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Because the Ethos has a built in Barlow, its reported field stop is only apparent.

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I wish people would not call the Smyth field flattener in some wider-field eyepieces a "built-in Barlow". Barlows act to increase the magnification of an eyepiece and may or may not help with flattening of the field. The negative Smyth lens set in the front of these eyepieces is an integral part of the eyepiece as a whole and cannot be removed without compromising the eyepiece performance. They are designed to compensate for the field curvature produced by a set of eyepiece optics that are correcting off-axis astigmatism. Also, these field stops for eyepieces such as these are *internal*, and the figures given by Tele Vue will result in true field figures which are dead-on accurate. Clear skies to you.

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David W. Knisely
Hyde Memorial Observatory
http://www.hydeobservatory.info
Prairie Astronomy Club
http://www.prairieastronomyclub.org

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