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Current Michelle PSF structure

Current Michelle PSF Structure

This page illustrates the observed Michelle PSF structure in good seeing conditions, using as an example an image of a standard star taken in the 12.5 micron Si-6 filter. This is a reasonable guide to the expected PSF now that the chop tail problem has been fixed thanks to work by Chris Carter and Matt Rippa along with others of the engineering staff. This image was taken in better than average seeing conditions, but should be representative of images taken in the better part of IQ=70% conditions in the optical.

The following images show:

  • the PSF displayed on a linear scale with IRAF
  • the PSF displayed on a logarithmic scale with IRAF, to show the low-level structure
  • an annotated version of the logarithmic scale image pointing out the diffraction rings
  • an annotated version of the linear scale image
  • a screen shot of the radial profile as determined in IRAF, scaled to show the faint PSF structure
  • and finally the radial profile plot with the various features marked as in the annotated images

The image has a full width at half maximum of 3.58 pixels = 0.36 arc-seconds according to IRAF. The core is slightly elongated. Fitting a two-dimensional gaussian to the core gives a size of 0.40 by 0.36 arc-seconds with a ellipticity of 0.09 and a position angle of 2 degrees east of north. These values are slightly different than what is given by IRAF; in particular the ellipticity from the 2-D gaussian fit to the core is smaller than what is reported in IRAF and the net FWHM, taken to be the square root of the product of the FWHM values along the two axes, is 0.388 arc-seconds. The estimated strehl of this image is 0.6. Under the best seeing conditions Michelle sometimes produces strehls of over 0.75 in this filter.

Outside the core one has several diffraction rings. The first of these is not an actual ring, but appears to be trefoil and possibly some coma. Outside this first ring there are three more rings as shown in the images and plots above.

The outer structure can be seen a bit better in the residual image obtained by subtracting the best fit 2-D gaussian from the original image. That is shown just below.

There is usually trefoil in the Michelle images. Low-level astigmatism and possibly coma may be seen in some instances.

Now that the chop tails are gone our goal is to have ellipticities 1 - b/a of less than 0.1. Larger ellipticities than this are grounds for an image to be set to "Usable" rather than "Pass", unless the program is only concerned with photometry and not image structure. In some instances, as in the example above, it appears that the IRAF ellipticity value is inaccurate.

In conditions of high wind we still sometimes observe a very low-level elongation along the elevation direction in some Michelle images. Programs that need the best PSF structure should request that their observations be made in lower wind conditions to avoid any problems of this sort.