it out-gas until between Xmas and New Year when we do the final commissioning and start real operations. We will also run sometime later 3 time-tagged command procedures to do electrical calibration of DIARAD and PMOs.

I would like to take this oportunity to thank all the VIRGO team for their efforts in our endavour - we have successully completed the first step.

Best regards, Claus

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VIRGO message # 94 on Tue Dec 12 19:32:34 GMT 1995

Mail from: cfrohlich@obsun.pmodwrc.ch

Subject: VIRGO-Status and Planning

Original Date: Tue, 12 Dec 1995 20:31:19 --100

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Dear All

This is an update of the VIRGO Status and information on the forthcoming actions:

Status: (i) The instrument works fine, the temperatures are reasonably constant. The radiation shield has increased slightly since yesterday. The reason could be a increase in the OSR temperature (we are checking with FOT) which in turn could be due to switch on of other instruments or subsystems. (ii) During the last three days we had some time tagged commands (TTC) in order to switch instrument modes and the reference. These are used to check the electrical calibrations and some instrument performance and are mainly the IcoIs. The following commands have been sent with the relevant times (they are in UTC and the execution in VIRGO should be during the block starting 3 minutes later). Planned activities (most important for the IcoIs): (i) The instruments were intended to be switched to operational on day 23 (radiometers) and day 30 (SPM and LOI), with a test of all covers on day 23.

This was planned to start after MCC2 (second midcourse correction) and related S/C activities. Now the MCC2 by itself is so small that it will not take place as such (only 76 cm/s, Centaur did such a good job, that also MCC1 was quite small 4 m/s and we have a lot hydracine left [possibly for 30 years of operations, some people say]). They will however make a pre-Halo injection maneuver which will then ease the real insertion and make it happen earlier (now planned for 17 March instead of early April). This maneuver will also be used for calibrating some thrusters not yet used in earlier orbit maneuvers. This 'MCC2' is planned for January 4, and the related S/C activities for January 5 and 6.

In view of these changes I have decided to postpone our actions until January, 7 and do the activities of both days (23 and 30) together (PROC20, PROC17 and PROC19). This means that the test of all covers around day 23 is no longer and I would like to propose to open them by TTC for 6-9 minutes each on December 22 or 23. Vicente volunteered to watch the EGSE during that time and signal to us any irregularities. For the instrument we have to put them in a mode where they can see the sun: the PMO's will be set to shutter auto and BU- Mux accordingly, for the SPM we need only to switch the BU-Mux, for the DIARAD we suggest to switch it to 'auto left' before the opening and back to 'long calibration' after and for the LOI we switch off the calibration before and back on after (comments on the proposed actions by the IcoI before Dec. 14). We will test the finally agreed sequence at PMOD/WRC with the VIRGO instrument to make sure that the commands are correct.

Our plans are to send Thomas Frohlich (for the FOT command interface) and have asked SSD to have Udo Telljohann for the EGSE operation during commanding. Thierry Appourchaux will participate also in the January activities as LOI representative mainly for the off-pointing tests. If some other CoI or technical managers would like to participate in the January activities, let me know. (ii) On January 11 and 12 the off-pointing tests are planned. These consists in stepping the pointing of the S/C E-W (1st day) and N-S (2nd day). The steps are 30 arcsec and cover ranges 0 to 10 arcmin E (N), back to 0 and to 10 arcmin W (S) and back to 0. It takes about 10h40 minutes (settling time of the S/C for each step 480 s). Before and after we will lock and unlock the pointing system of LOI in order to have the solar image moving over the detector. This test will be done together with MDI which will also let image move. The objective of this test is flat fielding information for the imagers and offsets for the SPM and may be radiometers (Question to the IcoI: should we open the covers of SPM-B and PMO-B during that test?).

(iii) The IcoI shall prepare a first report on the instrument performance by assessment of the level0 data of day 6 to 12 (7-13 Dec, day 5 is not very relevant because of synchronization only at the end and because of OBT phase corrections around 23h00 which messed-up some blocks). This evaluation will also be used to check the VDC algorithms for level1 calculations. The report should contain a comparison of the values from the last ground tests with the ones now in order to assess: (i) general performance (e.g. internal temperature distributions) (ii) electrical performance (e.g. resistors, electrical calibrations) (iii) noise performance (e.g. power spectral of the dark signals). If you have any questions concerning this report please let me know before Friday.

