Recollect Chandra X-Ray Observatory

( 今天带给大家的是我们 AstroLeaks 第一篇英文文章, 介绍Chandra X-ray 望远镜的历史. 王同学是南京师范大学物科学院的一名大四本科学生,正在紫金山天文台跟纪丽老师做本科毕业论文. 这篇文章就是她对 Chandra 望远镜的调研总结. 她正在申请美国天文专业的研究生,在这里我们祝她的申请一切顺利.)

TITLE:
Recollect Chandra X-Ray Observatory

AUTHOR:
Xiao Wang (Nan Jing Normal University)

Chandra X-Ray Observatory was launched at 12:31 pm on July 23, 1999 by the Space Shuttle Columbia in Kennedy Space Center, United States. Since it orbited successfully, Chandra has been working and showing us more and more information of the hot and turbulent regions in the universe with its unprecedented capabilities. No feelings but appreciation should we thank Chandra’s contributions to astrophysics. Besides admiration, it is also necessary for us to think about the unforgettable history of the building of Chandra telescope and to learn something.

A BRIEF HISTORY: PASSION AND FAITH VS CHALLENGES

As the old saying goes that Roma was not built in one day, the Chandra X-Ray Observatory didn’t, without any doubt, show up in Kennedy Space Center suddenly. To be specific, it included countless hard-work, enthusiasm and faith of many scientists and engineers for over twenty years.

In 1976, Dr. Giacconi and Dr. Tananbaum’s sent to NASA a proposal of a 1.2 meter X-ray Observatory, which was taken into consideration in the next year. Then NASA let Marshall Space Flight Center (MSFC) and Harvard-Smithsonian Center to take the responsibility of this mission and funded the group to test the feasibility. This observatory was named as the Advanced X-Ray Astrophysics Facility (AXAF), because they hoped it wouldn’t be taken as another Hubble Space Telescope (HST), an exorbitant instrument being built at that time.

Figure 1. The cartoon pictures they drew to make results in obviousness. From a poster shown on the talk:On the Making of Chandra—Martin C. Weisskopf, NASA/Marshall Space Flight Center

After 3 years’ concept design and preliminary analysis, AXAF did not manage to enter its designing and development phase, the most important phase called “new start”, until 1991. There were mainly two reasons “contributing” to this long-lasting delay. First and foremost, almost everyone knew that HST costed a larger amount of money each year and frequently ran into problems. In that case, how would the Congress like to fund a project, which seems to be another HST? Second, there were a lot of competitions that came from many other instruments serving at other fields in astronomy. On the circumstances, administrators promoted the program by letting people with general or casual knowledge about astronomy know more about AXAF’s value and significance and hence approved it. For example, they requested chief scientists in various disciplines to list top 10 interesting problems in their minds in astrophysics and to list the corresponding instruments for the solutions. Then they extracted the 10 most important ones from all the problems and 4 necessary instruments including AXAF and HST (See figure 1) were included.

Scientists had been engaging themselves in selling AXAF till 1988. They got three years time and money to prove the ability to build a pair of mirrors which must have a resolution of 0.5 arcsecond (the ability to discern words in newspapers about 2000 meters away). If they succeeded, AXAF would be funded, otherwise it would be canceled. That was called “Mirror Challenge”. Although they overcame this challenge, because of the retrenchment of funding, they still had to negotiate with some reconstructions, such as to reduce some instruments, disconnection with the ISS (International Space Station) astronauts, and so on. Facing the funding problem, scientists and engineers sometimes did seem pallid. What they could do that time was to set up some new goals and then spared no efforts to achieve them in order to make AXAF as powerful as possible.

In sum, through this brief history, we can find many struggles in promoting the program.

UNDERTAKING RISKS

Given a choice, most people may prefer to deal with what they are capable of rather than taking challenges. However, it is not the case in science, because scientists always need to explore new worlds, which need taking risks in doing something out of certain purposes. When building AXAF, scientists and engineers inevitably experienced a large amount of challenges and undertook many risks.

Building High Resolution Mirror Assembly

Figure 2. CXO Mirror Being Assembled at Eastman-Kodak(http://chandra.harvard.edu/graphics/ resources/illustrations/mirrorFabCirc-72.jpg)

It is the High Resolution Mirror Assembly (HRMA, Figure 2) that, in some sense, enables AXAF to make sharp images comparable to optical telescopes. In the “Mirror Challenge”, the mirror’s resolution tested out finally was 0.19 arc-second, even better than expected. However, engineers hadn’t possessed the ripe technology and scientists were not sure whether 0.5 arc-second could be achieved when “Mirror Challenge” was set in 1988, let alone in 1976 when the original version was proposed. So they acted not because they could do at that time but had to do. Here is an example: The mirror polishing was finished two weeks in advance thanks to the hard work of the manufacturers and leaders. Under such circumstances, engineers pointed that: since we have spare time, why not do more polish to make the mirror smoother? The leaders knew it was a risk and they had to make full use of time because of the urgency and uncertainty of the subsequent assignment. However, they chose to exchange the precious two weeks for smoother mirrors which meant that the higher resolution and more accurate data would be achieved.

