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Disagree With Focal Length

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Posted by Daniel Johnson on March 19, 2003 13:24:42 UTC

A 40mm eyepiece for a scope with focal ratio f/4.5 is a big mistake. It would give an exit pupil of almost 9 mm. Most human eyes cannot dilate that wide, and no human eye can give sharp focus at that dilation. Mind you, you can still see just fine with such an exit pupil, so you may have the impression that nothing is wrong, but you’re throwing away the light from a lot of your mirror—it just never enters your eye!
The maximum exit pupil worth considering is around 7 mm, and I personally stick with 6 mm for deep-sky views, since that central 6mmm of the eye focuses best and many eyes won’t reach 7mm except under pristine darkness. Anyone near or past retirement age may be limited to 5mm or less, since the eye loses dilation with age.
The exit pupil is the eyepiece focal length divided by the scope focal ratio. For a scope of f/4.5focal ratio, an eyepiece longer than 32mm is just a mistake, and I personally would stick with the 24-to26mm range for deep-sky, wide-field views.
Many manufacturers sell decent Plossl eyepieces. That design is a good all-around beginning choice, i.e. decent view quality and affordable. A set of eyepieces with focal lengths of about 24, 16, and 10 mm will serve you well for starters. Try Orion (at Telescope.com ). The best and most expensive eyepieces on the planet are sold by TeleVue. Their Panoptic 24mm (at about $400!) would be the ultimate for your scope if you want to stay with 1.25” eyepieces, and it will outperform most 2” eyepieces (except for the Nagler 2” eyepieces from TeleVue). The Radian, Panoptic, and Nagler lines sold by TeleVue, with prices ranging from $240 to perhaps $700, are spectacular. They will outperform Plossls, but Plossls are a good bang-for-the-buck choice.
If you ever get a Barlow, don’t get a cheap one. Spend the bucks—“bargain” Barlows are worthless. Celestron’s best Barlow is reasonable, as is TeleVue’s least expensive Barlow.

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