[Aavso-photometry] Re: Dark Sky Annulus

[Michael Newberry]

Wolfgang,

You are right about FWHM being independent of the brightness (exactly the 
same if everything is ideal). I showed a graph of the radial profile and 
FWHM for a star that peaked at 668 ADU above background. You asked about a 
bright star. The brightest star in that part of the same image peaks at 
13,200 ADU above background. So I measured that one, plus one super faint 
star just to illustrate this independence of FWHM on brightness. These were 
all measured in a similar area of the image so that focus differences, tilt, 
and field curvature do not change the actual PSF appreciably.

Here are 3 stars ranging from bright to faint, measured using Mira Pro 7:

Star peaks at 13,200 ADU, FWHM = 6.48:
    http:/www.mirametrics.com/pub/SkyAnn4.png

Star peaks at 668 ADU, FWHM = 6.50:
    http:/www.mirametrics.com/pub/SkyAnn1.png

Star peaks only 33 ADU above background, FWHM = 6.6:
    http:/www.mirametrics.com/pub/SkyAnn5.png

That is a range of 400 times in star brightness (6.5 magnitudes). Here is an 
image showing the very faint star, marked by the aperture set using Arne's 
criteria with aperture radii of 8, 16, and 26 pixels. The "very bright" star 
to the right is the 668 ADU star that I originally measured:
    http:/www.mirametrics.com/pub/SkyAnn6.png

This last image shows the contrast boosted way up to show just how faint 
that last star really is! Looking just at relatively faint stars, it is 
obvious that a person would be inclined to "eyeball" the FWHM as smaller 
than it really is. Even measuring it, without a robust FWHM tool, what would 
you get? Answer: It would be systematically smaller for fainter stars. And 
then you would use an inner sky aperture that actually rides up on the 
profile of star, rather than being at the edge of the sky. This would bias 
the sky to higher value. As a side point, one thing this exercise shows is 
how robust the FWHM calculation is in Mira. I am not trying to brag about 
Mira here, just make a point that your software should give you the same 
answer for FWHM independent of brightness, within variations caused by 
higher noise at the faint end. It is a common problem in the software world 
for different FWHM values to be reported for stars of different brightness.

The point of all this is to illustrate that Arne's R = 2.5 x FWHM criterion 
is also independent of the brightness of the stars.

Michael Newberry


----- Original Message ----- 
From: "Wolfgang Renz" <w_renz at onlinehome.de>
To: "Michael Newberry" <mnewberry at mirametrics.com>
Sent: Wednesday, January 04, 2006 4:25 PM
Subject: Re: [Aavso-photometry] Re: Dark Sky Annulus


Hello Michael

The FWHM example is one of a pretty faint star.
Do you also have one for a bright star with much
larger count values (say >= 30000 ADUs) ?

The FWHM should be about the same (if its from
the same image), but I'm sure it will extend much
farther outside significantly above the sky back-
ground.

Clear skies
  Wolfgang

-- 
Wolfgang Renz, Karlsruhe, Germany
Rz.BAV = WRe.vsnet = RWG.AAVSO