This series of posts will try to cover the science, instruments, astrophysics, and more that led to the imaging of Sagittarius A* black hole at the center of our Milky Way galaxy.
Taken from Kazunori et al. (2022)
Table of Contents
Important Note
These posts will cover the mentioned topics to the best of my ability and thus will be updated by removing or adding the information. The reader is encouraged to see the journal publications mentioned in the references (and references therein).
Instruments
The Event Horizon Telescope (EHT) project comprises eight telescopes (for 2017 observations) spread across the globe. Some geographical details of the instruments used in the 2017 observations are as follows,
| Facility | Diameter [m] | Location | Latitude | Longitude | Elevation [m] |
|---|---|---|---|---|---|
| ALMA | 12 (x54) 7 (x12) | Chile | $-23^\circ01^\prime 45.1^{\prime\prime}$ | $-67^\circ45^\prime17.1^{\prime\prime}$ | $5074.1$ |
| APEX | 12 | Chile | $-23^\circ00^\prime20.8^{\prime\prime}$ | $-67^\circ45^\prime32.9^{\prime\prime}$ | $5104.5$ |
| JCMT | 15 | Hawaii, USA | $+19^\circ49^\prime22. 2^{\prime\prime}$ | $-155^\circ28^\prime37. 3^{\prime\prime}$ | $4120.1$ |
| LMT | 50 | Mexico | $18^\circ59^\prime08. 8^{\prime\prime}$ | $-97^\circ18^\prime53. 2^{\prime\prime}$ | $45933$ |
| PV $30\,m$ | 30 | Spain | $37^\circ03^\prime58. 1^{\prime\prime}$ | $-3^\circ23^\prime33. 4^{\prime\prime}$ | $2919.5$ |
| SMA | 6 (x8) | Hawaii, USA | $+19^\circ49^\prime27. 2^{\prime\prime}$ | $-155^\circ28^\prime39.1^{\prime\prime}$ | $4115.1$ |
| SMT | 10 | Arizona, USA | $32^\circ42^\prime05. 8^{\prime\prime}$ | $-109^\circ53^\prime28. 5^{\prime\prime}$ | $3158.7$ |
| SPT | 10 | Antarctica | $89^\circ59^\prime22. 9^{\prime\prime}$ | $-45^\circ15^\prime00. 3^{\prime\prime}$ | $2816.5$ |
A very brief rundown of these instruments is listed below,
ALMA
The Atacama Large Millimeter and submillimeter Array (ALMA) is a collection of 66 radio telescopes in Chile. The ALMA setup has,
- observational wavelength from $350\,\mu\text{m}$ to $10\,\text{mm}$,
- spatial resolution of $10$ milliarcseconds, and
- velocity resolution of under $50\,\text{m/s}$.
The ALMA facility was upgraded with a hydrogen maser as a local oscillator under the ALMA Phasing Project (APP). Phasing cards were introduced and a high end fiber link was introduced to carry the transmission from the site to the data facility. A phase solver and ALAM VLBI Observing Mode (VOM) were introduced as software upgrades to this facility as well.
APEX
The Atacama Pathfinder Experiment (APEX) is a single unit radio telescope working at the submillimeter range of $0.2 – 1.5\,\text{mm}$ wavelength. This wavelength range lies between infrared and radio wavelengths.
Under the EHT upgrade, the APEX reflector sensitvity was incresaed from $17\,\mu\text{m}$ to $10\,\mu\text{m}$ at frequencies greater than $1\,\text{THz}$. Like ALMA, the APEX also has a hydrogen maser for frequency reference.
JCMT
The James Clerk Maxwell Telescope (JCMT) is a submillimeter telescope at Mauna Kea Observatory in Hawaii, USA. Its two instruments, the Submillimeter Common-User Bolometer Array (SCUBA) and SCUBA-2 are capable of making observations in $450\ -\ 850\,\mu\text{m}$ wavelength.
LMT
The Large Millimeter Telescope (LMT) is the largest single-aperture telescope in the $0.85 – 4\,\text{mm}$ observational range. Components of LMT consist of,
PV $30\,\text{m}$
The $30\,\text{m}$ telescope operates at the wavelength range of $0.8\,-\,3\text{\mm}$ with a precision of $55\,\mu\text{m}.$ Technical summary of the PV $30\,\text{m}$ telescope can be found here.
Under the EHT project, only the 1.3 and 0.87 mm bands were used. A hydrogen maser was introduced as well.
SMA, SMT, and SPT
The submillimeter array (SMA) consists of eight 6 meter radio telescopes operating in $1.545\,-\,0.735\,\text{mm}$ wavelength range.
The submillimeter telescope (SMT; also known as Heinrich Hertz Submillimeter Telescope) is a 10-m parabola dish operating in the radio and far-infrared region of the electromagnetic spectrum.
The South Pole Telescope (SPT) is also a 10-m diameter telescope used to make observations in microwave, millimeter, and submillimeter wavelength ranges.
great job