HiPERCAM is a state-of-the-art, quintuple-beam imager that saw first lightas a Visitor Instrument on the GTC in February 2018. The instrument uses re-imaging optics and 4 dichroic beamsplitters to record ugriz (300-1000nm) images simultaneously on its five CCD cameras. The detectors in HiPERCAM are frame-transfer devices cooled thermo-electrically to -90°C, thereby allowing both long-exposure, deep imaging of faint targets, as well as high-speed (over 1000 windowed frames per second) imaging of rapidly varying targets.
HiPERCAM was funded by a €3.5M European Research Council Advanced Grant awarded to Vik Dhillon, and was designed and built by a consortium from the Universities of of Sheffield, Warwick, Durham, the UKATC and the IAC.
As HiPERCAM is a Visitor Instrument, this web page gives only a minimal amount of information. Users are invited to visit the HiPERCAM web site, where all the different HiPERCAM observing resources are provided.
Index
- LATEST NEWS
- Observing Modes
- Technical Details
- Data acquisition system
- FITS data structure
- Data reduction
- On-sky performance
- Guaranteed Time - Reserved Targets
- HiPERCAM contacts at GTC
- Useful Documents
- Additional Information
Observing Modes
Imaging
HiPERCAM is able to image simultaneously in 5 channels (ugriz), rather than the 3 channels of its predecessor, ULTRACAM. HiPERCAM is able to frame at (windowed) rates of over 1 kHz, rather than the maximum of ∼400 Hz available with ULTRACAM. HiPERCAM has a field of view of 2.8’ x 1.4’ on the GTC (3.1’ across the diagonal), greater than that of ULTRACAM on the VLT. This ensures that more comparison stars are available for differential photometry, allowing brighter targets to be observed, such as the host stars of transiting exoplanets. In short, HiPERCAM provides an order-of-magnitude improvement over ULTRACAM, thereby revolutionising the field of high-speed optical astrophysics. Operationally, HiPERCAM has adopted the successful ULTRACAM model. Hence, HiPERCAM is a Visitor Instrument, seeing first light on the 4.2m WHT on La Palma during October 2018, and first light on the 10.4m GTC on La Palma during February 2018. Since 2023 the instrument has been placed in a new rotator at the G Folded Cassegrain focii of the GTC, thereby giving HiPERCAM a permanent home at the GTC. Apart from that, there are two extra enhancements for HiPERCAM namely: the COMParison star Pick-Off system (COMPO), and a diffuser lens to be placed in front of the collimator on demand (A-band ranked programs only).
| Feature | Value |
| Number of simultaneous colours | 5 (u' g' r' i' z') |
| Readout noise | 4.5e- at 263kHz |
| CCD temperature | 183 K |
| Dark current | 100e-/pix/hr |
| Longest exposure time | 1800s |
| Highest frame rate | >1050 Hz |
| FoV on GTC | 2.8' x 1.4' |
| Probability of r'=14 | 92% |
| Comparison star pick-off | Yes |
| Dummy CCD outputs | Yes |
| Deep depletion | Yes |
| QE at 700/800/900/1000nm | 88%/78%/53%/13% |
| Fringer suppression CCDs | Yes |
| Fringe amplitude in z | <1% |
In general terms, HiPERCAM provides images with a similar depth as OSIRIS+ (see more details here) but observing in the five filters simultaneously, being far more efficient in multicolour analyses. As an example, magnitudes up to 28 in g', 27.5 in r', 27 in i' and 26 in z' can be attained with HiPERCAM in a 4h integration under subarsec seeing conditions.
COMParison star Pick-Off system (COMPO)
To correct for transparency variations in the Earth’s atmosphere, astronomers use the technique of differential photometry, where the target flux is divided by the flux of one or more comparison stars observed at the same time, and in the same patch of sky, as the target. In order not to degrade the signal-to-noise ratio of the resulting light curve significantly, it is necessary to use comparison stars that are brighter than the target star. If the target star is particularly bright, it becomes difficult to find suitable comparison stars, especially if the field of view of the photometer is small.
