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Re: Coadd images that contain no stars [message #80325 is a reply to message #80316]

Sun, 27 May 2012 22:26

Russell Ryan
Messages: 122
Registered: May 2012

Senior Member

On May 27, 11:19 am, spasoklampa...@yahoo.com wrote:
> On May 27, 6:05 pm, Russell Ryan <rr...@stsci.edu> wrote:
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>> On May 27, 10:56 am, spasoklampa...@yahoo.com wrote:
>
>>> Hi,
>
>>> I have two images from GALEX from a molecular cloud, that need to
>>> be combined to create an image. The problem is that there are no stars
>>> present. There is only the nebula. How is it possible to coadd these
>>> two frames? On each image half of the cloud is present and there is
>>> some overlap of course. Is there a way in IDL?
>
>>> Thanks a lot
>
>> Why cant you just add them? I'm confused....
>
>> Do you need to align the images? If so, then you need to correlate
>> the two images to find the appropriate shifts. Even then, and I know
>> how careful you need to be, the GALEX PSF varies across the FoV
>> significantly. So coadding frames of anything can be very tricky if
>> they're not centered on precisely the same position in the sky.
>
> Well the problem is that in the first frame half of the cloud
> appears in the lower left of frame, while in the second frame the
> other half appears in the upper right part of the frame. Since there
> are no stars present on both images, how is it possible to estimate
> the shifting to make the coaddition?

You need to correlate the images.

http://en.wikipedia.org/wiki/Phase_correlation

Just follow the description in the Method section. Maybe you don't
want the Hamming window... Either way, I think there is code in the
astro library to do this.

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[Message index]

		Re: Coadd images that contain no stars By: Mats Löfdahl on Wed, 30 May 2012 06:03
		Re: Coadd images that contain no stars By: Helder Marchetto on Wed, 30 May 2012 04:53
		Re: Coadd images that contain no stars By: Mats Löfdahl on Wed, 30 May 2012 04:17
		Re: Coadd images that contain no stars By: Helder Marchetto on Wed, 30 May 2012 01:58
		Re: Coadd images that contain no stars By: Russell Ryan on Sun, 27 May 2012 22:26
		Re: Coadd images that contain no stars By: spasoklampanas on Sun, 27 May 2012 08:19
		Re: Coadd images that contain no stars By: spasoklampanas on Sun, 27 May 2012 08:15
		Re: Coadd images that contain no stars By: Russell Ryan on Sun, 27 May 2012 08:05

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