Best regards, Claus Frohlich

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VIRGO message # 95 on Fri Dec 22 17:51:18 GMT 1995

Mail from: cfrohlich@obsun.pmodwrc.ch

Subject: Pi-Message

Original Date: Tue, 19 Dec 1995 10:33:51 +0100

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Dear all

I had to write a VIRGO Status report, which is copied to the end of this message for your information. I hope you agree with my statements.

As to the plans over X-Mas: We will have a series of cover tests by time tagged commands on 24 December - a real X-Mas present for the European (timewise). Vicente will watch the tests from the EGSE at EOF. The data will be evaluated to level0 files by the VDC and will be available after about 14h00 (CET) on 25 December for further evaluation by the ICoIs.

The next action will be after the manoeuvres which are now planned for the first half of January. We are preparing the corresponding TSTOL procedures which are slightly different from the ones originally planned for execution on mission days 23 and 30. Both actions are now combined in one which is planned to be executed on 16 January.

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VIRGO Status Report

VIRGO was switched on during day 5 and the cover released the success of which was tested by opening the cover of one radiometer which delivered the first 'science data' of SOHO. Some electrical tests have been run on mission days 8, 9 and 10 with time tagged commands. The data of each instrument have been analyzed as far as this is possible with no sun. The performance of the data acquisition system is nominal. The temperatures of the sensor unit and the power supply are well within the range expected. There is an oscillation in the heat sink temperature of the sensor unit and the bottom of the power supply with a 195 minutes period and an amplitude of about 0.1 K. Also other temperatures show a similar oscillation but smaller amplitudes which could indicate that it comes from the outside. We have not yet been able to test whether this oscillation is in the VIRGO reference temperatures measured by the S/C.

Each ICoI has evaluated the data of his instrument:

In conclusion, the experiment is in good operational condition as far as the tests up to now have shown. The 6-minute oscillation has to be investigated further, but it will not probably not influence the real measurements. The tests of all cover is planned for 24 December and rest of the commissionning and start of solar measurements mid January.

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VIRGO message # 97 on Tue Jan 16 18:03:25 GMT 1996

Mail from: cfrohlich@obsun.pmodwrc.ch

Subject: VIRGO Science start

Original Date: Tue, 16 Jan 1996 18:52:37 --100

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PI-Message 17 January 1996

Dear all

Tomorrow will start the real measurements. Thomas Frohlich and Udo Telljohann will command VIRGO to start the measurements of PMOA, SPMA and LOI (after some tests of the guiding which can only be done with the sun). This is foreseen for 17 January 18h00-21h30 UT. DIARAD asked for another day of calibration tests with the cover open and will start in its mode left a day later (18 January 20h00 UT by time tagged commands).

So the first level1 data will be available some time during Friday for PMOA, SPMA and LOI. The DIARAD data have to wait still some time because the evaluation of the electrical calibration needed for the irradiance calculation is not yet available from IRMB. For the time being the daily values for the SOHO archive will be the irradiances of PMOA (until DIARAD is available, then the average of both), SPMA and LOI.

I think it is a good moment to thank the VDC for all their work done in the last few weeks and also the IcoIs who have helped them to get everything ready for the production of level1 data. Congratulations, and we all look forward to gather good and interesting data.

After 8 February some test of the S/C pointing will start and essentially continue until February 26. Within this time around the 20 we will have the two days of off-pointing tests together with MDI. The first few days of this period will not disturb us too much, so we will probably get a month of data to play with and to test our evaluation procedures. Between 27 Feb and the Halo Orbit insertion which starts at 13 March we will have another 2 weeks of data. Then the covers will be closed for protection of the instruments until 23 March when the first month and the real mission of SOHO Science starts.

By analyzing the closed data from the last 6 weeks we have found that the long calibration of DIARAD gives a 'nice' 192 minute oscillation of the temperature of VIRGO which is also seen in the signals - the first g-mode! We will never have this mode continuously so this effect should not influence our science. We are preparing a strategy for the DIARAD calibration with a minimum of influence on the other signals. Moreover, we find some more noise in the SPM and LOI data than we had during the TV tests here in Davos. It seems that this phenomenon was always there when mounted on the S/C as evaluation of some recent GSCTs have shown. We do not yet understand, but the noise level is still acceptable (well below the solar noise).

