[Aavso-photometry] Re: Dark Sky Annulus

[arne]

Actually, Michael has misquoted me.  I said 5xfwhm radius, not diameter,
for the inner sky annulus radius.  I'm more conservative than Michael is.
Typically, fwhm is 3 pixels for my telescope, and I set the inner radius
to 15 pixels, outer radius to 24 pixels (10pix width).  What you use is
a matter of preference; the further you go from the star, the less influence
the star wings have on the sky background but the less correlation the sky
background has with the sky underneath the star.  The important points
are to be concentric with your target and to have sufficient pixels included
in the annulus to get good signal/noise determination of the mean sky
background.
Arne

Michael Newberry wrote:
> 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
>