Post Reply 
 
Thread Rating:
  • 0 Votes - 0 Average
  • 1
  • 2
  • 3
  • 4
  • 5
Co-addition with proper velocity vector.
04-12-2008, 13:18
Post: #1
Co-addition with proper velocity vector.
I am attempting to coadd fits images in a way that will increase the signal of an object moving with respect to the star field, say an asteroid. So, if I know the proper motion velocity vector of the target object, I can add each subsequent fits image with the corresponding offset from the previous image - effectively smearing out the stars and brightening the moving object.

Is this possible in SWarp? I've looked through the documentation and it doesn't seem obvious to me.

Thanks in advance!
Find all posts by this user
Quote this message in a reply
04-13-2008, 19:04 (This post was last modified: 04-13-2008 19:07 by Emmanuel Bertin.)
Post: #2
RE: Co-addition with proper velocity vector.
This should not be too difficult. Once the input images have been astrometrically calibrated, the simplest is probably to create (or edit if they exist already) a .head file for each of them, where the CRVALi include the necessary shift and override the original ones. For instance, if the CRVAL1,CRVAL2 coordinates of the 2nd frame are (245.2345,-10.3456), and must be offset by (.0020,.0010), then the .head file should contain:
CRVAL1 = 245.2365
CRVAL2 = -10.3446
END

... and so on. Obviously one should not forget to include to apply a 1/cos(delta) factor in the RA component of the proper motion!
- Emmanuel.
Visit this user's website Find all posts by this user
Quote this message in a reply
04-13-2008, 19:11
Post: #3
RE: Co-addition with proper velocity vector.
Thanks. I will try that.

I was concerned that using this approach would get me into trouble when the offsets get too large (i.e. large proper motion vector). I feared this large "artificial shift" of the center pixel coordinate would cause the projection parameters to no longer be valid.
Find all posts by this user
Quote this message in a reply
04-13-2008, 20:12
Post: #4
RE: Co-addition with proper velocity vector.
If the images are astrometrically calibrated, the only thing you might fear for large offsets is a slight field rotation. What are the typical offsets?
- Emmanuel.
Visit this user's website Find all posts by this user
Quote this message in a reply
04-17-2008, 18:17
Post: #5
RE: Co-addition with proper velocity vector.
Thank you for your prompt and helpful responses.

It turns out to work very well. My proper motion vectors are small enough such that the plate rotation angle does not appear to be an issue (RA: -2.5e-06 deg/sec DEC:6.7e-07 deg/sec in 30 second exposures, stacked in sets of 90 images)


One more thing: is there a way to force the co-added output image to contain ONLY the region contained in ALL component images (as opposed to just being centered on that region)?

Thanks again!
Find all posts by this user
Quote this message in a reply
04-25-2008, 12:34
Post: #6
RE: Co-addition with proper velocity vector.
Quote:One more thing: is there a way to force the co-added output image to contain ONLY the region contained in ALL component images (as opposed to just being centered on that region)?

Unfortunately no. Sorry about that!
- Emmanuel.
Visit this user's website Find all posts by this user
Quote this message in a reply
Post Reply 


Forum Jump:


User(s) browsing this thread: 1 Guest(s)