One way to increase the brightness of comparison stars available for differential photometry is simply to increase the field of view of the instrument. COMPO, stands for COMParison star Pick-Off system, provides that extra FOV. Light from a bright comparison star that falls outside the 3.1' diagonal field of view of HiPERCAM, but within the 10' diameter field of view of the Folded Cassegrain focus of the GTC, is collected by a pick-off arm. The light is then redirected to a second arm, via a set of relay optics, which injects the starlight onto one of the corners of the HiPERCAM CCDs. In this way it is possible to use much brighter comparison stars than would usually be available for differential photometry. This will be of particular importance when observing bright targets, like exoplanet host stars, for which nearby comparison stars of comparable or greater brightness are rare. More quantitatively, without COMPO, there is a 90% probability of finding a comparison star of magnitude r = 14 in the field of view, whereas with COMPO one will be able to find comparison stars of magnitude r = 12 with the same probability. COMPO will also be of great benefit to most us-band observations, as many of the targets observed by HiPERCAM tend to be blue (e.g. white dwarfs) and hence bright in the us-band, whereas most comparison stars tend to be red and hence faint in the us-band. The use of COMPO, via hfinder, is accepted for all programs.
Diffuser
When observing the brightest sources with HiPERCAM, such as the host stars of transiting exoplanets, the signal-to-noise ratio in a differential light curve can be limited by variations in seeing or atmospheric scintillation, rather than the brightness of the target or comparison stars. In the case of seeing, the varying PSF alters the fraction of light falling outside the photometry software aperture in a way that differs between the target and comparison stars, due to the fact that the latter almost always lie outside the isoplanatic patch (only ∼2 arcsec in the optical on La Palma of the former. Simply increasing the size of the software aperture is not a solution due to the corresponding increase in sky and read noise, and profile fitting is unable to model the subtle changes in PSF due to rapid seeing variations. It is possible to create a more stable PSF by defocusing the telescope, but the most stable PSFs are only achievable using beam-shaping diffusers.
We have tested such a diffuser in HiPERCAM on the GTC. The diffuser was placed in front of the collimator and, as expected, gave a much more stable PSF compared to using telescope defocusing and it covers a large number of pixels in order to reduce the systematic photometric errors induced by seeing and pixel sensitivity variations. In November 2019, we successfully tested a prototype diffuser with HiPERCAM on the GTC, as shown in the photographs below. The use of the diffuser is now available for accepted A-band ranked regular programs (not including GTC Large Programmes).
By combining this new diffuser with COMPO, HiPERCAM on the GTC will become the perfect tool for ground-based, broad-band transmission-photometry studies of the atmospheres of transiting exoplanets. For more information on diffusers do please check the following paper.
Narrow band filters
HiPERCAM can use five different filters at a time, each mounted in a cartridge which must be manually loaded into the instrument. There is a set of narrowbands filters availables at GTC. Nearly all of the filters have been procured from Asahi Spectra Co., Japan. They are of dimension 50mmx50mmx~5mm and nearly all of them have an identical optical thicknesses, i.e. there is no change in focus when introducing a different filter in the f/2.3-2.5 beams of the HiPERCAM camera. The currenly available narrowband filters for HiPERCAM are shown in the table below. The use of these filters is only accepted for A-band ranked regular programs (not including GTC Large Programmes).
| # | name | λcen (Å) |
FWHM (Å) |
throughput |
| 11 | NaI No.1 | 5912 | 312 | plot, data |
| 48 | Halpha very narrow | 6577.6 | 28 | plot, data |
| 49 | Halpha very broad | 6579.6 | 152 | plot, data |
| 50 | Halpha extremely broad | 6583.0 | 350 | plot, data |
It is possible to use any of the narrow-band filters listed on this page: narrowband filters. All the filters listed are all par-focal and of the same size as the other HiPERCAM filters. However, in order to use one of these other filters, you should contact us since they are not currently at GTC. The use of these filters is only accepted for A-band ranked regular programs (not including GTC Large Programmes).
