DAG's Focal-Plane Instruments: KORAY, DIRAC, PLACID and DAGOS
Ramazan Şahin, PhD · · 4 min read · Updated
The instruments that turn a telescope's light into data sit at its focal plane. The roles and key specifications of four instruments built for DAG400.
A telescope's primary mirror collects light, but the instruments that turn that light into measurable data sit where the telescope forms its image: the focal plane. Cameras, spectrographs, coronagraphs and calibration units all belong here. For the 4-metre DAG400 telescope of the Eastern Anatolia Observatory, the institution's list highlights four instruments: KORAY, DIRAC, PLACID and DAGOS. The role of each is summarised below together with values from the institution's technical tables.
KORAY: a field derotator that holds the image still
DAG400 uses an altitude-azimuth mount. With this kind of mount the image at the focal plane slowly turns while the telescope tracks a target, and during long exposures the rotation smears stars into arcs. KORAY (K-mirror Optical RelAY) is a K-mirror field derotator that cancels this rotation. A K-mirror consists of three mirrors arranged in a shape reminiscent of the letter K; rotating the assembly turns the image by twice the assembly's own rotation angle.
According to the institution's page, KORAY can rotate at up to 5 degrees per second over a range of ±170 degrees with an angular precision of ±0.1 degrees. The displacement relative to the optical axis is given as 250 micrometres and the radius of its field of view as 3.5 arcminutes.
DIRAC: an infrared camera behind adaptive optics
DIRAC (DAG Infra Red Adaptive-Optics Camera) is designed for diffraction-limited near-infrared imaging and receives its light either from the adaptive optics module or from the coronagraph. The institution notes that such an adaptive-optics-fed infrared imager can serve many research topics in which fine detail is essential.
Its detector is a Teledyne H1RG with 1016 × 1016 pixels of 18 micrometres. It covers 0.9–2.4 micrometres. With a plate scale of 33 milliarcseconds per pixel, the field of view is 33″ × 33″. Read noise is 18 electrons and dark current is below 0.1 electrons per pixel per second. There are two six-slot filter wheels: the first holds the broadband Y, J, H and K filters plus two slots reserved for users, while the second holds narrow continuum filters for each of those four bands, a dark position and a lens for imaging the pupil.
PLACID: a programmable coronagraph
Seeing a faint planet or dust disc right next to a bright star requires suppressing the star's light, and a coronagraph is the instrument that does this. PLACID (Programmable Liquid-crystal Active Coronagraphic Imager) is described by the institution as an adaptive stellar coronagraph. At its heart is a liquid-crystal spatial light modulator (SLM) that can change the phase of light pixel by pixel, so the coronagraph mask can be reshaped in software without swapping any physical part. The institution stresses that this adaptability gives the instrument a notable place both nationally and internationally. PLACID also includes a mechanism that can route the optical path between the focal-plane instruments.
According to the technical table, PLACID works in the H (1.63 micrometres) and Ks (2.15 micrometres) bands. The SLM has a resolution of 1920 × 1152 pixels. The full width at half maximum (FWHM) of the images is 85 milliarcseconds in H and 115 in Ks, in line with the diffraction limit of a 4-metre mirror at those wavelengths. Contrast is given as better than 1.1 × 10⁻² at 2 λ/D from the star and better than 5.3 × 10⁻³ at 5 λ/D. Total optical throughput is above 22 per cent.
DAGOS: an optical simulator
DAGOS (the DAG telescope optical simulator) is a test source that mimics the telescope's light without looking at the sky. It produces calibration images for all focal-plane instruments and provides artificial sources at many wavelengths and at different positions in the field for alignment tests and for keeping the system in shape. It also offers a test bed for new optical systems that may be added to DAG400 in the future. According to the institution, DAGOS is to be integrated so that it works behind the adaptive optics system and the coronagraph.
How the instruments work together
The four instruments can be pictured as links in one chain. KORAY removes the image rotation caused by the altitude-azimuth mount. The adaptive optics system TROIA repairs the wavefront distorted by the atmosphere. PLACID suppresses the light of a bright star when needed, and DIRAC records the resulting sharp infrared image. DAGOS serves to check that every link in the chain is aligned and calibrated. The institution's history page states that the observatory is planned to offer photometric and coronagraphic observations first, followed by spectroscopy.
References
- DAG400 Telescope · Türkiye National Observatories (trgozlemevleri.gov.tr)
- History · Türkiye National Observatories (trgozlemevleri.gov.tr)
Last accessed: 24 September 2026.