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Realuminising and other maintenance at the LT 10 Oct 2019

Late September saw the Liverpool Telescope (LT) taken offline to realuminise the primary mirror and undertake other essential maintenance. Realuminising the primary is a massive undertaking but it went swimmingly, and throughput of the telescope improved by over 40%.

Why realuminise?

Telescope mirrors differ from normal domestic mirrors in that they use a reflective layer deopsited on the front surface of glass instead of having it encapsulated behind a layer of glass. Front surface mirrors like this are better for precision scientific optics and provide optimum reflection quality, but because the reflection surface is exposed to the atmosphere it degrades faster than, for example, a bathroom mirror. Periodic re-coating is therefore an expected maintainance task for any telescope mirror. See the news story about the 2015 mirror coating for more detailed discussion of how optical mirrors like the LT's are constructed and maintained.

Mirror transit
primary mirror
LT Engineering Manager Stuart Bates by the primary mirror covered in protective tissue.
(Image © 2015 M. Crellin)

The telescope needs to be dismantled to remove the mirror. Fortunately we do not have to ship the mirror off La Palma to get it recoated. The nearby William Herschel Telescope (WHT) has a vacuum mirror coating chamber and provides the recoating service for many of the telescopes at Observatorio del Roque de los Muchachos.

Primary mirror being lifted by crane.
The primary mirror suspended from the crane. (Image © 2019 R. Smith)

The LT primary mirror is 2 metres in diameter and weighs 1.3 tonnes. As you can imagine, manoeuvering such an unwieldy yet fragile and expensive piece of equipment out of the enclosure, into a truck, and 500 metres down the road to the WHT, is quite a tense undertaking. It was skilfully handled however by the on-site team of LT site manager and senior mechanical engineer Stuart Bates, LT project scientist Robert Smith, New Robotic Telescope mechanical engineer Ali Ranjbar, and maintanance support and site engineer Dirk Raback, all in collaboration with a local La Palma crane operator and IAC and WHT staff.

The primary mirror in its mirror cell was slid out from underneath the telescope and immediately covered in lint-free tissue for protection and to prevent dangerously focused solar reflections. The mirror was then attached to the crane hoist and carefully lifted out of its cell, out of the top of the open enclosure, and into its special transit box in the shade of the LT Annexe building. The transit box was sealed and craned onto the truck, which then very slowly drove to the WHT.

Realuminising
primary mirror
The giant WHT vacuum chamber used for mirror coating. (Image © ING)

At the WHT, the mirror's old aluminium layer was carefully removed with powerful acids under strict safety supervision. The glass "blank" was washed and placed in the WHT's huge realuminising chamber.

primary mirror
The newly coated mirror dominates the foreground with the partially dismantled telescope structure in the background. (Image © 2019 R. Smith)

Aluminising went fine, and a few days later the primary was put back in the same way it came out. Moderate winds during the mirror hoist caused a slight level of concern, but the hoisting went fine nonetheless and the mirror was placed perfectly onto the pneumatic actuators in its mirror cell.

After that the cell was connected to the telescope, and refitting of the instruments followed in short order. In two days the telescope and all instruments were reassembled, and Robert Smith and Stuart Bates could begin recommissioning the whole system.

Recommissioning

The telescope had had its major optical components dismantled and reassembled, and all instruments had been removed and remounted. So from 30 September to 2 October, a full end-to-end test of the telescope's electrical, hydraulic, pneumatic and optical systems was made, along with similar tests for each instrument. This lengthy sequence of tests proved the telescope was operating nominally and that all of its instruments were in focus and working properly.

Some robotic observations were allowed to be made during recommissioning partly for testing purposes, but proper robotic observing essentially recommenced on 2nd October.

Other Maintenance

Realuminising the primary and recommissioning the telescope afterwards were not the only things done on this maintenance trip. The LT being in pieces afforded a rare opportunity to access normally out-of-reach components for maintenance. Extra tasks completed by the on-site team included:

  • Cassegrain rotator mechanism:
    When the telescope is fully assembled, the cassegrain derotator bearing is one of the most inaccessible systems. Both of the Cassegrain rotator's gearboxes were replaced, the drive motors realigned and the optical encoder tape cleaned.
  • SkycamZ:
    SkycamZ, itself an 8-inch Newtonian reflecting telescope mounted piggyback on the LT's top end ring, was removed and dismantled. Rather than recoating, this time the SkycamZ primary only required careful cleaning. This was done and the telescope reassembled. Its camera is due to be replaced soon, so until that happens SkycamZ remains offline. See the Skycam page for further details.
  • Primary mirror support:
    While observing, the LT's primary mirror is held in place to micron accuracy by an array of pneumatic supports. These supports were serviced and the accompanying pipework and fittings were all replaced.
  • SkycamT:
    SkycamT was removed from the LT's top end ring, cleaned, realigned, refocussed, and refitted. It remains in nightly use. See the Skycam page for further details.
  • Instruments:
    Whilst they were off the telescope, various routine servicing and minor repairs were performed on the science instruments.
  • Other optical cleaning:
    Other optical components cleaned were The LT's tertiary (science fold) mirror, the autoguider pickoff mirror, and all the filters. The telescope's secondary mirror was not recoated this time.

Summing up, the two weeks of site work was very successful. A very big thank you goes to everyone involved; our own staff, those from IAC and WHT and the specialists contracted locally on La Palma.

The Death Throes of a Stripped Massive Star 20 May 2019
position of supernova
Liverpool Telescope IO:O background image of the field around the host galaxy, with inset showing closeup taken by Canada-France-Hawaii Telescope (CFHT). Position of supernova SN2018gep in its host galaxy is marked by the white crosshairs in the CFHT inset. Click image for bigger version.
spectra of supernova
Optical spectra of SN2018gep taken from the ground by the LT (highlighted in yellow) and other telescopes. Numbers next to spectra denote time elapsed in days since supernova. Click image for bigger version.

