The Hubble Space Telescope needs no introduction. It’s become well known for its stunning images of nebulae and star-fields, and it wouldn’t be amiss to say the telescope has even become synonymous with images of strange beauty often from distant cosmic shores. No doubt saying something is like the Hubble Space Telescope simplifies the task of communicating that object’s potential and significance, especially in astronomy, and also places the object in stellar company and effortlessly elevates its public perception.

It’s for the latter reason that the comparison shouldn’t be made lightly. Not all telescopes are or can be like the Hubble Space Telescope, which sports some of the more cutting-edge engineering at play in modern telescopy, undoubtedly necessary to produce some of the images it produces (here’s a list of stunners). The telescope also highlighted the role of aestheticism in science: humans may be how the universe realises itself but the scope of that realisation has been expanded by the Hubble Space Telescope. At the same time, it has become so famous for its discoveries that we often pay no heed to the sophisticated physics at play in its photographic capabilities, in return for images so improbable that the photography has become irrelevant to our realisation of their truth.

ASTROSAT, on the other hand, is an orbiting telescope whose launch on September 28 will place India in the small cohort of countries that have a space-borne observatory. That’s insufficient to claim ASTROSAT will be akin to the Hubble as much as it will be India’s debut on the road toward developing “Hubble-class” telescopes. ASTROSAT’s primary science objectives are:

Understand high-energy processes in binary systems

Search for black hole sources in the galaxy

Measure magnetic fields of neutron stars

Study high-energy processes in extra-galactic systems

Detect new transient X-ray sources

Perform limited high angular-resolution deep field survey in UV

The repeated mentions of high-energy are synonymous with the parts of the electromagnetic spectrum ASTROSAT will study – X-ray and ultraviolet emissions have higher frequencies and thus higher energies. In fact, its LAXPC (Large Area X-ray Proportional Counter) instrument will be superior to the NASA NuSTAR X-ray telescope: both will be logging X-ray emissions corresponding to the 6-79 keV* energy range but LAXPC’s collecting area will be almost 10x the collecting area of NuSTAR’s. Similarly, ASTROSAT’s UV instrument, the Ultraviolet Imaging Telescope, studies wavelengths of radiation from 130 nm to 320 nm, like the Cosmic Origins Spectrograph on board the Hubble spans 115-320 nm. COS has a better angular and spectral resolution but UVIT, as well as the Scanning Sky Monitor that looks for transient X-ray sources, tops with a higher field of view. The UVIT and LAXPC double up as visible-wavelength detectors as well.

In contrast, the Hubble makes observations in the infrared, visible and UV parts of the spectrum. Its defining feature is a 2.4-m wide hyperbolic mirror that serves to ‘collect’ photons from a wide field of view onto a secondary hyperbolic mirror, which in turn focuses into the various instruments (the Ritchey-Chrétien design). ASTROSAT also has a primary collecting mirror; it is 30 cm wide.

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On the road to Hubbleness

But it’s quite wrong to think ASTROSAT could be like Hubble when you consider two kinds of gaps between the instruments. The first is the technical-maturity gap. Calling ASTROSAT “India’s Hubble” will imply that ISRO has reached that level of engineering capability when it has not. And making that reference repeatedly (here, here, here and here) will only foster complacency about defining the scale and scope of future missions. One of ISRO’s principal limitations is payload mass: the PSLV rocket has been the more reliable launch vehicle at our disposal and it can lift 3,250 kg to the low-Earth orbit. The GSLV rocket can lift 5,000 kg to the low-Earth orbit (10,000 kg if an upper cryogenic stage is used) but is less reliable, although promising. So, the ASTROSAT weighs 1,500 kg while the Hubble weighs 11,110 kg – the heaviest scientific satellite launched till date.

A major consequence of having such a limitation is that the technology gets to define what satellite is launched when instead of astronomers laying out what they want to find out and technology setting out to achieve it, which could be a useful impetus for innovation. These are still early days for ISRO but it’s useful to keep in mind even this component of the Hubble’s Hubbleness. In 1974, NASA and ESA began collaborating to build the Hubble. But before it was launched in 1990, planning for the James Webb Space Telescope (JWST) – conceived from the beginning to be Hubble’s successor – began in the 1980s. In 1986, an engineer named Pierre Bely published a paper outlining how the successor will have to have a 10-m primary mirror (more than 4x the width of the Hubble’s primary mirror) and be placed in the geostationary orbit so Earth doesn’t occlude its view of space, like it does for the Hubble. But even four years later, NASA didn’t have a launch vehicle that could heft 6,500 kg (JWST’s weight) to the geostationary transfer orbit. In 2018, Europe’s Ariane 5 (ECA) will be doing the honours.

