[Aavso-photometry] Re: Dark Sky Annulus

[Michael Newberry]

Not to beat to death this thread, but I wanted to add one more picture 
related to FWHM.

Here is the faintest star I can find near the other stars I measured. This 
star peaks at 17.7 ADU above the sky. The sky noise is 3.67 ADU, which makes 
the peak only 4.8 sigma's above the background. The FWHM measurement gives 
6.56 pixels. I am not sure I would put much significance in the slightly 
higher FWHM measured at this brightness level because noise causes variation 
in the centroid position, which then affects the FWHM measurement. This star 
is so faint you can easily see integer quantization in the values of the sky 
pixels.
    http://www.mirametrics.com/pub/SkyAnn8.png

I hope this sequence of measurements illustrates the mistake one can make by 
gauging the FWHM by eye or by using other simple techniques.

Michael

----- Original Message ----- 
From: "Michael Newberry" <mnewberry at mirametrics.com>
To: "Aavso-Photometry" <aavso-photometry at mira.aavso.org>
Sent: Wednesday, January 04, 2006 6:16 PM
Subject: Re: [Aavso-photometry] Re: Dark Sky Annulus


> Arne,
>
> Yes, it appears I did misquote you. Sorry about that. If the wings are 
> large
> then you need to use a larger aperture. As you know, the relative 
> importance
> of the wings is strongly affected by the seeing---the worse the seeing, 
> the
> more extended are the wings. On the other hand, too large a radius and you
> may not be measuring the same background as that underneath the star. A
> person really has to understand their instrumentation and know the extent 
> of
> the wings on the stellar profile. This brings us into the black art part 
> of
> photomety that you referred to. Said another way, doing really good
> photometry takes some time. Dave Crawford and I used to get a real kick 
> out
> of comparing pairs of published papers measuring the same stars in a 
> cluster
> and seeing that the two sets of supposedly precise measurements were
> sometimes 10 times (or more!) different than the quoted errors.
>
> To illustrate the 3 stars for which the FWHM was measured, here is an 
> image
> that shows all 3 of them using apertures set to 8, 32.5, and 42.5 pixels
> based on your quoted sky annulus criterion. Personally, I would put the 
> sky
> apertures closer in this case, but I think you'll agree that somewhere
> between your rule of thumb and the one I mentioned is the "sweet spot" for
> this image.
>    http://www.mirametrics.com/pub/SkyAnn7.png
>
>
> Michael Newberry
>