This conceptual design for Handel's  Jephtha imagines that the events of the opera take place within a precisely controlled environment, engineered by heaven as a test of Jephtha's morality - Jephtha the subject, heaven the archive that records his every response.

Xingyi Chen is a designer and illustrator working across theatre and opera. They have a BA in Design for Performance from the Royal Central School of Speech and Drama.

www.xingyichenwork.com

Handel's Jephtha is the last major piece of music he wrote. In the margin of the chorus "How Dark, O Lord, Are Thy Decrees" he wrote, "unable to go on owing to weakening of the sight of my left eye".

Like Milton before him, Handel went blind whilst writing about divine judgment. As with Paradise Lost, Jephtha is filled with an anxiety about how inescapable God's gaze is. As these two men lose their sight, they become increasingly troubled by God's omniscience – his infinite sight.

Jephtha's story is taken from the Old Testament. The Israelites are losing a war with the Ammonites and so Jephtha, a great warrior, is called out of exile to lead the army. Before battle he promises, should he win, to sacrifice the first living thing he sees. On returning, his daughter, Iphis, rushes out to welcome him. Jephtha refuses to go back on his vow but, as he is preparing to kill Iphis, an angel comes down and saves her.

As Handel mourns the loss of his sight, Jephtha wishes he had never seen. You can feel that in the music: the terror and confusion of men who know they have passed the point of no return.

Xingyi's designs for Turandot and The Tales of Hoffmann (both directed by Ella Marchment) boldly reimagine where those operas take place. Jephtha all takes place in the Israelites' camp and risks being overly static. In the libretto, however, there are constant references to a second place: Heaven. Heaven hangs over the characters, listening, remembering and judging. It felt natural to bring Heaven onto the stage.

The question we began with was: "what might this Heaven look like?"

Jephtha's Heaven is a place of memory. Jephtha sings that "recorded stands my vow in Heav'n above" – that "Heav'n hear [his] thoughts, and wrote them down". The language is bureaucratic; Heaven sounds like an archive, writing down and recording what takes place in the world. It made me think of the work of Lars Tunbjörk. His offices find a strange holiness in the rhythmic mundanity of office life.

We decided to have Heaven be an office like this that surrounds the desert. Throughout the whole opera, angels would be recording what the characters sing – these words are typed onto a huge scroll, cut up, and stored in a vast archive. Heaven becomes a busy – slightly sinister – presence on stage, constantly at work as the action takes place. We feel that sense of Heaven's omniscience.

One particular inspiration for this expression of Heaven was Vicki Mortimer and Katie Mitchell's set for George Benjamin's Written on Skin, where a modern Heaven surrounds an ancient world – this contrast making the two feel at odds with one another.

With this concept, the Israelites in the "real" world wouldn't see the heavenly archive all around them. That separation would collapse in the third act when the angel comes to save Iphis.

We wondered how we could break this separation earlier.

A problem the libretto presents, to be solved through dramaturgy and scenography, is how passive Iphis is. In the context it was written in, an audience could be expected to believe that Jephtha following through with his promise is an act of piety. In fact, it is an act of extreme barbarism and extreme misogyny. Jephtha treats Iphis' life as his property, just as Agamemnon does with Iphigenia's in Aeschylus. Iphis' reaction to this is strangely accepting. Xingyi and I wanted to find a way to make Iphis more powerful in the story – to find a concept that centred the story on her.

Having already decided on having Heaven on stage, we wondered what would happen if we combined Iphis and the angel (both soprano roles) into one role. We imagined that Iphis' passivity was deliberate – she has been sent from Heaven to Earth to test Jephtha's morality. Her passivity is a form of the scientific method; she is giving Jephtha a fair moral test, only revealing herself to be an angel once she is sure he would have killed her.

This meant that throughout the opera we can blur the lines between Heaven and Earth by having angels disguise themselves to enter Earth, as Iphis does. These angels would gather evidence of the sins being committed to store in the heavenly archive. This opens up an opportunity to split the chorus in two, with some parts of the music for them sung by the angels in Heaven. So now at the end of the opera, when the chorus sings of Heaven's righteousness, it is not the Israelites singing but angels, as Jephtha is presented with the evidence against him and condemned. The final image being the scroll from Heaven falling down to Earth and Jephtha trying and failing to climb up it.

Aesthetically, a very important point of reference for our Heaven was the work of the late Erwin Olaf. Olaf constructs spaces that are so ordered and clean that they feel uninhabitable except by figures who appear equally purified.

Inspired by the work of Erwin Blumenfeld – and wanting to abstract the angels' forms as they move through space – we used fluted glass to fracture or frame their bodies depending on whether they pass behind it or in front of it. This allows the angels to feel as though they never fully settle into a natural humanoid form. We also drew on Vivienne Westwood's asymmetric suits in the angels' costumes to reinforce this sense of instability.

By contrast, we wanted the Israelites to feel as though they exist almost before the beginning of time. At the end of Paradise Lost, Milton describes Adam and Eve being sent east from Eden to the "subjected Plain". The story of Jephtha takes place only a few generations later, so we imagined a world that is barren; never having been meant to be inhabited. The work of Kay Sage, the surrealist painter known for landscapes that seem to belong to both the beginning and the end of time, was a particularly useful reference for imagining this strange early Earth. As were Gustave Doré's illustrations to Paradise Lost, with their feeling of harsh emptiness.

The aim of this design is not to redeem Jephtha, but to expose him. By making Heaven visible and active we give ourselves the opportunity to turn the work away from religious parable and toward being an exploration of how dogma and bravado escalate into violence and misery.

Jephtha would be Handel's last major work. At the end of the final chorus, brass rises in layers to form one vast final chord and then there is silence. In that silence I imagine an empty Heaven. An angel walks through the office and turns off the lights – the fluorescent tubes flicker and go out, first in Heaven and then, as if the sun is one of them, on the Earth too.