The photons are coming

MOONS has reached a major milestone. After years of planning, photons are about to pass through many metres of fibre, millions of pounds of optics and finally arrive on the detectors. This will be the first test of the full light path of the instrument, and indeed the first time many of the different components have been used together. All this has to be tested at the final operating temperatures, so this is also the first time the cryogenic system has been tested with a significant fraction of the optical components in place.

The majority of optical components for one side of the spectrograph are now in place. It has taken many years to manufacture some of these components and many months to precisely install them inside the cryostat. The state of the work can be seen in the photograph below which was captured shortly before the cryostat was closed up to start the pump down and then the cool down.

The light path begins with the bundles of fibre outside the cryostat that will eventually bring light in from the telescope. These fibre bundles pass through the cryostat wall into the cold (-130 degree) chamber of the cryostat. Here they are aligned to form the entrance “slit” of the spectrograph. The alignment of the slit is extremely precise and has required many dedicated months of work in Paris. This was the first major component to be installed in the optical path.

The expanding beam of light leaving the fibres is brought back under control by the large collimator mirror that is located at the base of the cryostat. For safety’s sake, this was the last component to actually be installed – since it is at the bottom of the cryostat, we didn’t want to risk dropping anything on it! After leaving the collimator, the photons journey back past the slit and are then sorted into the correct wavelength channels by the two large fixed dichroics. More discussion of how these mounts were designed in Arcetri and tested in Edinburgh can be found here. Once split into the correct channels the photons reach the dispersive elements, before being image by dedicated cameras. The cameras have already been painstakingly internally aligned at Cambridge before installation in Edinburgh.

In this cool down we have two of the three optical channels installed – the RI band and the YJ band. These two use very different detector technologies, which have to operate at two very different temperatures, so it is an excellent test to have both of these working at the same time. Since this is the first cool down with detectors installed, we are using engineering grade devices.

The completeness of the instrument is captured in the table below, which lists the main optical components for the first spectrograph. The table gives an indication of what has been installed, but also how they have been tested – giving some indication of how few of these components have been tested together. There are numerous reasons why this is the case, but the main one is that since the MOONS optics are so large, any tests on individual components would probably require the manufacture of huge optical components – rather like the other components in the instrument itself. Designing and building suitable tests to look at the components in isolation would rapidly become just as complex as just putting MOONS together and testing. This is therefore what we have chosen to do!

What next?

As discussed above this cool down is going to provide information about a wide range of properties of the instrument. Many of these aspects are functional, i.e. cooling times and temperature stabilities. We fully expect these all to work, we just need to verify and understand performances.

The more interesting tests are to look at the image quality that can be achieved with the spectrograph. We will illuminate fibres outside the cryostat with the correct optical beam and this will deliver light to the detectors. The first and most basic test will be to get the cameras into focus. Due to the speed of the cameras, the depth of focus is very small, which makes focus a real challenge. Once the optimum focus has been achieved the next test will be to analyse the image quality achieved across the field.

There are other interesting tests that will be performed along the way. For instance, within the instrument there are a number of cryogenic mechanisms: a shutter to block light, a focus mechanism for the slit, large sliders to switch dispersers between high and low-res modes and finally each camera has three motors to allow tip/tilt of the detector. Any cryogenic mechanism is difficult to build, so simply the testing of these components is also interesting. Assuming all function correctly, having in-focus cameras will also let us test the precision and repeatability of these mechanisms in detail.

As this article hopefully captures, there is a great deal of testing to be done. This “first-light” moment for MOONS is going to be very exciting, but the interpretation of everything is going to take considerably longer than a moment!