arXiv subjects

Erik Høg

Publications and source records attributed to Erik Høg.

At least 19 recordsLinked to original sources

arXiv2026

The Last Meridian Circles -- an Epilogue

The aim is to document in some detail the last 35 years of meridian circles, a type of instrument with a fundamental role in astronomy for a very long time, and to do so while witnesses are still alive and can contribute. This is about finding facts. Meridian circles provided fundamental star positions for centuries. These positions were tied to a well-defined celestial coordinate system of right ascension and declination, and accurate proper motions ensured a transformation of the positions over long periods of time. This function of the meridian circles has been taken over by space astrometry and VLBI. The Hipparcos astrometric satellite was approved by ESA in 1980 and launched to a successful 3-year mission in 1989. Its successor, Gaia, completed a mission of 10.5 years in January 2025. An account is given of the last 18 meridian instruments, which were active for some part of the 35 years between 1980 and 2015. This account is based on input kindly supplied in correspondence with many colleagues.

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arXiv2024

GaiaNIR: Note on processing and photometry

Some ideas for onboard processing and photometry with an astrometry satellite are presented, especially designed for GaiaNIR which may be launched about 2045 as a successor of Gaia. - Increased sensitivity, reduced image overlap, and simpler PSF calibration in GaiaNIR will result if the proposed initial processing of data from the detectors is implemented, because the across-scan smearing will become insignificant. - Filter photometry is required for high angular resolution as needed for astrometric and astrophysical reasons. Low-dispersion spectra are questioned because they fail at high star density. This will be a much greater problem with GaiaNIR than it is with Gaia because of the larger number of stars expected. It was the aim to collect in this note all arguments about GaiaNIR photometry which can be stated with words only, in correspondence with readers. The remaining work to be done for the definition of photometric equipment on the satellite requires further quantitative assessments and comparison of various options. Finally, 1) an advantage of filters is that the photometric observations can also be used for astrometry, 2) the XP spectra in Gaia will give very good astrophysical data for about 400 million single stars with G <~ 18.5 mag, but filters would have been better for all fainter and for all multiple stars, and 3) it is presently not clear which advantages for astrophysics low-dispersion spectra in the NIR might have over filters.

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arXiv2024

Astrometric Accuracy of Positions

The great development of astrometric accuracy since the observations by the Greek astronomer Hipparchus about 150 BC has often been displayed in diagrams showing the accuracy versus time. Two new diagrams are provided here, one for positions only, another including parallaxes. New information is included on the Gaia DR3 data and the coming data releases, and about two great astronomers from the distant past, Wilhelm in Kassel and al-Sufi in Isfahan.

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arXiv2024

A review of 70 years with astrometry

In 1953 I heard of an experiment in 1925 by Bengt Strömgren where he observed transit times with the meridian circle at the Copenhagen University Observatory measuring the current in a photocell behind slits when a star was crossing. In 1954 just 22 years old I was given the task as a student to make first test observations with a new meridian circle of the observatory. I became fascinated by the instrument and by the importance of astrometry for astronomy. Work at four meridian circles, two in Denmark, one in Hamburg, one in Lund, and Pierre Lacroute's vision of space astrometry in France had by 1973 created the foundation for development of the Hipparcos satellite, and Gaia followed. In 2013 I proposed a successor satellite which has gained momentum especially thanks to the efforts of David Hobbs and it has a good chance to be launched by ESA about 2045. But 70 years ago, optical astrometry was considered a dying branch of astronomy, unattractive compared with astrophysics. The following growth built on the still active interest in astrometry in Europe in those years and it was supported by ESA, the European Space Agency. This review is only about astrometry where I was personally involved.

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arXiv2023

Archives on astronomy from the 1950s

Information on archives from the 1950s of 15 astronomical observatories is provided beginning with a list of correspondence and other information related to astronomy of the Copenhagen University Observatory in the 1950s. The Appendix contains information from the 14 other observatories about their archives from those years, most of them having no archive at all. Public links are given to most of the files. - Print of the present list and the Danish astronomy archive itself will be placed at the Rigsarkivet, the Danish National Archives.

