Data reduction details

Introduction

The following steps are done by IOP4 to reduce data from one epoch (when invoked as iop4 --epoch-list <epochname>):

1. Registering the epoch in the database, listing, registering and downloading the raw files.

2. Classifying the files from their FITS header (FLAT, BIAS, DARK, LIGHT -science-; image sizes, band, exposure time). Many of these are telescope or instrument dependent and the implementation is relegated to the corresponding telescope or instrument class.

3. Create MasterBias, MasterDark and MasterFlat images.

   Following standard procedures, the master calibration frames are created by grouping all images of that type in the epoch (for each image size, exposure time, band, etc).

   Currently, MasterDark are created only for DIPOL. The dark current in the other instruments is negligible.

4. Calibrate science images, which creates a ReducedFit for each RawFit of type LIGHT. This includes:

   1. Apply the MasterBias, MasterDark (if any) and MasterFlat to the RawFit.

   2. Astrometrically calibrate the images (giving it a correct WCS).

5. Compute relative photometry and relative polarimetry results.

6. Correct the resulting flux and polarization to account for host galaxy contamination (Nilsson et al. [2007]).

CAHA T220 Information

Information about the telescope: https://www.caha.es/CAHA/Telescopes/2.2m.html

• Information about the camera:

  • https://www.caha.es/es/telescope-2-2m-2/cafos

  • https://www.caha.es/CAHA/Instruments/CAFOS/detector.html

• Reduction of polarization is done according to Zapatero Osorio et al. [2005].

OSN T090 Information

Information about the telescope: https://www.osn.iaa.csic.es/page/telescopio-90-cm

• RoperT90 instrument (photometry, polarimetry with filters). Retired 23rd October 2021.

  More information about the RoperT90 camera: https://www.osn.iaa.csic.es/page/camara-versarray-t90-retirada

• AndorT90 instrument (photometry, polarimetry with filters):

  Information about the AndorT90 camera: https://www.osn.iaa.csic.es/page/camaras-ccdt150-y-ccdt90

• DIPOL polarimeter: Piirola, V. et al. [2020], Otero-Santos et al. [2024].

OSN T150 Information

Information about the telescope: https://www.osn.iaa.csic.es/page/telescopio-15-m

• Old camera (RoperT150): https://www.osn.iaa.csic.es/page/camara-ccd-roper

• AndorT150 instrument (photometry, polarimetry with filters).

  Information about the camera: https://www.osn.iaa.csic.es/page/camaras-ccdt150-y-ccdt90

References

[1]

K. Nilsson, M. Pasanen, L. O. Takalo, E. Lindfors, A. Berdyugin, S. Ciprini, and J. Pforr. Host galaxy subtraction of TeV candidate BL Lacertae objects. \aap , 475(1):199–207, November 2007. arXiv:0709.2533, doi:10.1051/0004-6361:20077624.

[2]

M. R. Zapatero Osorio, J. A. Caballero, and V. J. S. Béjar. Optical Linear Polarization of Late M and L Type Dwarfs. \apj , 621(1):445–460, March 2005. arXiv:astro-ph/0411531, doi:10.1086/427433.

[3]

Piirola, V., Berdyugin, A., Frisch, P. C., Kagitani, M., Sakanoi, T., Berdyugina, S., Cole, A. A., Harlingten, C., and Hill, K. High-precision polarimetry of nearby stars (d < 50 pc) - mapping the interstellar dust and magnetic field inside the local bubble. A&A, 635:A46, 2020. URL: https://doi.org/10.1051/0004-6361/201937324, doi:10.1051/0004-6361/201937324.

[4]

J. Otero-Santos, V. Piirola, J. Escudero Pedrosa, I. Agudo, D. Morcuende, A. Sota, V. Casanova, F. J. Aceituno, and P. Santos-Sanz. Characterization of High-polarization Stars and Blazars with DIPOL-1 at Sierra Nevada Observatory. \aj , 167(3):137, March 2024. arXiv:2312.05312, doi:10.3847/1538-3881/ad250d.

[5]

Ferdinando Patat and Martino Romaniello. Error Analysis for Dual-Beam Optical Linear Polarimetry. \pasp , 118(839):146–161, January 2006. arXiv:astro-ph/0509153, doi:10.1086/497581.

[6]

Ccd data reduction guide. URL: https://www.astropy.org/ccd-reduction-and-photometry-guide/.

[7]

Larry Bradley, Brigitta Sipőcz, Thomas Robitaille, Erik Tollerud, Zè Vin\'ıcius, Christoph Deil, Kyle Barbary, Tom J Wilson, Ivo Busko, Axel Donath, Hans Moritz Günther, Mihai Cara, P. L. Lim, Sebastian Meßlinger, Simon Conseil, Azalee Bostroem, Michael Droettboom, E. M. Bray, Lars Andersen Bratholm, Geert Barentsen, Matt Craig, Shivangee Rathi, Sergio Pascual, Gabriel Perren, Iskren Y. Georgiev, Miguel de Val-Borro, Wolfgang Kerzendorf, Yoonsoo P. Bach, Bruno Quint, and Harrison Souchereau. Astropy/photutils: 1.8.0. May 2023. URL: https://doi.org/10.5281/zenodo.7946442, doi:10.5281/zenodo.7946442.

[8]

Dustin Lang, David W. Hogg, Keir Mierle, Michael Blanton, and Sam Roweis. Astrometry.net: Blind Astrometric Calibration of Arbitrary Astronomical Images. \aj , 139(5):1782–1800, May 2010. arXiv:0910.2233, doi:10.1088/0004-6256/139/5/1782.