CFHT, Instruments, Imaging, AOB, PSF Reconstruction
The CFHT Adaptive Optics Bonnette :
The CFHT DPH2PSF package
The CFHT DPH2PSF package is intended for the PUEO observers who wish to
retrieve the spatial point spread function (PSF) associated with their
different objects. The package allows to fully compute the PSF and is
available either in IDL or in C language. For detailed instructions on
how to use this package, please read the documentation below.
The latest version of the package can always be obtained from the CFHT
- The IDL version of the package
consists of a single IDL procedure and we advise to use this one whenever
IDL is available at your site.
- Otherwise, you need to retrieve the C
language package. However, before compiling this package, you also need
to retrieve the NASA
FITSIO library, that handles the FITS input/output. Refer to the
accompanying README file for more information about this.
The following documentation apply for any of these two versions.
During a PUEO run, only the atmospheric corrected PSF is computed on-line.
This PSF includes all the atmospheric residual aberrations, but not the
non-common path aberrations, which mainly occur on the imagery channel,
past the AO beamsplitter. These aberrations are mostly of static nature but
their calibration is rather time-consuming and can not reasonnably carried
out during the run set-up. In addition, experience shows that these
aberrations may slightly vary with time, perhaps depending on the
temperature or on the direction of the pointing of the telescope. Also,
very often, one object is taken as several exposure which are then combined
for a better signal-to-noise ratio. Then there is advantage to combine the
data used to estimate the PSF and this can not be done easily on-line. For
these reasons, the PSF is not fully retrieved during the acquisition at the
telescope. This task is left to the observer as part of the data reduction
During a normal PUEO run, the PSF computation facility in the data
acquisition system allows to compute, for each exposure, the
atmspheric corrected PSF and to save the structure fonction of the
atmospheric corrected phase. For instance if the exposure is named
123456o.fits the related structure function is saved as
dph123456o.fits. These dph files must have been taken by
the observers because the PSF computation is based on their content.
Also during the run, the observer must have acquired some point source
(reference) images, at least on for each filter he/she wishes to make PSF
computations. These images are used to determine the non-common path
aberrations. Of course, more reference images are useful, knowing
that for optimal PSF reconstruction, the reference source
The last requirement may seem very stringent as one may think that
if one needs to image a point source often, there is no point in doing
complicated calculations to have the PSF. But it is not so: first even
a reference image, taken several nights before or after the object will
usually give a good result. Second, even if the observer is willing to
take a reference just before and/or after each of his/her object,
there is no need to make sure that the AO correction for the reference
is the same as for the object. Any bright star is good (but be
careful not to saturate the wavefront-sensor). In practice, we
recommand taking a reference source from time to time (say every hour),
especially if the seeing has changed significantly. Note that
a photometric standard for instance can be used as reference.
To summarize, in order to compute the PSF associated to an object, the
data required are:
- bright, to have a good signal-to-noise ratio image,
- bright again, so that the correction is good. The best is to
have a reference for which the correction is better than for the
object for which the user wishes to compute the PSF. However, this
is not a strict requirement.
- taken at a time as closed as close as possible to the object for
which the PSF is required.
- The dph files related to the acquisition of the object.
- One or several images of a point source (reference source),
taken with the same
filter as the object. See above for how to choose good reference
sources. These images must be:
- of size 128x128,
- fully reduced, that is dead-pixel, dark, flat and background corrected.
- The dph files related to the acquisition of each reference source.
When the data described in the previous section are available, the PSF can
be easily computed using the CFHT DPH2PSF package. The user must just
indicate the name of the different data files in a parameter file and run
the IDL or C program. For a given object, the parameter file must have the
xxxxxxx <- Name of the output file for the calculated PSF
xxxxxxx <- Name of the first dph file for the object
...... <- Name of other dph files for the object (optional)
* REFERENCE 1
xxxxxxx <- Name of the image of the first reference
xxxxxxx <- Name of the first dph file for the first reference
....... <- Name of other dph files for the first reference (optional)
* REFERENCE n
xxxxxxx <- Name of the image of the nth reference
xxxxxxx <- Name of the first dph file for the nth reference
....... <- Name of other dph files for the nth reference (optional)
Path informations can added to the name of the file.
The following example illustrates this structure:
* REFERENCE 1
* REFERENCE 2
In this example, we want to compute the PSF associated to the object taken
with exposures 970214_083, 970214_084, 970214_085 and save the result in
file psfP464_h_2.fits. The name of the output file is indicated
after the * PSFFILE keyword. The dph files associated with the
object are thus dph970214_083.fit, dph970214_084.fit and
dph970214_085.fit which must be given after the * OBJECT keyword
and followed by an empty line. To compute the PSF, we have selected two
references to insure a better precision (although one reference would have
been enough). The name of the FITS file containing the reduced image of the
first reference source is P300_h_c128.fits and must be given after
the * REFERENCE 1 keyword. The dph file related to the acquisition
of this reference source are then indicated (here
dph970214_052.fits, dph970214_053.fits and
dph970214_054.fits, followed by an empty line. The same
informations for the second reference source are given after de * REFERENCE
2 keyword: the reduced image is P361_h_c128.fits and the
associated dph files are dph970214_086.fits,
dph970214_087.fits and dph970214_088.fits.
When the parameter file is ready, the PSF computation is performed by
invoking the CFHT DPH2PSF package. For the example above, the command is:
- For the IDL program:
from the IDL command line.
- For the C language program:
from the Shell prompt.
Methodology and expected performance
From the dph files associated to the acquisition of the reference source,
the atmospheric corrected PSF of the reference source can be computed. By
comparing the latter to the actual image of the reference source, the
non-common path aberrations can be estimated and added to the atmospheric
corrected PSF of the object, computed from the dph files associated to the
acquisition of the object.
The computed PSF should be accurate when the magnitude of the guide
source is 13 or brighter. By guide source, we mean the source that was
used to guide the AO system for the correction during the object
acquisition. Usually, the guide source is the object itself or a star
close-by if the object is too faint or too extended. Further informations
can be obtained in reference .
Beside allowing the computation of PUEO's PSF, the dph files have
in their header different informations related to the acquisition,
including an estimation of the uncorrected atmospheric seeing.
- J.-P. Véran,
F. Rigaut, H. Maıtre, D. Rouan. Estimation of the adaptive optics
long exposure point spread function using control loop data. J. Opt.
Soc. Am. A, 14(11), 3057-3069, 1997.
Created 18 August 1998.
Page maintained by Jean-Luc Beuzit.
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