how does IRAF/daophot calculate offsets between images?
Karsten Brogaard wrote on Dec 08, 2006
Hi,
how does IRAF/daophot calculate offsets between images? Daophot uses a task called offset which is not present in IRAF/daophot. I found a manual saying that the equivalent in IRAF/daophot is pcalc but I do not see how I can use this task for that. Please help. Thanks in advance.
Karsten
how does IRAF/daophot calculate offsets between images? Daophot uses a task called offset which is not present in IRAF/daophot. I found a manual saying that the equivalent in IRAF/daophot is pcalc but I do not see how I can use this task for that. Please help. Thanks in advance.
Karsten
Mike Fitzpatrick wrote on Dec 08, 2006
Hi Karsten,
I've been avoiding your question since I don't know all that much about the photmetry tasks. However, PCALC is simply a utility for adding some offset to the positions in a table (the same function the OFFSET task does I believe). If you question then is how to find what this offset is between two images, there might be several methods to try. The XYXYMATCH task (see examples in help page) can be used to match pixel coordinates between two lists, e.g. the output of DAOFIND on to images can be matched to find the shift between those images using just the pixel positions by XYXYMATCH. Similarly, given an image with a WCS and a star position (several actually) you can compute the WCS position of the objects in each image and then crossmatch that way.
I'm sure this is a fairly common procedure and hopefully somebody who does this routinely can give a better answer. Hope this helps.
Cheers,
-Mike
I've been avoiding your question since I don't know all that much about the photmetry tasks. However, PCALC is simply a utility for adding some offset to the positions in a table (the same function the OFFSET task does I believe). If you question then is how to find what this offset is between two images, there might be several methods to try. The XYXYMATCH task (see examples in help page) can be used to match pixel coordinates between two lists, e.g. the output of DAOFIND on to images can be matched to find the shift between those images using just the pixel positions by XYXYMATCH. Similarly, given an image with a WCS and a star position (several actually) you can compute the WCS position of the objects in each image and then crossmatch that way.
I'm sure this is a fairly common procedure and hopefully somebody who does this routinely can give a better answer. Hope this helps.
Cheers,
-Mike
Karsten Brogaard wrote on Dec 08, 2006
Hi,
I found a task myself called imalign which does the job using a starlist from daofind.
Karsten
I found a task myself called imalign which does the job using a starlist from daofind.
Karsten
Last post on Dec 08, 2006