Ecosyste.ms: Awesome
An open API service indexing awesome lists of open source software.
https://github.com/kbarbary/sfdmap
Get E(B-V) values from SFD dust map data
https://github.com/kbarbary/sfdmap
Last synced: 19 days ago
JSON representation
Get E(B-V) values from SFD dust map data
- Host: GitHub
- URL: https://github.com/kbarbary/sfdmap
- Owner: kbarbary
- Created: 2016-10-25T22:02:41.000Z (about 8 years ago)
- Default Branch: master
- Last Pushed: 2024-04-29T23:22:01.000Z (7 months ago)
- Last Synced: 2024-10-13T23:32:08.620Z (about 1 month ago)
- Language: Python
- Size: 115 KB
- Stars: 33
- Watchers: 2
- Forks: 12
- Open Issues: 3
-
Metadata Files:
- Readme: README.md
- License: licenses/ASTROPY_LICENSE
Awesome Lists containing this project
README
# sfdmap
[](https://travis-ci.org/kbarbary/sfdmap)
[](https://coveralls.io/r/kbarbary/sfdmap?branch=master)
[](https://pypi.python.org/pypi/sfdmap)
A minimal, fast, MIT-licensed Python module for getting E(B-V) values from
[Schlegel, Finkbeiner & Davis (1998)](http://adsabs.harvard.edu/abs/1998ApJ...500..525S) dust map FITS files.
```python
>>> import sfdmap
>>> m = sfdmap.SFDMap()
>>> m.ebv(100., 40.) # Get E(B-V) value at RA=100 degrees, Dec=40 degrees
0.10739716819557897
```
## Install
Requirements: numpy and a FITS reader (either fitsio or astropy).
```
pip install sfdmap
```
The FITS files comprising the map must be downloaded separately. Among other
locations, they are available from http://github.com/kbarbary/sfddata. On UNIX
systems, run the following to download the maps (93 MB download size):
```
wget https://github.com/kbarbary/sfddata/archive/master.tar.gz
tar xzf master.tar.gz
```
A directory "sfddata-master" will be created. Move or rename as you like.
## Detailed Usage
#### Initialize map:
```python
>>> import sfdmap
>>> m = sfdmap.SFDMap('/path/to/dustmap/files')
>>> m = sfdmap.SFDMap() # get directory from SFD_DIR environment variable
```
By default, a scaling of 0.86 is applied to the map values to reflect
the recalibration by Schlafly & Finkbeiner (2011). To get the original
values, use `scaling=1.0` when constructing the map:
```python
>>> m = sfdmap.SFDMap(scaling=1.0)
```
Get E(B-V) value at RA, Dec = 0., 0. (ICRS frame)
```python
>>> m.ebv(0., 0.)
0.031814847141504288
```
Get E(B-V) at three locations (first argument is RA, second is Dec):
```python
>>> m.ebv([0., 5., 10.], [0., 1.5, 2.1])
array([ 0.03181879, 0.02374864, 0.01746732])
```
By default the coordinates are assumed to be in degrees in the ICRS coordinate
system (e.g., "J2000"). This can be changed with the `frame` and `unit`
keywords:
```python
>>> m.ebv(1.68140, -1.0504884, frame='galactic', unit='radian')
0.031820329230751863
```
The dust map values are linearly interpolated by default. Change this with the
`interpolate` keyword:
```python
>>> m.ebv(1.68140, -1.0504884, frame='galactic', unit='radian', interpolate=False)
0.031526423990726471
```
You can pass an astropy `SkyCoord` instance:
```python
>>> from astropy.coordinates import SkyCoord
>>> coords = SkyCoord([0., 5., 10.], [0., 1.5, 2.1], frame='icrs', unit='degree')
>>> m.ebv(coords)
array([ 0.03181879, 0.02374864, 0.01746732])
```
Finally, there is a convenience function in the module so that you
don't have to construct a `SFDMap` instance if you just want to query
the map once:
```python
>>> sfdmap.ebv(0., 0.) # get map directory from SFD_DIR environment variable
0.031818788521008
>>> sfdmap.ebv(0., 0., mapdir='/path/to/dust/files')
0.031818788521008
```
### How do I get extinction at a specific wavelength or in a specific filter?
The E(B-V) values from the map give information about the *amplitude*
of dust extinction in a given direction. To get the extinction at a
given wavelength or through a given filter, one needs information
about the relative extinction between different wavelengths: an
"extinction law". One can use the
[extinction](http://extinction.readthedocs.io) package for this. For
example, the following code gets the extinction in magnitudes at RA,
Dec = (0., 0.) and wavelengths (4000, 5000):
```python
>>> import extinction
>>> ebv = m.ebv(0., 0.)
>>> wave = np.array([4000., 5000.])
>>> extinction.fitzpatrick99(wave, 3.1 * ebv)
array([ 0.12074424, 0.09513746])
```
To get the extinction in a given bandpass, one needs to know the
source spectrum, as different wavelengths in the bandpass will have
different extinction. With an assumed source spectrum, one would
integrate the source spectrum with extinction applied through the
bandpass and then compare to the unextincted spectrum integrated
through the same bandpass. Some papers provide extinction values in a
number of common bandpasses; these assume some source spectrum.
### Performance Note
Note that while passing an astropy `SkyCoord` object works, if you
have coordinates in the ICRS or FK5 (epoch 2000) (e.g., J2000) systems, it
is far faster to pass latitute and longitude directly. This is particularly
true for small numbers of coordinates or scalar coordinates:
```python
In [1]: from astropy.coordinates import SkyCoord
In [2]: import sfdmap
In [3]: m = sfdmap.SFDMap()
In [4]: m.ebv(0., 0.) # evaluate once to trigger reading the FITS file
Out[4]: 0.03181878852100873
In [5]: coord = SkyCoord(0., 0., unit='degree')
In [6]: %timeit m.ebv(coord) # time with SkyCoord object
100 loops, best of 3: 18.1 ms per loop
In [7]: %timeit m.ebv(0., 0., unit='degree') # pass ra, dec directly
10000 loops, best of 3: 80 µs per loop
```
## Alternatives
There are a couple other packages that support getting dust
values from this map. Both these packages have a bigger scope than
this one and include several other 2-d or 3-d galactic dust maps.
Check them out if you want to compare between different maps or need 3-d
maps. Below, I note a few relevant differences from this package.
#### [mwdust](http://github.com/jobovy/mwdust) [[docs](https://pypi.python.org/pypi/mwdust)]
`mwdust.SFD` gives the extinction in a given band rather than E(B-V).
The API is geared towards 3-d maps, so a distance must be given. Python 3
is not currently supported. The license is BSD.
#### [dustmaps](http://github.com/gregreen) [[docs](http://dustmaps.readthedocs.io/en/latest/)]
`dustmaps.sfd.SFDQuery` uses astropy's SkyCoord for all coordinate
conversions and therefore has suboptimal performance for small numbers
of coordinates (see "Performance Note" above). The license is GPLv2.