IQUEYE

The photon counter

IQUEYE is designed to be the fastest ever photon-counting photometer, consisting of 100 parallel channels, each sampling a 1/100th portion of the telescope pupil and able to time-tag each detected photon with an accuracy of the order of 10 ps. It collects light within a field of view of a few arcseconds (from 1” to 6”), divides the telescope's light beam into four equal parts, and focuses each sub-beam onto an independent single-photon-counting diode. The most innovative feature of IQUEYE is the data acquisition system, coupled with ultrafast detectors, a rubidium oscillator, and a GPS receiver, which allows us to time-tag the detected photons with a final absolute UTC-referenced rms time accuracy superior to 0.5 ns over one hour of observation.

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Photon-counting detectors

IQUEYE uses Geiger-mode SPAD detectors, produced by the Italian company Micro Photon Devices. They can tolerate full daylight also when powered, their quantum efficiency is ~60% at 550 nm, they are thermoelectrically regulated at a nominal temperature of -10°, to ensure a dark current lower than a few tens of counts per second. The integrated timing circuit provides an extremely accurate signal with a time accuracy of the order of 35 ps. They have a diameter-sensitive area of 100 µm, which defines a field of view of 6.1 arcsec.

Optical subsystem

IQUEYE is a fixed–aperture, non-imaging photometer optimized for ultra–fast timing of point sources. The instrument collects light from a selectable field of view (typically a few arcseconds) defined by interchangeable pinholes and splits the incoming beam into four equal sub-pupils using a reflective pyramid. Each sub-beam is independently re–imaged onto a dedicated SPAD detector, allowing parallel photon counting and partially mitigating detector dead-time losses. Two motorized filter wheels placed along the optical path host broad– and narrow–band filters as well as polarizers, enabling flexible photometric and polarimetric observations. A dedicated field camera provides real-time visualization of the target region for acquisition and guiding.

Timing and data acquisition system

Each detected photon is individually time-tagged using a high-resolution time-to-digital converter referenced to a rubidium oscillator and a GPS receiver. This architecture provides a relative timing precision of about 100 ps and an absolute accuracy better than 0.5 ns over one hour of observation. Photon arrival times are recorded without binning, enabling full post-processing flexibility.

Filters/Polarizers

Instrument performance

• Relative time accuracy: 100 ps (for 1h of continuous observation)
• Absolute time accuracy: 500 ps (for 1h of continuous observation)
• Dark current rate: less than 100 Hz
• Maximum count rate: 8 MHz
• Dynamic range: >40 dB
• Limiting magnitude: mV = 24 with 2 h exposure time and S/N = 10
• Effective field of view: (selectable) 3.5, 5.2, or 6.1 arcsec
• Operative spectral range: Δλ = [350, 925] nm (see Table 1 for filter details)
• System total efficiency: 33% (peak ~ 550 nm)
  18% (average over Δλ = [350, 925] nm spectral range)

Scientific applications

IQUEYE is designed to investigate astrophysical phenomena on microsecond to second timescales, including optical pulsars, fast variability in compact binaries, rapid transients, and high-time-resolution polarimetry. The instrument also acts as a technological pathfinder for future quantum photometers on extremely large telescopes. The observing programmes presently active include:

• High time resolution observations of optical pulsars
• Timing of optical transients and counterparts of binary systems with compact objects
• Searches for prompt/delayed optical flashes from Fast Radio Bursts and Magnetars
• Monitoring the intranight variability of Blazars
• Lunar and asteroidal occultations
• Stellar intensity interferometry experiments

The observations are often carried out within the framework of multiwavelength campaigns, in synergy with facilities operating from the radio up to the gamma-ray bands.

