A Powerful Twin Arrives: First Images from FORS2 at VLT KUEYEN on Paranal

The first, major astronomical instrument to be installed at the
ESO Very Large Telescope (VLT) was FORS1
(FOcal Reducer and Spectrograph) in September
1998. Immediately after being attached to the Cassegrain focus of the
first 8.2-m Unit Telescope, ANTU, it produced a series of
spectacular images, cf. ESO PR
14/98. Many important observations have since been made with
this outstanding facility.

Now FORS2, its powerful twin, has been installed at the
second VLT Unit Telescope, KUEYEN. It is the fourth major
instrument at the VLT after FORS1, ISAAC and UVES..

The FORS2 Commissioning Team that is busy installing and
testing this large and complex instrument reports that “First Light”
was successfully achieved already on October 29, 1999, only two days
after FORS2 was first mounted at the Cassegrain focus. Since
then, various observation modes have been carefully tested, including
normal and high-resolution imaging, echelle and multi-object
spectroscopy, as well as fast photometry with millisecond time
resolution. A number of fine images were obtained during this work,
some of which are made available with the present Press Release.

The FORS instruments

ESO PR Photo 40a/99 [Preview – JPEG: 400 x 345 pix – 203k] [Normal – JPEG: 800 x 689 pix – 563kb] [Full-Res – JPEG: 1280 x 1103 pix – 666kb]

Caption to PR Photo 40a/99: This digital photo shows the twin instruments, FORS2 at KUEYEN (in the foreground) and FORS1 at ANTU, seen in the background through the open ventilation doors in the two telescope enclosures. Although they look alike, the two instruments have specific functions, as described in the text.

FORS1 and FORS2 are the products of one of the most
thorough and advanced technological studies ever made of a
ground-based astronomical instrument. They have been specifically
designed to investigate the faintest and most remote objects in the
universe. They are “multi-mode instruments” that may be used in
several different observation modes.

FORS2 is largely identical to FORS1, but there are a
number of important differences. For example, it contains a Mask
Exchange Unit (MXU) for laser-cut star-plates [1]
that may be inserted at the focus, allowing a large number of spectra
of different objects, in practice up to about 70, to be taken
simultaneously. Highly sophisticated software assigns slits to
individual objects in an optimal way, ensuring a great degree of
observing efficiency. Instead of the polarimetry optics found in
FORS1, FORS2 has new grisms that allow the use of higher
spectral resolutions.

The FORS project was carried out under ESO contract by a consortium
of three German astronomical institutes, the Heidelberg State
Observatory and the University Observatories of Göttingen and Munich. The
participating institutes have invested a total of about 180 man-years
of work in this unique programme.

The photos below demonstrate some of the impressive possibilities
with this new instrument. They are based on observations with the
FORS2 standard resolution collimator (field size 6.8 x 6.8
armin = 2048 x 2048 pixels; 1 pixel = 0.20 arcsec). In addition,
observations of the Crab pulsar demonstrate a new observing
mode, high-speed photometry.

Protostar HH-34 in Orion

ESO PR Photo 40b/99 [Preview – JPEG: 400 x 444 pix – 220kb] [Normal – JPEG: 800 x 887 pix – 806kb] [Full-Res – JPEG: 2000 x 2217 pix – 3.6Mb] The Area around HH-34 in Orion

ESO PR Photo 40c/99 [Preview – JPEG: 400 x 494 pix – 262kb] [Full-Res – JPEG: 802 x 991 pix – 760 kb] The HH-34 Superjet in Orion (centre)

PR Photo 40b/99 shows a three-colour composite of the young
object Herbig-Haro 34 (HH-34), now in the protostar stage of
evolution. It is based on CCD frames obtained with the FORS2
instrument in imaging mode, on November 2 and 6, 1999.

This object has a remarkable, very complicated appearance that
includes two opposite jets that ram into the surrounding interstellar
matter. This structure is produced by a machine-gun-like blast of
“bullets” of dense gas ejected from the star at high velocities
(approaching 250 km/sec). This seems to indicate that the star
experiences episodic “outbursts” when large chunks of material fall
onto it from a surrounding disk.

HH-34 is located at a distance of approx. 1,500 light-years,
near the famous Orion Nebula, one of the most productive star
birth regions. Note also the enigmatic “waterfall” to the upper left,
a feature that is still unexplained.

PR Photo 40c/99 is an enlargement of a smaller area around
the central object.

