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Re: PSF Energy inside circle [message #61576]

Wed, 23 July 2008 13:52

wlandsman
Messages: 743
Registered: June 2000

Senior Member

On Jul 23, 2:03 pm, Michael Aye <kmichael....@googlemail.com> wrote:
> Dear all,

> So how could I find and integrate the next "ring" of pixels? How would
> I even calculate the ever growing circumference correctly, taking into
> account that I have to sum up ever more pixels?

The options mentioned so far seem fine for large radii. For small
radii, the use of integer pixel values can introduce "noise" since a
pixel must be either entirely within or outside the circle. The
program aper.pro
( http://idlastro.gsfc.nasa.gov/ftp/pro/idlphot/aper.pro )
sums using partial pixels, using the exact overlap area of a pixel
with a circle.

The program is somewhat awkward to use, in part because it is very
old, and in part because it propagates error sources. But if you
want to compute the flux within 10 concentric circles centered at the
position [500.2, 500.5] with radii of 3, 3.5, 4, 4.5, ... 7.5 then the
following is a start:

IDL> apr = 3. + findgen(10)/2.
IDL> aper,im, 500.2, 500.5, flux, eflux, sky,skyerr, 1, apr,/
flux,setsky=0
IDL> plot, apr, flux ;plot encircled flux vs. radius

In this case I force the "sky" (background) to have a value of
zero. You can have aper.pro compute the background by giving it an
inner and outer "sky" radii far from the central source. --Wayne

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[Message index]

		Re: PSF Energy inside circle By: wlandsman on Wed, 23 July 2008 13:52
		Re: PSF Energy inside circle By: pgrigis on Wed, 23 July 2008 13:24
		Re: PSF Energy inside circle By: Kenneth P. Bowman on Wed, 23 July 2008 12:17
		Re: PSF Energy inside circle By: pgrigis on Wed, 23 July 2008 11:15
		Re: PSF Energy inside circle By: Bob[3] on Thu, 24 July 2008 05:43
		Re: PSF Energy inside circle By: Kenneth P. Bowman on Fri, 25 July 2008 10:18
		Re: PSF Energy inside circle By: Bob[3] on Tue, 29 July 2008 08:38
		Re: PSF Energy inside circle By: maye on Wed, 30 July 2008 15:06

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