Technical Details
HiPERCAM is a high-speed, quintuple-beam CCD camera for the study of rapid variability in the Universe. Light from the telescope is first collimated and then split into five beams using four dichroic beamsplitters, as shown below. The beam in each arm then passes through a re-imaging camera, which focuses the light through a bandpass filter and detector-head window onto a CCD. The HiPERCAM collimator has been designed for use on both the 4.2m WHT on La Palma and the 3.5m NTT on La Silla, giving a platescale of 0.3''/pixel and 0.35''/pixel, and a field of view of 11.4' and 13.4' across the diagonal of the detector, respectively. The same collimator also gives excellent optical performance on the GTC, giving a platescale of 0.081''/pixel and a field of view of 3.1' (diagonal). A new collimator designed specifically for HiPERCAM on the GTC, giving a platescale of 0.113''/pixel and a field of view of 4.3' (diagonal), has been designed but not yet procured.
HiPERCAM uses frame-transfer CCDs cooled to 183 K, with deep-depletion CCDs in the red channels, each equipped with anti-etaloning, resulting in low dark current, high QE and low fringing. Hence, as well as high-speed work, HiPERCAM is ideal for scientific applications requiring deep (i.e. long exposure), single-shot spectral-energy distributions (SEDs), such as supernova light-curves, gamma-ray bursts and other transients.
The figures below show the hardware architecture of the HiPERCAM data acquisition system. It can be seen that the instrument is composed of 4 components which are located in 4 different places at the telescope: the instrument on the Folded Cassegrain port; the electronics rack on the elevation ring (and close to the instrument); the data reduction PC (DRPC) and its peripherals in the control room; and the GPS antenna mounted externally to the dome.
The ESO New General detector Controller (NGC) is mounted on the instrument in order to minimise the lengths of the video and clock/bias cables to the 5 HiPERCAM CCDs. The power for the NGC comes from a Power Supply Unit (PSU) mounted in the HiPERCAM electronics rack, which is located adjacent to the instrument on the GTC elevation ring. Control of the NGC and receipt of data from the CCDs is via a fibre running between the ESO rack PC in the electronics rack (marked LLCU) and the NGC. The trigger cable running between the LLCU and the NGC is used for GPS timestamping. The external GPS antenna is connected via fibre to the LLCU. Another fibre connects the LLCU with the data reduction PC in the GTC control room.
Data acquisition system
The data acquisition system (DAS) in HiPERCAM is detector limited, i.e. the throughput of data from the output of the CCDs to the hard disk on which it is archived is always greater than the rate at which the data comes off the CCDs. This means that the instrument is capable of running continuously all night at its maximum data rate without ever having to pause for archiving of data. All CCDs are read out simultaneously and have identical exposure start and end times. In order to change the exposure times of the CCDs with respect to each other, it is possible to skip the readout of selected CCDs using the NSKIP parameter. For example, if NSKIP is set to 3,2,1,2,3 for the ugriz CCDs, and the exposure time is set to 10 s, then the CCD controller will read out only the r-band CCD on the first readout cycle (giving r a 10 s exposure), then the g, r and i-band CCDs on the second cycle (giving g and i a 20 s exposure), then the u, r and z-band CCDs on the third cycle (giving u and z a 30 s exposure), etc.
The HiPERCAM graphical user interface (GUI) is shown below. The top right of the GUI shows three buttons that allows the astronomer to change the readout mode: full frame, windowed or drift mode. Below this are the various CCD readout parameters, giving the astronomer complete control over the detector setup. The number of exposures is usually set to zero in the GUI and the green Start button is then pressed: the data acquisition system will then take data continuously until the red Stop button is pressed. All frames of a HiPERCAM run on a target are written to a single, custom-format FITS file.
FITS data structure
HiPERCAM writes its data to FITS cubes, with one cube representing one run on a target. Each plane in the cube contains data from all 5 CCDs at a particular time; since all 5 CCDs in HiPERCAM are exposed and read out simultaneously, only a single timestamp is written for all 5 frames. The FITS cubes also contain a full set of standard headers, such as the CCD and telescope parameters.