Lets hope for a good start and good data.

Best regards, Claus Frohlich

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VIRGO message # 98  on  Mon Jan 22 16:24:34 GMT 1996

Mail from: cfrohlich@obsun.pmodwrc.ch

Subject: LOI Cover Problems

Original Date: Mon, 22 Jan 1996 17:17:28 --100

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Dear all,

Last Wednesday when we started measuring we get very good data from the SPM  and the radiometers. But we have a problem with the opening the LOI cover.   It did open on the 24 December, but from all we know now only half way, and  seems to open still now, but it bounces back and remains closed. This is  known because LOI sees the sun shortly, but long enough for locking its pointing to the sun. So the instrument works fine; but .....

In the following I send you a description of what we know or think to know or what we simply assume. This is a report which is the basis for a planning meeting today after the daily meeting at 10h00. On Wednesday morning we will  have another oportunity to execute commands during 4 hours. At the moment it  is not clear whether we can do what we propose to do for a definitive opening of the cover.

Any further ideas are obviously welcome. Let me know.

Best regards, Claus Frohlich

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VIRGO message # 100  on  Mon Feb 12 19:38:12 GMT 1996

Mail from: cfrohlich@obsun.pmodwrc.ch

Subject: VIRGO Status

Original Date: Mon, 12 Feb 1996 20:36:49 --100

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Dear all
 

Before I leave the EOF for Switzerland I would like to give you a short status of what was done and seen in last few days.

During the thermoeleastic tests the temperature of the radiation shield  cooled by about 2 degrees. We tried the LOI cover several times during the  cooling down and at low temperatures (hoping to get thermal stress in the actuator and thus friction). As expected these small temperature changes  were not able to increase the friction and all tests showed a essentially constant exposure time as usual (about 565 ms). The plans are as follows:

That is the story of the LOI cover. We have another problem with PMO-B you may remember since 17 January when we did the opening. We do not understand it in detail, but the effect is the following: when the shutter is actuated to open the electronic fuse is triggered and any further shutter movements
are inhibited. Yesterday the same happened to PMO-A and that is really bad.  We will be able to use both of them as back-ups for occasional measurement but no longer as operational instruments. May-be we can find a way around the problem and can run the shutter again continuously; but this is uncertain - first we have to understand what is going on. We will have to review the startegy and in the next few days when I am back to Davos I will
discuss this with the radiometrists.

Best regards, Claus Frohlich

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VIRGO message # 101  on  Tue Feb 27 18:29:59 GMT 1996

Mail from: cfrohlich@obsun.pmodwrc.ch

Subject: Status report by PI

Original Date: Tue, 27 Feb 1996 19:12:00 --100

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to: virgo@solar.stanford.edu
thomas@pmodwrc.ch
utelljoh@vmprofs.estec.esa.nl
ml@radio.oma.be
wolfgang@pmodwrc.ch
sanchez@esa.nascom.nasa.gov

Dear All

After reactions from some of you and from outside I see a strong need to inform about the status of VIRGO. It is not at all as bad it may sound when I am informing about problems; we have also very nice data.

The SPM work just fine and the data profit from the nice and stable environment of SOHO and the continuity of the observations. The degradation  is much smaller than in former experiments: about 0.07% per day for the green and the blue compared to the EURECA green channel with 0.5% per day during the first 50 days. The red VIRGO channel at the beginning was at about the same rate, but when we reduced the temperature of the SPM-A head by about 7 degrees the rate decreased by about a factor 2.5 being now at 0.03% per day.   The solar noise is behaving as the model of Bo (he said; a plot is on OBSUN.PMODWRC.CH anonymous ftp in /user/pub/virgo/ as nois_1st.ps): above the five minute region the power decreases in all colors as about 1/nue^4.5, above them it varies as 1/nue^1.5 in the red and green and flattens out in blue to a nearly constant power around 0.6 mHz. A similar behavior is
observed in the PMO data: the 5 minutes are readily seen but influenced by the bad sampling and a reliable power spectrum can be assumed below about
1.5 mHz (12 minutes period) which varies first as 1/nue^0.5 and flattens out also as the blue does at frequencies below about 0.6 mHz. The p modes from a time series of about 15 days show just a marvelous spectrum (probably the best ever see for irradiance observations, and that is just the beginning. At the same place you can find a plot as pmod_1st.ps): very low noise (for the short period of time already a signal-to-noise ratio (amplitude) for the highest peaks of about 6) and one may see even l=3 at 3.1 mHz. The ratio of (l=0)/(l=2) and also (l=1)/l=3) varies with wavelength as expected due to the weighing by the limb darkening.

