One disadvantage to angled eyepieces for some people (such as myself), not
mentioned by Jack Stevens, involves watery eyes. If your eyes are prone to
water, looking downward tends to make the condition worse, and may make the
scope effectively useless at times, particularly on cold days; when looking
into the wind; or when your allergies are acting up. Just something to
keep in mind -- I learned the hard way.
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P W (Bill) Smith
Grays Harbor, Washington USA
birdman at techline.com