Technical information: Photo 40b/99 is based
on a composite of three images taken through three different filters:
B (wavelength 429 nm; Full-Width-Half-Maximum (FWHM) 88 nm; exposure
time 10 min; here rendered as blue), H-alpha (centered on the hydrogen
emission line at wavelength 656 nm; FWHM 6 nm; 30 min; green) and S II
(centrered at the emission lines of inonized sulphur at wavelength 673
nm; FWHM 6 nm; 30 min; red) during a period of 0.8 arcsec seeing. The
field shown measures 6.8 x 6.8 arcmin and the images were recorded in
frames of 2048 x 2048 pixels, each measuring 0.2 arcsec. The Full
Resolution version shows the original pixels. North is up; East is
left.

N 70 Nebula in the Large Magellanic Cloud

ESO PR Photo 40d/99 [Preview – JPEG: 400 x 444 pix – 360kb] [Normal – JPEG: 800 x 887 pix – 1.0Mb] [Full-Res – JPEG: 1997 x 2213 pix – 3.4Mb] The N 70 Nebula in the LMC

ESO PR Photo 40e/99 [Preview – JPEG: 400 x 485 pix – 346kb] [Full-Res – JPEG: 986 x 1196 pix – 1.2Mb] The N70 Nebula in the LMC (detail)

PR Photo 40d/99 shows a three-colour composite of the N
70 nebula. It is a “Super Bubble” in the Large
Magellanic Cloud (LMC), a satellite galaxy to the Milky Way
system, located in the southern sky at a distance of about 160,000
light-years. This photo is based on CCD frames obtained with the
FORS2 instrument in imaging mode in the morning of November 5,
1999.

N 70 is a luminous bubble of interstellar gas, measuring
about 300 light-years in diameter. It was created by winds from hot,
massive stars and supernova explosions and the interior is filled with
tenuous, hot expanding gas. An object like N70 provides
astronomers with an excellent opportunity to explore the connection
between the lifecycles of stars and the evolution of galaxies. Very
massive stars profoundly affect their environment. They stir and mix
the interstellar clouds of gas and dust, and they leave their mark in
the compositions and locations of future generations of stars and star
systems.

PR Photo 40e/99 is an enlargement of a smaller area of this
nebula.

Technical information: Photos 40d/99 is based
on a composite of three images taken through three different filters:
B (429 nm; FWHM 88 nm; 3 min; here rendered as blue), V (554 nm; FWHM
111 nm; 3 min; green) and H-alpha (656 nm; FWHM 6 nm; 3 min; red)
during a period of 1.0 arcsec seeing. The field shown measures 6.8 x
6.8 arcmin and the images were recorded in frames of 2048 x 2048
pixels, each measuring 0.2 arcsec. The Full Resolution version shows
the original pixels. North is up; East is left.

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The Crab Nebula in Taurus

ESO PR Photo 40f/99 [Preview – JPEG: 400 x 446 pix – 262k] [Normal – JPEG: 800 x 892 pix – 839 kb] [Full-Res – JPEG: 2036 x 2269 pix – 3.6Mb] The Crab Nebula in Taurus

ESO PR Photo 40g/99 [Preview – JPEG: 400 x 444 pix – 215kb] [Full-Res – JPEG: 817 x 907 pix – 485 kb] The Crab Nebula in Taurus (detail)

PR Photo 40f/99 shows a three colour composite of the
well-known Crab Nebula (also known as “Messier 1”), as
observed with the FORS2 instrument in imaging mode in the
morning of November 10, 1999. It is the remnant of a supernova
explosion at a distance of about 6,000 light-years, observed almost
1000 years ago, in the year 1054. It contains a neutron star near its
center that spins 30 times per second around its axis (see
below). PR Photo 40g/99 is an enlargement of a smaller
area. More information on the Crab Nebula and its pulsar is available
on the web, e.g. at a dedicated
website for Messier objects.

In this picture, the red light is predominantly produced by
hydrogen emission from material ejected by the star that exploded. The
blue light is predominantly emitted by very high-energy
(“relativistic”) electrons that spiral in a large-scale magnetic field
(so-called syncrotron emission). It is believed that these
electrons are continuously accelerated and ejected by the rapidly
spinning neutron star at the centre of the nebula and which is the
remnant core of the exploded star. This pulsar has been identified
with the lower/right of the two close stars near the geometric center
of the nebula, immediately left of the small arc-like feature, best
seen in PR Photo 40g/99.

Technical information: Photo 40f/99 is based
on a composite of three images taken through three different optical
filters: B (429 nm; FWHM 88 nm; 5 min; here rendered as blue), R (657
nm; FWHM 150 nm; 1 min; green) and S II (673 nm; FWHM 6 nm; 5 min;
red) during periods of 0.65 arcsec (R, S II) and 0.80 (B) seeing,
respectively. The field shown measures 6.8 x 6.8 arcmin and the images
were recorded in frames of 2048 x 2048 pixels, each measuring 0.2
arcsec. The Full Resolution version shows the original pixels. North
is up; East is left.

