arXiv subjects

Michael Debus

Publications and source records attributed to Michael Debus.

3 recordsLinked to original sources

arXiv2026

Second-Generation Fabry-P\'erot Unit for CARMENES

CARMENES consists of two high-resolution spectrographs located at the Calar Alto 3.5m telescope, covering the wavelength range from 520nm to 1710 nm. The original Fabry-Perot (FP) units have been essential to track and remove instrumental drift while also improving the wavelength solution provided by the hollow cathode lamps. In this paper we present the second-generation FP unit that has begun operation at Calar Alto in April 2024. It features drastically improved temperature stabilization, additional temperature monitoring, and enhanced optomechanics. Temperature stabilization is improved through better vacuum vessel design, thermal shielding and improved thermal control. The temperature is now monitored with Pt-100 sensors inside the vacuum vessel, one of which is used to drive the temperature control. The new purpose-built optomechanics allow for an improved alignment procedure leading to higher line contrast. The etalon coatings now cover a broader wavelength range, the optics have been upgraded, and the fibers have been updated to an octagonal shape. In laboratory measurements with a Fourier transform spectrometer we compare the FP radial velocity (RV) drift to that of an iodine cell. In a two hour binning we achieve an RMS of the differential RVs of 6cm/s. The new FP unit has already been successfully integrated at CARMENES, with first calibration data indicating improved RV precision.

astro-ph.IM↗

arXiv2024

Accurate calibration spectra for precision radial velocities -- Iodine absorption referenced by a laser frequency comb

Astronomical spectrographs require frequency calibration through sources like hollow-cathode lamps or absorption-gas cells. Laser frequency combs (LFCs) provide highest accuracy but are facing operational challenges. We aim to provide a precise and accurate frequency solution for the spectrum of molecular iodine absorption by referencing to an LFC that does not cover the same frequency range. We used a Fourier Transform Spectrometer (FTS) to produce a consistent frequency scale for the combined spectrum from an iodine absorption cell at 5200--6200\AA and an LFC at 8200\AA. We used 17,807 comb lines to determine the FTS frequency offset and compared the calibrated iodine spectrum to a synthetic spectrum computed from a molecular potential model. In a single scan, the frequency offset was determined from the comb spectrum with an uncertainty of $\sim$1 cm s$^{-1}$. The distribution of comb line frequencies is consistent with no deviation from linearity. The iodine observation matches the model with an offset of smaller than the model uncertainties of $\sim$1 m s$^{-1}$, which confirms that the FTS zero point is valid outside the range covered by the LFC, and that the frequencies of the iodine absorption model are accurate. We also report small systematic effects regarding the iodine model's energy scale. We conclude that Fourier Transform Spectrometry can transfer LFC accuracy into frequency ranges not originally covered by the comb. This allows us to assign accurate frequency scales to the spectra of customized wavelength calibrators. The calibrators can be optimized for individual spectrograph designs regarding resolution and spectral bandwidth, and requirements on their long-term stability are relaxed because FTS monitoring can be performed during operation. This provides flexibility for the design and operation of calibration sources for high-precision Doppler experiments.

astro-ph.IM↗

arXiv2023

Towards 10 cm/s radial velocity accuracy on the Sun using a Fourier transform spectrometer

The IAG solar observatory is producing high-fidelity, ultra-high-resolution spectra (R>500000) of the spatially resolved surface of the Sun using a Fourier Transform spectrometer (FTS). The radial velocity (RV) calibration of these spectra is currently performed using absorption lines from Earth's atmosphere, limiting the precision and accuracy. To improve the frequency calibration precision and accuracy we plan to use a Fabry-Perot etalon (FP) setup that is an evolution of the CARMENES FP design and an iodine cell in combination. To create an accurate wavelength solution, the iodine cell is measured in parallel with the FP. The FP can then be used to transfer the accurate wavelength solution provided by the iodine via simultaneous calibration of solar observations. To verify the stability and precision of the FTS we perform parallel measurements of the FP and an iodine cell. The measurements show an intrinsic stability of the FTS of a level of 1 m/s over 90 hours. The difference between the FP RVs and the iodine cell RVs show no significant trends during the same time span. The RMS of the RV difference between FP and iodine cell is 10.7 cm/s, which can be largely attributed to the intrinsic RV precisions of the iodine cell and the FP (10.2 cm/s and 1.0 cm/s, respectively). This shows that we can calibrate the FTS to a level of 10 cm/s, competitive with current state-of-the-art precision RV instruments. Based on these results we argue that the spectrum of iodine can be used as an absolute reference to reach an RV accuracy of 10 cm/s.

astro-ph.IM↗