Image Quality for the Gemini Telescopes


The Gemini telescopes will achieve near diffraction-limited images at 2.2µm and beyond, with minimal degradation of the best seeing conditions at shorter wavelengths. The telescope optics are compatible with adaptive optics. A graph of image diameter versus encircled energy illustrates the wide-field optical and IR image specification for an 8-m telescope.

This graph illustrates the possibilities using no compensation, tip/tilt correction, and adaptive optics.

Image Quality Diagram

For adaptive optics, image quality is quantified in terms of two profile independent parameters, the Strehl width and the Strehl ratio. The Strehl width is the diameter of a cylinder, which is the same height as the peak intensity of the delivered image, with a volume equal to that contained in 100% of the delivered point spread function (PSF). The Strehl width has units of arcseconds in the focal plane. The Strehl ratio is the ratio of the central intensity of delivered image to that of the central intensity of a perfect, diffraction-limited image.

Factors to consider for image quality are:

Recent advances in polishing and testing techniques, combined with the advent of active and adaptive optics techniques, are what makes it possible to specify near-diffraction-limited imaging at 2.2µm with the Gemini telescopes. It is also now possible to establish a quantitative relation between the results of different optical testing techniques and the final image delivered by the telescopes.

High-Angular Resolution Configuration

The intention is to achieve near-diffraction-limited images at infrared wavelengths for a field of view limited by the correlation lengths of atmospheric distortions. At optical wavelengths, the intention is to have the image quality be limited by the external seeing.

Wide-Field Configuration

Image performance goals have been discussed at length, particularly for this case. The outcome was that the wide-field optical system should have 0.25 arcsec (FWHM) image quality. This is to be achieved over a 45 arcmin field within 20 degrees of the zenith. Away from the zenith, the system performance may follow the effects of atmospheric seeing.


To obtain the best image quality, the free atmosphere seeing reaches a FWHM of 0.2 arcseconds at V and 0.12 arcseconds at K with wavefront tilt correction on excellent sites like Mauna Kea and Cerro Pachon.

The requirements are:

The Gemini approach to the above is:


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Ruth A. Kneale / web@gemini.edu / January 2, 1998