Re: fun with co-adding images on Disk Set 19/more effects...

[Chris Albertson <chrisalbertson90278@yahoo.com>]

>   So, there _are_ tools outside of IRAF to do some of these tasks :-)
> On the other hand, the documentation of IRAF is a heck of a lot
> better
> than that of my code :-(  Oh, and IRAF handles floating-point images,
> too, which can be important in some situations (like this one).  
> 
>   Maybe I should just be quiet ...

Yes, that is one thing that should be done early on in the "pipe":
conversion to floating point pixels.

Also note that the mapping of the sky onto pixels by the Mk IV
optics in not linear.  (If we care about percents of pixels out
at the edge)  Translate and rotate all you want but if the
image scale is spacialy variant you will never get perfect
image co-addition.

sub-pixel interpolation must preserve total flux.  A star
shifted, rotated and scaled by fractional pixels must still
be as "bright".  (2D interpolation must use inverse non-linear
transform.)

In a production system one must handle bad pixels, ice crystals,
and comic rays either by filling in these bad spots with
interpolated data or (my preference) keeping track of how
many "good pixels" contributed to each final pixel.  Note the
number of "good pixels" could be fractional.  Then divide the total
by the number of contributing pixels (which could be < the
number of frames used.)


I would not want to program the above myself.  I'd never get it
right.


=====
Chris Albertson 
  Home:   310-376-1029  chrisalbertson90278@yahoo.com
  Cell:   310-990-7550
  Office: 310-336-5189  Christopher.J.Albertson@aero.org

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