Event Horizon Telescope
Memo Series

EHT Memo 2023-L1-01

L1 Processing and Validation
Metadata for the Flux Density Calibration of the April 2018
Event Horizon Telescope Data
J. Y. Koay1 , C. Romero-Cañizales1 , L. D. Matthews2 , M. Janssen3 , L. Blackburn4,5 , R. P. J. Tilanus6 ,J. Park7 , K. Asada1 , S. Matsushita1 , A.-K. Baczko3 , N. La Bella8 , C.-K. Chan6,9,10 , G. B. Crew2 ,V. Fish2 , N. Patel4 , V. Ramakrishnan11,12 , H. Rottmann3 , J. Wagner3 , K. Wiik13 ,P. Friberg14 , C. Goddi8,15 , S. Issaoun4 , G. Keating4 , J. Kim6,16 , T.P. Krichbaum3 , D. Marrone6 ,G. Narayanan17 , A. Roy3 , I. Ruíz18 , S. Sánchez18 , P. Torne18,3 J. Weintroub4,5
Dec 05, 2023 – Version 1.1
1

Academia Sinica Institute of Astronomy and Astrophysics, No.1, Sec. 4, Roosevelt Rd, Taipei 10617, Taiwan, R.O.C.
Massachusetts Institute of Technology Haystack Observatory, 99 Millstone Road, Westford, MA 01886, USA
3
Max Planck Institut für Radioastronomie (MPIfR), Auf dem Hügel 69, 53121 Bonn, Germany
4
Center for Astrophysics | Harvard & Smithsonian, 60 Garden St., Cambridge, MA 02138, USA
5Black Hole Initiative at Harvard University, 20 Garden Street, Cambridge, MA 02138, USA
6
Steward Observatory and Department of Astronomy, University of Arizona, 933 N. Cherry Avenue, Tucson, AZ 85721, USA
7
Department of Astronomy and Space Science, Kyung Hee University, 1732, Deogyeong-daero, Giheung-gu, Yongin-si, Gyeonggi-do
17104, Republic of Korea
8
Department of Astrophysics / IMAPP, Radboud University, P.O. Box 9010, 6500 GL Nijmegen, The Netherlands
9
Data Science Institute, University of Arizona, 1230 N. Cherry Ave., Tucson, AZ 85721
10Program in Applied Mathematics, University of Arizona, 617 N. Santa Rita, Tucson, AZ 85721
11
Finnish Centre for Astronomy with ESO, FI-20014 University of Turku, Finland
12
Aalto University Metsähovi Radio Observatory, Metsähovintie 114, FI-02540 Kylmälä, Finland
13
Tuorla Observatory, Department of Physics and Astronomy, University of Turku, Finland
14
East Asian Observatory, 660 N. A’ohoku Place, Hilo, HI 96720, USA
15
INAF - Osservatorio Astronomico di Cagliari, Via della Scienza 5, 09047, Selargius, CA, Italy
16
California Institute of Technology, 1200 East California Boulevard, Pasadena, CA 91125, USA
17
Department of Astronomy, University of Massachusetts, 01003, Amherst, MA, USA
18
Institut de Radioastronomie Millimétrique, Avenida Divina Pastora 7, Local 20, E-18012, Granada, Spain
2

Abstract
The Event Horizon Telescope (EHT) observations carried out in 2018 April at 1.3 mm wavelengths included 9 stations in the array, comprising 7 single-dish telescopes and 2 phased arrays. The metadata package
for the 2018 EHT observing campaign contains calibration tables required for the a-priori amplitude calibration of the 2018 April visibility data. This memo is the official documentation accompanying the release of
the 2018 EHT metadata package, providing an overview of the contents of the package. We describe how telescope sensitivities, gain curves and other relevant parameters for each station in the EHT array were collected,
processed, and validated to produce the calibration tables.

Contents
1

Introduction
1.1 Scope of this Memo . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2
3

2

Basic Concepts for EHT Flux Calibration3

3

Collection and Contents of the Raw Metadata Packages

4

4

Station Sensitivities and Gain Curves
4.1 ALMA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2 APEX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.3 GLT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.4 LMT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.5 SMT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.6 JCMT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.7 IRAM 30 m . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.8 SMA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.9 SPT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

5
6
6
7
7
7
8
9
10
10

5

Processing and Validation of System Temperatures and Phased Array SEFDs
5.1 Processing Pipeline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2 Single Dish Stations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.3 Additional Processing for Phased Arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.4 Validation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.5 ANTAB Table Generation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

11
11
11
14
17
17

6

L1 Metadata Package: Contents and Data Properties
6.1 Package Contents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.2 Comparing 2017 and 2018 Station Sensitivities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.3 Remaining Issues in the EHTmetadata_2018April_v1.0 Package . . . . . . . . . . . . . . . . . . .

18
18
19
20

A Naming Conventions and Identifications for Stations and Tracks

22

B Formats and Contents of Raw Metadata Tables

23

1

Introduction

The Event Horizon Telescope (EHT) 2018 campaign was carried out over six tracks observed on 2018 April 21, 22, 24, 25,
27 and 28. There were a total of nine participating stations, of which two are phased arrays (ALMA and SMA) and sevenare single dish stations (APEX, IRAM 30 m, SPT, LMT, SMT, JCMT and GLT). The Greenland Telescope (GLT; Inoueet al., 2014; Chen et al., 2023) is a new addition to the 2018 EHT array. The EHT array also observed at double the bandwidth than in 2017. Four frequency bands were used, each with bandwidths of 1.875 GHz for ALMA and 2.0 GHz at theother stations. These bands were centered at frequencies of 213.1 (band 1), 215.1 (band 2), 227.1 (band 3), and 229.1 GHz(band 4). Due to the mismatched sampling rates between ALMA and the other stations, a portion of each bandwidth islost during correlation, resulting in correlated data of 1.856 GHz per band (comprising 32 spectral windows of 58 MHzeach). Note that the GLT only observed in bands 3 and 4 in 2018. With the exception of ALMA, which observes in linear
polarization, all the other EHT stations observed in circular polarizations. Visibility data on ALMA baselines are converted2

from mixed to circular polarization basis via the PolConvert software package (Martí-Vidal et al., 2016) after correlation. Additionally, the JCMT observed only a single polarization (right circular polarization, RCP) throughout the 2018
campaign. In 2017, the JCMT switched between RCP and left circular polarization (LCP) observations on different tracks.
The processing of the EHT data from 2018 onwards is now separated into Level 1 (L1) and Level 2 (L2) tasks, each producing corresponding data packages. The L2 data package is still being defined at the time of writing. The L1 data package
mainly comprises validated correlation products, and an associated metadata package containing information on telescopesensitivities and other parameters necessary for the flux density calibration of the correlated visibilities. The L1 data products are released to the EHT collaboration and the respective project principal investigators (PIs), including PIs external to
the EHT collaboration, for further calibration and for archiving purposes. Within the EHT, the additional calibration steps
required to produce science-ready data are referred to as the L2 processing stage; these L2 calibrated data are internal EHT
collaboration products.
Prior to 2018, each telescope site provided the metadata required for the flux density calibration in different formats andwith varying levels of refinement (for details, see Janssen et al., 2019; Issaoun et al., 2017a). Processing the metadata to
generate calibration tables (in the standard, widely used ANTAB1 format) was thus time-consuming and labor-intensive.
For 2018, steps were taken to standardize both the format and contents of the requested metadata packages across all stations, where possible, as well as the scripted pipeline reading and processing these packages. This allows for a streamlined
approach that is easy to maintain and reuse for future EHT epochs. The long term goal is for the metadata collection to be
automated as much as possible, and incorporated into the VLBI Monitor2 system.

1.1

Scope of this MemoIn this memo, we describe the collection, processing and validation of the 2018 metadata necessary for the absolute fluxdensity calibration of the 2018 EHT observations, and the contents of the metadata products released as part of the L1
package. In Section 2, we give a brief overview of the basic concepts for the flux calibration of EHT data. We then provide
details on the raw metadata collection process for 2018 in Section 3. Section 4 describes the determination of the DPFUs
and gain curves for each station, while Section 5 describes additional processing and validation of the system noise temperatures and phased array SEFDs, as required for the generation of the ANTAB calibration tables. Section 6 summarizes thecontents of the L1 metadata package, as well as a brief summary of the metadata properties. Telescope acronyms and codesused in this memo are found in Appendix A. The codes used to reference the different observing tracks, and details on theparticipating stations during each track, are also provided in Appendix A.