The Liverpool Telescope's SPRAT spectrograph obtained the first spectra of a broad-lined stripped-envelope supernova last year, just seven hours after discovery by the Zwicky Transient Facility (ZTF).

The SPRAT spectra contributed to the study of the supernova, named “SN2018gep”. The results of the study are presented in a recent paper by Ho et al submitted to the Astrophysical Journal, entitled “The Death Throes of a Stripped Massive Star: An Eruptive Mass-Loss History Encoded in Pre-Explosion Emission, a Rapidly Rising Luminous Transient, and a Broad-Lined Ic Supernova SN2018gep”.

The supernova was identified as a rapidly rising (1.3 mag/hr) and luminous transient, and was discovered extremely early in its evolution — within an hour of the shock breakout.

The robotic Liverpool Telescope (LT) is ideally suited to the follow-up of fast transients such as this one, and the first spectrum of SN2018gep was obtained with SPRAT. The authors believe this is the earliest-ever spectrum of a stripped-envelope SN, in terms of temperature evolution.

This was followed by an intensive spectroscopic monitoring campaign using telescopes from around the world.

A retrospective search through pre-explosion data showed emission in the days to weeks leading up to the event, which is the first definitive detection of precursor emission for a supernova of this class.

The authors of the paper conclude that the data are best explained by shock breakout in a massive shell of dense circumstellar material at large radii that was ejected in eruptive pre-explosion mass-loss episodes.

LT helps discover huge nova "super-remnant" in another galaxy 9 January 2019

This is a composite image of Liverpool Telescope data (bottom left) and Hubble Space Telescope data (top right) of the nova super-remnant. M31N 2008-12a is in the middle of the image. Credit: Matt Darnley / LJMU.

An international team of astrophysicists have uncovered an enormous bubble currently being "blown" by the regular eruptions from a binary star system within the Andromeda Galaxy.

As reported in this week's Nature, recent observations with the Liverpool Telescope and Hubble Space Telescope, supported by spectroscopy from the Gran Telescopio Canarias, and the Hobby-Eberly Telescope (some of the largest astronomy facilities on Earth) discovered this enormous shell-like nebula surrounding ‘M31N 2008-12a’, a recurrent novae located in our neighbouring Andromeda Galaxy. At almost 400 lightyears across and still growing, this shell is far bigger than a typical nova remnant (usually around a lightyear in size) and even larger than most supernova remnants.

Dr Matt Darnley, lead author on the study and Reader in Time Domain Astrophysics at Liverpool John Moores University's Astrophysics Research Institute explains: “Each year ‘12a’ (as we lovingly refer to it) undergoes a thermonuclear eruption on the surface of its white dwarf. These are essentially hydrogen bombs, which eject material equivalent to about the mass of the Moon in all directions at a few thousand kilometres per second. These ejecta act like a snow plough, piling the surrounding ‘interstellar medium’ up to form the shell we observe – the outer ‘skin’ of the bubble, or the ‘super-remnant’ as we have named it.”

These new observations coupled with state-of-the-art hydrodynamic simulations (carried out at LJMU and the University of Manchester) have revealed that this vast shell is in fact the remains of not just one nova eruption but possibly millions – all from the same system.

Despite its uniqueness and staggering scale, the discovery of this super-remnant may have further significance.

Dr Matt Darnley continued: “Studying 12a and its super-remnant could help is to understand how some white dwarfs grow to their critical upper mass and how they actually explode once they gets there as a ‘Type Ia Supernova’. Type Ia supernovae are critical tools used to work out how the universe expands and grows.”

In a related work, also led by Matt Darnley, this team has predicted that 12a will ultimately explode as a Type Ia Supernova in less than 20,000 years – a very short time in cosmological terms.

Dr Rebekah Hounsell, second author on both studies and a post-doctoral researcher at the University of Pennsylvania, explains: “These are some of the largest explosions in the Universe (type Ia supernovae). Such an event in the Andromeda galaxy (M31) would be one of the closest supernovae observed by telescopes (the last one in M31 was in 1885 and in the Large Magellanic Cloud in 1987). The last one in our own galaxy (that we actually saw) was 1604. Although we’ve predicted that 12a will undergo a supernovae explosion in less than twenty thousand years – that sounds a long time, but of course that could still mean within the next decade or so.”

New Robotic Telescope website launched 28 Feb 2019
Locations of LT and NRT Location of the LT and NRT in the northern part of the Observatorio del Roque de Los Muchachos, on the summit of the island of La Palma in the Canary Islands, Spain. Image credit: Google (aerial pic), J.Marchant (annotation).

We have recently launched a new website for the Liverpool Telescope 2 or "New Robotic Telescope (NRT)" project. The webpages at www.robotictelescope.org detail the science case, NRT team and latest news items in relation to the new telescope.

The NRT team are currently preparing the Phase A design of the new 4-metre fully robotic and autonomous telescope, ready for a design board review in the Spring. The NRT will slew faster than the LT and be on target taking data within 30 seconds of trigger, allowing us to explore more rapidly fading targets.

The proposed location for the NRT is 400 metres from the LT on La Palma at the disused Carlsberg Meridian Telescope site, 100m from the William Herschel Telescope. There are also plans in place to repurpose the LT to support the scientific operations of the NRT and facilitate gravitational wave follow-up, along with facilitating a variety of new projects for the National Schools Observatory. The re-design of the LT is to allow a 2°×2° wide-field camera and a fibre-fed spectrograph as the main two instruments.

Keep an eye on the NRT website's latest news for updates on funding and design board reviews.