The High Optical Resolution Ultra-stable Spectrograph (HORuS) is a high-resolution spectrograph on GTC. HORuS has a FWHM resolving power of about 25,000. It collects light at the Nasmyth focal plane, shared with OSIRIS, using a 3x3 IFU (2.3x2.3 arcsec) with microlenses, into optical fibers that form a pseudo-slit at the spectrograph entrance, providing almost continuous coverage between 380 and 690 nm.

As HORuS is a Visitor Instrument, this web page gives only a minimal amount of information.


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icon Observing Modes

icon Spectroscopy

The High Optical Resolution Spectrograph (HORuS) is a high-resolution spectrograph on GTC based on components from UES, a spectrograph which was in use at the 4.2-m William Herschel Telescope (WHT) between 1992 and 2001. HORuS is quite simple and provides GTC with the ability to obtain moderately high resolution spectra (R=25000) with broad spectral coverage (approximately 380-690nm) for point sources.

HORuS collects light at the Nasmyth focal plane, shared with OSIRIS, using a 3x3 IFU (2.3x2.3 arcsec) with microlenses, into optical fibers that form a pseudo-slit at the spectrograph entrance. The light is dispersed with a 79 gr/mm echelle grating and cross-dispersed with three prisms, providing almost continuous coverage between 380 and 690 nm. The introduction of a mirror bender greatly reduces the area of the instrument, and a bundle of fibers (together with an array of microlenses) makes possible to cover the seeing disc (<20% loss of light from point sources for a seeing FWHM ~1.5 arcsec). HORuS will operate on the GTC as a general-purpose high-resolution spectrograph. HORuS is a Visitor Instrument on GTC, and its commissioning was completed in December 2017. The instrument will be operating at the GTC until at least early 2020.

The following table summarizes some basic parameters of the instrument.

Spectral range 377-691 nm
Spectral resolution R~25,000
F.O.V. (IFU) 2.25 x 2.25 arcsec2 (3x3 fibers)
Sky Calibration 1 fiber 2 arcsec off the IFU
Fiber size 0.75 arcsec
Detector 1 x 4096 x 4096 Fairchild CCD486 BI
Pixel Size 15 µm/pix


HORuS IFU configuration at the GTC focal plane (left) and pseudoslit for calibrations purposes (right).


HORuS signal-to-noise ratio per pixel for a 15 min. exposure on HD 140283. For comparison, the gray curves show the predictions for UVES (1 arcsecond slit, 1 arcsecond seeing on the 8m VLT), and HARPS (fiber-fed on a 3.6m telescope).


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icon Technical Details

An array of microlenses is used to modify the focal ratio of the telescope (f/17) to a suitable value for optical fibers (f/2.5). In the pseudo-slit of the spectrograph, a second array of microlenses adapts the focal output ratio to that of the instrument's collimator (f/11). The new instrument is coupled with optical fibers to the focal plane of GTC, using the Nasmyth-B focus where OSIRIS is currently located, sharing focus with it but without interfering in any way with its usual operation. The optical design of HORuS involves only one main modification of UES, the introduction of a flat mirror (folder) just after the collimator in order to produce a more compact design.


Top: Ray-trace through the HORuS optics. Bottom: View of the light path of HORuS without the protection cover.


Top panel: View of the entrance of the optical fiber in OSIRIS (through the main connection panel) and the two cables of the calibration mechanism (motor and zero switch). Bottom panel: View of the supporting structure for the cable that communicates OSIRIS with the HORuS spectrograph.

The CCD camera is a "plug and play" system, supplied by the company Spectral Instruments (SI). It consists of a main module, or "camera head" which contains both the CCD itself (in this case, a model Fairchild CCD486 BI of 4096 x 4096 pixels), proximity electronics, and a coupled shutter. In the case of HORuS this shutter is located in a different place from the optical path (inside of the spectrograph) but its control, despite changing the model (original 90mm, new 1 arcsecs), it is the same one that comes with the complete camera.


Left panel: Camera head module with the shutter open showing the CCD486 of Fairchild, as mounted by Spectral Instruments. This shutter is dismantled in HORuS. Right panel: Rear view and connection of the CCD camera already in operation.

The acquisition arm consists of a mechanical support, two optical fibers (the one that comes from the calibration lamps and the one that goes to the spectrograph) and a positioning system for the calibration mirror, which introduces this element in the optical path in such a way that the light that comes from any one of the lamps of calibration enters the science fiber. The movement of the mirror between the positions of "SKY" and "CALIBRATION" is carried out by a PI-NI 310 NEXACT piezoelectric actuator (Miniature Linear Actuator), with nominal travel of 10mm (something more in practice, ~ 12mm). It is managed via serial port USB for a PI driver model E-861 (NEXACT Controller) in open loop.