2

Basic Concepts for EHT Flux Calibration

We define here the basic terms and concepts for EHT and (sub-)mm VLBI flux calibration, which will be used throughout
this memo. The EHT memos by Issaoun et al. (2017a) and Janssen et al. (2019) for the calibration of the EHT 2017 data
provide more detailed descriptions of these concepts.
To convert the visibility amplitudes from correlation coefficients to units of flux density, taking into account the different
telescope sensitivities across the array, the system equivalent flux density (SEFD) of each telescope needs to be determined.
The SEFD is given by (e.g., Event Horizon Telescope Collaboration et al., 2019):
SEFD =∗T sys
1,
ηph DPFU × gE

1 http://www.aips.nrao.edu/cgi-bin/ZXHLP2.PL?ANTAB
2 https://vlbimon1.science.ru.nl/

3(1)

where the degrees per flux density unit, DPFU, is a conversion factor from units of temperature (K) to units of flux density(Jy), and gE is the normalized elevation-dependent gain curve of the telescope. ηph is the time-dependent phasing efficiency
for a phased array, and has a value of unity for single dish telescopes. The DPFU can be estimated as:
DPFU =

ηA Ageom,2k(2)where ηA is the aperture efficiency of the antenna, Ageom is the geometrical area of the antenna dish in units of m2 , andk = 1.38 × 103 is the Boltzmann constant in units of Jy m2 /K.
∗
T sys
is the effective system noise temperature, corrected for atmospheric attenuation, given by:
∗
T sys
=

eτ
T sys ,
ηl(3)

where τ is the line-of-sight atmospheric opacity, and ηl is the forward efficiency (implicitly assumed to be the same as
the feed efficiency, accounting for rearward efficiency loss due to ohmic losses, spillover and scattering; see Kutner &∗Ulich, 1981, for instance). It is important to note that T sys
differs from the standard T sys value used in cm-wavelength VLBI,∗
which does not account for atmospheric attenuation. Single dish (sub-)mm telescopes typically measure T sys
directly byplacing a hot load of known temperature in the signal path. This is done regularly during or in between target scans to∗account for the time and elevation (airmass) dependence of T sys.
The gain curve of each station is parameterized as a function of elevation, E, following the equation (Janssen et al., 2019):
gE = 1 − B(E − E0 )2 .

(4)

This equation is fit to measurements of antenna temperature or antenna efficiency (normalized by their mean values at thepeak of the curve) as a function of elevation at each station, to derive the best fit values of the parameters B and E0 , bothof which are coefficients. These two parameters are then converted to the polynomial gain coefficients used in the standard
ANTAB calibration table format, given by:
a0 = 1 − BE0 2 ,a1 = 2BE0 ,a2 = −B .

(5)

The coefficients beyond the second order are set to 0, following the parameterization of Equation 4 which, based on measurements in 2017 considers that a second order polynomial is sufficient to fit the gain curve of existing EHT stations. Notethat there is still an uncertainty about the shape of the LMT gain curve (especially considering the dish has an active surface3 to correct for gravitationally induced deformations), due to a lack of measurements (see Section 4.4), especially since
the completion of its upgrade to a 50 m diameter antenna in 2017 December.

3

Collection and Contents of the Raw Metadata Packages

In 2020 October, a plan was put in place to compile the 2018 EHT metadata according to specific standards. The aim wasto streamline the tasks during and after metadata collection, including metadata validation and the production of ANTAB
calibration tables. After receiving feedback from site representatives during 2020 November, the checklist of required
metadata and submission formats was finalized, and the majority of telescope representatives submitted their metadata
packages in 2021 January. Cross-checks and validation of the raw metadata packages were conducted throughout 2021.
Any technical and/or formatting issues discovered were reported back to the telescope representatives, so that the issues
could be fixed and new versions submitted.
The raw metadata from each station comprise the following files:
3 http://lmtgtm.org/telescope/telescope-description/4

∗• Effective system temperature (T sys) table per observed track, with associated timestamps, VEX scan number andname of observed source for each scan. For validation purposes, additional columns include elevation, azimuth,
opacity at zenith, effective temperature of the atmosphere (T atm ) and ambient temperature in the receiver cabin (T amb ).
∗For single-dish stations, there is one T sys
column per polarization (RCP, LCP), per observing band (lower sidebands,∗i.e., band 1 & band 2; and upper sidebands, i.e., band 3 & band 4); this gives eight columns with T sys
values per∗scan for single-dish stations. For the SMA, RCP and LCP T sys values are provided for each of the 7 antennas in the∗∗array, giving a total of 14 columns with T sys
values. For ALMA, phased T sys
values (and corrected for the phasing
efficiencies) are provided in the tables. Since the bandpass information is also used when estimating the phased
∗
∗
T sys
, entries are provided for each of the 32 spectral windows in 32 columns. Therefore, for ALMA, one T sys
file is∗provided per band and per observing track, to avoid putting T sys for all four bands in a single file with 32×4 columns.

• Flag table per observed track, denoting the status and viability of each observed scan, with comments from thetelescope operators on possible technical or weather issues. Each scan is assigned a flag code, which can be used toassist downstream processing in determining the data quality and in the excision of bad data. The flag codes stand forthe following possible scenarios: success/no problems (S), not observed/unusable data (N), partial observation or late∗on source (P), not good or uncertain data quality (U), missing T sys(T). More than one code can be applied to eachscan record.
• Phasing efficiency table per observed track, for phased arrays only. For the SMA, the phasing efficiencies are provided per band and per polarization, for a total of 8 columns in each file. For ALMA, the values contained in thephasing efficiency tables are in linear polarization, and are not the values used to produce the final calibration tables,
so are solely for diagnostic checks. The ALMA phasing efficiencies are also provided per band and per (linear, X andY) polarization.
• Antenna efficiency and gain table for single dish stations (with associated elevation and timestamps if available),by which the station DPFUs and elevation dependent gain curves can be derived. The stations also had the option to
directly provide DPFUs and gain parameters that they had estimated themselves.
The format of these tables was devised such that the header of each file and its name unequivocally identify it with a given
observing track and antenna site. Examples of the table filenames, formats and contents are provided in Appendix B. Each∗row of T sys
values and their corresponding flag code should ideally be associated with valid VEX scan numbers and source
names. We acknowledge that in some stations the local software from where measurements are obtained, has certain restrictions such as the number of characters allowed for source names. Thus, sometimes source names will be shorter versions of
the VEX names, which is dealt with by the processing pipeline (Section 5).
When available, the telescope representatives also provided supporting data and documents, including antenna temperature
measurements, observer logs, and descriptions of how the main quantities in the metadata were measured.

4

Station Sensitivities and Gain Curves

The DPFUs and gain curve coefficients of each station in the array are provided in the ANTAB table headers, which arethen used by standard calibration software to perform absolute flux density calibrations on visibility data. A summary of
the station DPFUs and gain curve coefficients for the EHT array in 2018 is provided in Table 1, together with their estimated uncertainties. The DPFU uncertainties are expected to be the dominant source of systematic errors in the flux density
calibration of EHT data, which includes uncertainties in the planet models used to derive their flux densities and between∗
planet models used in different calibration software (typically 5 to 10%). T sys
values are not expected to vary much within
the typical scan duration of a few minutes, so their uncertainties are expected to be no larger than a few percent. The GLT
is an exception (details in Section 4.3), due to sub-optimal performance in 2018 when it was still only partially commis∗
sioned, where T sys
uncertainties are also significant. The following subsections describe how the DPFUs, gain curves, andtheir corresponding uncertainties were determined for each station.
5

Table 1: EHT station DPFUs and gain curve parameters. DPFU values are quoted for both lower (LSB) and upper (USB) sidebands andboth polarizations (RCP and LCP). For phased arrays (ALMA and SMA), the DPFUs represent the combined sensitivity of all phased
dishes. The parameters listed in the table are defined in Section 2.

Station
ALMAa
APEXb
GLTc
LMT
SMT
JCMTd
IRAM 30 m
SMAe
SPT
a

LSB-RCP
1.19
0.02530.188
0.01880.02960.144
0.040
0.00698

LSB-LCP
1.19
0.02620.126
0.01880.138
0.040
0.00731

DPFU (K/Jy)
USB-RCP USB-LCP
1.19
1.19
0.02590.02700.008850.008850.188
0.126
0.01820.01820.0296
0.142
0.137
0.040
0.040
0.00698
0.00731

uncertainties± 10%± 5.5%
± 7%
± 22%
± 6%
±(11–14)%± 19%±(5 – 15)%
± 10%

gE
B
0
0.00002 ± 3.6%
000.000082 ± 10.4 %00.00018 ± 5.3%
0
0

E0
0
36.6 ± 1.0%0057.6 ± 2.0%043.7 ± 1.3%
00

The ALMA DPFU uncertainty is based on the overall 10 % systematic uncertainties associated with the QA2 flux calibration tables.

b APEX measures DPFUs for each of the 4 bands separately. LSB and USB DPFUs shown here are the mean of the band 1 and 2 DPFUS, and band 3 and4 DPFUs, respectively.
c GLT DPFU uncertainties are based on the scatter of 3 measurements, inclusive of flux density model errors, but may be larger due to significant antenna∗ uncertainty contributing to the total SEFD uncertainty.
astigmatism. There is also a ∼ 15% T sys
d For the JCMT DPFU, the night-time value is shown.
e The SMA DPFU uncertainty is based on the dominant 5-15 % uncertainty on the phasing efficiency.

