EdgeHD is an aplanatic Schmidt telescope designed to produce
aberration-free images across a wide visual and photographic field of
view. The optical system was designed to reduce more than just off-axis
star coma, but also to give an astrograph quality flat focal plane all
the way to the edge of the field of view.
True Astrograph Quality
Many optical designs that advertise themselves as "astrograph" quality
actually only produce pinpoint stars across a curved focal plane. While
this may be acceptable for visual observing, stars will appear out of
focus at the edge when used with a flat chip sensor of a digital
camera. EdgeHD optics produce a focal plane more than three-times
flatter than a standard Schmidt Cassegrain telescope and dramatically
flatter than other competing coma-free designs. This guarantees you
visibly sharp stars across some of the largest ccd chips available
today.
Above: A curved focal plane does not perfectly coincide with a flat CCD chip and therefore will appear out of focus at the edges.
Even when focused sharply in the center of the field of view, a curved
focal plane will produce out of focus stars at the edges. The wider the
field of view the more pronounced the defect becomes. Compare the two
images below. Both images were taken with the same camera under the
same conditions. When focused on-axis both appear to be sharp in the
center of the image. But as you view the stars closer and closer to the
edge, the stars in the first image (taken with a competing coma-free
design) become larger and less focused until finally, at the very
corners, the star images are de-focused so much that they appear as
rings instead of pinpoints.
Competing Coma Free Optical Design
Celestron EdgeHD Optics
Fig
1 - Competing Coma Free design focused on axis, shows sharp stars in
the center of the image, but dramatically out of focus stars (donuts)
at the edges.
Fig 2 - EdgeHD focused on axis, focuses sharply across the entire FOV.
Both M3 images taken with a 35 mm chip (42 mm diameter focal plane) camera
Improved Performance
Superior edge performance not only creates rounder, more pleasing stars
but actually improves the resolution and limiting magnitude when
compared to telescopes of equal aperture.
Celestron's High Definition optics let you resolve
smaller spot sizes all the way to outer edge, revealing finer detail
and greater contrast.
Fig
1 - Off-axis Coma inherent in many optical design, (such as SCT's shown
above) distort star images and compromise resolution.
Fig 2 - EdgeHD focus sharply across the entire FOV revealing fine detail lost with other systems.
Both images cropped down and taken with a 37 mm chip (52 mm diameter focal plane).
Brighter Images
EdgeHD optics give you smaller (more concentrated) stars that create
brighter images and allow you to see down to a fainter magnitude than
other equally sized telescopes. Poor edge quality can spread out
starlight so much that the brightness of a star appears the same as the
sky background, making it undetectable to your eye (or camera). With
EdgeHD optics you will be able to capture the faintest of galaxies all
the way to the corner of your full frame camera chip.
Fig 1 - Faint starlight can go unnoticed at the edge of field of telescopes that exhibit off-axis coma.
Fig 2 - EdgeHD optics can resolve faint detail missed in telescopes of equal aperture.
Both images cropped down and taken with a 37 mm chip (52 mm diameter focal plane).
