- Map size
- Make an image about twice the size of the primary beam (e.g.
\(2\times55''\)
at 90 GHz and
\(2\times22''\)
at 230 GHz for NOEMA antenna)
to ensure that all the area of the primary beam (inner quarter of the
dirty map) will be cleaned whatever the deconvolution algorithm is used.
However, avoid making a too large dirty image because the CLEAN
algorithms will then try to deconvolve region outside the primary beam
area where the noise dominates.
- Support
- Start your first deconvolution without any support to
avoid biasing your clean image. If the source is spatially bound, you
can define a support around the source and restart the deconvolution with
this a priori information. Be careful to check that there is no
low signal-to-noise extended structure that could contain a large
fraction of the source flux outside your support... Avoid defining a
support too close to the natural edges of your source. Indeed,
deconvolving noisy regions around your source is advisable because it
ensures that you do not bias your deconvolution too much.
- Stopping criterion
- Choose the right stopping criterion.
Use the stability
CLEAN_NKEEP parameter preferentially, combined
with a
CLEAN_STOP = r SIGMA with r between 1 or 2.
That keeps
CLEAN_ARES,
CLEAN_FRES and
CLEAN_NITER to zero. If it does not work, then
- Estimate an empirical noise on your first deconvolved cleaned image
with
STATISTIC,
CLEAN, or
SHOW NOISE.
- If this empirical noise value is similar to the value computed from the
visibility weights (this noise value is one of the outputs of the
UV_MAP command), your observation is not dynamic range limited.
Apart from using a user-specified support (
SUPPORT or
MASK)
there is not much you can do to improve your result,
- If not, you are dynamic range limited. You may use
CLEAN_STOP = r % where r depends on the dynamic range as stopping criterium. Alternatively,
you can select the effective noise level as the true Sigma,
CLEAN_STOP = r 'clean%rms' with r being 1 to 2. The problem in such cases is that the noise level may be channel
dependent, an issue that is not well handled.
- Convergence checks
- Ensure that your deconvolution converged by
checking that:
- The cumulative flux as a function of the number of clean component
has reached a roughly constant level (use
/FLUX option of the
deconvolution commands to see this curves, or
SHOW CCT or
VIEW CCT).
- The residuals are similar or smaller in the source region (where Clean
components were found) compared to elsewhere.
If not, change the values of the stopping criterion, whichever you used.
- Deconvolution methods
- If you want a robust result in all cases,
start with
HOGBOM. If you prefer obtaining a quick result, use
CLARK but you then first need to check that the dirty sidelobes are
not too large on the dirty beam. If you obtain stripes in your Clean
image:
- First check that your deconvolution converged.
- Then check that there are no spurious visibilities that should
be flagged : use command
UV_FLAG as a last resort.
- If it is clear that you have an extended source structure, you
should first ask yourself whether you are in the wide-field imaging
case and act accordingly (see next chapter). Else you can try a
CLEAN variant which better deals with cases that implies a large
spatial dynamic. This is rare at NOEMA, but may happen with ALMA.
- Outside help
- Always consult an expert until you become one.