4.1

ALMA

The ALMA flux calibration parameters were determined by the ALMA Level 2 Quality Assurance (QA2) team for the
EHT observations, who directly produce and submit the calibration tables in ANTAB format. The ALMA SEFDs are deter∗
mined based on self-calibration gains and measured T sys
converted to circular polarizations via PolConvert (Martí-Vidal
et al., 2016). Just as for 2017, an average DPFU value of 0.031 K/Jy corresponding to that of a single 12 m dish is factored
out of the SEFD estimates, and used in the ANTAB table header. Variations in the sensitivity of the phased ALMA arraydue to the total number of observing antennas and phasing losses are incorporated into the time-dependent system temperature values in the EHT ANTAB tables. A flat gain curve is assumed, since any elevation and time-dependent antenna gains
are solved indirectly through self-calibration. Details on the ALMA QA2 calibration for 2017 are presented by Goddi et al.
(2019) and are also discussed briefly by Janssen et al. (2019) and Event Horizon Telescope Collaboration et al. (2019).

4.2

APEX

The APEX DPFUs are determined by the APEX team from observations of Mars and Jupiter, based on planet flux densities
derived using the ASTRO program within the IRAM GILDAS4 software package. The given uncertainties of 5.5% include
both the measurement errors and the 5% quoted flux density uncertainties of the planet models. The determination of
APEX calibration parameters is described in detail in the accompanying documentation provided with the raw APEX
metadata package, contained within in the L1 metadata package (see Section 6).
Although no gain dependence on elevation was found based on the 2018 planet antenna temperature measurements, quasar
data from 2017 showed a curve with small gains (with no more than 5% deviations from a flat gain curve), which was used
for the 2017 EHT data calibration. We use the same gain curve for 2018, with parameters shown in Table 1.

4 https://www.iram.fr/IRAMFR/GILDAS

6

4.3

GLT

The GLT was only partially commissioned during the 2018 observations, resulting in the telescope performance being
sub-optimal at the time. A systematic, saddle-shaped deformation in the GLT antenna dish imprinted when the dish waslifted onto the support cone, resulted in significant astigmatism and a low measured aperture efficiency of 21.6 ± 0.9% at
230 GHz for the upper sidebands. As noted, the GLT did not observe in bands 1 and 2 in 2018. Due to RCP output instabilities in the efficiency measurement data, only the LCP values could be used; the LCP aperture efficiency is applied toboth polarizations. This aperture efficiency was derived based on scans of Venus, using the flux density model from the
CASA data reduction software (Butler, 2012), and taking into account the extended size of the planet and the beam size of
the GLT. With a geometric dish diameter of 12 m, this translates into a DPFU value of 0.0088 ± 0.0004 K/Jy, considering only
the measurement errors. Adding the quoted 5% uncertainties of the Venus flux density model (Butler, 2012) in quadrature
gives a 7% total uncertainty. Note that these uncertainties are derived based only on the three available efficiency measurements in March 2018, before the dish surface improvements were carried out. The uncertainties are thereby likely to belarger and are difficult to quantify, due to additional systematic errors arising from the dish astigmatism. Nevertheless, due∗∗
to instabilities in the output of the continuum detector used for the T sys
measurements in 2018, the T sys
measurements are
estimated have uncertainties of up to ∼ 15% , thus dominating the total SEFD uncertainties. Detailed descriptions of theseissues, and the full derivation of the GLT calibration parameters are described in a separate memo (Koay et al., 2023).
Due to the extreme Northern latitude location of the telescope, observed sources typically do not vary significantly inelevation throughout an observing track. Therefore a flat gain curve is assumed.

4.4

LMT

Due to an emergency situation, LMT observations were cancelled midway through the 2018 EHT campaign, so observers
were unable to measure the aperture efficiency after the campaign as planned. LMT aperture efficiencies were subsequently
measured in 2020 January, and are provided by the telescope team in the gain tables. We use these 2020 measurements tobe representative of the 2018 DPFU values, since no work was done on the antenna between the two dates.
The LMT aperture efficiencies between 28-80◦ are shown in Figure 1, normalized over the mean of the aperture efficiencies
at the peak/plateau range of elevations, i.e., between 40 ◦ to 60◦ , assuming that the measurement at 35◦ is an outlier. Theseaperture efficiencies were determined based on observations of R Leo. Assuming a 50 m diameter dish, we used the mean
aperture efficiencies of 26.5% in RCP and 17.7% in LCP to derive a DPFU of 0.188 K/Jy in RCP and 0.126 K/Jy in LCP.
The gain curve cannot be reliably characterized due to insufficient data points at low elevations. In particular, there is anuncertainty regarding how the active surface panels, designed to compensate for elevation-dependent deformations of thedish, actually affect the gain curve. We therefore tentatively assume a flat gain curve for the LMT with large uncertainties
in the DPFU, up to 22%, considering the 1σ standard deviation of the measured antenna efficiencies. Additional fixes may
be needed downstream to improve the LMT calibration, at least until the gain curve can be characterized once the telescoperesumes operations, which have stopped since March 2020 due to the Covid-19 pandemic.

4.5

SMT

Measurements of the SMT aperture efficiency and gain curves for the EHT 2017 observations are described in detail inthe memo by Issaoun et al. (2017b). Since no additional work was carried out at the telescope between 2017 and 2018, thegain curve used for the 2017 observations is retained for 2018. The aperture efficiency data were reprocessed by the SMT
team, using the Butler 2018 planet models of Jupiter and Mars (Bryan Butler, private communication), giving a beam sizeof 34.00 8 and an aperture efficiency of 0.66 ± 0.04 for the lower sidebands (both polarizations), and 0.64 ± 0.04 for the upper
sidebands (both polarizations). These translate into DPFUs of 0.0188 ± 0.011 K/Jy for bands 1 and 2, and 0.0182 ± 0.011 K/Jy
7

LMT 2020 Gain Curve

1.2

Normalized A_eff

1.0
0.8
0.6
0.4
gain curve
RCP
LCP

0.2
0.0

10

20

30

40
50
60
Elevation (deg)

70

80

90

Figure 1: The LMT aperture efficiencies in 2020, normalized by the mean aperture efficiency at the gain curve plateau (at elevations of
40-60◦ ) for each polarization. The dashed line shows the best fit gain curve using the normalized aperture efficiencies at both polarizations.

for the upper sidebands, consistent with the 2017 values. We note that these DPFU values were provided directly by the
SMT team in the ANTAB header format, together with the gain curve polynomial coefficients.

4.6

JCMT

Long-term characterization of the JCMT antenna reveals no significant elevation-dependent gains, so a flat elevation-gain
curve is used. However, there is a known time-dependence of the gains (Figure 2), determined from antenna aperture efficiency measurements between 2006 and 2017 (Issaoun et al., 2018). The aperture efficiencies remain flat during night-time(1930 to 0730 Hawaii Standard Time, HST), and then dip during the day. These time variations in the aperture efficiencies are caused by deformations in the dish due to thermal gradients induced by solar heating during the day. For the 2018
observations, we use the same piece-wise model-fit of the gains determined from the 2006 to 2017 data by Issaoun et al.
(2018), assuming that the gains behavior has not changed in 2018. The 2018 antenna efficiency measurements obtained in
the daytime indeed show consistent trends (Figure 2). However, we normalize the night-time gains (gt ) to unity, giving:gt = 1.0
gt = 1.938 − 1.161 × exp −for 1930 to 0730 HST
2(HST − 13.550)
167.701(6)!
for 0730 to 1930 HST∗
In the ANTAB calibration tables, these time-dependent gains are accounted for by dividing the T sys
values of each JCMT
scan by the corresponding gains value.

