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Key project milestone: fold system review complete!

Overview: critical design review

It was an exciting end to the year for NRT as the project team passed an important design milestone; the fold system critical design review, the first major assessment since the Preliminary Design Review (PDR) in late 2021.


The fold system Critical Design Review (CDR) was a two-day hybrid meeting with external experts to assess the readiness for manufacture of the fold mirror and related support structure.


Graphical figure showing the NRT project timeline from 2018 to 2026. Different milestones are noted on the timeline including the conceptual, preliminary, detailed and manufacturing/final design periods. The fold system CDR is highlighted as having occurred in December 2022
Diagram showing the timeline of the NRT project and where the fold system CDR sits

As parts of the telescope design mature, the project team will start to perform more of these CDRs on the various telescope systems. The detail required at a CDR stage is far greater than for preliminary design reviews and, after a successful outcome, usually serves as a gateway to the manufacture of the system.


A graphical figure showing a timeline from 2021 to 2024 focusing on the key milestones for the fold system review covering design, manufacture and testing.
Fold system focused timeline showing the different stages of fold mirror design and review

The fold system was the first to be reviewed in this way and served as a great experience for the team. They were able to refine the review process before moving onto larger systems further into the detailed design phase of the project.


What is the NRT fold mirror system?

The fold mirror is a key part of the telescope's optical system. It is the third mirror that the light hits on its path through the telescope, for this reason it is sometimes referred to as the 'M3 mirror'. The fold mirror directs light from the secondary mirror towards instruments around the acquisition and guidance (A&G) box at the bottom of the telescope. The mirror also moves out of the light path when the instrument at the straight-through port is in use.



Cartoon showing the NRT and the location of the A&G box at the base of the telescope tube. Image of the A&G box shows where the fold mirror is located inside the box.
NRT structure and A&G box, showing location of fold mirror and assembley.

  • Fold mirror: The fold mirror reflects light at a 45 degree angle from the secondary mirror (M2) towards the instrument. The mirror needs to be very flat and not warp or bend if there are temperature changes to the support structure, otherwise the image will be distorted. This mirror also needs to be small enough to not block the autoguider instrument.

  • Mirror support structure: The support structure holds the mirror in place, so that it doesn't move or tilt when the telescope moves. The support structure should not deform the mirror surface during temperature changes or through pressure on the mirror.

  • Fold system mechanisms: These mechanisms allow the mirror to move to the middle of the A&G box or return to the home position. The mechanisms receive position-change commands from the telescope control system.


Cartoon showing the mirror support structure sitting on the fold mechanism, along with the separate fold mirror which will be attached to the support.
Diagram of the mirror support structure, mechanism and fold mirror.

How was the review done and what were the outcomes?

The review was divided into four main stages;

  • Design work and documentation

  • Pre-review

  • Main review

  • Post review – reporting, actions and further work

Graphic showing the timeline from October to December 2022 focusing on the fold mirror review process specifically.
Timeline of 2022 showing the detailed process of the fold review.

Review Panel

The project team selected a range of experts for the external review panel to ensure feedback was independent, honest, and so the project would benefit from a wide range of expertise in different areas.

Professor Habib Khosroshahi of the Iranian National Observatory (INO; a long time advisor to the project) was selected as chair. The other members of the panel were:

  • David Lee – Science and Technology Facilities Council, UK Astronomical Technology Centre

  • Marco Riva - Italian National Institute for Astrophysics (INAF)

  • Mauro Ghigo - INAF

Documentation stage

Ahead of the main review session, the project team prepared all the required documentation covering the mechanical, optical and opto-mechanical design of the system. The documentation also covered project management related issues, manufacturing plans and maintenance considerations.


Pre-review phase

The panel were given a month to review the documentation. During this time, clarification and requests for more information were provided back to the project team. The panel also posed questions and suggested points for discussion during the review.


Fold system review:

  • When: 14-15th December 2022

  • Where: Astrophysics Research Institute, Liverpool (hybrid meeting)

The two day review followed the agenda below:

Table showing the meeting schedule for the fold mirror review
Day 1 schedule for the fold review meeting

Table showing the meeting schedule for the fold mirror review
Day 2 schedule for the fold mirror review
Screenshot showing different members of the NRT team and review panel on a zoom call.
The NRT team and panel discussion during the fold system review

Review outcomes and follow-up

  • Review points and actions

The panel raised 93 individual points; many were closed through discussions between the NRT team and the panel prior to the review. The panel recommended an accurate alignment process or fixture for the mirror stand. As a result the team have redesigned this component so that it can be disassembled and reassembled with high repeatability.


Since the review, 62% these points have been closed through further development and analysis; 24% are in progress and 14% remain open.


Pie chart showing the completion rate of the points raised by the review panel. 62% of points are closed, 14% are open and 24% are in progress.
Completion statistics for points raised during the view. At date of publication, 62% of the 93 points raised have been closed.

  • Main topics for consideration

The panel highlighted a few areas where further work is needed, with priority given to topics which need to be progressed before the fold mirror order is placed.

  • The path to manufacture for the mirror and support system (who is responsible for each part and timelines)

  • A sensitivity study of the mounting pads at the back of the mirror (for gluing/assembly process)

  • To prototype the lateral mirror support to confirm manufacturing process

  • Confirm polishing and testing requirements to ensure specification and verification is successful.

  • Comments from panel and chair

The panel provided a report summarising key topics to be addressed by the team and included the following statement:

“The panel was satisfied with the technical expertise of the NRT project team and believed that the subsystem could be developed and tested at LJMU.

…Therefore, the fold mirror subsystem can proceed to the procurement and development stage. ”


  • Further work

The project team are now busy closing items from the review, with the intention of ordering the fold mirror and its support structure in the next few month. This will allow early performance tests of the system in the labs at LJMU prior to the integration with the A&G box in 2024.


Engineering diagram showing the fold support, mechanism and mirror in more detail. The mirror is separate from the support system with the attachment mechanism showing at the back of the mirror.
Support mechanism and structure along with fold mirror

The NRT project team would like to offer their thanks to the expert panel who supported the whole review process and in particular to Professor Habib Khosroshahi for chairing the panel.

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