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The High Time Resolution mode (HIT) of FORS2

ESO PR Photo 40h/99 [Preview – JPEG: 400 x 304 pix – 90kb] [Normal – JPEG: 707 x 538 pix – 217kb] Time Sequence of the Pulsar in the Crab Nebula

ESO PR Photo 40i/99 [Preview – JPEG: 400 x 324 pix – 42kb] [Normal – JPEG: 800 x 647 pix – 87kb] Lightcurve of the Pulsar in the Crab Nebula

In combination with the large light collecting power of the VLT
Unit Telescopes, the high time resolution (25 nsec = 0.000000025 sec)
of the ESO-developed FIERA CCD-detector controller opens a new
observing window for celestial objects that undergo light intensity
variations on very short time scales. A first implementation of this
type of observing mode was tested with FORS2 during the first
commissioning phase, by means of one of the most fascinating
astronomical objects, the rapidly spinning neutron star in the Crab
Nebula. It is also known as the Crab pulsar and is an
exceedingly dense object that represents an extreme state of matter –
it weighs as much as the Sun, but measures only about 30 km
across.

The result presented here was obtained in the so-called trailing
mode, during which one of the rectangular openings of the
Multi-Object Spectroscopy (MOS) assembly within FORS2 is placed
in front of the lower end of the field. In this way, the entire
surface of the CCD is covered, except the opening in which the object
under investigation is positioned. By rotating this opening, some
neighbouring objects (e.g. stars for alignment) may be observed
simultaneously. As soon as the shutter is opened, the charges on the
chip are progressively shifted upwards, one pixel at a time, until
those first collected in the bottom row behind the opening have
reached the top row. Then the entire CCD is read out and the digital
data with the full image is stored in the computer. In this way,
successive images (or spectra) of the object are recorded in the same
frame, displaying the intensity variation with time during the
exposure.

For this observation, the total exposure lasted 2.5 seconds. During
this time interval the image of the pulsar (and those of some
neighbouring stars) were shifted 2048 times over the 2048 rows of the
CCD. Each individual exposure therefore lasted exactly 1.2 msec
(0.0012 sec), corresponding to a nominal time-resolution of 2.4 msec (2
pixels). Faster or slower time resolutions are possible by increasing
or decreasing the shift and read-out rate [2].

In ESO PR Photo 40h/99, the continuous lines in the top and
bottom half are produced by normal stars of constant brightness, while
the series of dots represents the individual pulses of the Crab
pulsar, one every 33 milliseconds (i.e. the neutron star rotates
around its axis 30 times per second). It is also obvious that these
dots are alternatively brighter and fainter: they mirror the
double-peaked profile of the light pulses, as shown in ESO PR Photo
40i/99. In this diagramme, the time increases along the abscissa
axis (1 pixel = 1.2 msec) and the momentary intensity (uncalibrated)
is along the ordinate axis. One full revolution of the neutron star
corresponds to the distance from one high peak to the next, and the
diagramme therefore covers six consecutive revolutions (about 200
milliseconds).

Following thorough testing, this new observing mode will allow to
investigate the brightness variations of this and many other objects
in great detail in order to gain new and fundamental insights in the
physical mechanisms that produce the radiation pulses. In addition,
it is foreseen to do high time resolution spectroscopy of rapidly
varying phenomena. Pushing it to the limits with an 8.2-m telescope
like KUEYEN will be a real challenge to the observers that will most
certainly lead to great and exciting research projects in various
fields of modern astrophysics.

Technical information: The frame shown in Photo
40h/99 was obtained during a total exposure time of 2.5 sec
without any optical filtre. During this time, the charges on the CCD
were shifted over 2048 rows; each row was therefore exposed during 1.2
msec. The bright continuous line comes from the star next to the
pulsar; the orientation was such that the “observation slit” was
placed over two neighbouring stars. Preliminary data reduction: 11
pixels were added across the pulsar image to increase the
signal-to-noise ratio and the background light from the Crab Nebula
was subtracted for the same reason. Division by a brighter star (also
background-subtracted, but not shown in the image) helped to reduce
the influence of the Earth’s atmosphere.

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Notes

[1] The masks are produced by the Mask
Manufacturing Unit (MMU) built by the VIRMOS
Consortium for the VIMOS and
NIRMOS
instruments that will be installed at the VLT MELIPAL and YEPUN
telescopes, respectively.

[2] The time resolution achieved during the
present test was limited by the maximum charge transfer rate of this
particular CCD chip; in the future, FORS2 may be equipped with
a new chip with a rate that is up to 20 times faster.

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