Contribution by Dr. Gerold Holtkamp, June 2025

The measurements were taken from 7:20 PM UTC on March 4, 2025, to 12:16 AM UTC on March 5, 2025, in Osnabrück, Sonnenhügel. The moon was 28% illuminated and 63° distant. It set at 12:20 AM UTC. During the measurement, some cirrus clouds passed through.

The measurement technique used:
Telescope: Skywatcher Newton 250/1200 mm
Mount: Skywatcher AZ-EQ6
Filter: Luminanz (Antlia)
Camera: QHY268M with gain 60, offset 20, chip temperature -10° C
Guiding: Skywatcher Guidescope Evoguide 50 ED  with camera ZWO ASI120mm

NINA was used for camera and mount control.

The present measurement and the subsequent evaluation with the hops software of the Exoclock-Project resulted in:
Rp/Rs = 0.1168 +/- 0.0089 (expected 0.10693 +/- 0.00095) *
–> Rp = 0,1168 x 972.091 = 113.540 +/- 8.651 km
The exoplanet radius determined in this measurement is thus close to the value of Rp = 111,800 +/- 25,719 km already measured by the author on July 29, 2024, but slightly above (outside its error limits) the Rp = 101,580 km cited by Exoclock as a literature value.** For comparison: R (Jupiter) = 69,911 km. The exoplanet is thus considerably larger than Jupiter.*** The system is 965 light-years from Earth.

The value of the transit's temporal center, which is important for the Exoclock project and is expressed as the difference between the observed and the expected value, is in good agreement with the expected value at O-C = -0.55 +/- 3.17 minutes****, although with a rather large error bar.

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* [Rp = Rplanet, Rs = Rstar]

** There is already a measurement from the author on July 29, 2024.  https://kosmos-os.de/tres-3b-hat-p-18b-toi-2154b-und-toi-1251-01/?customize_changeset_uuid=1e92cf67-5763-4f5f-b7e9-ed767a9618bc#TOI-2154b-und-sein-Mutterstern-TOI-2154
The sources for the star system data listed here can also be found there.

Star: TOI-2154
Spectral Type: F
Apparent brightness: 11.1 mag (V)
Distance: 965 light years
Surface temperature: 6,280 K
Mass: 1.233 times the mass of the Sun
Radius: 1,396-facher Radius der Sonne = 1,396 x 696.340 km = 972.091 km
Constellation: Cepheus

Exoplanet TOI-2154b
Discovery: 2023
Distance to the parent star: 7.67 million km (large semi-axis)
Orbital period: 3.82 days
Surface temperature: 1,580 K
Mass: 0.92 times the mass of Jupiter
Radius: 101,580 km (radius of Jupiter 69,911 km)

***The planetary radius is determined directly from the dip in the light curve, given a known star radius. (see also https://kosmos-os.de/messung-der-transitlichtkurven-der-exoplaneten-wasp-84b-und-kps-1b-am-7-und-8-maerz-2024)

**** [O = Observed, C = Calculated]

The author already has a measurement on July 29, 2024. For the data sources for the star system, please refer to the corresponding article on this website: https://kosmos-os.de/tres-3b-hat-p-18b-toi-2154b-und-toi-1251-01