The six measurements of Mars and Uranus obtained by the JCMT in 2018 show a systematic decrease in the aperture
efficiencies compared to the mean of the 2006-2017 values (Figure 2). Using only the mean night-time aperture efficiency
8

JCMT Antenna Efficiency vs HST

0.8
0.7
0.6
A_eff

0.5
0.4
0.3
model-daymodel-night2006-2017
2018

0.2
0.1
0.0

0

5

10

HST (hr)

15

20

Figure 2: The JCMT antenna efficiencies measured between 2006 and 2017 (red points) and that measured in 2018 (blue crosses). The
dashed line shows flat gains during night-time HST, and the dip in antenna efficiencies during the day time.

value of 46.2% in 2018, provided in the JCMT gain tables, we estimate the 2018 night-time JCMT DPFU to be 0.0296 K/Jy
in RCP (in 2018, the JCMT observed only in RCP). While the aperture efficiencies are measured at 221 GHz, we assume
this value applies to all four bands. Due to the lack of measurements in 2018, we assume DPFU uncertainties of ±11% in
the night-time, and ±14% during the day, similar to that between 2006-2017, and consistent with the scatter in the 2018
aperture efficiency measurements.

4.7

IRAM 30 m

The IRAM 30 m telescope team provided DPFUs with associated uncertainties for the upper and lower sidebands and forboth polarizations, which they determined from Mars and 3C279 observations in 2018. The two calibrators give systematically offset DPFU values, but are still consistent within the 1σ errors, so they are deemed insignificant. For the ANTAB
tables, we use the mean DPFU of these two calibrators for each band and polarization, as given in Table 1.
Due to bad weather during the 2018 observations, the antenna temperature measurements show a large scatter and havelarge uncertainties, and are thus not ideal for the estimation of the gain curve. Only antenna temperature measurementsfrom M87 observations are available at high elevations (Figure 3), and the scatter is large. Additionally, some sources
including Mars, are observed over a limited range of elevations, making it difficult to normalize the antenna temperaturesconsistently for all sources. In Figure 3 for example, the normalization is done using the mean antenna temperature valuesof each source within the elevation range of 25 - 50◦ , which is not ideal. We therefore re-use the 2017 IRAM 30 m gain
curve (Janssen et al., 2019) for the 2018 observations, since they are not expected to vary significantly.

9

IRAM-30m Gain Curve

1.6
1.4
Normalized TA*

1.2
1.0
0.8
gE (2017)
M87 (2018)
3C279 (2018)
MARS (2018)
OJ287 (2018)
J1337-1257 (2018)

0.6
0.4
0.2
0.0

10

20

30

40
50
Elevation (deg)

60

70

80

Figure 3: The IRAM 30 m telescope antenna temperatures (T A∗ ) measured in 2018, normalized for each source by their mean values
within the elevation range of 25 - 50◦ . Only T A∗ values obtained when τ < 0.5 and for measured FWHM beam sizes less than 1500 are
used. Large T A∗ outliers have also been flagged. The black dashed line shows the 2017 gain curve.

4.8

SMA

The SMA DPFUs are provided by the SMA team for each observing track, antenna, band and polarization. They are estimated based on observations of Callisto for the April 21, 22, 24 and 25 tracks, and of Ganymede for the April 27 track.
Due to poor weather, no good calibration solutions were available to derive the DPFU for the April 28 track, so we use theApril 27 DPFU values for Apr 28, as recommended by the SMA team. The phased DPFU for all 7 antennas observing in2018 is 0.040 K/Jy, for a single-dish aperture efficiency of ∼75%.
For the ANTAB calibration tables, the DPFU of the SMA is set to 1.0. The actual measured DPFUs are used to modify the
∗
∗
T sys
measurements for each antenna, before the T sys
values (in units of Jy) are added together in phase (see Equation 3 inJanssen et al., 2019). More details are provided in Section 5.3.
The SMA has a flat gain curve as a function of elevation (Matsushita et al., 2006). This is because each of the array antennas adjusts its subreflector focus position as a function of elevation in real-time, based on the measured elevation-dependent
focus curves. The focus curves are different from antenna to antenna, but a common focus curve (i.e., averaged over all
antennas) is used for each of the antennas to avoid introducing any path length differences, and thus delays, between antennas.

4.9

SPT

Four measurements of the SPT aperture efficiency were provided by the station representatives, determined from observations of Saturn and Jupiter on 2018 April 28 at elevations of 16 to 22◦ . The 2018 flux density models for Saturn and Jupiterat 221.1 GHz (Bryan Butler, private communication) were used to characterize the SPT beam shape. The mean aperture
efficiencies are 24.5±0.8% in RCP and 25.7±0.8% in LCP. Assuming a 10 m diameter dish, this gives DPFU values of
0.0069±0.0002 K/Jy and 0.0073±0.0002 K/Jy in RCP and LCP, respectively. With uncertainties of up to 10% in the flux
density model of Saturn, adding this in quadrature to the measurement uncertainties gives 10% uncertainties in total for the
SPT DPFUs. The total error budget is thus dominated by the flux density model uncertainty. We assume that these DPFU
10

values are consistent across both the lower and upper sidebands. The low aperture efficiencies are a result of the telescope
being under-illuminated, operating at sensitivities closer to that of a 6 m dish.
As for the 2017 calibration of SPT, we ignore the elevation-dependent gains of the SPT, since sources typically do not varymuch in elevation throughout the observing track due to its geographical location.

5

Processing and Validation of System Temperatures and Phased Array SEFDs

5.1

Processing Pipeline

We built a python pipeline (available on a private GitHub repository5 ) to process the metadata from both the single-dishand phased array stations, while performing a number of validation routines. An independent pipeline6 has also beendeveloped for processing of the EHT metadata in the future. Calibration tables are then generated for the whole EHT arrayin the standard ANTAB format used in many radio astronomical data reduction software packages.
After defining some global parameters pertaining to an EHT run, the pipeline automatically draws information from VEX
files and performs a cross-match with the metadata tables submitted by each station. Hence, only some code sections(written specifically for antennas needing special processing in a particular observing run) would need to be re-written orcommented out when running the pipeline for different observing runs (see Section 5.2).

5.2

Single Dish Stations

The pipeline cross-matches available VEX files with the files submitted by each station (see Section 5.1). This helps verify
that every expected file in the metadata packages actually exists, as well as to identify erroneous information associatedwith metadata measurements. During verification, the pipeline issues warnings and provides both lists of missing files andlists of affected scans. Relevant metadata files automatically open for inspection in real time so that a clarification requestfrom telescope representatives is made promptly, and/or solutions are implemented when needed.
While an issue is resolved, the user can decide on-the-fly whether the affected track/antenna/scan rows should be flagged∗
or if the issue can be safely ignored. If flagging the data, the options are to delete the row completely, or to have their T sys
values reset to NaN (not a number in computing notation) and later replaced by modelled values.
Here we describe the main processing and validation steps applied to single dish stations (including issues detected andhow these were resolved).

5.2.1

System Noise Temperature Timestamps∗
We applied a shift to T sys
timestamps to correspond to the middle of their respective scans. We do this only for stations∗
providing a single T sys measurement per scan, i.e., all single-dish stations observing in 2018. This eases the a-priori flux∗
calibration process downstream, considering that different stations use different strategies for observing T sys; some stations∗
measure T sys before the target scan, while others measure after, or sometimes a mixture of both. Standard data processing∗
software like AIPS (Greisen, 2003) requires that the T sys
timestamp be between the start and stop time of the scan. Typi∗
cally, a time ‘offset’ parameter in the ANTAB table is used to shift all the T sys
timestamps of a station by a constant value
5 https://github.com/cristinaroca/EHTmd. Send your GitHub ID to cromero@asiaa.sinica.edu.tw to receive an invitation.
6 https://github.com/mpifr-vlbi/l1-calibration. Send your GitHub ID to mjanssen@mpifr-bonn.mpg.de to receive an invitation.

11

in time to move them in between the appropriate scans. However, due to different sources having different observing scan∗
lengths, as well as inconsistencies in the time gaps between the actual T sys
measurements and the scan start/stop times∗
for a particular station, applying a single, constant time offset value to each station is insufficient for adjusting all the T sys
timestamps in the observing track correctly in between the station’s corresponding scan start/stop times.

