REPORT FROM ISAS 13-Apr-1992 15:20 JST L. Acton SCIENCE OPERATIONS During the week we continued to follow solar regions in support of the Max '91 observing campaign with the VLA. This work was supported at ISAS by Dr. Nariaki Nitta and it appears that the coordination was fully effective although the sun was quieter than might have been desired. There were no operational problems with SXT this week. On Saturday, 11 April, Krith Strong organized a well-attended SXT science and operations discussion at ISAS. It is gratifying to see the great range of scientific activity underway with Yohkoh data. The video disk loaded with all complete SXT full frame images that Keith Strong continues to build turns out to be a tremendously valuable research tool and an amazing movie to watch! Because of reformatter backup there is still a gap between 21 December 1991 and 23 February 1992 and before 1 November 1991. The data reformatter operation has experienced problems to numerous to mention so has not progressed as we had hoped. Most of these problems have to do with accessing the Sirius database on the ISAS mainframe computer and our ISAS colleagues are working hard to fix the problems. The problem of shutdown of the ISAS computers at night and on weekends continues to be a vexation even though SXT has been given permisson to keep them on during reformatting catch up. During the week Greg Slater and Jim Lemen strove to complete a perfectly aligned movie of desaturated SXT images for public information release to the media. Because the knowledge of exact Yohkoh pointing is less than we had hoped and planned pre-launch perfection escaped them but the movie they produced is really very acceptable. They are now working to produce the final product with title pages, etc., to be released during an ISAS/NASA parallel press release on or about 23 April. EXPERIMENT OPERATIONS The appended memo summarizes the results from the successful CCD warm-up test of last week. We have learned a lot and will now take steps to change Yohkoh pointing so as to extend the life of the CCD. Jim Lemen will be copying across an IDL save file with 3 figures which illustrate the results of the test. The MDI folks will want to have a look. I was quite surprised to discover that the CCD was again (or still) contaminated with some sort of condensate. I now know how to monitor this to see if it returns. It appears as little horizontal "dashes" (aligned along the CCD transfer gate ridges as would be expected for a liquid) in optical full resolution images. The effect is averaged out in half resolution images and only appears as noise in the image. The contamination did not return within the first hours following recooling the CCD to -21 C. Now, going back to the January "TEC off" event I can see that the condensate did not completely disappear. This time I think we removed it all as the image is creamy smooth as it was in September before the TEC was turned on. The condensate appeared very quickly after TEC turn on in October so I guess we did not allow enough time for outgassing. The optical signal increased by only about 5% when the condensate cleared off. I surmise that the loss of signal related to this contamination, which is only a fraction of the total loss, is caused by visible light, that would have fallen onto the virtual gate half of the CCD pixel, being scattered or refracted onto the polysilicon gate half--which is opaque to visible light. It is tempting to surmise that the same material which condenses onto the CCD is what polymerizes on the entrance window of the aspect telescope to cause the general loss of signal. This is only a supposition but something is certainly coming from somewhere to cause this effect. With the great increase in dark current caused by warming up the CCD we can now see that the predicted damage from ionizing radiation is present. A dark current "image" is present on the device. Also, the visible light ghost image is now seen to be a direct reflection of this radiation damage because it perfectly matches the increased dark current. This permits monitoring the radiation effects without having to warm up the CCD. During CCD developement work for SXT Janesick produced a curve of increase in dark current for a TIJ split A and a split C part versus amount of Al-K irradiation (time axis). I would appreciate it if someone would fax me a copy of that curve. [ISAS fax 81-427-69-4532.] I believe that it was handed out with the radiation working group notes as well as being sent out in a memo. What I am interested in is how much the dark current in a C split part increased before the knee of the curve where it became unpinned. As noted below, in the worst places we now see a 6-fold increase in dark current. These regions are quite localized to the solar limb so that by shifting the pointing of Yohkoh from time to time we should be able to maintain a functional CCD for several years if other problems do not come into play. This is extremely good news as the telescope continues to return superb images, now over a quarter of a million exposures! ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ To: Yohkoh Operations Team From: L. Acton Date: 13-April-1992 Subj: SXT CCD Warm-up Test This test was conducted between 8 and 11 April. It was done in 4 steps with 24 hours between each step. The transitions were quite well observed. 1. Thermo_electric cooler (TEC) off [CCD temp increased to -3 C.] 2. Thermal strap heater on [CCD temp increased to +22 C.] 3. Heater off [CCD back to -2 C.] 4. TEC on [CCD cooled to -21 C.] As a result of this test we have learned: 1. The CCD was contaminated with some condensate (water?) that diappearaed when the temperature went from -3 to +20 C. 2. The CCD shows a dark current pattern exactly like the famous visible light ghost image. 3. The dark current in the worst spots is 6 times the average dark current in the CCD. The ghost image at these locations is about 75% as bright as unaffected areas. 4. There is evidence of low-level CTE effects in the short exposure "read noise" image from the x-ray damaged regions. 5. None of these effects are yet detectable in solar x-ray images. It is important to make some changes to extend the life of the CCD. For this reason we suggest that the standard pointing of Yohkoh be shifted 1 arcmin to the EAST for normal operation. An alternate position is 1 arcmin to the WEST of the present position. This would be used for those situations when we wish to observe the west limb crossing of a big region. I request that Yohkoh be repointed to the 1 arcmin EAST position as soon as possible. This will place the limb x-rays in a new position and extend the life of the CCD. Thank you, Loren Acton