The other is the public-outreach gap. As historian Patrick McCray has repeatedly noted, telescopes are astronomers’ central research tools and the quality of astronomy research is a reflection of how good the telescopes are. This doesn’t just mean large reflecting mirrors, powerful lenses and – as it happens – heavy-lift launch vehicles but also the publication of raw data in an accessible and searchable format, regular public engagement and, most importantly, effective communication of discoveries and their significance. There was a hint of ISRO pulling off good public outreach before the Mars Orbiter Mission launched in November 2013 but that evaporated soon after. Such communication is important to secure public support, political consensus and priority funding for future missions that can expand an existing telescope’s work. For the perfect example of what a lack of public support can do, look no further than the India-based Neutrino Observatory. NASA, on the other hand, has been celebrated for its social media efforts.

And for it, NASA’s missions are more readily recognisable than ISRO’s missions, at least among people who’ve not been following ISRO’s launches closely since the 1960s. Not only that, while it was easier for NASA’s scientists to keep the JWST project from being cancelled, due to multiple cost overruns, thanks to how much its ‘predecessor’ the Hubble had redefined the images of modern astronomy since the late 1990s, the Hubble’s infamous spherical aberration fault in its first years actually delayed the approval of the JWST. McCray writes in a 2009 essay titled ‘Early Development of the Next Generation Space Telescope‘ (the name of JWST before it was changed in 2002),

Years before the Hubble Space Telescope was launched in 1990 a number of astronomers and engineers in the US and Europe were thinking hard about a possible successor to the HST as well as working to engage a broad community of researchers in the design of such a new observatory. That the launch of any such successor was likely to be many years away was also widely accepted. However, the fiasco of Hubble’s spherical aberration had a serious effect on the pace at which plans were advancing for the Next Generation Space Telescope. Thus crucially for the dynamics of building the “Next Big Machine,” the fate of the offspring was intimately tied to that of the parent. In fact, … it was only when in the mid-1990s that the NGST planning was remade by the incorporation of a series of technology developments in infrared astronomy that NASA threw its institutional weight and money behind the development of a Next Generation Space Telescope.

Autonomous shouldn’t mean opaque

But even for all the aestheticism at play, ISRO can’t be said to have launched instruments capable of transcending their technical specifications, either: most of them have been weather- and resource-monitoring probes and not crafted for the purpose of uncovering elegance as much as keeping an eye out. But that doesn’t mean, say, the technical specifications of the ASTROSAT payload shouldn’t be readily available, that there shouldn’t be one single page on which one can find all info. on ISRO missions (segregated by type: telecom, weather-monitoring, meteorology, resource-monitoring, astronomy, commercial), that there shouldn’t be a channel through which to access the raw data from its science missions**, or that ISRO continue to languish in its misguided conflation of autonomy and opacity. It enjoys a relative abundance of the former, and does not have to fight for resources in order to actualise missions it designs based on internal priorities. On the other hand, it’s also on the cusp of making a habit of celebrating frugality***, which could in principle provide the political administration with an excuse to deny increased funding in the future, and surely make for a bad idea in such an industry that mandates thoroughness to the point of redundancy as space. So, the day ought to come when the bright minds of ISRO are forced to fight and missions are chosen based on a contentious process.

There are multiple ways to claim to be the Hubble – but ASTROSAT is definitely not “India’s Hubble”. ISRO could in fact banish this impression by advertising ASTROSAT’s raw specs instead of letting people abide by inadequate metaphors: an amazing UV imager, a top-notch X-rays detector, a first class optical observer. A comparison with the Hubble also diminishes the ASTROSAT by exposing itself to be not like the Hubble at all and, next, by excluding from conversation the dozens of other space-borne observatories that it has already bested. It is more exciting to think that with ASTROSAT, ISRO is just getting started, not finished.

Quelle: WIRE

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