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arXiv2022

Interviews about modern astrometry

Michael Perryman has interviewed some of the scientists and project leaders in the Hipparcos and Gaia missions, the interviews with photos of the persons are given at his site: https://www.michaelperryman.co.uk . Michael has also written essays -- 84 to date ! -- about results from the Gaia mission and they are placed at his site. Three of the interviews are with me and transcriptions, co-authored with Michael, are provided below with the titles: #1. An interview about astronomy and astrometry up to 1980. #2. An interview about the revival of astrometry after 1980. #3. The billion-star astrometry after 1990. The third interview begins in 1990 when I had the first ideas for a Hipparcos successor. In 1992 I made a detailed design with direct imaging on CCD detectors in a satellite proposal called Roemer. In 1993 a supposedly better option was proposed with the acronym GAIA where the capital "I" stood for Interferometer. In 1998, however, interferometry was shown to be unsuited for the purpose and we returned to the original idea from 1992 for the further development. The name was later changed to Gaia -- for the sake of continuity.

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arXiv2021

Faint objects in motion: the new frontier of high precision astrometry

Sky survey telescopes and powerful targeted telescopes play complementary roles in astronomy. In order to investigate the nature and characteristics of the motions of very faint objects, a flexibly-pointed instrument capable of high astrometric accuracy is an ideal complement to current astrometric surveys and a unique tool for precision astrophysics. Such a space-based mission will push the frontier of precision astrometry from evidence of Earth-mass habitable worlds around the nearest stars, to distant Milky Way objects, and out to the Local Group of galaxies. As we enter the era of the James Webb Space Telescope and the new ground-based, adaptive-optics-enabled giant telescopes, by obtaining these high precision measurements on key objects that Gaia could not reach, a mission that focuses on high precision astrometry science can consolidate our theoretical understanding of the local Universe, enable extrapolation of physical processes to remote redshifts, and derive a much more consistent picture of cosmological evolution and the likely fate of our cosmos. Already several missions have been proposed to address the science case of faint objects in motion using high precision astrometry missions: NEAT proposed for the ESA M3 opportunity, micro-NEAT for the S1 opportunity, and Theia for the M4 and M5 opportunities. Additional new mission configurations adapted with technological innovations could be envisioned to pursue accurate measurements of these extremely small motions. The goal of this White Paper is to address the fundamental science questions that are at stake when we focus on the motions of faint sky objects and to briefly review instrumentation and mission profiles.

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arXiv2021

The Gaia Successor in 2021

A new mission about twenty years after Gaia with similar astrometric performance would be important for all branches of astronomy. The two missions together would, e.g., give much more accurate motions of the common objects due to the large epoch difference. By adding a Near-InfraRed (NIR) capability to the new mission we will be able to peer into the obscured regions of the Galaxy and measure up to 10 or 12 billion new objects and reveal many new sciences in the process. ESA has now ranked the development of this mission so high that a launch about 2045 is quite probable. A brief history of the project is included.

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arXiv2020

GaiaNIR: Combining optical and Near-Infra-Red (NIR) capabilities with Time-Delay-Integration (TDI) sensors for a future Gaia-like mission

ESA recently called for new "Science Ideas" to be investigated in terms of feasibility and technological developments -- for technologies not yet sufficiently mature. These ideas may in the future become candidates for M or L class missions within the ESA Science Program. With the launch of Gaia in December 2013, Europe entered a new era of space astrometry following in the footsteps of the very successful Hipparcos mission from the early 1990s. Gaia is the successor to Hipparcos, both of which operated in optical wavelengths, and Gaia is two orders of magnitude more accurate in the five astrometric parameters and is surveying four orders of magnitude more stars in a vast volume of the Milky Way. The combination of the Hipparcos/Tycho-2 catalogues with the first early Gaia data release will give improved proper motions over a long ~25 year baseline. The final Gaia solution will also establish a new optical reference frame by means of quasars, by linking the optical counterparts of radio (VLBI) sources defining the orientation of the reference frame, and by using the zero proper motion of quasars to determine a non-rotating frame. A weakness of Gaia is that it only operates at optical wavelengths. However, much of the Galactic centre and the spiral arm regions, important for certain studies, are obscured by interstellar extinction and this makes it difficult for Gaia to deeply probe. Traditionally, this problem is overcome by switching to the infra-red but this was not possible with Gaia's CCDs. Additionally, to scan the entire sky and make global absolute parallax measurements the spacecraft must have a constant rotation and this requires that the CCDs operate in TDI mode, increasing their complexity.

