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Celestron EdgeHD 8" SCT
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The EdgeHD marks a milestone in commercial SCT development. In the standard SCT design, the primary and secondary mirrors are both spherical. As designed, the standard SCT has inherent off axis coma and a fairly steeply curved focal plane. To put these off axis aberrations in perspective, it is vital to understand that this is a VERY old design. The original formula was decided in an era when 1.25” eyepieces were JUST coming into mainstream. Prior to the first commercial SCTs, many telescopes were still equipped with .965 eyepieces, and a “Wide Field” eyepiece was usually a 15mm or 20mm Erfle with 60 to 65 degrees of apparent field, or a 32mm Plossl with a 50 degree apparent field.
The designer(s) rightfully fixed the design to be optimized for these 1.25” eyepieces and when used with high quality eyepieces of this type, the coma and field curvature of the standard SCT design is a VERY acceptable tradeoff to the goals of having a compact, light, and affordable, but large aperture telescope.
What has changed though is EVERYTHING! I mean it has been 40 YEARS since Celestron first introduced the C8 that we know and love. THINK about ALL of the changes in technology that have occurred in 40 YEARS! How about the PC, the Internet, Antilock brakes, and GIANT FREAKING 2” WIDEFIELD EYEPIECES and 50Bigabit CCD Cameras! Oh my, the world really as changed.
But though that all, the SCT remained exactly what it was. Yes, there were minor tweaks and at even some special improvements, but mostly, the design stayed unchanged. Only in recent years has the combination of ultra-wide eyepieces and giant megapixel cameras exposed the underlying weakness of the old design. It was a 1.25” telescope living in a 2” world. And folk were starting to notice.
Meade stepped up to the bat first with its ACF design. The ACF design addressed the issue of off axis coma. Now the coma in the SCT design is not severe, however it is not presented as the only aberration. In the SCT, the coma is MIXED with field curvature. If you have an raw comatic blur, it is not always easy to see. MOST of the light from a comatic blur is concentrated in the head of the “Comet” and only on bright stars will the comatic tail have enough energy to really become obnoxious. The problem however is that the coma in the SCT design was NOT presented as a pure, stand alone aberration. The comatic blur was also presented to the observer in a defocused blur, which expanded the head of the comet to the point that the brighter parts of the “V” shaped tail very close to the central part of the comatic blur suddenly became enlarged angularly to the point that it could be EASILY RESOLVED. Throw in your Uber-wide field 2” eyepiece and OH, things could get ugly fast. A bright star at the edge of a 31mm T5 Nagler when views though an SCT could be a Comet Halley experience!
The Meade ACF addressed a BIG part of this problem. In the ACF design, the scope was far better corrected for coma. The result was that now, when you put the bright star at the edge of the field of that same 31mm Nagler, even though the star may not be in perfect focus due to the field curvature of the scope, the unfocused blur remains ROUND. Even though it might actually be resolved as a tiny slightly out of focus image on a brighter star, the absence of the comatic tail gives a MUCH more pleasing round blur rather than the spikey blur of a comatic tail! Still, while the ACF did improve the design, the field curvature remained.
To be fair, younger observers could also apply their visual accommodation to minimize the effects of the curved focal plane of the ACF design. To accommodate the field visually, the observer focuses the telescope on a star near the outside edge of the field. Now, when they look back at a star in the center of the field, their eye will attempt to bring the center into perfect focus. If the field curvature is minor and the observers accommodation is good, an ACF scope can indeed provide a very sharp across the field performance. Still, with the proliferation of 82 degree apparent field eyepieces and their much higher magnification as compared to narrower field eyepieces, even young observers were often at the limits of what they could COMFORTABLY accommodate. Accommodation introduces eye strain if you push yourself, and eyestrain is the LAST thing that a dedicated observer wants to deal with in a long observing session.
The CCD chip is even less forgiving. It cannot perform ANY accommodation. If the field is curved, stars become progressively defocused the further you go off axis, and the effect is to limit the size of the CCD chip that can be used unless one chooses to employ field flatteners. And hey, this is a GREAT way to get a flat field for someone that likes to image. In addition, the flattener could be left on the telescope for visual observers that could not fully accommodate the curvature of the native focal length.
The marketing challenge though is finding a solution for the growing number of aging visual observers using high power wide field eyepieces and becoming more aware of both the off axis coma and field curvature, and also finding a solution for the imagers that wanted to image small targets where a long native focal length and serious aperture become your friends.
Celestron’s response was the EdgeHD. The EdgeHD design incorporates a sub aperture field corrector that both reduces coma and flattens the field. With the EdgeHD, both the needs of the high power ultra-wide field eyepiece observers and the imagers wanting to go after small targets with long focal lengths have telescope that presents a field that is well enough corrected for both coma and field curvature that even the most demanding visual observers using the widest field eyepieces and the dedicated imager using the now common larger format CCD cameras to be happy with the SCT.
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