[Paper Review] The Herschel-PACS photometer calibration: Point-source flux calibration for scan maps

Z. Balog, Thomas Müller+10 more|Repository of the Academy's Library (Library of the Hungarian Academy of Sciences)|Sep 24, 2013

Stellar, planetary, and galactic studiesPhysics and Astronomy9 references92 citations

TL;DR

This paper presents a high-precision flux calibration for the Herschel-PACS photometer's scan map mode using five fiducial stars (α Boo, α Cet, α Tau, β And, γ Dra) as absolute flux references. By correcting for instrumental effects—specifically evaporator temperature drift and telescope mirror flux variations—the relative photometric accuracy reaches 0.5% in the blue and green bands and 2% in the red band, with the absolute calibration limited primarily by 5% uncertainty in stellar atmosphere models.

ABSTRACT

This paper provides an overview of the PACS photometer flux calibration concept, in particular for the principal observation mode, the scan map. The absolute flux calibration is tied to the photospheric models of five fiducial stellar standards (alpha Boo, alpha Cet, alpha Tau, beta And, gamma Dra). The data processing steps to arrive at a consistent and homogeneous calibration are outlined. In the current state the relative photometric accuracy is around 2% in all bands. Starting from the present calibration status, the characterization and correction for instrumental effects affecting the relative calibration accuracy is described and an outlook for the final achievable calibration numbers is given. After including all the correction for the instrumental effects, the relative photometric calibration accuracy (repeatability) will be as good as 0.5% in the blue and green band and 2% in the red band. This excellent calibration starts to reveal possible inconsistencies between the models of the K-type and the M-type stellar calibrators. The absolute calibration accuracy is therefore mainly limited by the 5% uncertainty of the celestial standard models in all three bands. The PACS bolometer response was extremely stable over the entire Herschel mission and a single, time-independent response calibration file is sufficient for the processing and calibration of the science observations. The dedicated measurements of the internal calibration sources were needed only to characterize secondary effects. No aging effects of the bolometer or the filters have been found. Also, we found no signs of filter leaks. The PACS photometric system is very well characterized with a constant energy spectrum nu*Fnu = lambda*Flambda = const as a reference. Colour corrections for a wide range of sources SEDs are determined and tabulated.

Motivation & Objective

• To establish a consistent and homogeneous flux calibration for PACS scan maps using well-defined stellar standards.
• To identify and correct instrumental effects degrading photometric accuracy, particularly temperature drift and mirror flux variations.
• To quantify the intrinsic photometric repeatability of the PACS instrument after correction of systematics.
• To assess the limitations of the absolute calibration, primarily due to uncertainties in stellar atmosphere models.
• To provide a reference framework for users to reprocess data with customized reduction scripts to ensure calibration accuracy.

Proposed method

• The calibration uses five K- and M-type stars as fiducial flux standards with well-defined photospheric models.
• Measured fluxes from PACS scan maps are compared to model-predicted fluxes at reference wavelengths to derive correction factors.
• Instrumental effects—evaporator temperature drift and telescope mirror flux variation—are modeled and corrected using time-series data from the fiducial stars.
• The correction process involves normalizing flux ratios to a reference point (f(x)/f(c) = 1) to remove systematic trends over time.
• A single, time-independent response calibration file is used, validated by the absence of aging effects or filter leaks.
• Color corrections for a wide range of source SEDs are derived and tabulated using the constant νFν = λFλ = const reference spectrum.

Experimental results

Research questions

• RQ1 What is the intrinsic photometric repeatability of the PACS photometer after correcting for instrumental systematics?
• RQ2 How do temperature drift and telescope mirror flux variations affect the calibration of scan map observations?
• RQ3 To what extent do discrepancies between K-type and M-type stellar models limit the absolute flux calibration accuracy?
• RQ4 Can the relative calibration accuracy be improved to below 1% in the blue and green bands through instrumental corrections?
• RQ5 What is the impact of stellar atmosphere model uncertainties on the final absolute flux calibration?

Key findings

• After applying corrections for evaporator temperature and telescope flux variations, the relative photometric repeatability improves to 0.5% in the blue and green bands and remains at 2% in the red band.
• The observed flux ratios of the five fiducial stars to model predictions are consistent with unity, with mean ratios of 1.003 ± 0.019 (blue), 1.005 ± 0.017 (green), and 1.006 ± 0.018 (red), confirming calibration consistency.
• The intrinsic uncertainty of the photometry is now below 0.5% in the blue and green bands, indicating high stability of the PACS bolometer response.
• No aging effects on the bolometers or filters, nor signs of filter leaks, were detected over the mission duration.
• The absolute calibration accuracy is limited by a 5% uncertainty in the stellar atmosphere models, leading to a conservative total uncertainty of 7% in absolute fluxes.
• The correction process effectively removes time-dependent trends in the blue band, confirming the success of the instrumental correction model.