18 I The Celestron EdgeHD
The spot matrix shows that on-axis images are diffraction
limited at all three wavelengths, and remain diffraction-limited
over the central 15mm. While blue and red are slightly enlarged,
in green light images are fully diffraction-limited over a 38mm
image circle. The size of the off-axis blue and red spots remain
nicely balanced.
On a night of average seeing, stars will display a FWHM of
23µm, comparable in size to the spot pattern at the very edge
of a 42mm field. Relative illumination in the EdgeHD 925 is
excellent. The central 12mm is completely free of vignetting,
while field edges receive 90% relative illumination. For most
imaging applications, flat fielding would be optional.
For full-field imaging on a tight budget, the EdgeHD 925 is an
excellent choice. It offers nearperfect on-axis performance and
outstanding images over a full 42mm image circle.
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Celestron EdgeHD 925
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