Left: assembly of the arm within the OSIRIS focal plane. Right: OSIRIS focal plane in Nasmyth-B, showing its useful field of view and the shadow introduced by the folder vignetting and three of the filter wheels. The HORuS arm is placed in the vignetted area of the OSIRIS folder including the optical fibers and the array of microlenses.


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icon Data acquisition system

The HORuS Control Software runs on a Windows XP PC. This is due to the incompatibility of the drivers of the CCD camera with more recent operating systems. The PC is located in Cabinet A, near the spectrograph. The connection to the PC is made through Ethernet using VNC (Virtual Network Computing) or a virtual windows remote desktop.


A screenshot of the windows-based HORuS interface, used by astronomers at the telescope to control the instrument. There also exists an engineering interface for low-level control and telemetry of the CCD controller.

The HORS control software is very easy to use. The user interface shows different labels associated with the different functions performed by the software. These tabs are only enabled when the corresponding function can be performed. For example, the Camera label will only be available when its electronics are on. Note that the "Camera Temperature & Pressure" and "Spectrograph Temperatures" tabs are always displayed (same as the MCT that was enabled from the beginning of the program), even when the associated equipment was not turned on. This is so for in case of switching off these devices, you can continue to see the values ​​of the previously acquired sensors. One can acquire images from the CCD camera according to different parameters that can be selected by the user.


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icon Data reduction


A data reduction pipeline for HORuS is under development.

icon On-sky performance


The completed HORuS instrument was installed and tested on the sky at end of June 2018. The instrument was used for the first time in conjunction with the final fiber link, which had been previously demonstrated to provide about 80% of transmission in laboratory tests. The acquisition arm, which was linked to some of the performance issues found two years earlier, had also been replaced with a new one, designed from scratch to provide more rigidity and stability, and which had been checked at the IAC to return to the same SKY/CAL positions within specifications. The acquisition software had also been upgraded in many regards, including calibration sequences, the ability to compute 3x3 images for the IFU, and thorough checks at every step of the operations.

The data secured in June reveals that the HORuS image quality is appropriate, with a constant level bias level in the detector (around 500 counts in the 'slow' readout mode), very little background light, and modest scattered light within the instrument. This can be appreciated in the figure below, where the spectra of the blue sky and a bias exposure, are shown. The fiber-to-fiber throughput variationsamount to only a few percent, as expected from data taken at the IAC.


Images showing a bias (top left-hand panel), a spectrum of the blue sky at twilight (top right- hand panel), a ThAr spectrum (bottom-left) and a spectrum of the metal-poor star HD 140283 (V=7.2). The detector was read using 4 channels, which leaves a distinct signature (at the level of a few electrons) among the quadrants, as can be seen in the bias frame.

The spectral coverage was confirmed from the sky spectrum. Some 27 orders were measured, from order 59 in the blue end, spanning approximately 379.9- 386.4 nm, to order 33 in the red end, spanning 674.5-695.3 nm. The dispersion direction is aligned quite well with the detector, especially in the red. We nevertheless trace the distortion with polynomials to extract the spectra, as illustrated in the figure below. The orders have a fairly constant half-width across the cross-dispersion direction, about 33.8 pixels.


Spectrum of the sky where the tracing or the orders has been overlaid: in dash for the left-side and solid for the right side of each order.


Emission lines due to thorium in the HORuS calibration lamp (left). The right-hand panel shows the measured FWHM of many lines as a function of wavelength. The measurements indicate that the instrument has a resolving power of R=λ/FWHM = 23,400, very close to the design of 25,000. The spectrum is heavily oversampled in both the spatial and the spectral directions.

Top: The instrument throughput comparation between the observations obtained with GTC+HORuS (black line) and the output from the exposure time calculator for the UVES spectrograph on the VLT (red line) for the reference metal-poor star HD 140283 (V=7.2, SN~600). Bottom: Same for the Red Giant Branch star HD 122563 (V=6.4, SN~900). Note that UVES is not fiber-fed, and has two optimized arms, but less spectral coverage than HORuS.

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icon HORuS contact persons at GTC


contact email @ gtc.iac.es
David Garcia - main contact david.garcia
Riccardo Scarpa riccardo.scarpa

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Last modified: 09 January 2019