Data Format

The scientific data are acquired (in binary format) together with two reference signals, the first one is 40 MHz coming from an atomic clock with long term frequency stability (small Allan variance) and the second one is pulse-per-second (PPS) provided by a GPS needed to remove any residual drift of the clock reference frequency and for synchronization to the UTC. This is actually the only “calibration” needed for such a system. Thanks to these signals the de-rollover of the data is performed and the final output is a time tagged list of event (photon detections) referenced to the UTC (up to 500 ps absolute accuracy). The files for the final users are in fits format which means that they comprehend a header, and the event list with the arrival times of the photons in fraction of days. In the header all the information about the reference epoch (MJD), the name of the telescope and its location (geocentric coordinates), and other useful information on the processing of the raw data are provided.

IQUEYE Observing Overheads and Pointing Procedure

• There are no additional slew-time restrictions when using IQUEYE.
• The primary instrument configuration concerns filter selection. Filters must be chosen by the user, and the observers will apply the selected configuration to the instrument. The filters are mounted in a filter wheel mechanically attached to IQUEYE’s optical head, and a dedicated procedure is available for changing them when required. Note: the use of filters may be mandatory for certain pointing sources and flux calibrators, as the detected photon count could otherwise be too high and lead to detector saturation.
• It is recommended to perform the initial pointing calibration using a nearby bright source, directly monitoring the increase in photon counts recorded by IQUEYE. Users should therefore provide one or two calibration sources located close to the science target. Once the maximum signal is reached on the calibration source, a pointing offset can be applied to move the telescope to the science target. This procedure typically requires up to 30 minutes.
• This overhead also includes the use of additional filters, since filters are mandatory when observing bright sources.
• At the end of the procedure, IQUEYE will be accurately positioned on the target source and can safely begin a long-duration recording.

Data Archiving and Delivery

• The data volume depends strongly on the target source and its photon flux, since only photon events are recorded. For a typical one-hour observation, the data size is generally up to approximately 1 GB (or less).
• Raw data are initially stored in a proprietary binary format and subsequently converted by the IQUEYE team into FITS files, with timestamps referenced to either local time or UTC.
• For pulsar studies (or any other timing analysis), users are responsible for applying the barycentric correction using the Gemini Observatory coordinates and the target source position. The IQUEYE science team will not provide these corrections.
• After the completion of an observing night, the data are manually transferred from Gemini Observatory to the IQUEYE team, and processing begins.
• The expected data-processing turnaround time is no more than 8 weeks.

Limiting magnitudes

Below are reported the minimum and maximum magnitudes achievable without filters for the instrument mounted at Gemini-South.

Acknowledgments

We gratefully acknowledge the support of Gemini South staff, mechanical team, electronics, software and operations for their invaluable support, expertise, and help before and during instrument commissioning.

The IQUEYE@Gemini project has been funded by the Department of Physics and Astronomy of the University of Padova, with a grant of the PARD 2024 Call. GN, PO, and LZ wish to thank all the personnel of the Asiago observatory who contributed to the IQUEYE@Gemini project. T.C. gratefully acknowledges support by the ANID BASAL FB210003.

L.Z., M.F., A.S., and S.C. also acknowledge support from the INAF Grants “Uncovering the optical beat of the fastest magnetized neutron stars (FANS)" and “Coordinated Multi-wavelength Exploration of Fast Radio bursts (COMEFAR)".

The IQUEYE@Gemini project is part of the AQUEYE+IQUEYE project (https://web.oapd.inaf.it/zampieri/aqueye-iqueye/).

Relevant publications

Naletto G., et al. "Iqueye, a single photon-counting photometer applied to the ESO new technology telescope." Astronomy & Astrophysics 508.1 (2009): 531-539.

Cassanelli T., Marcone Puga P., Naletto G., Zampieri L., Ochner P., Fiori M., Spolon A., et al., “IQUEYE at Gemini South: instrument, science commission, and first results”, 2025, SPIE, 13624, 136241R. doi:10.1117/12.3066537

Zampieri L., Naletto G., Barbieri C., Verroi E., Barbieri M., Ceribella G., D'Alessandro M., et al., “Aqueye+: a new ultrafast single photon counter for optical high time resolution astrophysics”, 2015, SPIE, 9504, 95040C. doi:10.1117/12.2179547