With the PMO radiometers we have a problem with their shutters, the one of PMO-A failed on the 11 February (electronic fuse problem) and PMO-B was not working when we started the observations on January 17. Both worked at Xmas.   Before its failure PMO-A worked fine as the power spectrum shows. We started last week to operate them in a different way with the covers: PMO- A is open for about 8h and then closed for 21 minutes. 9 minutes before it closes PMO-B is opened for 39 minutes. In this way we have really continuous observations with a sampling every minute (instead of every 2 minutes during the shuttered operation). From the time series point of view this is much better, but we obviously reduce the absolute accuracy due to the much fewer reference values. We are presently developing a new algorithm to evaluate data with the operational scheme (major problem is to improve the thermal model to enable accurate calculation of the 'closed' power during the long open phases. The absolute accuracy is now fully under the responsibility of the DIARAD which works fine, I was told, but the evaluation is not completed, so I have not seen any results yet. We have a period of about 2 weeks with real comparisons between the two types of radiometers which will be used to determine the way the VIRGO irradiance will be calculated. Very
preliminary  comparison show a rather large difference between DIARAD and PMO-A. As the evaluation of DIARAD is not yet finalized and the PMO thermal model is still under investigation this difference will hopefully be resolved soon.

As to the operation of the VDC: It works fine and struggles mostly with data transmission problems over the net. You probably wonder why no level 1 data of the SPM are available. The reason is simple: the orbit data had some problem some time ago and have still some for the velocity. In order to overcome this problem we will calculate the velocity data from the distance differences (velocity correction are now in any case in the sub- ppm range) and Antonio will make the level1 data of SPM available soon. The PMO algorithm for the 'old' data needs some refinement (thermal model, see above) and the new one is under development. DIARAD is still under development. It is hoped that all these data will be available before end of March.

The present planning for the LOI cover opening looks as follows: at the moment the mechanics people at ESTEC are looking into the model Thierry has
developed which is constrained by a lot of measurements on the Spare Model here in Davos (e.g. position recordings with TV camera under different conditions, motor performance, cover properties). The model seems to work quite well to explain the behavior on ground. For the moment being there is
no real explanation for the behavior in space (several hypothesis exist but could not be proven on ground). Nevertheless, we are confident that shortening of the pulse will open the eventually. All technical problems which could lead to a blocking which would never allow to open it fully could be excluded (hopefully not too much by our hopes and fears). The project in co-operation with MMS and SAAB has developed a method which will allow to shorten the pulse in a well defined manner. This needs a patch of the on-board S/W, which is being developed and will then be loaded in an existing patch area reserved for tests. Tests of the S/W patch are planned at SAAB with our Spare Model and support in the week of 19 March and the upload of the S/W and the LOI cover opening on the 26 and 27 March. This is the planning and I am confident that we will open the cover.

When asked about the VIRGO status, you should now know what is going on and what is planned and distribute this knowledge to who ever would like to know.

