Wide Field Eyepieces

Hi,

Thanks for your reply jeyjey, have a couple of questions - don't really understand why an f/4 scope is hard on wide field eyepieces? Is it because the scope would already deliver an wide field of view ? Would buying a regular 40mm or 56mm with 52* FOV be better than a wide field eyepiece with 70-80* FOV for my SNT?

Thanks

Neill

Hi Neill,
Your 26mm gives you (800mm/26mm) = 31 magnification.

This gives an exit pupil of (200mm/31) = 6.5mm.

All this means you are near the limit, if you go much longer in eyepiece focal length you will see the shadow (of an already fairly large) secondary.

You may be go a larger apparent field eyepiece , but they are expensive. Also (as mentioned in the previous post) edge details suffers in a f/4 (this is due to Coma, even in a perfect mirror).

An Coma Corrector (such as TeleVue Paracor) may help, but unfortunately this is further expense.

John

Hi,

Thanks for your reply jeyjey, have a couple of questions - don't really understand why an f/4 scope is hard on wide field eyepieces? Is it because the scope would already deliver an wide field of view ? Would buying a regular 40mm or 56mm with 52* FOV be better than a wide field eyepiece with 70-80* FOV for my SNT?

Thanks

Neill

Neill --

The "faster" or "shorter" a scope (ie: the lower the f/ number), the steeper the light cone is. For instance, an f/10 8" scope has a light cone 8" in diameter and 80" long. An 8" f/4 scope has a light cone 8" in diameter but only 32" long. Because the light cone is shorter (and steeper), the lenses in the eyepiece have to bend the light more, which generally requires more curvature in the lens. With more curvature comes a higher manufacturing cost (and much closer tolerances).

More curvature also means that the blue light is bent more than the red light (or is it the other way around?). Anyway, this is what produces chromatic abberation (or unwanted "color"). It can be reduced or even eliminated with more glass surfaces, exotic glass (ED, flourite, etc.), or air-spaced designs (allowing the facing surfaces of glass to have different curvatures). All of these things add cost and complexity to the eyepiece.

Many of these abberations vary linearly with the distance from the center of the field. In other words, the farther out in the field of view, the worse they become. These two reasons together are why fast scopes are brutal on inexpensive eyepieces.

John also has some good points about exit pupil. With an f/4 scope at my age (41), I wouldn't want to go much over 24mm (for a 6mm exit pupil). As I age further (and my iris becomes less flexible), I'll want to drop down to roughly a 5mm exit pupil (20mm in an f/4 scope).

Of course, these two goals are at odds: you can't use a long focal length Plossl because of exit pupil, and you don't want to use inexpensive wide-field eyepieces because of the fastness of your scope. I'd save up for a single, used, quality wide-field occular (a Panoptic if it were me -- but others swear by Pentax's). Barring that, I'd max out your exit pupil with a decent Plossl (say, a 26mm -- which you already have ).

-- Jeff.

Just to give an example,
At the WSP observing this year on the saturday night I tried my 26mm series 4000 meade plossel (52 degree) on Phil Lardners 20 inch Dob (F5 I think) and there was significant distortion in the outer half of the image.
I then swapped it for my 20mm type 2 nagler (84 degree) and the image was absolutely perfect (and amazing btw), like looking at a photograph.

Moral of the story, relatively cheap eyepieces might (and prababily won't) work well on a fast scope.

I got my sky at night mag for nov 05 last week there and there was a group test on wide angle eyepieces based on that - have ordered a 30mm Moonfish Ultrawide - it has 80* FOV - okay its only costing £60 and prob will not be as good as the very expensive ones like Naglers and some of the Meade 5000 series - but was never going to spend £300-400 - willing to take the chance on the Moonfish.

Haven't tried their eyepieces, but I ordered my Bob's Knobs from them. Great service -- fast, friendly, and they didn't whinge about shipping internationally (OK, they are in Spain, so I imagine most of their trade is international, but still).

-- Jeff.