Help needed for manually aligning images
jcely wrote on Sep 11, 2009
Hey everybody, undergrad here new to research needing some help.
I need to run Multidrizzle on a rather large number of images from many different orbits and observing runs. And as i will be looking for subtle changes in a nebula over time, my adviser thinks we shouldn't trust HST's pointing accuracy and measure the image shifts manually.
I have no idea where to start with this. the images sometimes have large shifts and often have large rotations across the detector. There are two bright stars in the image that i can use.
All assistance is helpful
Thanks
Justin
I need to run Multidrizzle on a rather large number of images from many different orbits and observing runs. And as i will be looking for subtle changes in a nebula over time, my adviser thinks we shouldn't trust HST's pointing accuracy and measure the image shifts manually.
I have no idea where to start with this. the images sometimes have large shifts and often have large rotations across the detector. There are two bright stars in the image that i can use.
All assistance is helpful
Thanks
Justin
Francisco Valdes wrote on Sep 11, 2009
Hello,
If by manually you also mean interactively and the only goal is add small tweaks to the world coordinate system (WCS) already provided then I would suggest msczero from the mscred package. This task will display an image to ximtool or ds9. You can then select and centroid on one or more stars and enter the RA/DEC. When you exit it will update the WCS for a shift (error) in the tangent point.
mscred is an external package that you would need to download and install if you don't already have it. Since you are new you might want to look at http://iraf.noao.edu/projects/ccdmosaic/astrometry/astrom.html
Yours,
Frank Valdes
If by manually you also mean interactively and the only goal is add small tweaks to the world coordinate system (WCS) already provided then I would suggest msczero from the mscred package. This task will display an image to ximtool or ds9. You can then select and centroid on one or more stars and enter the RA/DEC. When you exit it will update the WCS for a shift (error) in the tangent point.
mscred is an external package that you would need to download and install if you don't already have it. Since you are new you might want to look at http://iraf.noao.edu/projects/ccdmosaic/astrometry/astrom.html
Yours,
Frank Valdes
jcely wrote on Sep 11, 2009
that seems like what i want, but what do you mean by "small"? my shifts include large rotations and shifts of up to half the detector (ACS and WFPC2) size. If this task can work with these shifts then that would be perfect.
Francisco Valdes wrote on Sep 11, 2009
My assumption is that the WCS is fairly accurately defined. This is independent of how the pixels are actually laid out in the image; i.e. rotations and shifts. A task like multidrizzle (I believe but have not used it) or mscimage figure out which pixels are needed for every output pixel based on the WCS and it is fine for the image rasters (the pixel coordinates) to not match. In this case what I meant was that if you used ds9 or ximtool (or other tasks) to query the celestial coordinate of a source, like a star, you get the RA and DEC to within some small error. Therefore, if you get the RA and DEC to be exactly the same for multiple images with small adjustments to the RA/DEC zeropoint, called the tangent point, the everything will align well in multidrizzle.
You may be thinking that you first need to resample each image to a common orientation and gridding and then call multidrizzle to stack them. But I believe that all this can be done in one step and all you need is the WCS.
If it is not the case that there is a good WCS already with the images then it is a much more complicated situation.
Frank
You may be thinking that you first need to resample each image to a common orientation and gridding and then call multidrizzle to stack them. But I believe that all this can be done in one step and all you need is the WCS.
If it is not the case that there is a good WCS already with the images then it is a much more complicated situation.
Frank
Last post on Sep 11, 2009