Building High Energy Transmission Grating

Figure 3. the Low Energy Transmission Grating (http://chandra.harvard.edu/graphics/resources/illustrations/gratingsLow1-72.jpg)

High Energy Transmission Grating (HETG) was made by MIT and was set at the rear of HRMA together with the Low Energy Transmission Grating (LETG) designed by Space Research Organization of Netherland (See Figure 3). These two instruments can be separately rotated into the track of the incident X-ray photons. Owing to these transmission gratings, it is available to make more precise measurements of the energy spectrum.

The largest difficult in making transmission gratings was to manufacture hundreds of identical gratings. To make things more difficult, the preliminary strategy broke down due to bankruptcy of a corporation, the only one could provide them with a key machine generating hard X-ray photons steadily. This happened less than a month before the preliminary design review and meant that the team would fall behind the schedule. However, within 3 weeks, a new way was raised and this new technique also led to several industrial spinoffs promptly. In fact, we had to admit that this was a big risk for the team. If they hadn’t come to the rescue, HETG might have not come true, let alone the promotion in technique and social development.

Building Advanced CCD Imagining Spectrum

Figure 4. View of Focal Plane connected to backplate (non-text version). The flex prints from the CCDs wrap around and are attached to the feed thru connectors on the detector housing backplate, shown below the focal plane. (http://chandra.harvard.edu/graphics/ resources/illustrations/acis nolabels.jpg)

Locating on the focal plane of HRMA, the Advanced CCD Imagining Spectrum (ACIS) can show us images and spectrums and provide more precise information of the incident photons (Figure 4). In February of 1984 when the technology of the CCD was in its infancy, Dr. Garmire of Pennsylvania State University proposed that CCD Imaging spectroscopy should be used in AXAF. Building this instrument would surely bring some troubles and risks, but it would, at the same time, not only strengthen AXAF but also propel the development of CCD.

Each coin has two faces. Nowadays, we appreciate their choice because of the progresses both AXAF and CCD industry has enjoyed.

ELABORATED TESTINGS AND CALIBRATIONS

It is the rule of thumb of AXAF’s managers that everything depends on proof rather than confidence. Failing in final testing means there’s no necessary to launch the instrument. So, during the process of construction, you can find testing any time and any where.

Since the completion of a small part, tests have been adopted. Engineers cut the mirror while measuring its roundness together with slope and polished it while testing its smoothness. After ultimate aligning, the mirrors needed to be verified to have 0.5 arc-second resolution for X-ray.

Figure 5. the High Resolution Cameral (HRC) (http://chandra.harvard.edu/graphics/ resources/illustrations/HRClabel-72.jpg)

Everyone should be very cautious of each step, because an instrument might be useless with a slack electric wire or several loosen bolts. When testing the High Resolution Cameral (HRC) (See Figure 5), engineers found that 4 screws were not set appropriately which caused potential differences of about 4000V directly and made, without any doubt, the results confusing.

Figure 6. From a poster shown on the talk:On the Making of Chandra---Martin C. Weisskopf, NASA/Marshall Space Flight Center

For the final test in “Mirror Challenge”, MSFC built an X-ray Calibration Facility (XRCF), the vacuum in whose one end could create an almost identical atmosphere with the space and an X-ray source in other end imitated incident X-ray photons for AXAF (See figure 6). By setting director on the mirrors’ focal plane, engineers could test the mirrors’ resolution with the help of a “X-ray tracing code”. However, the outcome, 3.8 arc-second worse than expected 0.5 arc-second, made everyone down and tense. Thinking about each possible reason, they focused on that mirrors were out of shape due to gravity which would not exist in outer-space. So they had to either build a special framework to counteract the effect of gravity or revise the “X-ray tracing code”. By adopting the latter method, they proved that the mirrors had 0.19 arc-second resolution much better than planned.

