CFHT, Instruments, Imaging, AOB, PSF Reconstruction

The CFHT Adaptive Optics Bonnette :
PSF Reconstruction


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 web site:

Documentation

The following documentation apply for any of these two versions.

Introduction

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 process.

Data acquisition

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 must be: 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:

PSF computation

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 following structure:
* PSFFILE
xxxxxxx        <- Name of the output file for the calculated PSF
 
* OBJECT
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:
* PSFFILE
psfP464_h_2.fits

* OBJECT
dph970214_083.fits
dph970214_084.fits
dph970214_085.fits

* REFERENCE 1
P300_h_c128.fits
dph970214_052.fits
dph970214_053.fits
dph970214_054.fits

* REFERENCE 2
P361_h_c128.fits
dph970214_086.fits
dph970214_087.fits
dph970214_088.fits
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:

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 [1].

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.

References

1
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.
Please send comments to : beuzit@cfht.hawaii.edu