[Aavso-photometry] Re: Dark Sky Annulus

[Wolfgang Renz]

Thx a lot.

Using inappropriate radii is a common mishap.
Having such a large inner sky annulus radius and such a
large gap probably looks uncommon for many photometrists.

One major source of confusion in this area is that radii and
diameter values are often mixed and not clearly distinguished/
labeled and some therefore use the wrong values.

Clear skies
  Wolfgang

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



----- Original Message ----- 
From: "Michael Newberry" <mnewberry at mirametrics.com>
To: "Wolfgang Renz" <w_renz at onlinehome.de>; "Aavso-Photometry" <aavso-photometry at mira.aavso.org>
Sent: Thursday, January 05, 2006 1:37 AM
Subject: Re: [Aavso-photometry] Re: Dark Sky Annulus


> 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