5.2.2

Metadata Clean-up and Flagging

When comparing metadata and VEX files, the pipeline identifies source name discrepancies and a warning is raised. In the2018 metadata we found seven such discrepancies. Five of these corresponded to the use of alternative VEX source names
for which we directly implemented source name replacements in the pipeline. The additional discrepancies were due to∗scans reflecting T sys
information for a calibrator (OMC1 in the case of the SMT, and Saturn in the case of the LMT) used
for pointing instead of information for the target source (OJ287 and SgrA*, respectively). After inspection of the affected∗
metadata file, we flagged the associated T sys
measurements to be later replaced by modelled values.
Some of the early metadata submissions also showed scan number discrepancies when compared to the corresponding
VEX files. Telescope representatives were informed and requested to submit revised metadata packages.
Additional series of checks were aimed at finding contiguous scans on different sources sharing the same timestamps and/or∗
T sysvalues, due to e.g., errors in data entry or during the production of the raw metadata package at each station. A coupleof LMT scans had such a problem, and after careful inspection, we deemed these values as unreliable and decided to flagthem for replacement.
Using information from the VEX files, the pipeline obtains global variables and calculates expected antenna elevations
per scan by taking into account the telescope location and source position in the sky at the time of observing. Doing this isuseful in two ways:• We can compare calculated elevations with those provided in the raw metadata packages and obtain a list of track/antenna/scan entries where discrepancies are larger than 5◦ . For the LMT, we found seven scans in track e18c21(corresponding to April 21, see table 4) with elevation discrepancies of 8◦ up to 28◦ and we noticed that the largest∗
difference between T sys
and VEX timestamps in these scans was shorter than 20 minutes. Looking at the trend of ele∗vation and T sys variation in time for neighbouring scans on the same source, we determined that these entries should∗∗have their T sys
flagged for replacement. In addition, a total of 235 SMA T sys
measurements in different tracks had
their original elevation values differing from the calculated ones by about 8◦ up to 65◦ . Most of these corresponded
to values measured during the long gaps between scans of target sources. After verifying that there were other measurements for the same scans closer in time to the actual VEX scan times, these entries were flagged to be completely
deleted.
∗• Some scans were observed by a particular station, but do not have any associated T sys
nor elevation entry in thesystem noise temperature table. This includes three LMT scans having an “S” flag code in the flag table. We thus
assigned a calculated elevation to such entries. In addition, we obtained an opacity value by interpolating τ fromneighbouring scans, to accompany the new elevation information.

Having reliable elevation values for scans whose original elevation values were dubious, as well as for scans with no eleva∗
tion information, allows us to obtain substitute model T sys
values for them (see Section 5.2.4).
Finally, the pipeline attempts to automatically detect outliers. For this, we implement an initial elevation and opacity curve
fitting (see details in section 5.2.4), and deviations from the modelled curves are obtained. To ensure outlier candidates
are narrowed down automatically as much as possible, we also used the modified z–score as an additional outlier indicator. This method uses the median absolute deviation, which is more robust against outliers than the median or the mean.

12∗
Once an outlier is identified, its associated T sys
is flagged. This section of the code for automated flagging requires further
improvement with better flagging algorithms.

In addition, interactive plots are generated to aid the identification of outlier candidates (see Section 5.4). Since all thevalues are tagged by track/antenna/scan, we can easily identify the origin of suspicious values and inspect them in theircorresponding metadata files.
∗
The outlier candidates identified by the code and then visually inspected, mostly occur when there are rapid T syschanges.
This is the case in many SMT scans (see details in section 5.2.4). After inspection, we did not deem any of the candidates
found in SMT nor any other antenna as true outliers.

Detailed information on the scans affected by elevation discrepancies and other issues described above, are noted in theauxiliary files generated by the pipeline, included in the metadata package (Section 6).

5.2.3

Additional Processing for Specific Antennas

LMT – Band and polarization labels have been corrected. After interaction with telescope representatives, it was deter∗
mined that the T sys
values provided under the column headers Tsys_b1r, Tsys_b1l, Tsys_b2r, and Tsys_b2l in
the .tsys tables (an example of this table is provided in Appendix B) from the LMT raw metadata package corresponded
instead to upper and lower sideband in LCP, and to lower and upper sideband in RCP.
∗JCMT – The JCMT T sys
values are measured at the intermediate frequency (IF) of 4 GHz, corresponding to bands 2 and∗3 in Double-sideband (DSB) mode. We have assigned the same T sys
values to the other two bands, assuming they are∗comparable across the bands. Note that this may lead to larger uncertainties in the T sys
values in bands 3 and 4. The DSB∗
T sysvalues are then converted to single-sideband (SSB) equivalent values, using the empirically measured7 sideband ratio∗∗of 0.9. This is done by multiplying a factor of 1.9 to the T sys
of bands 3 and 4 (USB), and a factor of 2.11 to the T sys
in∗bands 1 and 2 (the LSB corresponding to the image sidebands). We then divided the T sys
values by the gains in night-timeand day-time, as shown in equation 6.

5.2.4

Modelling of System Noise Temperatures∗
After identifying scans affected by different issues, some T sys
entries will now appear as NaN and need to be replaced withalternative values.
∗
We model T sys
as a function of elevation (E) and opacity (τ). We first consider Equation 3 with τ = τ0 /sin(E), whereτ0 is the opacity at zenith. We note that at some stations, the zenith opacities are measured by a radiometer operating at
a slightly different frequency from that of the actual EHT observing frequencies. However, the differences in τ are notexpected to be significant.

Taking into account the receiver temperature (T rx ), the temperature that an antenna should measure when pointing at theempty sky, at a given elevation is given by:
T sys = T rx + T atm ηl (1 − e−τ ) + T amb (1 − ηl ),where the contribution from the Cosmic Microwave Background radiation is regarded as negligible. Substituting T sys inEquation 3, we have (Rohlfs & Wilson, 2006):
∗
T sys
=


eτ
T rx + T atm ηl (1 − e−τ ) + T amb (1 − ηl )
ηl

7 https://www.eaobservatory.org//jcmt/wp-content/uploads/sites/2/2019/05/RxA3m-SB-Notes-2018.pdf

13∗=⇒ T sys=eτ(T rx + T atm ηl + T amb (1 − ηl )) − T atm .
ηl

By considering Q0 = (T rx + T atm ηl + T amb (1 − ηl )) η−1
and Q1 = −T atm , we have:
l
∗
T sys
= eτ0 /sin(E) Q0 + Q1 ,

(7)

∗
∗
which is the function we have used in the pipeline to find T sys
outliers, and to model missing T sys
values. This approach
alleviates the problem of not having T atm and T amb values (they were not provided by some stations) as well as other quantities. We note that this equation is valid when T atm = T amb and also in the cases when T atm , T amb (e.g., for the GLT and the
SPT).
∗Missing values in the original single-dish tables (e.g., VEX scans that were observed but are missing T sys
counterparts,∗and scans deemed unreliable and thus flagged) are modelled by curve-fitting equation 7 to the available T sys
values in each∗track. Once Q0 and Q1 are obtained, we applied them to the scans missing T sys, using their corresponding elevation andopacity values from the metadata tables, or when missing, using the calculated elevation based on observing time, source
position and antenna location, and the interpolated opacity from neighbouring scans. We note that the fitted parameters Q0
and Q1 in equation 7, are obtained per track and per antenna while excluding those values with flag code “U”. This approxi∗
mation does not allow for an exact representation of T sys
in time, given that Q0 and Q1 (which incorporate quantities suchas T atm , T amb and ηl ) are treated as constants for each combination of track and antenna, although strictly speaking, they∗can vary in time. This representation results in modeled T sys
values which are close enough to the original ones for most of∗
the metadata (see Figure 4). However, we note that the model performs poorly for scans in tracks where the T syschanges
rapidly for a given source (see Figure 5).

5.3

Additional Processing for Phased Arrays∗
For the phased arrays, i.e. ALMA and SMA, the DPFUs and phasing efficiencies are factored into the T sys
values in the∗
ANTAB calibration tables. Therefore, for these two stations, the T sys sections of the ANTAB tables in fact contain their
SEFD values as a function of time.

5.3.1

ALMA SEFDs

For ALMA, ANTAB format calibration tables are provided by the team responsible for the QA2 and Polconvert processing of the ALMA EHT data. For each track and observing band, the tables contain 32 columns of SEFD values (with
0.031 K/Jy factored out as a single-dish DPFU), one for each of the 32 spectral windows.
These SEFDs are provided at a rate of less than a second (with rates as high as 0.4s), interpolated by Polconvert tomatch the time resolution of the VLBI correlation times.
However, since the AIPS software is unable to handle such a high cadence, our processing pipeline averages the values
over 6s before writing them into the combined ANTAB table for all stations.

5.3.2

SMA SEFDs∗
The SMA provides DSB T sys
measurements for each antenna and each polarization, measured at a cadence of once every minute. The phasing efficiencies are provided for each band and polarization, at a shorter cadence of 20 to 30s per
measurement. These data are processed via the following steps:

14∗Figure 4: Comparison between original T sys
values (blue circles) and modelled ones (orange crosses) for antennas LMT (top), SMT(middle), and SPT (bottom) in RCP, band 1. The latter two antennas are the ones for which the model is less accurate. This could be
related to significant time variations in T atm and T amb within a track, or due to directional or time dependent changes in opacity. However,∗as a first approximation and with the aim of finding missing T sys
values, we consider this approach as reasonable. Note that originalvalues with flag code “U” do not have a modelled counterpart (this is more noticeable in the last observing track for SMT).

