Snapshots of the universe

Four newly-released images of the universe as captured by an overhauled and upgraded Hubble Space Telescope, provide some cosmic surprises.

THE Hubble Space Telescope (HST), launched in April 1990 as a joint project of the National Aeronautics and Space Administration (NASA) of the United States and the European Space Agency (ESA), has been pushing the frontiers of astronomy. Instruments aboard the low-earth orbiting telescope have been generating new science all these 12 years with astounding images of the cosmos. These images include beautiful ones of new galaxies and colliding galaxies and the birth and death of stars and even those that provide proof of the existence of black holes. However, the first new images of the universe - four spectacular and breathtaking ones released on April 30 - taken by the overhauled and upgraded HST suggest, as Ed Weiler, the associate administrator for space science at NASA put it, that "we haven't seen anything yet".

These images taken with the new Advanced Camera for Surveys (ACS), which according to Holland Ford of Johns Hopkins University (JHU), who headed the ACS project that began in December 1994, "are among the best images of the distant universe humans have ever seen". The ACS was installed in March during Servicing Mission 3B (SM3B), replacing the faint object camera (FOC), one of the original instruments of the HST. The new instrument, which has given the telescope a ten-fold greater observational efficiency, marks the beginning of a new era of exploration of the universe with the HST.

The ACS is expected to open up new capability for discovery, especially of the early universe. The four images are evidence of the ACS' capability to discover celestial objects far beyond the reach of currently available instruments, and that too in a fraction of the time. It will enable the deepest images of the universe for the foreseeable future, until 2010 when the Hubble Project will come to an end and its successor, the next generation space telescope (NGST), trains its 8 metre diameter mirror (as against Hubble's 2.4 m) from a vantage point 1.5 million km from the earth (as against Hubble's orbit 600 km above) to catch the first light of the universe. The ACS has taken astronomers to the 'twilight zone' period when galaxies were just beginning to form out of the blackness following the cooling of the universe after the big bang.

The ACS is a camera of superlatives. It is expected to go beyond the sensitivity range of the largest ground-based telescope eventually to see the faintest of objects. It delivers a panoramic and high resolution image with an amazing 16 million picture elements per snapshot. A typical digital camera delivers two to four megapixels. The camera, which was designed at JHU and built by Ball Aerospace Technologies, is a phone booth-sized instrument and comprises three different cameras operating in three specialised channels: the wide field channel, the high resolution channel (with a smaller field of view) and the solar blind channel (which is sensitive to ultra-violet rays). The camera is equipped with a coronagraph (which is four to six times better than the existing imaging spectrograph) which blocks out the glare of quasars and stars so that fainter objects near them become observable. The wide wavelength range and its three modes of operation make the ACS a very versatile camera.

The ACS can survey a field on the sky twice as large as that was covered in the legendary hubble deep field (HDF) surveys of 1995, to the same exposure depth, three to four times faster than the HST's workhorse camera that took the HDF, the wide field and planetary camera 2 (WFPC2). A deep "core sample" from the "twilight zone" period had taken about 10 days of dedicated observations with WFPC2 whereas the ACS would take less than three days. The ACS is expected to become Hubble's new workhorse. It is sensitive to wavelengths ranging from the ultra-violet to the far infrared (115-1050 nanometre).

The ACS image of the Tadpole Galaxy reflects the dramatic gains over WFPC2 resulting from doubling area and resolution and demonstrates a five-fold improvement in sensitivity. The image brought an unexpected bonus in the form of the spectacular backdrop of an enormous number of galaxies - over 6,000 - like a wallpaper pattern beyond the Tadpole. It resembles a galaxy-filled HDF image taken by the HST in 1995. The difference, however, is that the ACS image was taken in one-twelfth the time taken by the original HDF. In fact, in blue light it shows even fainter objects than the HDF. The number of galaxies seen in the field of view is about twice the number in the HDF. Like the HDF, the ACS image stretches back to nearly the origins of time and contains a variety of shapes that are snapshots of galaxies throughout the universe's 13 billion-year evolution. The ACS images are so sharp that astronomers can identify "building blocks" of galaxies, colliding galaxies and extremely distant galaxies in the field - a superb sampler of galaxies.

Servicing Mission SM3B was the fourth one to the HST, the third having been split into two. SM3A was conducted in December 1999. During the five most challenging spacewalks, or extra vehicular activities (EVAs), over an 11-day mission, the crew of seven astronauts upgraded the HST with the ACS, a new power unit and new rigid solar arrays (which together now generate 27 per cent more electric power) and an experimental cooling unit - cryocooler - for an infrared camera called near infrared camera and multi-object spectrometer (NICMOS). The new high-tech mechanical "refrigerator" pumped the heat out of the interior of NICMOS, achieving the target temperature for neon gas passing through the instrument of 700Kelvin (- 2030C). The temperature is now maintained within a few hundredths of a degree. The resuscitated NICMOS, with its new phase of self-regulated operation, is being checked out, and by early June the first infrared images taken with NICMOS since 1998 are expected to be released. Earlier, the instrument was being cooled by rapidly depleting blocks of solid nitrogen ice. According to the Space Telescope Science Institute (STScI), the telescope has been operating superbly since March after its refurbishing during SM3B.