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New Exposure Time Calculators 28 Nov 2019
The new ETC for imaging with the LT. A similar ETC for spectroscopy is on the same page.

New Exposure Time Calculators (ETCs) for the LT have been installed on the website at the Exposure Time Calculator page.

Between the two ETCs (one for imaging, the other for spectroscopy), existing and prospective users can answer questions on what exposure times are necessary to achieve a required signal to noise ratio. Users can select any of the many instruments mounted on the LT and adjust their settings, as well as the effect of atmospheric turbulence ("seeing") and background sky brightness.

The original idea to upgrade the ETCs' usability with Google Charts was Dr Marco Lam's, who is Scientific Software Developer for the LT. The new ETCs were developed by Lam and Dr Doug Arnold, DevOps Engineer for the New Robotic Telescope.

"Interfaces are becoming more web based, and the new exposure time calculator, using Google Charts and extended JavaScript functionality, has really improved the usability of this tool" said Dr Arnold.

Feel free to use the ETCs for yourself and discover how the LT can obtain images and spectra of your target:

A Milestone Gamma Ray Burst Study: GRB190114C 26 Nov 2019

(adapted from LJMU press release)

Hubble Space Telescope image of gamma-ray burst afterglow (circled) in its host galaxy. Credit: NASA, ESA, V. Acciari et al 2019.

Liverpool John Moores University astrophysicists and the Liverpool Telescope contributed to a study published in Nature recently of a gamma-ray burst caused by the collapse of a massive star 5 billion light years away.

Analysis of the minutes immediately after the burst reveals emission of photons a trillion times more energetic than visible light.

“These are the highest energy photons ever seen from a gamma-ray burst,” stated Dr Daniel Perley, a senior lecturer at LJMU's Astrophysics Research Institute involved with the study.

Gamma-ray bursts are the most powerful explosions in the Universe, emitting more energy within seconds than the Sun provides in its entire life cycle. Most of this energy is in the form of gamma-rays, a form of electromagnetic radiation much more energetic than visible light or even the X-rays that are used in medicine.

On January 14 this year, an extremely bright and long gamma-ray burst, known as GRB 190114C, was detected by a suite of telescopes, including NASA’s Swift and Fermi satellites in space as well as the Major Atmospheric Gamma Imaging Cherenkov (MAGIC) telescopes and the ARI-operated Liverpool Telescope on the ground.

It is believed the burst is produced when material is expelled from a collapsing star at virtually the speed of light. This material collides with gas that surrounds the star, causing a powerful shock wave. The new study attributes this shock wave as the source of the ultra-high-energy photons, which were detected by MAGIC for a period lasting almost an hour following the initial explosion of the burst. The LT simultaneously observed the source in optical light.

Scientists have been searching for this type of signal for many years, so this detection is considered a milestone in high-energy astrophysics.

Dr Perley added: “Gamma-ray bursts have been known about for decades but many aspects of them are still a mystery. How can a single explosion produce so much energy so fast? So, these new results help us understand what really happens under these extreme conditions.”

Dr Perley was accompanied in the publication by his PhD student Allister Cockeram as part of a team of more than 100 astronomers across the globe.

Andrew Levan of the Institute for Mathematics, Astrophysics & Particle Physics at Radboud University in the Netherlands, said: “The observations suggest that this particular burst was sitting in a very dense environment, right in the middle of a bright galaxy 5 billion light years away. This is really unusual, and suggests that might be why it produced this exceptionally powerful light.”

In addition to the Nature study, a further paper based on measurements from the event has also been submitted by the Liverpool Telescope Gamma-Ray Burst team in the Astrophysical Journal for publication soon. In this paper, a team of scientists currently or formerly based at Liverpool John Moores University and their students, including PhD student Jordana Mitjans at University of Bath and Prof Shiho Kobayashi at LJMU, used the Liverpool Telescope data from this burst to study the properties of the shock-wave in detail during the critical first minutes after the explosion.

A third paper, prepared by Antonio de Ugarto Postigo of the IAA-CSIC in Granada, Spain with contributions by Perley and others, studies the nature of the distant galaxy in which the burst exploded.

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.