Hi,
I am trying to produce a noise map with one of our 5.5GHz continuum observations. There is a sfind.rms image produced after the miriad task sfind to detect sources, but that map just has a constant value over the whole image (for all rmsbox values I tried: 50, 1000, 5000, 10000, 23494); also, the sfind.log catalogue has a rms(bg) column with all values being the same, which are weird...
I am wondering if there's a problem with that noise estimate and is that the right way to produce a noise map? I'm thinking of the residual map, but that might be different with the noise map?
Thanks!
Tai-An
How to produce the noise map?
Moderator: Mark.Wieringa
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- ATCA Expert
- Posts: 297
- Joined: Mon Feb 08, 2010 1:37 pm
Re: How to produce the noise map?
Hi Tai- An,
I was hoping someone else would answer this one, as I've never run sfind before.
However when I ran it, with only 2 parameters set: sfind in=myimage options=rmsimg
I get an output list of 19 sources and an rms image that is not constant - there are peaks at the location of the strongest sources and small variations from block to block.
Make sure you are giving it the restored image (not a clean model).
If you're still having problems, post the task inputs so we can try to reproduce the problem.
Cheers,
Mark
I was hoping someone else would answer this one, as I've never run sfind before.
However when I ran it, with only 2 parameters set: sfind in=myimage options=rmsimg
I get an output list of 19 sources and an rms image that is not constant - there are peaks at the location of the strongest sources and small variations from block to block.
Make sure you are giving it the restored image (not a clean model).
If you're still having problems, post the task inputs so we can try to reproduce the problem.
Cheers,
Mark
Re: How to produce the noise map?
Hi Mark,
Thanks for the suggestion, It turns out that it's the "rmsbox" which I set it too large... But if I set rmsbox the default value (20 pixels), it works for one of our sources (G345) but not for the other (G018). For G018, miriad returns an error message "### Fatal Error: No convergence in GCF" at the beginning of the FDR analysis. Fortunately it work for rmsbox=40 and the resultant image is shown in the attachment (left is the restored image; right is the rms map). I wonder if there is any rule to decide the rmsbox value to input? There's this sentence from miriad manual but I found it confused to interpret practically: "It is suggested that this (rmsbox) be several to many times the beam size to prevent sources from artificially skewing the background estimates."
Thanks a lot!
Tai-An
P.S. The sfind task inputs that I use:
Task: sfind
in = (restored map)
type =
region =
xybin =
chan =
slev =
levs =
range = 0,0,log
cutoff =
rmsbox = 40
alpha =
xrms =
device = /xs
nxy =
labtyp = hms,dms
logfile =
options = normimg,rmsimg,fdrimg,mark,pbcorr,psfsize
csize =
Thanks for the suggestion, It turns out that it's the "rmsbox" which I set it too large... But if I set rmsbox the default value (20 pixels), it works for one of our sources (G345) but not for the other (G018). For G018, miriad returns an error message "### Fatal Error: No convergence in GCF" at the beginning of the FDR analysis. Fortunately it work for rmsbox=40 and the resultant image is shown in the attachment (left is the restored image; right is the rms map). I wonder if there is any rule to decide the rmsbox value to input? There's this sentence from miriad manual but I found it confused to interpret practically: "It is suggested that this (rmsbox) be several to many times the beam size to prevent sources from artificially skewing the background estimates."
Thanks a lot!
Tai-An
P.S. The sfind task inputs that I use:
Task: sfind
in = (restored map)
type =
region =
xybin =
chan =
slev =
levs =
range = 0,0,log
cutoff =
rmsbox = 40
alpha =
xrms =
device = /xs
nxy =
labtyp = hms,dms
logfile =
options = normimg,rmsimg,fdrimg,mark,pbcorr,psfsize
csize =
- Attachments
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- g018_noise_161215.jpeg (221.69 KiB) Viewed 7922 times
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- ATCA Expert
- Posts: 297
- Joined: Mon Feb 08, 2010 1:37 pm
Re: How to produce the noise map?
Hi Tai An,
great you got it working.
I think the GCF error is due to a failure to fit the background noise distribution - this may be due to a large source (compared to the rmsbox size).
I agree the instructions for setting the rmsbox size are a bit vague - I think that it is trying to find a balance between fitting variations in the noise across the field (rmsbox not too big) and not being affected by individual sources (rmsbox not too small).
Cheers,
Mark
great you got it working.
I think the GCF error is due to a failure to fit the background noise distribution - this may be due to a large source (compared to the rmsbox size).
I agree the instructions for setting the rmsbox size are a bit vague - I think that it is trying to find a balance between fitting variations in the noise across the field (rmsbox not too big) and not being affected by individual sources (rmsbox not too small).
Cheers,
Mark