Best regards, Claus Frohlich

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VIRGO message # 103  on  Mon Mar 25 16:49:26 GMT 1996

Mail from: cfrohlich@obsun.pmodwrc.ch

Subject: PI Message

Original Date: Mon, 25 Mar 1996 17:47:45 --100

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PI-Message 25-March-1996

Dear all

First point I want to inform you is that we will open the LOI cover tomorrow. The project has provided a S/W patch for the on-board computer which enable us to switch off VIRGO at a given time accurately. In this way we can change the pulse length to what ever value below the maximum length we wish. This S/W patch was tested last week successfully with the VIRGO spare instrument at SAAB in Goteborg.   The test with the real LOI cover starts tomorrow around 15h30 UTC. First we will send the normal open command and see whether it behaves as usual. I do not expect do see any difference, but who knows it may even open. If not we will start with a pulse length of 450ms which is the most probable value for opening if we can assume that it behaves as on ground. This is somewhat hypothetical as from all tests we made and from the models of the cover we have no explanation of the behavior other than that the pulse length is not 720ms but 520ms. This would explain the behavior with only a few adjustments of the parameters (within reasonable limits). It is,
however, very unlikely because a failure in the Logic Cell Array which controls the sequences, is needed. The pulse length is made by counting the 10Hz signal from 8 down to 0 and 100ms after the start of the count down power is enabled to the cover motor yielding the 700ms nominal and 720ms with the delays. A change in the pulse length would be common to all covers. The small covers have no problem with such a pulse length, so they work without problem as shown (the PMO covers work 6 times each every day since mid of February as replacement of the shutters) and the DIARAD cover is
heavier than the LOI cover and is most likely just on the other side of the range (we will not try again in any case). Other scenarios with some obstacles in the way have to be rejected, Hansjurg checked the proper opening when he did late October last year the alignement check during which the LOI cover is opened manually.

The second point I would like to touch is more general and relates to the presentation of early results. For the moment being we have only preliminary results and nothing which can or has to be published yet. Probably, this will be true still for some time. But the VIRGO team is asked to show results during meetings of any kind as e.g. scientific discussions, presentations to our funding agencies, more formal ones as AGU Spring meeting and COSPAR where
abstracts had to be submitted already some time ago. There is also the IRIS meeting next week where Thierry A will present some p mode results. In order to handle these things in a concerted way I propose the following common policy for all future activities:

        (i) inform the VIRGO team of such intentions by e-mail to virgo@solar.stanford.edu in advance;
        (ii) put the viewgraphs you intend to show as postscript files in the VDC /virgo/doc/ directory (see NB1 for instructions);
        (iii) if you have an abstract put it as ascii or ps file in the VDC /virgo/doc/ directory (indicate format by extension .asc or
            .ps); as an example I have put the COSPAR and AGU abstract allready submitted to /virgo/doc/;
        (iv) label the material as "VIRGO preliminary Results" and put the ICoI name concerned, your name if you want and "the VIRGO
            team" as reference.
        (v) tell the organizer that this material is not for publication yet; this may change after a first VIRGO team publication with the results (one is planned
            for Solar Physics, organized by SOHO, see NB2 below).

The third point is the next VIRGO team meeting: We discussed this point last September and agreed in principle to have a meeting in Tenerife in March 96. Due to the situation with the LOI cover I have implicitely postponed the meeting. Now, we should start thinking about it again. It will probably not be possible to have it before the big PR event on May 2nd which is at the same time in Paris and at GSFC about the first month of SOHO (I will be at ESA-HQ
during this event). Would like to get your feelings about that.

For the moment that is it. Lets cross fingers for tomorrow.

Best regards, Claus Frohlich

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VIRGO message # 104  on  Fri Mar 29 13:46:19 GMT 1996

Mail from: thierrya@so.estec.esa.nl

Subject: LOI status report

Original Date: Fri, 29 Mar 1996 14:42:00 +0100

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Dear colleagues,

Now the LOI has been working fine for more than 2 days.  It now almost fully commissioned, I just have to get the off pointing data next week and something with it.

The instrument is slowly setting in temperature and we can see in the high voltage guiding signal: we had a drift of 1 arcsec over one day!  The short term
guiding stability is very good about 0.03 arcsec at 1 sigma; it is even better as we make a short integration for the guiding pixels.

The temperature stability on the detector is about 0.002 deg C peak-to-peak!

Overall Udo and I are very pleased with the performance of the instrument.

Oh! By the way we did see the 5-min oscillations in the integrated signal over the 12 pixels.

We should start to get level_1 data in about 1 month from now after cross checking everything with the Spanish tribe.

See some of you in Nice.

ThierryA (and Udo of course)
 
 

Last updated by Thierry Appourchaux on Wenesday, January 21, 1998 at 13:28