RE: Focus indication

["Gutzwiller, Michael" <mgutzwiller@lanvision.com>]

Another thing to consider is that the PSF changes shape from the center to
the edges in the Mark IV, even in the corrected lenses.  If you are fitting
the PSF to a 2 dimensional gaussian I recommend the stars you use should be
close to the center of the image to avoid being misled by the remaining coma
at the edges.

Mike G.


-----Original Message-----
From: Doug Welch [mailto:welch@physics.mcmaster.ca]
Sent: Thursday, August 02, 2001 5:30 AM
To: TASS List
Subject: Re: Focus indication



A two-dimensional Gaussian is a fairly standard fit for
this sort of situation. It is not uncommon for optics
to show some astigmatism which results in the axis of
unequal FWHM changing by 90 degrees through the point of
best focus.

Cheers,
Doug

On Wed, 1 Aug 2001, Chris Albertson wrote:

> 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...
> >