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RE: Stacking
One other problem with stacking our Mark IV images is that the
PSF and maybe the image scale is not constant over the entire
frame. So you do not want to stack a start seen in a corner
over the same star seen in the center. OK, this effect is
very small (but still present) if we are talking about
stacking images with small offsets. The rotation issue is
there even for small offsets.
I think the way to go, if you want to stack images is to
make two passes. First process each image into a star list
and do an astrometric catalog match with the 100 or so
"best" stars in each image. Next compute the
transformation between all the star lists. This transformation
will include rotation and possibly non-linear terms. next
apply these transformations to the images and stack them.
Now re-process the stacked image. IRAF can do the stacking.
There is a task that accepts a list of images and a list
of transformations
--- Michael Gutzwiller <deepsky@fuse.net> wrote:
> Just for the record, Image Scientist does do sub-pixel stacking but
> it does
> not do rotational adjustments, just linear. Tom, do the stars at the
> edge
> show rotational trailing after you stacked the images?
>
> Mike G.
>
> -----Original Message-----
> From: owner-tass@listserv.wwa.com
> [mailto:owner-tass@listserv.wwa.com]On
> Behalf Of Tom Droege
> Sent: Monday, December 17, 2001 12:43 AM
> To: tass@listserv.wwa.com
> Subject: Stacking
>
>
> Just for fun I used Image Scientist to stack 8 images from Data Set
> 20. The result was as expected. A reduction of signal to noise of
> the
> square root of 8. It seems to me that this is worth doing. This
> gives the
> equivalent of an eight hundred second exposure. There are more stars
> to be
> found at any one S/N setting. It did not seem to take all that long.
>
> This would give 7 data points over the 2 hour, 19 minutes that the
> sky is
> tracked for these runs. This would seem to be enough to show any
> trend.
>
> Taking longer real exposures really reduces the dynamic range since
> the sky
> brightness is so large.
>
> I suppose that one could get the same accuracy result averaging the
> stars
> after detection, but then this results in fewer detections. Any
> comments?
>
> With the stack, the bright pixels sure show up. Since there was
> about a 10
> pixel drift over the 1200 seconds of time covered by the eight 100
> second
> exposures, the bright pixels show up as little single pixel streaks
> where
> the bright pixel is more or less constant.
>
> Tom Droege
>
>
>
>
>
>
=====
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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