HST ACS weight file and mask file?
fornax wrote on Dec 01, 2009
First, I am not an observer, hence may not know common(trivial) tasks in Iraf.
I am trying to perform isophotal analysis using the IRAF task "ellipse" on ACS WFC images (that have NOT been dithered but have been drizzled). The resulting file is a multi-extension fits file [MEF] that has a science image(SCI), a weight image(WHT) and a context image (CTX).
Due to the gap, ellipse gives erratic results when the semi-major axis falls on the gap between the 2 WFC chips.
Is there a solution to this?
I thought, the weight map (WHT in the MEF) can be used to mask out the "bad pixels" which includes the "gap". However the WHT file is not made of 0's and 1's. Consequently, ellipse fails when I provide this WHT file in the parameter field for "dqf" in ellipse.
Alternatively, one can create a .pl (pixel mask) file. But how does one create that from the WHT (weight) map? The iraf task ellipse also says that if a .pl file is used (in an automated run) then the image has to have a 3-letter file extension. FITS is 4-letter.
http://stsdas.stsci.edu/cgi-bin/gethelp.cgi?ellipse
[Scroll down to the section "Bad Pixel flagging can be....."]
I also think merely blanking out the pixels will not help. Instead for "ellipse" to work properly some sort of interpolation has to be done across the "gap".
Please give me some inputs.
Thanks for your help!
PS: I am using a 3-sigma clipping, but the problem arises when ellipse selects the first point on an isophote (i.e. progresses the semimajor axis) on the gap. It probably finds all neighbouring points as bad pixels and throws out a "sector" or so. Whatever it does, it "rotates" the ellipse [i.e. with some unrealistic PA and ellipticity] and this effect stays for the rest of the frame past the gap.
If it helps. Here is the ACS handbook:
http://www.stsci.edu/hst/HST_overview/documents/datahandbook/v7/hstdh_introTOC.html
I am trying to perform isophotal analysis using the IRAF task "ellipse" on ACS WFC images (that have NOT been dithered but have been drizzled). The resulting file is a multi-extension fits file [MEF] that has a science image(SCI), a weight image(WHT) and a context image (CTX).
Due to the gap, ellipse gives erratic results when the semi-major axis falls on the gap between the 2 WFC chips.
Is there a solution to this?
I thought, the weight map (WHT in the MEF) can be used to mask out the "bad pixels" which includes the "gap". However the WHT file is not made of 0's and 1's. Consequently, ellipse fails when I provide this WHT file in the parameter field for "dqf" in ellipse.
Alternatively, one can create a .pl (pixel mask) file. But how does one create that from the WHT (weight) map? The iraf task ellipse also says that if a .pl file is used (in an automated run) then the image has to have a 3-letter file extension. FITS is 4-letter.
http://stsdas.stsci.edu/cgi-bin/gethelp.cgi?ellipse
[Scroll down to the section "Bad Pixel flagging can be....."]
I also think merely blanking out the pixels will not help. Instead for "ellipse" to work properly some sort of interpolation has to be done across the "gap".
Please give me some inputs.
Thanks for your help!
PS: I am using a 3-sigma clipping, but the problem arises when ellipse selects the first point on an isophote (i.e. progresses the semimajor axis) on the gap. It probably finds all neighbouring points as bad pixels and throws out a "sector" or so. Whatever it does, it "rotates" the ellipse [i.e. with some unrealistic PA and ellipticity] and this effect stays for the rest of the frame past the gap.
If it helps. Here is the ACS handbook:
http://www.stsci.edu/hst/HST_overview/documents/datahandbook/v7/hstdh_introTOC.html
Mike Fitzpatrick wrote on Dec 01, 2009
I hope you'll get a reply here from a knowledgeable user, but I think your best bet may be the Help Desk at STScI (help@stsci.edu) since ELLIPSE is their task and they know better how to deal with ACS data.
-Mike
-Mike
Last post on Dec 01, 2009