Re: Focus indication

[Chris Albertson <chrisalbertson90278@yahoo.com>]

Here's my opinion:

A "Radial Profile" would work better.  From the centrod you compute:

For each pixels within (say) a radius of 5 pixels of the centriod
  1) Compute distance from centriod:  sqrt( (x1-x2)^2 + (y1-y2)^2 )
  2) Plot this vs. the pixel's value in counts.

Fit gaussian to above.

The advantage here is that you've used ALL the pixels around the
centriod not just a "cross" which wastes a lot of infomation.

I don't think you need to subtract the background.  It would only
translate the gaussian not change it's shape.

"Roundness" is usful not only durring setup but as a way to clasify
objects.  Non-round objects could be blends or moving objects.
"Roundness" is the ratio of FWHM[x}/FWHM[y}  
FWHM[y} would be the radial profile of only those points withine 45 deg 
of vertical.

Stupendous Man wrote:
> 
>   Rob asked:
> 
> > I have the centroid, but am unsure on the interpolation from the centroid to
> > calculate the FWHM.  Theoretically, I believe I should fit a gaussian to the
> > marginal sums (x and y, from star pixels only).  Is this the only way, the
> > wrong way, or is there a simpler way?  Strongly looking for the simpler
> > way...
>
--
   Chris Albertson             
   chrisalbertson90278@yahoo.com
   Redondo Beach, California
   home: 310-376-1029
   cell: 310-990-7550