RE: Focus indication

["Creager, Robert S" <CreagRS@LOUISVILLE.STORTEK.COM>]

Right now, I'm trying to talk Tom into letting me modify the program so that
you launch it once, point it at a directory, and then as fits file pairs are
placed into that directory, it'll calculate and display away.  Currently,
it's launched after every image is downloaded, calculates stuff and lives
for 15 seconds more, then quits.

Thanks for the more detailed description.  I believe this is a doable piece
for me now.

Rob

> -----Original Message-----
> From: aah@nofs.navy.mil [mailto:aah@nofs.navy.mil]
> Sent: Wednesday, June 13, 2001 11:56 AM
> To: tass@listserv.wwa.com
> Subject: RE: Focus indication
> 
> 
> Well, my real question was:  when you start talking
> real-time, I assume you are adding features to Tom's
> observing code which is written in Qbasic.  Now I guess
> what you are doing is letting Tom spawn off to a C-code
> program similar to what you are doing with the fits
> file writer.
>   So some basic information:
>     (a) you do need to locate some star.  The obvious
>         way is for Tom to use the cursor and select
>         a star on some image display.  The less obvious
>         way is to find a star in the central 200x200 region.
>         That second method is not always easy; you have
>         to be able to reject saturated stars,
>         non-stars (such as cosmic rays or satellites),
>         and stars that are contaminated by neighbors.
>         Good luck -- this is the hard part.  The star you
>         want has peak intensity about half-well and no
>         neighbors within ~8 pixels in any direction.
>     (b) once a star is located, you form a 'box' around
>         the star, usually about 16x16 pixels.  In the
>         x direction, you add up all pixels inside the box
>         at each y position; that gives you a 16-element
>         vector, which is called the y marginal sum vector
>         (marginal comes from the 'margins' of the box).
>         Similarly, for the y direction, you add up all pixels
>         inside the box at each x position; this forms a
>         16-element vector that is orthogonal to the first
>         vector.  Once the vectors are formed, you can
>         find the star's centroid in each vector (direction),
>         and then the half-intensity points from that centroid
>         by interpolation.  This centroiding method is described
>         in any text that talks about centroiding, such as AIP
>         or Ron Stone's 1970's AJ centroiding article.  It is
>         quicker than fitting a full 2-D function and is adequate
>         for Tom's needs.
> Arne
>