∗
values are multiplied by a factor 2 to obtain the SSB equivalent values (see Issaoun et al., 2017a), for
1. The SMA T sys
a sideband ratio of 1.
∗
2. The T sys
values are then divided by the DPFUs of the corresponding track, antenna and polarization, to convert them∗from units of K to units of Jy. Since the DPFU values for each band are roughly comparable, and the T sys
values are15

∗
Figure 5: Error percentage between T sys
modelled and original values vs elevation, for antennas LMT (top), SMT (center), and SPT∗(bottom) in RCP, band 1. SMT in particular had abrupt changes in T sys
which cannot be handled properly by the fit, therefore resulting inlarger error percentages compared to other antennas. The largest error (> 60 %) occurs in track e18d28, scan No0033, which comes rightafter a scan with flag code “U” on the same source.

the same for all 4 bands, we average the DPFU values across all 4 bands as well.
∗(in units of Jy) over all 7 antennas is then estimated following Equation 3 in the memo by Janssen et3. The phased T sys
al. (2019).
∗
4. The SEFDs for each band and polarization are estimated by dividing the phased T sys
by the corresponding phasing∗
efficiencies. The phasing efficiencies are first averaged over the one minute cadence of the T sys
measurements.

5. SEFD outlier values and those measured during gaps in observing scans (e.g., when the antennas are slewing) are
flagged.

16

5.4

Validation

The pipeline (Section 5.1) generates 2D and 3D plots (using the Seaborn and Plotly data visualization libraries inpython) which allow us to visually assess the quality of the data from single-dish antennas. The validation plots are provided per antenna, colour-coded by target source and symbol-coded by polarization or observing track: (1) 2D plots: opac∗∗
ity vs. time and T sys
vs. time. (2) 3D interactive plots of T sys
vs. elevation and time, per polarization. The 3D interactive∗
plots are particularly useful because they allow a direct identification of the origin of suspicious T sys
values by hovering themouse above them, after which annotations become visible. By double-clicking on particular labels, one can also isolate
different tracers, thus enhancing the visibility of different data at a time. Here we show some examples based on APEX
metadata for all the 2018 tracks (Figure 6), and GLT metadata for track e18c21 only (Figure 7). All the validation plots canbe found in the L1 metadata tarball (see Section 6 for details).

∗
Figure 6: Snapshots of interactive 3D plots for APEX T sys
values as a function of both time and elevation for RCP (left panel) and LCP(right panel). Each source is marked with a different color, and each frequency band is denoted by a different marker shape.

∗
Similar 2D validation plots of T sys, phasing efficiencies and SEFDs as a function of time and elevation are also produced
for the phased arrays, and are included in the metadata package.

5.5

ANTAB Table Generation

Having processed the metadata as described above, and having calculated the DPFU values and gain curve coefficients, wecreated ANTAB tables in the standard format8 (see also Janssen et al., 2019, for detailed descriptions). The DPFU values∗
and gain curve polynomial coefficients are attached as headers, and the T sys
for single dish stations and SEFD values forphased arrays, ordered by station, are attached in the main body. This process generates one ANTAB table per observing
track, per band. We produce two sets of ANTAB tables:∗
1. raw ANTAB tables: contains single-dish station T sys
values without any flagging or corrections applied, using theoriginal timestamps provided by the stations. The only ‘processing’ steps applied to this version are for the determi∗
nation of the phased array SEFDs (as described in Section 5.3), the conversion of JCMT T sys
values from DSB to∗
SSB equivalents, and the factoring of the JCMT time-dependent gains into the T sys values.
8 http://www.aips.nrao.edu/cgi-bin/ZXHLP2.PL?ANTAB

17

Figure 7: Snapshots of interactive 3D plots for GLT similar to those in Figure 6. Here we isolate track e18c21 to visualize clearly how∗
T sys
in both RCP and LCP, vary with time and elevation. Band 3 is represented by circles, and band 4 by diamonds. The observed
sources in the isolated track are OJ287 (blue), M87 (red) and 3C279 (green).
∗
2. processed ANTAB tables: In addition to the processing of the phased-array SEFDs and JCMT T sys
as done for the∗
raw ANTAB tables, this version contains T sys values for which different flagging steps have been applied, and for∗which observed scans with missing T sys
values have been replaced with modelled ones. In addition, a shift in thetimestamps has been applied (to coincide with the center of each scan) for single dish stations.

We expect project PIs and the EHT L2 calibration teams to mainly use the processed ANTAB tables for calibration. The
raw ANTAB tables can be used for diagnostic checks.

6

L1 Metadata Package: Contents and Data Properties

6.1

Package Contents

As part of the 2018 L1 data release, the metadata is packaged as a single tarball (version X.X):
EHTmetadata_2018April_vX.X
This tarball package contains the following products each in a separate directory:• ANTAB calibration tables, separated by tracks and frequency bands. Each table contains DPFUs and gain curve∗coefficients for all stations in the header, and T sys
values as a function of time for all single-dish stations, and SEFD
values as a function of time for both ALMA and the SMA. As mentioned above, two versions of ANTAB tables areprovided for each track and band, (1) the raw ANTAB files in the directory antab/raw/; and (2) the processed
ANTAB files in the directory antab/processed/.
• Validation plots used for quality assessment, as described in Section 5.4. They are located in the plots/ directory.
• Auxiliary files produced by the metadata processing pipeline (Section 5.1), including tables of flagged or problem∗∗
atic T sys, tables comparing raw and modeled T sys, as well as python dictionaries of names and codes for tracks,
18

Table 2: Median sensitivities for primary targets in the EHT 2017 (from Event Horizon Telescope Collaboration et al., 2019) and 2018
campaigns. The antenna efficiencies (ηA ) and DPFUs are aggregated over frequency bands and polarizations, where necessary.

Station
ALMA
APEX
GLT
LMT
SMT
JCMT
IRAM 30 m
SMA
SPT
a

diameter (m)
2017
2018
73a (37)79a (43)
12
12
12
32.5
50
10
10
15
15
30
30
14.7a (6)15.9a (7)1010ηA (%)
2017
2018
68b
68b
61
64
22
28
22
60
65
52
46
47
55
75b
75b
e
60
25

∗
Median T sys
(K)
2017
2018
76
113
118
163
410
371
208
291
375
345c
379c
226
351
285c
283c
118
95

DPFU (K/Jy)
2017
2018
1.030
1.190
0.025
0.026
0.0088
0.083
0.157
0.017
0.019
0.033
0.030
0.034d
0.140
0.046
0.040
0.00610.0071

Median SEFD (Jy)
2017
2018
74
106
4,800
6,259
46,2874,500
1,362
17,10020,33010,50012,816
6,900
2,539
6,400
8,458
19,30013,365The diameter for phased arrays reflects the total collecting array of all antennas. Values in parentheses represent the number of participating antennas.

b The quoted aperture efficiency for phased arrays are that for each single dish.
c The quoted T ∗ are SSB equivalent values, assuming sideband ratios of 0.9 for JCMT and 1.0 for SMA.
sys
d A correction factor of 3.663 was applied to the 2017 IRAM 30 m DPFU, due to losses from maser frequency instabilities.
e The SPT antenna efficiency for 2017 was estimated assuming a 6m diameter dish, considering only 6m illumination of the dish by the receiver optics.
While the antenna is still under-illuminated in 2018, the antenna efficiencies in 2018 are estimated with respect to the full 10 m dish.

stations and sources. Descriptions of these individual files are provided in the README contained within the directory aux/, where these files are also located.
• Raw metadata files collected from each station, as described in Section 3. They are located in the directory raw/.
This metadata package is available online in the public domain 9 . The tarball is also staged and archived together with thecorrelator products for the 2018 observing campaign. For reference, tables summarizing identification codes used in thenaming of files associated with the various observing stations and tracks are provided in Appendix A.

6.2

Comparing 2017 and 2018 Station Sensitivities

The sensitivity parameters of each station in 2018 are summarised in Table 2, in comparison with their values in 2017. A
few notable differences include:∗• The median T sys
values are higher at ALMA, APEX, SMT and IRAM 30 m stations, due to overall poorer weather in2018 compared to 2017.

• The LMT sensitivity has improved in the 2018 observations, after the upgrade from a 32.5 m to a 50 m diameter dish.
We note also that the LMT switched to 2SB receivers just before the April 2018 EHT campaign; the telescope wasoperating with an interim 1mm DSB receiver in the 2017 which has since been decommissioned.
• The IRAM 30 m telescope DPFU has also increased in 2018, after the installation of a new maser. Amplitude losses
due to decoherence, attributed to excess noise in the maser frequency reference in 2017, required an ad-hoc downscaling of the telescope DPFU by a factor of 3.6.

