Transformation relations for UBV photometric system of 1m telescope at the T\"{U}B\.{I}TAK National Observatory

TL;DR

This study establishes transformation relations for the UBV photometric system of the T100 telescope at TÜBİTAK National Observatory, based on extensive observations from 2012 to 2024, enabling standardized photometry despite some systematic differences.

Contribution

The paper provides the first detailed transformation relations for the T100 UBV photometric system, including atmospheric extinction coefficients and systematic comparison with Landolt standards.

Findings

Transformation relations enable standardized UBV photometry for T100.

No significant seasonal variation in atmospheric extinction coefficients.

Small systematic differences in U-B compared to Landolt standards.

Abstract

UBV CCD observations of standard stars selected from Landolt (2009, 2013) were performed using the 1-meter telescope (T100) of the T\"{U}B\.{I}TAK National Observatory equipped with a back-illuminated and UV enhanced CCD camera and Bessell UBV filters. Observations span a long time from the years 2012 to 2024, 50 photometric nights in total. Photometric measurements were used to find the standard transformation relations of the T100 photometric system. The atmospheric extinction coefficients, zero points and transformation coefficients of each night were determined. It could not be found time dependence of the secondary extinction coefficients. However, it was determined that the primary extinction coefficients decreased until the year 2019 and increased after that year. It could not be found a strong seasonal variation of the extinction coefficients. Small differences in seasonal median values of them were used to attempt to find the atmospheric extinction sources. We found calculated minus catalogue values for each standard star, $\Delta(U-B)$, $\Delta(B-V)$ and $\Delta V$. Means and standard deviations of $\Delta(U-B)$, $\Delta(B-V)$ and $\Delta V$ were estimated to be 1.4$\pm$76, 1.9$\pm$18 and 0.0$\pm$36 mmag, respectively. We found that our data well matched Landolt's standards for $V$ and $B-V$, i.e. there are no systematic differences. However, there are systematic differences for $U-B$ between the two photometric systems, which is probably originated from the quantum efficiency differences of the detectors used in the photometric systems, although the median differences are relatively small ($|\Delta(U-B)|$< 50 mmag) for stars with $-0.5<U-B~{(\rm mag)} <1.6$ and $0.2<B-V~{(\rm mag)} <1.8$. As an overall result, we conclude that the transformation relations found in this study can be used for standardized photometry with the T100 photometric system.