HiPERCAM can generate up to 20 MB of data per second. In the course of a typical night, therefore, it is possible to accumulate up to 720 GB of data, and up to 5 TB of data in the course of a typical week-long observing run.
Data reduction
In order to handle the high data rates, HiPERCAM has a dedicated pipeline data reduction system, based on the successful ULTRACAM pipeline. The data reduction pipeline grabs raw data as soon as it has been read from the CCDs and written to disk, and then processes it in real time, keeping up with all but the highest frame-rates. The HiPERCAM data reduction pipeline has been designed to serve two apparently conflicting purposes. Whilst observing, it acts as a quick-look data reduction facility, with the ability to display images and generate light curves in real time. After observing, the pipeline acts as a fully-featured photometry reduction package, including optimal extraction.
To enable quick-look reduction whilst observing, the pipeline keeps many of its parameters hidden to the user and allows the few remaining parameters to be quickly skipped over to generate images and light curves in as short a time as possible. Conversely, when carefully reducing the data after a run, every single parameter can be tweaked in order to maximise the signal-to-noise ratio of the final data for detailed analysis and publication.
The HiPERCAM data reduction software also provides an “Application Programmers Interface” (API) to allow users to access and manipulate HiPERCAM data. This will be of interest to anyone who wants to code their own scripts. You can download and obtain all of the information related to the HiPERCAM data reduction software here.
The HiPERCAM pipeline stores individual multi-CCD exposures in files with extension .hcm. These are in fact FITS files that can be examined with e.g. fv, ds9, fitshead, but the extension .hcm is used to distinguish them clearly from the raw .fits files and to reduce the chances of overwriting the latter. Further information can be found here.
On-sky performance
HiPERCAM was commissioned on sky for the first time on the WHT during October 2018. This was a short run of only 1 commissioning night and 4 science nights, and was intended primarily to be a shake-down of the instrument prior to commissioning on the GTC. The instrument was then moved to the GTC, where it was allocated 3 commissioning nights and 10 science nights in February 2018. Due to terrible weather, the instrument was used on sky for only the last 3 nights, but this did allow all of the commissioning to be completed. Three more observing runs followed in April, May and June 2018, totalling 16 nights. A wide range of science was performed during this time, including the observation of black holes, white dwarfs, neutron stars, brown dwarfs, extrasolar planets/asteroids, AGN, FRBs, GRBs, SNe and ultra-diffuse galaxies.
An example star field observed with HiPERCAM on the GTC is shown below. The FWHM of the stars in these images are u = 0.56'', g = 0.44'', r = 0.41'', i = 0.37'', u = 0.36'', with no discernible variation with field angle. This indicates that HiPERCAM on the GTC can provide seeing-limited images across the whole field of view in even the very best seeing conditions on La Palma. Since the dichroics operate in a collimated beam and have anti-reflection coatings on their rear surfaces, no ghosting is expected in the images: a careful inspection of the brightest stars in the images below reveals that there is indeed no discernible ghosting. The pixel positions of the stars at the corners of the field of view are the same on all 5 CCDs, to within approximately 5 pixels (75μm), indicating that there is no discernible variation of platescale with wavelength and that the CCD heads are well aligned with respect to each other. Twilight-sky flat fields show no discernible vignetting in the corners of the field of view.
The measured photometric zero points of HiPERCAM on the GTC, defined here as the magnitude of a star that would give 1 electron per second above the atmosphere, are: u = 28.17 (25.76), g = 29.25 (28.71), r = 28.76 (29.05), i = 28.43 (28.60), z = 27.95 (28.04). The numbers in brackets show the corresponding values for OSIRIS, the common-user, single-channel optical imager at the GTC. These two sets of zero points were measured within a few days of each other during May 2018, using observations of SDSS standard stars.
Guaranteed Time - Reserved Targets
| Target Name | RA(2000) | Dec(2000) |
| TBD | TBD | TBD |
HiPERCAM contact persons at GTC
| contact | email @ gtc.iac.es |
| David Garcia - main contact | david.garcia |
| Gianluca Lombardi | gianluca.lombardi |
Useful Documents
More information
Last modified: 05 April 2025
