J/ApJS/280/29 Difference image analysis from DECam & Swope (Angulo+, 2025)
Optimizing convolution direction and template selection for difference image
analysis.
Angulo R., Rest A., Blair W.P., Jencson J., Coulter D.A., Wang Q.,
Foley R.J., Kilpatrick C.D., Li X., Piro A.L., Rojas-Bravo C.
<Astrophys. J. Suppl. Ser., 280, 29 (2025)>
=2025ApJS..280...29A 2025ApJS..280...29A
ADC_Keywords: Photometry, ugriz; Supernovae
Keywords: Astronomy image processing ; Astronomy data analysis ;
Time domain astronomy
Abstract:
Difference image analysis (DIA) is a powerful tool for studying
time-variable phenomena, and has been used by many time-domain
surveys. Most DIA algorithms involve matching the spatially varying
point-spread function shape between science and template images, and
then convolving that shape in one image to match the other. The wrong
choice of which image to convolve can introduce one of the largest
sources of artifacts in the final difference image. We introduce a
quantitative metric to determine the optimal convolution direction
that depends not only on the sharpness of the images measured by their
FWHM, but also on their exposure depths. With this metric, the optimal
convolution direction can be determined a priori, depending only on
the FWHM and depth of the images. This not only simplifies the
process, but also makes it more robust and less prone to creating
suboptimal difference images due to the wrong choice of the
convolution direction. As an additional benefit, for a large set of
images, we define a Figure-of-Merit based on this metric, which allows
us to rank a list of images and determine the ones best suited to be
used as templates, thus streamlining and automating the data reduction
process.
Description:
We used images taken with DECam on the Blanco 4m telescope located at
Cerro Tololo Inter-American Observatory (CTIO) in Chile.
The field of view (FOV) of DECam is about 3deg2 with a pixel scale of
0.27". In order to probe fields with different stellar density, we
selected extragalactic fields from the Young Supernova Experiment
(YSE; Jones+ 2021ApJ...908..143J 2021ApJ...908..143J) DECam survey (fields 257A and 403C;
PI: Rest, Program IDs: 2021A-0275 and 2023A-237157) and fields in the
Galactic plane from the DECam Eta Car survey (fields ec0814 and
ec0915; PI: Rest, Program IDs: 2021B-0325 and 2023A-643849). For this
analysis, we use images taken in the griz filters.
We also apply the same analysis for data from the Swope telescope, a
1m telescope located at Las Campanas Observatory in Chile,
in Section 3.2.
The FOV is 29.7x29.8arcmin2 with a pixel scale of 0.435". We chose two
extragalactic fields with moderate stellar density from the Swope
Supernova Survey (2022xkq and 2018jag; PI: Piro).
The analysis of Swope data also uses images in the griz filters.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table3.dat 78 383 Blanco/DECam fields
table4.dat 75 150 Swope fields
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See also:
J/ApJ/795/44 : PS1 SNe Ia (0.02<z<0.7) griz light curves (Rest+, 2014)
Byte-by-byte Description of file: table3.dat
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Bytes Format Units Label Explanations
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1- 6 A6 --- Field Name of field
8- 14 I7 --- ID [155041/1094432] Exposure ID
16- 34 A19 "datime" Date Observation Date
36- 43 F8.4 deg RAdeg [53/161.4] Right ascension (J2000)
45- 52 F8.4 deg DEdeg [-61/-4.2] Declination (J2000)
54- 58 F5.1 s Exp [50/300] Exposure time
60 A1 --- Filt [griz] Filter used
62- 70 F9.6 pix FWHM [2.47/11.16] Full-width at half-maximum
72- 78 F7.4 mag mag [17.61/24.16] Apparent magnitude,
m5σ, in Filt (G1)
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Byte-by-byte Description of file: table4.dat
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Bytes Format Units Label Explanations
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1- 7 A7 --- Field Name of field
9- 12 I4 --- ID [1047/9166] Exposure ID
14- 32 A19 "datime" Date Observation Date; YYYY-MM-DD Thh:mm:ss
34- 40 F7.4 deg RAdeg [16/76.5] Right ascension (J2000)
42- 49 F8.4 deg DEdeg [-12/10.8] Declination (J2000)
51- 56 F6.1 s Exp [45/1200] Exposure time
58 A1 --- Filt [gri] Filter used
60- 67 F8.6 pix FWHM [2.26/7.9] Full-width at half-maximum
69- 75 F7.4 mag mag [19.84/23.6] Apparent magnitude, m5σ,
in Filt (G1)
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Global notes:
Note (G1): Defined as the magnitude of point sources for which the effective
counts are at S/N of 5.
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History:
From electronic version of the journal
License: CC-BY-4.0
(End) Prepared by [AAS], Emmanuelle Perret [CDS] 10-Jun-2026