The brightness of the sky of the Caucasian Mountain Observatory of MSU in the near infrared

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Abstract

The results of measurements of background brightness in the near-infrared range (bands J, H, K), carried out in 2016–2023 at the Caucasus Mountain Observatory of Moscow State University was analyzed. It is shown that the instrumental background associated with the thermal radiation of the telescope is noticeable only in the K band, and at operating temperatures its contribution mainly determines the level of the overall background in this band. The coefficients of a polynomial taking into account the contribution of instrumental and extra-atmospheric backgrounds are presented. It is shown that the brightness of the sky background does not depend on air temperature, but a weak dependence on the water vapor content is observed, close to that expected from model calculations: in the J and H bands, the background brightness decreases at a rate of ≈1%/1 mm, and in the K band it grows at a rate of ≈2.5%/1 mm. The maximum amplitude of background brightness variability on short time scales (~30 minutes) has been estimated: ≈10% in the J and K bands and ≈30% in the H band. The maximum contribution of Moon’s radiation scattered in the atmosphere to the overall background level has been determined. It is shown that this contribution can be ignored at an angular distance of the observation point from the Moon greater than ~10° even during a full moon. The average background surface brightness mag/arcsec² in the J, H and K bands was calculated: mJ = 15.7, mH = 13.9 and mK = 13.1.

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About the authors

A. M. Tatarnikova

Lomonosov Moscow State University

Author for correspondence.
Email: andrew@sai.msu.ru
Russian Federation, Moscow

S. G. Zheltoukhov

Lomonosov Moscow State University

Email: andrew@sai.msu.ru
Russian Federation, Moscow

G. E. Nikishev

Lomonosov Moscow State University

Email: andrew@sai.msu.ru
Russian Federation, Moscow

A. N. Tarasenkova

Lomonosov Moscow State University; Institute of Astronomy of the Russian Academy of Sciences

Email: andrew@sai.msu.ru
Russian Federation, Moscow; Moscow

A. V. Sharonova

Lomonosov Moscow State University

Email: andrew@sai.msu.ru
Russian Federation, Moscow

References

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Supplementary files

Supplementary Files
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2. Fig. 1. Spectrum of the night sky at an altitude of 70° above the horizon, obtained on 21.09.2023 in KGO in different operating ranges of the camera. The purple line shows the spectrum of radiation of the black body with a temperature equal to the temperature of the metal tube of the telescope truss Ttube = 10 °C.

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3. Fig. 2. Dependence of the observed flux on the background in the K band on the air mass Mɀ for different telescope tube temperatures: 15–20°C (a), 10–15°C (b), 5–10°C (c), 0–5°C (d), from -5°C to 0°C (d), from –10°C to –5°C (e). Gray dots show individual measurements, blue lines are approximations of the data by straight lines.

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4. Fig. 3. Dependence of the sum of the instrumental and extra-atmospheric background in the K-band on the temperature Ttube (red squares) and its approximation by the model function (1) (blue circles).

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5. Fig. 4. Dependence of background brightness in the J, H and K bands on the time elapsed since sunset.

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6. Fig. 5. Dependence of the observed background brightness in the J, H and K bands on the air mass Mɀ at a depth of the Sun’s immersion below the horizon greater than 18° (for J and H) and greater than 6° (for K).

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7. Fig. 6. Dependence of the observed background brightness in the J, H and K bands on the temperature Ttube (gray dots) when the depth of the Sun's immersion below the horizon is greater than 18° (for J and H) and greater than 6° (for K). The background brightness in the K band corrected for the contribution of the instrumental background (pink dots).

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8. Fig. 7. Dependence of the observed background brightness in the J, H and K bands on the water content in the atmosphere when the depth of the Sun's immersion below the horizon is greater than 18° for J and H and greater than 6° for K (data in the K band are corrected for the contribution of the instrumental background). Blue lines show the approximation of gray dots by a straight line, black triangles show the results of model calculations.

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9. Fig. 8. Dependence of the brightness of the sky background in the vertical of the Moon on the air mass for the J, H and K bands.

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10. Fig. 9. Dependence of the relative contribution of scattered lunar radiation to the total brightness of the sky background in the vertical of the Moon on the angular distance between the measurement point and the Moon for the J, H and K bands. F is the brightness at a distance d, F1 is the brightness at the zenith.

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11. Fig. 10. Variability of the sky background brightness during monitoring observations in the J, H and K bands. The data are taken for different dates, the curves are arbitrarily shifted in time.

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