9 https://doi.org/10.25739/2x5r-xd34

19

6.3

Remaining Issues in the EHTmetadata_2018April_v1.0 Package

We summarize here outstanding issues in the release of the EHTmetadata_2018April_v1.0 package, that may affectthe a-priori amplitude calibration of EHT 2018 data. More careful analysis is required downstream to mitigate these issues.
If possible and required, they will be corrected in an updated version of the metadata package.
• There remains a large scatter in the SMA SEFDs on some tracks (e.g., at the beginning of tracks e18c25 on April 25,
e18g27 on April 27, and almost the entire track of e18d28 on April 28). Extreme SEFD outliers have been flagged,
but there are borderline cases where more careful flagging and analyses are needed after applying the a-priori calibration to fringe-fitted data. Ad-hoc SEFD corrections may also be needed on some range of scans, as was done for2017 SMA data.
• Due to very poor weather (including periods of snow) at the ALMA site on tracks e18g27 (April 27) and e18d28(April 28), the QA2 results are less reliable. For example, as a result of poor phasing, there are some jumps in the
ALMA SEFDs on some scans, which may need to be flagged pending further analysis.
• As mentioned, the LMT gain curve is not yet well-characterized. Large gain errors of up to 20% (or possibly higher)
are expected at low elevation scans, which may need to be corrected downstream via self-calibration or other methods. The gains can also be determined in the future once the telescope operations resume.
• The APEX DPFUs may need an ad-hoc rescaling (or network calibration) to account for an underestimation of flux
densities on APEX baselines, when compared to the flux densities of similar ALMA baselines to the same stations.
Additionally, for band 1, we found 25% differences between RCP and LCP amplitudes on all APEX baselines. Forband 4, there is a smaller but still significant 13% difference between the RCP and LCP amplitudes. These issues
were discovered after conducting preliminary a-priori calibration tests. Follow-up diagnostics found this issue to becaused by low IF power levels and too large block-down converter attenuation settings for the R2DBE associatedwith RCP in bands 1 and 4.
• Due to the small number of measurements, the GLT DPFU uncertainties are not well characterized, and are likely∗
to be larger than the quoted 7%. T sys
uncertainties are also large, ∼15%. Pointing errors at low elevations in 2018
can cause amplitude losses of up to 18% at 10◦ elevation, up to as much as 96% at 6◦ (Koay et al., 2023); this affects
scans observed at elevations below 10◦ , e.g. for 3C279.
∗∗• The frequency dependence of T sys
at the JCMT is unknown, and we have simply applied the T sys
measured in bands
2 and 3 to bands 1 and 4. Network calibration may be able to correct for these errors in bands 1 and 4.

6.3.1

Minor updates in the EHTmetadata_2018April_v1.1 Package

We have made minor modifications in v1.1 of the metadata package. These minor modifications include:• Adding the raw JCMT (Mm) metadata files, which were missing in the v1.0 of the release. The new raw metadata
release now includes all the files provided by the telescope representatives.
∗• Using updated sideband ratios measured in 2018 to convert JCMT DSB T sys
values into their SSB ones equivalents(as described in section 5.2.3). Previously we used the sideband ratio of from 2017 (see Issaoun et al., 2017a).

• Updating the validation plots and SEFD tables provided in the auxiliary files. These were affected by a bug in theprocessing pipeline which did not implement properly the needed updates after flagging and removing extreme
outliers in the SMA (Sw). This bug did not affect any of the ANTAB tables used for calibration.
All the issues described above for v1.0 are still present in v1.1.
20

References
Butler, B. 2012, ALMA Memo 594
Event Horizon Telescope Collaboration, et al., 2019, ApJL 875, L3
Goddi, C., Martí-Vidal I., Messias, H., et al. 2019, PASP, 131, 075003
Greisen, E. W. 2003, in Information Handling in Astronomy-Historical Vistas, Vol. 285, ed. A. Heck (Dordrecht: Kluwer),
109
Inoue, M., Algaba-Marcos, J. C., Asada, K., et al. 2006, Radio Science, 49, 564
Chen M.-T., Asada K., Matsushita S., Raffin P., Inoue M., Ho P. T. P., Han C.-C., et al., 2023, PASP, 135, 095001
Issaoun S., Folkers, T. W., Blackburn, et al. 2017, EHT memo series, 2017-CE-02
https://eventhorizontelescope.org/files/eht/files/EHT_memo_Issaoun_2017-CE-02.pdf
Issaoun S., Folkers, T. W., Marrone, D. P., Kim, J., Tilanus, R., and Falcke, H. 2017, EHT memo series, 2017-CE-03
https://eventhorizontelescope.org/files/eht/files/EHT_memo_Issaoun_2017-CE-03.pdf
Issaoun S., Falcke, H., Friberg, P., Janssen, M., Tilanus, R., and Wouterloot, J. 2018, EHT memo series, 2018-CE-01
https://eventhorizontelescope.org/files/eht/files/EHT_memo_Issaoun_2018-CE-01.pdf
Janssen, M., Blackburn, L., Issaoun, S., Krichbaum, T. P., Wielgus, M. 2019, EHT memo series, 2019-CE-01
https://eventhorizontelescope.org/files/eht/files/EHT_memo_Janssen_2019-CE-01.pdf
Koay, J. Y., et al. 2023, EHT memo series, 2023-L1-02, arXiv:2312.02759 [astro-ph.IM]
Kutner M. L. & Ulich B. L., 1981, ApJ, 250, 341
Martí-Vidal I., Roy, A., Conway, J., & Zensus, A. J. 2016, A&A, 587, A143
Matsushita, S., Saito, M., Sakamoto, K., et al. 2006, in Proceedings of the SPIE, Vol. 6275, Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, 62751W
McMullin, J. P., Waters, B., Schiebel, D., Young, W., and Golap, K. 2007, in Shaw, R. A., Hill, F., Bell, D. J. eds, ASP
Conference Series, Vol. 376, Astronomical Data Analysis Software and Systems XVI. p. 127
Rohlfs, K. & Wilson, T. L. 2006, Tools of Radio Astronomy, Fourth Revised and Enlarged Edition, Corrected Second
Printing, Springer

21

A

Naming Conventions and Identifications for Stations and Tracks

We provide a summary of the naming conventions for telescopes (Table 3) and observing tracks (Table 4) used in thismemo and in the naming of files in the metadata package. We show also the single-letter HOPS station code, used in the
HOPS data reduction pipeline as well as many custom EHT data validation and plotting libraries.
Table 3: Names and identifications for all the telescopes that participated in the 2018 EHT observing campaign.

Full Telescope Name
Atacama Large Millimeter/submillimeter Array
Atacama Pathfinder Experiment
Greenland Telescope
Large Millimeter Telescope
Submillimeter Telescope
James Clerk Maxwell Telescope
Institut de Radioastronomie Millimétrique 30 m Telescope
Submillimeter ArraySouth Pole Telescope

Abbreviation
ALMA
APEX
GLT
LMT
SMT
JCMT
IRAM 30 m
SMA
SPT

VEX Site Name
ALMA
APEX
THULE
LMT
SMTO
JCMT
PICOVELSMAPSPTVEX Code
Aa
Ax
Gl
Lm
Mg
Mm
Pv
Sw
Sz

HOPS Code
A
X
G
L
Z
J
P
S
Y

Table 4: VEX and EHT experiment identification numbers for each track of the 2018 EHT observing campaign. Participating stations foreach track are also shown.

Observing Datea (UT)2018 Apr 21
2018 Apr 222018 Apr 24
2018 Apr 252018 Apr 27
2018 Apr 28

VEX IDe18c21
e18e22
e18a24e18c25e18g27e18d28EHT Experiment No.
3644
3645
3646
3647
3648
3649

a

Participating Stations
Aa, Ax, Lm, Pv, Sw, Mm, Mg, Gl, Sz
Aa, Ax, Lm, Sw, Mm, Mg, Gl, Sz
Aa, Ax, Lm, Pv, Sw, Mm, Mg, Sz
Aa, Ax, Pv, Sw, Mm, Mg, Gl, Sz
Aa, Pv, Sw, Mm, Mg, Gl
Aa, Pv, Sw, Mm, Mg, Gl, Sz

These dates correspond to the day on which the majority of the track runs. Some of the tracks actually start a few hours prior to the start of specified datein UT.

