[AstroPy] Aperture photometry

[Sergio Pascual]

2011/4/14 Rene Breton <superluminique at gmail.com>:
> The plan is to deliver comparable functionality as the IDL astrolib routine.
> This means been able to perform the aperture photometry from an image array,
> given a list of coordinates, apertures, etc. The actual function arguments
> might be different to make it more modern and the algorithms might be
> improved, if possible.
>
> The aper.pro (http://idlastro.gsfc.nasa.gov/ftp/pro/idlphot/aper.pro)
> routine deals specifically with circular aperture photometry (polygonal
> could eventually be added if someone wants to extend the circular aperture
> one). According to the documentation there are two methods to deal with
> subpixels, a polygonal approximation and an exact calculation. Here is the
> quote:
>
> ; /EXACT -  By default, APER counts subpixels, but uses a polygon
> ; approximation for the intersection of a circular aperture with
> ; a square pixel (and normalizes the total area of the sum of the
> ; pixels to exactly match the circular area).   If the /EXACT
> ; keyword, then the intersection of the circular aperture with a
> ; square pixel is computed exactly.    The /EXACT keyword is much
> ; slower and is only needed when small (~2 pixels) apertures are
> ; used with very undersampled data.
>
> So my idea is also to provide different algorithms. The first is the crude
> aperture, with no subpixel. The second will be the polygonal approximation,
> which is fine in most situations, and last will be the exact. I'm still at
> the stage of taking apart the code and reorganizing it so I can't tell much
> more about the algorithm.
>

I have implemented a exact elliptical aperture (with axis not rotated)
and yes, it's very slow. I use a modified mid-point algorithm to trace
the quadrants of the ellipse (and compute the subpixel contribution),
but I think I should try a polygonal approximation and compare
performances.

Regards, Sergio

> Rene
>
>
>
> On 11-04-14 05:23 PM, Sergio Pascual wrote:
>>
>> It would be great to have routines directly comparable with those from
>> Iraf or IDL. What kind of apertures does Astrolib support? How it
>> handles subpixel fractions?
>>
>> I think there's a lack of information about the algorithms used to
>> effectively compute the subpixel fraction in the apertures.
>>
>> Regards, Sergio
>>
>> 2011/4/14 Rene Breton<superluminique at gmail.com>:
>>>
>>> Hi Sergio,
>>>
>>> I'm currently working on porting the IDL astrolib aperture photometry
>>> routines to python (which you can find in pyastrolib). I'm not done with
>>> them yet. Hopefully it should be done soon as I have some data reduction
>>> to
>>> perform. I can let you know if you're interested.
>>>
>>> If you want something more basic for now, have a look at the attached
>>> python
>>> script. It does the aperture photometry but doesn't handle subpixel
>>> contributions. If the radius of your aperture isn't too small it
>>> shouldn't
>>> be too far off a more careful reduction. I haven't made the script to
>>> handle
>>> multiple source positions/apertures but this could easily be fixed.
>>>
>>> Cheers,
>>>
>>> Rene
>>>
>>>
>>>
>>> On 11-04-14 01:18 PM, Sergio Pascual wrote:
>>>>
>>>> Hi
>>>>
>>>> Let's say I have an image as a  numpy array and I want to obtain the
>>>> summed values of the array inside an ellipse inside the image. In
>>>> other words, I wanto to make aperture photometry. Do you know of any
>>>> python astronomy package that implements this. It's a fairly basic
>>>> think, but I haven't found anything.
>>>>
>>>> Regards
>>>>
>>>
>>
>>
>>
>



-- 
Sergio Pascual                        http://guaix.fis.ucm.es/~spr
gpg fingerprint: 5203 B42D 86A0 5649 410A F4AC A35F D465 F263 BCCC
Departamento de Astrofísica -- Universidad Complutense de Madrid (Spain)