Re: [Phys-l] some photometry help

[Bernard Cleyet <bernardcleyet@redshift.com>]

I don't follow either poster as I'm a film person.  However, one may 
prevent saturation by "defocusing" -- any sources of defocused objects?

Another advantage of defocusing is colour balance is better preserved, 
so one may do B-V and get the class.

Hej, you have a polar drive and any old camera including digital, 
thereby, collecting the data yourself? -- much more satisfying than from 
the "web".

One may make a drive w/ two pieces of wood, a hinge, and long screw and 
nut.  Mount the camera on one piece and turn the screw to follow a 
bright star.  Since you're in the range ~  2 =>10  w/ fast film or CCD 
you probably won't need a drive and the earth's motion will do the 
"defocusing".

bc, wishes he had the time to do it himself!

Brian Whatcott wrote:

> At 08:18 PM 11/13/2006,  Josh you wrote:
>  
>
> > ///
> > - download jpeg photo of constellation (Ursa Major and minor so far, sans
> > connecting lines and names, of course) from wherever I can find it - some
> >    
> >
> > from Hubble site
>  
>
> > - load the images into Iris
> >
> > - Using the 'aperture photometry' feature, measure several "background"
> > spots, record the intensity (the sum of the intensities of the pixels in
> > the circle).  Average this out, record as "Background Level"
> >
> > - Bring the aperture over a star in the constellation, record the
> > intensity.  Repeat for all of the stars in the constellatoin (using same
> > size aperture each time).
> >
> > - Subtract background level from each star's intensity, calculate
> > "Instrumental Magnitude" as -2.5log(intensity)
> >
> > - Pick one star, find the difference between its accepted magnitude and my
> > instrumental magnitude, call the difference the "offset," and add this to
> > the magnitudes of the other stars, giving me "calculated magnitudes."
> > I've also tried several algorithms to find an "average offest," though
> > this doesn't really decrease my % difference much from the other offset
> > method.
> >
> > This works pretty well (<5-6% difference) with dimmer stars (2-6
> > magnitude), but not as well with magnitude <2 stars (up to 30% at times) .
> > Any ideas where I'm butchering the physics?  I'm a little suspicious
> > about a few things in particular:
> > - should I keep the same aperture size each time, or calculate some sort
> > of intensity per pixel, as the bright stars often appear larger than the
> > dim ones?
> >    
>
> ///
>
>  
>
> > Josh Gates
> >    
>
> The apparent size increase of brighter stars is an artifact (of course)
> of the recording method -  photographic or solid state -  which
> allows light to bleed out to adjacent image areas.
> I imagine this gives a burned out image center (where linear  photo
> response is lost) surrounded by a graduated halo depending on the
> central intensity.
> It would be interesting to have a series of exposures of the same star
>  features taken at different apertures/exposure times, in order to
> provide a linear response measurement range for stars of various
>  magnitudes.
>
> Even better, if you could vary the exposure applied to a given star
> in order to produce a standard reading, your exposure settings would
> provide a good measure of star intensity.
>
> But I am dreaming in technicolor. A more practical avenue might be
> for you  to plot measured intensities against the known intensities
>  of a range of dim to bright stars. These will certainly plot on a
> curve at the bright end.   But that curve might then allow you
>  to assign standard intensity values to any further
> bright star image.
>
>
>
> Brian Whatcott    Altus OK    Eureka! 
>
>
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