22

23∗
Example T sys
table format and contents.
Filename: e18c21_SZ.tsys

Formats and Contents of Raw Metadata Tables################################################################## Tsys/Tsys* table for each station and track################################################################### Track Start Date (UT): 2018-04-20# Experiment name: e18c21# Station ID: SZ# Operators/observer/contact person: eventhorizon telescope (eht@eht.edu)# Tsys* or Tsys (inclusive of opacity or not): Tsys*# Central observing frequencies in GHz (DSB/2SB): 213.1, 215.1, 227.1, 229.1, (2SB)# Sideband ratio (if available, for DSB): NA# Sideband coupling coefficient (if available, for 2SB):# Tau observing frequency in GHz: 225.0################################################################## * Timestamp gives the date and time at which Tsys*/Tsys was measured (can be during, before or after each scan)# * Scan column gives the VEX scan number associated with each Tsys*/Tsys measurement# * Tau values should be that measured at zenith# * Empty cells should have an ’NA’
# * If the same Tsys/Tsys* values are used for multiple bands, then include repeated values in the appropriate columns# * Tsys*/Tsys values should be provided without sideband correction## Timestamp(UT)
Scan
SourcePos_Az Pos_El Tsys_b1r Tsys_b1l Tsys_b2r Tsys_b2l Tsys_b3r Tsys_b3l Tsys_b4r Tsys_b4l
Tau
Tamb
Tatm
# YYYY-MM-DD HH:MM:SS(VEX)(deg) (deg)
(K)
(K)
(K)
(K)
(K)
(K)
(K)
(K)
(zenith)(K)(K)##########################################################################################################################################################
2018-04-21 06:51:21
No0051 3C279
240.2
5.9
222.6
218.5
226.3
223.0
218.5
214.8
216.6
209.2
0.053
271.7 220.9
2018-04-21 07:49:57
No0054 NRAO530 296.4
13.1
126.6
124.1
129.5
129.2
126.2
123.8
127.3
122.9
0.054
254.0 221.0
2018-04-21 07:58:18
No0055 SGRA
297.4
29.0
90.2
88.6
92.6
93.8
91.1
88.8
93.8
89.8
0.054
261.3 220.8
2018-04-21 08:10:04
No0056 SGRA
294.1
29.0
89.6
87.8
91.8
93.3
90.5
88.3
92.9
89.3
0.054
265.0 220.7
2018-04-21 08:22:41
No0057 NRAO530 287.7
13.1
130.8
128.1
133.9
134.2
130.3
128.0
131.2
127.0
0.052
268.0 220.8

B24################################################################## Flag tables################################################################### Track Start Date (UT): 2018-04-20# Experiment name: e18c21# Station ID: SZ# Operators/observer/contact person: eventhorizon telescope (eht@eht.edu)# Known bad IFs and channels:################################################################## * Scan column is the VEX scan number# * ScanStart and ScanStop are start and stop times for each scan following VEX file
# * FlagCode - assign appropriate code(s) for each scan, using multiple if necessary (e.g. PT, UT):#
S: Success, no problems#
N: Not observed and data totally unusable (total flag of scan, use this only if 100% sure)#
P: Partial observation (flag partial time-range) e.g late on-source#
U: Not so good or uncertain data quality (keep for now, note for downstream users) e.g., bad pointing#
T: Missing Tsys*/Tsys for this scan (interpolation from other scans required)# * Limit operator comments to 100 characters#################################################################### Scan
ScanStart(UT)
ScanStop(UT)
SourceFlagCode
Observer/Operator Comments# (VEX) YYYY-MM-DD HH:MM:SS YYYY-MM-DD HH:MM:SS##################################################################################################
No0051
2018-04-21 06:59:00 2018-04-21 07:03:00
3C279
S
#
No0052
2018-04-21 07:23:00 2018-04-21 07:33:00
SGRA
T# Tsys scan at wrong elevation.
No00532018-04-21 07:36:00 2018-04-21 07:46:00
SGRA
TP# No Tsys scan. Late on scan.
No00542018-04-21 07:51:00 2018-04-21 07:55:00
NRAO530
S#
No0055
2018-04-21 08:00:00 2018-04-21 08:10:00
SGRA
S#
No00562018-04-21 08:13:00 2018-04-21 08:22:00
SGRA
S#
No0057
2018-04-21 08:26:00 2018-04-21 08:30:00
NRAO530
S#

Example flag table format and contents.
Filename: e18c21_SZ.flag

25################################################################## Phasing efficiencies for each phased array and track################################################################## Track Start Date (UT): 2018-04-21# Experiment name: e18c21# Station ID: SW# Operators/observer/contact person: eventhorizon telescope (eht@eht.edu)################################################################## *Timestamp gives time for each phasing efficiency measurement# *Cadence and precision of phasing efficiencies should follow that produced by the station# *N is the number of telescopes observing in the array## Timestamp(UT)
N peff x 4 bands x 2 pols
# YYYY-MM-DD HH:MM:SS
Band-1-RCP Band-1-LCP Band-2-RCP Band-2-LCP Band-3-RCP Band-3-LCP Band-4-RCP Band-4-LCP##############################################################################################################################
2018-04-21 05:56:09
7 0.24024871 0.05376614 0.26618701 0.10487495 0.18657796 0.37746889 0.24862157 0.18657796
2018-04-21 05:56:14
7 0.07714367 0.14246480 0.32104154 0.13491297 0.07742733 0.37199276 0.23416090 0.07742733
2018-04-21 05:57:02
7 0.31014805 0.05887395 0.15769393 0.12820571 0.05147650 0.41022153 0.27828420 0.05147650
2018-04-21 05:57:51
7 0.50892541 0.17867515 0.07574473 0.30626215 0.13132703 0.56593394 0.53427692 0.13132703
2018-04-21 05:58:21
7 0.38716008 0.12616070 0.16030566 0.20038906 0.10372790 0.64648049 0.59182322 0.10372790
2018-04-21 05:58:51
7 0.15929065 0.18521967 0.26011389 0.24312429 0.03929938 0.84489059 0.79754373 0.03929938
2018-04-21 05:59:20
7 0.23347205 0.13258306 0.26167665 0.21430481 0.11962959 0.54098511 0.40466134 0.11962959
2018-04-21 05:59:50
7 0.14592671 0.13400968 0.35303846 0.20403350 0.18310070 0.47837284 0.35911362 0.18310070
2018-04-21 06:00:20
7 0.29739306 0.12892525 0.21754603 0.21462931 0.14821269 0.66865731 0.60594267 0.14821269
2018-04-21 06:00:49
7 0.20361152 0.19952152 0.19580552 0.25275614 0.06050972 0.83088507 0.82691807 0.06050972
2018-04-21 06:01:19
7 0.11900591 0.10304374 0.33619428 0.14566428 0.11043349 0.55654936 0.41239379 0.11043349
2018-04-21 06:01:49
7 0.25732087 0.76051137 0.84108035 0.79557787 0.72244056 0.86797338 0.79854751 0.72244056
2018-04-21 06:02:18
7 0.72950628 0.71199815 0.62463551 0.75040914 0.69852207 0.71572688 0.66894971 0.69852207
2018-04-21 06:02:48
7 0.62666174 0.37809111 0.57887363 0.62199385 0.67937367 0.62098385 0.60202166 0.67937367

Example phasing efficiency table format and contents.
Filename: e18c21_SW.pheff26################################################################# Gain curve table and measurements################################################################## Range of dates of measurements: 2018-03-20 to 2018-10-05# Station ID: SZ# Antenna Mount Type: Offset Gregorian, Alt-Az mount# Antenna Diameter (m): 12# Operators/observer/contact person: eventhorizon telescope (eht@eht.edu)################################################################### Timestamp gives the date and time for each antenna efficiency measurement# Freq is the observing frequency# errAeff_r and errAeff_l are the errors/uncertainties of the measured antenna efficiencies# Tau should be the value measured at zenith## Aeff_r and Aeff_l from if0 and if1 channels, respectively.
# Timestamp(UT)
Freq
SourceEl Aeff_r errAeff_r Aeff_l errAeff_l
Tsys*
Tau
Tamb
Tatm
Notes/Comments# YYYY-MM-DD HH:MM:SS (GHz)(deg)(K) (zenith) (K)(K)############################################################################################################################
2018-04-28 13:03:15
221
Saturn22.2 0.253
0.03
0.263
0.03
83.8
0.046 247.7 207.6
2018-04-28 13:38:43
221
Saturn22.2 0.250
0.03
0.263
0.03
83.8
0.046 246.7 207.6
2018-04-28 15:00:00
221
Jupiter 16.4 0.236
0.02
0.247
0.02
94.0
0.047 248.7 207.7
2018-04-28 15:33:26
221
Jupiter 16.4 0.242
0.02
0.255
0.02
94.0
0.047 247.7 207.5

Example gains table format and contents.
Filename: SZ_2018-04.gc