forked from lsst/psfex
-
Notifications
You must be signed in to change notification settings - Fork 0
/
Copy pathREADME
112 lines (95 loc) · 4.88 KB
/
README
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
PSFEx stands for ``PSF Extractor'': a software that makes PSF models for
use with the SExtractor program or QualityFITS by Frederic Magnard.
Here is a short and preliminary cookbook.
1:
First of all, a binary FITS catalog of the bright sources in the image to
analyse must be created with SExtractor.
- CATALOG_TYPE in the .sex SExtractor configuration file must be set to
FITS_LDAC. This will allow PSFex to have access to the content of the
original FITS image header.
- The .param SExtractor file must contain at least the
following parameters:
NUMBER
X_IMAGE
Y_IMAGE
FLUX_RADIUS
FLAGS
FLUX_APER(1)
FLUXERR_APER(1)
ELONGATION
VIGNET(35,35)
The VIGNET size (here, 35x35 pixels) sets the maximum size of the fittable
zone around each PSF candidate star.
The extraction parameters in the .sex configuration file do not have to be
refined much; however it is important to check that GAIN, SATUR_LEVEL and
PHOT_APERTURES (which will act as the reference aperture for profile-fitting
photometry) are all set to the correct values.
2:
Now the PSF model can be created with psfex from the previously SExtracted
catalog. In the PSFEx configuration file ("default.psfex" by default), one
should mostly care about the following parameters:
PSF model
---------
PSF_ACCURACY: Accuracy to expect from pixel values. This represents more
or less the pixel-to-pixel stability of the detector
response or its calibration. You might want to increase the
value of PSF_ACCURACY if the intra-pixel response is a bit
bumpy (e.g. old NICMOS detectors). The default is 0.01 (1%).
BASIS_NUMBER: sqrt of the maximum number of super-resolved pixels. Pixels
are super-resolved in priority in the brightest areas of the
PSF, and then in the darker portions.
PSF selection
-------------
* SAMPLE_FWHMRANGE:the range in FWHM in which the PSF candidate stars are
selected. Allows the bigger galaxies and artifacts such as
cosmic-ray impacts to be rejected early in the processing.
* SAMPLE_VARIABILITY:
The larger this parameter, the wider the fluctuations in
FWHM are allowed among PSF candidate stars. For instance,
0.2 means that a 20% peak-to-peak variation of the FWHM is
allowed.
* SAMPLE_MINSN: The minimum Signal-to-Noise Ratio (on aperture fluxes) for a
non-saturated candidate star to be eligible for contributing
to the PSF model.
* SAMPLE_MAXELLIP: Maximum ellipticity (A-B)/(A+B) for a non-saturated candidate
star to be eligible for contributing to the PSF model.
Modeling the PSF variability
----------------------------
* PSFVAR_KEYS: List of "keys" (SExtractor parameters) on which the PSF is
supposed to depend (e.g. X_IMAGE,Y_IMAGE for a spatial
mapping of the PSF). Use an empty string ("") to specify a
constant PSF.
* PSFVAR_GROUPS: Some context keys can be grouped together in a such way that
the associated polynom degree does not excede a given value.
PSFVAR_GROUPS defines the index of the group to which each
PSFVAR_KEYS belongs. This index designates the position of
the group in the list of PSFVAR_DEGREES. For instance, to
make the PSF vary with pixel coordinates x and y as a 3rd
order polynomial (that is, 10 parameters: cste,x,y,x2,xy,y2,
x3,x2y,xy2,y3), one should use the following configuration:
PSFVAR_KEYS X_IMAGE,Y_IMAGE
PSFVAR_GROUPS 1,1
PSFVAR_DEGREES 3
If one wants to allow the PSF to vary with both x,y in a
quadratic way, and the airmass in a linear way, one should
use
PSFVAR_KEYS X_IMAGE,Y_IMAGE,:AIRMASS
PSFVAR_GROUPS 1,1,2
PSFVAR_DEGREES 2,1
The colon before the AIRMASS parameter indicates that
AIRMASS should be interpreted as a FITS keyword of the
SExtracted image header, and not as a SExtractor catalog
parameter (this example supposes of course that the AIRMASS
keyword was present in the image header, and that it
contains the average airmass during the exposure).
A typical session
-----------------
Building a model of the PSF:
% sex -c psf.sex foo.fits -CATALOG_NAME mypsf.cat
% psfex mypsf.cat
Using it in SExtractor (this is not recommended for serious science use at
the moment):
% sex -PSF_NAME mypsf.psf foo.fits
Please report any bug or strange behaviour to the AstrOmatic forum:
http://astromatic.net/forum
Emmanuel Bertin.