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arXiv2020

Astrometric accuracy during the past 2000 years

The great development of astrometric accuracy since the observations by Hipparchus about 150 BC was documented in 2008 in the first version of the present report. This report was updated in Høg (2017d), e.g. with recent information on the catalogues before 1800 AD. The development has often been displayed in diagrams showing the accuracy versus time. A new simplified diagram is provided in the present report as Fig.2 and in a .png file and this information will presumably be the main interest for some readers.

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arXiv2019

Voyage 2050 White Paper: All-Sky Visible and Near Infrared Space Astrometry

A new all-sky visible and Near-InfraRed (NIR) space astrometry mission with a wavelength cutoff in the K-band is not just focused on a single or small number of key science cases. Instead, it is extremely broad, answering key science questions in nearly every branch of astronomy while also providing a dense and accurate visible-NIR reference frame needed for future astronomy facilities. For almost 2 billion common stars the combination of Gaia and a new all-sky NIR astrometry mission would provide much improved proper motions, answering key science questions -- from the solar system and stellar systems, including exoplanet systems, to compact galaxies, quasars, neutron stars, binaries and dark matter substructures. The addition of NIR will result in up to 8 billion newly measured stars in some of the most obscured parts of our Galaxy, and crucially reveal the very heart of the Galactic bulge region. In this white paper we argue that rather than improving on the accuracy, a greater overall science return can be achieved by going deeper than Gaia and by expanding the wavelength range to the NIR.

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arXiv2019

Astro2020 Activity, Project of State of the Profession Consideration (APC) White Paper: All-Sky Near Infrared Space Astrometry. State of the Profession Considerations: Development of Scanning NIR Detectors for Astronomy

Gaia is a revolutionary space mission developed by ESA and is delivering 5 parameter astrometry, photometry and radial velocities over the whole sky with astrometric accuracies down to a few tens of micro-arcseconds. A weakness of Gaia is that it only operates at optical wavelengths. However, much of the Galactic centre and the spiral arm regions, important for certain studies, are obscured by interstellar extinction and this makes it difficult for Gaia to deeply probe. This problem can be overcome by switching to the Near Infra-Red (NIR) but this is not possible with silicon CCDs. Additionally, to scan the entire sky and make global absolute parallax measurements the spacecraft must have a constant rotation and this requires the detectors operate in Time Delayed Integration (TDI) mode or similar.

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arXiv2019

All-Sky Near Infrared Space Astrometry

Gaia is currently revolutionizing modern astronomy. However, much of the Galactic plane, center and the spiral arm regions are obscured by interstellar extinction, rendering them inaccessible because Gaia is an optical instrument. An all-sky near infrared (NIR) space observatory operating in the optical NIR, separated in time from the original Gaia would provide microarcsecond NIR astrometry and millimag photometry to penetrate obscured regions unraveling the internal dynamics of the Galaxy.

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arXiv2018

Astrometry history: Hipparcos from 1964 to 1980

Here follow three reports covering different aspects of the early history from 1964 to 1980 of the Hipparcos satellite mission. The first report "Interviews about the creation of Hipparcos" contains interviews from 2017 with scientists about how the mission was conceived up to the begin of technical development. The second report "From TYCHO to Hipparcos 1975 to 1979" is about the Hipparcos development based on new material from my archive. From my 65 years dedicated to the development of astrometry, I argue that very special historical circumstances in Europe were decisive for the idea of space astrometry to become reality: Hipparcos did not just come because astrophysicists needed the data. The third report "Miraculous 1980 for Hipparcos" documents how the approval of the astrometric mission in January 1980 in competition with an astrophysical mission was only achieved with very great difficulty, even after an outstanding astrophysicist had presented overwhelming arguments that the astrometric mission would be scientifically much more important.

