11.1.2 Angular Resolution

The primary HIPO detectors are e2v CCD47-20 1024x1024 pixel frame transfer CCDs with plate scales of 0.33''x0.33'' pixels at low resolution and 0.05''x0.05'' pixels at high resolution. The HIPO field of view (FOV) is a 5.6' square, the 8' diagonal of which corresponds to the 8' diameter SOFIA FOV. Pixels will normally be binned to best match the seeing blur size and to reduce the effect of read noise. The high resolution mode includes no re-imaging optics. It is possible to replace one or both of the CCD47's with CCD67's having half the field of view, twice the pixel size, and much faster imaging operation.

The HIPO image quality is dominated by seeing and image motion effects. The red curve in Figure 11-1 is the nominal image quality expected at first light for SOFIA, based on the expected shear layer seeing, the as-built optical performance, and 2'' rms image motion. The blue curve represents the ultimate combined optical quality and image motion requirement (80% encircled energy in a 1.6'' diameter circle) convolved with the expected shear layer seeing. Also plotted are representative photometry aperture diameters likely to be used for processing occultation frames under both conditions described above. The image motion assumed is larger than will be experienced when observing at high frame rates. Occultation photometry will be extracted from data frames using effective aperture sizes comparable to the 80% enclosed light diameter plotted here.

HIPO FWHM beam diameter as a function of wavelength

Figure 11-1: Plotted here is the expected instrument FWHM beam diameter as a function of wavelength.

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