What’s more, when finally calibrating the mirrors with a particular data, one scientist of Harvard-Smithsonian found the mirrors were not concentric, which made the efficiency of catching incident photons of upper side 10 percent higher than the lower. Admittedly, but for this calibration, results would be away from reality. So, for certain X-ray observatories, this calibration should have be an indispensable one. In fact, one leader of Harvard-Smithsonian said:“ The existing observatories may have these problems, but we’ll never know.” That is to say, the certain X-ray telescopes launched before hadn’t had this calibration and the data they got was suspect. But for AXAF, this one was just only one of the over 250,000 calibrations listed by MSFC team and because there were too many tasks, one engineer thought it was reasonable when an instrument used for testing got breakdown.

While testing ACIS, the well-designed door couldn’t be open strangely. Although this problem was overcome at last, scientists and engineers were really tense, because if the door wouldn’t be open when launched into space, ACIS would have lost its purpose, wasting large amounts of time and money.

From all examples listed above, we have to admit the significance of all tests and calibrations made. Without them, what could AXAF have brought to scientists and the development of astronomy?

SPLENDID COOPERATIONS

Figure 7. The Second SWG:Andrew Wilson,Andy Fabian,Jeff Linsky,Harvey Tananbaum,Alan Bunner,Steve Holt,Martin Weisskopf,Riccardo Giacconi,Bert Brinkman,Steve Murray,Gordon Garmire,Leon van Speybroeck,Claude Canizares,Richard Mushotzky From a poster shown on the talk:On the Making of Chandra---Martin C. Weisskopf, NASA/Marshall Space Flight Center

Dr.Tananbaum once said that AXAF had a dozen outspoken scientists and engineers who had been working together like great buddies while building Einstein X-ray Observatory and would have end-to-end support on the project, which was its superiority over HST. But for the gorgeous cooperation, according to him, AXAF not only would not have glorious achievements but also could not be finished in a good and timely fashion.

First consider cooperation in MSFC and Harvard-Smithsonian. MSFC took responsibility in overall management and systems-engineering while Harvard-Smithsonian provided science and technique supports. This teamwork had already been formed during building Einstein X-ray Observatory, said by Dr.Tananbaum. In that case, they were deeply aware of the importance of collaboration and necessity of communication.

What’s more, we can easily see cooperation’s significance in the “Mirror Challenges”. TRW Corporation, once in charge of Einstein X-ray Observatory and Compton Gamma Ray Observatory, was HRMA’s contractor. Its Space and Electron Department took responsibility in cutting, grinding, polishing, aligning and final testing. In fact, these five tasks were cooperations done by three institutions: the first three were in Perkin-Elmer Corporation (now Raytheon Optical System), while the third one was in Eastman Kodak Corporation and the last one proceeded in MSFC. What’s more, for P-E Corporation, missions were not only just cutting, grinding and polishing but also measuring the roundness, the slope and the smoothness. As a result, considering the tight schedule, P-E Corporation assigned its own part to 40 subcontractors and many small businesses. It was due to this huge mixed team that mirrors could be finished timely.

While we feel grateful to the teamwork referred above, we should also appreciate the international cooperation described below. In order to compensate for the retirement of two engineers during the time waiting for funds, the director of LETG chose to work together with his competitor- Max Plank Institute in Germany. By cooperating with those who had had more than 20 years experiences in making X-ray transmission gratings, they could make sure the engineers were so professional and experienced that could complete tasks within schedule successfully.

As far as AXAF’s concerned, the completion was a result of cooperation of many independent institutions. Then it appeared significant to ensure harmonious association and open communication within those institutions. The major managers really did a good job in that due to working as interpreters when different institutes had conferences together. This can be exemplified by a behavior of “not speaking”. People in some areas use it to express agreement while others deliver their soundless resistance. Apart from interpretation, one manager of TRW group members recalled that once in trouble, leaders from NASA, MSAF, and Harvard-Smithsonian would face difficulties together with them and teleconferences could be made anytime to discuss those problems, which made him feel more interesting in contrast with working as a single one. So almost everyone was willing to give up rest and try their best to accomplish AXAF, a program cohering a prodigious amount of people’s hard work. From this instance, we can see that the senses of responsibility and passion caused by cooperation could enhance the efficiency in working.

“Chandra” , in honor of a famous astrophysicist Subrahmanyan Chandrasekhar, was a result of a public name contest sponsored by NASA. Chandra X-ray Observatory does reveal many secrets in universe, besides that, its history also tells us many precious experience as well as many characters such as passion, teamwork, meticulousness, braveness and insistence needed in great work. Let’s enjoy these from the endless recollection of the Chandra.

REFERENCE

Wallace H. Tucker & Karen Tucker, Reaving the Universe: the making of the Chandra X-ray Observatory, Harvard University Press, 2001.

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