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arXiv2017

Absolute astrometry in the next 50 years

With Gaia in orbit since December 2013 it is time to look at the future of fundamental astrometry and a time frame of 50 years is needed in this matter. A space mission with Gaia-like astrometric performance is required, but not necessarily a Gaia-like satellite. It should be studied whether this can be obtained within the budget of a medium-size ESA mission. A dozen science issues for a Gaia successor mission in twenty years, with launch about 2033, are presented and in this context also other possibilities for absolute astrometry with milliarcsecond (mas) or sub-mas accuracies are discussed. The three powerful techniques: VLBI, the MICADO camera on the E-ELT, and the LSST are described and documented by literature references and by an extensive correspondence with leading astronomers who readily responded with all the information I needed. In brief, the two Gaia-like missions would provide an astrometric foundation for all branches of astronomy from the solar system and stellar systems, including exo-planet systems, to compact galaxies, quasars and dark matter (DM) substructures by data which cannot be surpassed in the next 50 years. - In April 2017 ESA selected our proposal Hobbs et al. (2016) for study of a detector with NIR sensitivity for a Gaia successor mission, called GaiaNIR.

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arXiv2017

Solar system and small-field astrometry

Astrometric issues for solar system studies are discussed. An overview gives references and cover all aspects of the solar system where astrometry is important: orbits of planets, moons, asteroids and NEOs, masses of asteroids, occultations of asteroids and KBOs, and families of asteroids and KBOs. The roles of astrometry from the ground, from Gaia and from a Gaia successor are discussed, but not small-field astrometry from space. It appears from work with CCD cameras at the 1.55 m astrometric reflector in Flagstaff that an accuracy of 1 mas is the best possible from the ground during one night observing when using ordinary telescopes, i.e. without wave-front correctors, and for field sizes larger than 2 arcmin. It has been seen that the same accuracies can be reached with the much larger 4-m class telescope on Hawaii although it is not specifically designed for astrometry. The accuracy of 1 mas from the ground refers mainly to non-moving point sources, but it is expected that 1 mas can be reached from the ground for solar system bodies from many nights of observations when phase effects are taken into account.

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arXiv2017

Young astronomer in Denmark 1946 to 1958

This is a personal account of how I became an astronomer. Fascinated by the stars and planets in the dark sky over Lolland, an island 100 km south of Copenhagen, the interest in astronomy was growing. Encouraged by my teachers, I polished mirrors and built telescopes with generous help from the local blacksmith and I observed light curves of variable stars. Studies at the Copenhagen University from 1950 gradually led me deeper into astronomy, especially astrometry (the astronomy of positions), guided by professor Bengt Strömgren and my mentor dr. phil. Peter Naur. I was lucky to take part in the buildup of the new observatory at Brorfelde during the first difficult years and the ideas I gathered there have contributed to the two astrometry satellites Hipparcos and Gaia launched by the European Space Agency (ESA) in respectively 1989 and 2013.

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arXiv2017

The Baltic Meetings 1957 to 1967

The Baltic meetings of astronomers from Northern Germany and Scandinavia began in 1957 and gathered up to 70 participants. Reports of the presentations are available from all meetings, providing an overview of the interests of astronomers in this part of the world 50 years ago. Most interesting to see for a young astronomer in our days, I think, is that a large part of the time was about astrometry. This focus on astrometry was the basis for the scientific knowhow which made the idea of space astrometry realistic, resulting in the approval by ESA of the first astrometry satellite Hipparcos in 1980 which brought a revolution of high-precision astrometry of positions, motions and distances of stars. The correspondence with ten observatories shows that only one of them has any archive of letters at all from the 1950s, that is in Copenhagen where about 7000 letters on scientific and administrative matters are extant. - These letters have now been stored in the Rigsarkivet.

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