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# Eyepieces : fast/slow, FOV ??? - Newbie question

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I'm after a wideangle eyepiece for my f10" f4.7 Dob. I've read a few posts which talk about scopes being fast and slow and have guessed that f4.7 is fairly fast. I understand that this has an effect on the size of eyepieces you can use before the edge starts vinagretting?? My scope will take 2" and 1.25" eyepieces.

Could someone please explain fast/slow and the impact on eyepieces.

I want an eyepiece with a wide FOV for searching for Messier objects and other DSOs. What is the maximum fov I could reasonably use, is 2" better than 1.25", what is the best size of eyepiece for this kind of thing 30mm, 40mm or bigger ?

Brian.

As requested

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The fast / slow bit refers to the focal ratio of the 'scope, the f/ number. Anything in the f/5 or f/6 is considered fast and f/10 or f/14 is considered slow. The difference WRT eyepieces is the angle of the cone of light that is entering the eyepiece.

If you consider a refractor (its easier to describe is all), the f/ ratio is the ratio between the diameter of the 'scope to the length of the 'scope. Specifically it's the aperture and focal length, but they're near enough the same for talking about it. An f/10 'scope will be ten times as long as it's diameter.

If the 'scope is f/5 you can see that the light cone from the lens to a point where the eyepiece is will be very much steeper than if the 'scope were an f/20 version. In the f/20 'scope the light has four times as far to travel along the 'scope before converging as compared to the f/5 'scope.

Eyepiece design becomes difficult / expensive when catering for a steep light cone so some eyepieces don't perform well with fast 'scopes and their steep light cones.

HTH

Captain Chaos

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What is the maximum fov I could reasonably use

60-70 degrees is the widest field the eye can take in without tilting the head to see the edges. 82 degrees are available and perhaps even 100 degrees. The wider the FOV, the more complex the eyepiece so quality drops unless you spend considerable money on the likes of Pentax and Televue. If you keep the FOV to a more manageable 60-70 degrees, good eyepieces are available for £50-£100.

is 2" better than 1.25"

Above approx 25mm, the 1.25" eyepiece barrel restricts the FOV so most 25mm+ focal lengths are supplied with 2" barrels.

what is the best size of eyepiece for this kind of thing 30mm, 40mm or bigger ?

I wouldn't be without a 30-40mm 2" wide-field eyepiece.

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So looking for a 2" widefield (within by budget) I came across the MoonFish Superview series where is says

This new Moonfish® SuperView Serie features a modern optical design that allows you to use them on "fast" telescopes - telescopes with a short focal ratio down to f/5 - with respectable field sharpness (but, please, keep in mind that the exit pupil does not get too large with the long focal length Moonfish® eyepieces, though. For example, at f/5 you may well use the Moonfish® SV32mm, but the Moonfish® SV42mm and SV50mm will render a not-quite-useful exit pupil of 8.4mm or 10mm, respectively. The exit pupil should never grow bigger than your own eyes' pupils can, otherwise you will waste light.)

I guess this recommends a 32mm, but I think that with a Dob I need a wider field to more easily find DSOs so I guess I would need lower magnification. This is where I get confused about magnification and FOV. If you use 2 lens with the same FOV but different magnifcation what does the eye see differently?

Going for a SV42 or SV50 would mean a would 'waste light' but would it be better? Would it mean having to tilt my head to see the whole FOV?

Another option is the Moonfish 32mm Wide Scan 70° 2" Eyepiece

Any Dob users our there with some tips?

Thanks again,

Brian.

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I've got the 80 deg and its very poor at f4.5 in my Dob, I'd be extremely surprised if it was anywhere near acceptable at f5 either....

I've got the 70 deg model on its way at the moment which is supposed to be better at short focal ratios, I'll do a side by side on the two eyepieces when I get the chance.

In answer to your question, 2 lenses with different magnifications you will see objects larger/ smaller. The FOV you will see is tied into the magnificationand the eyepeices apparent field of view (the AFOV). The AFOV of an eyepiece and the true field of view (TFOV) you see are not the same thing , which is where I think you are getting confused?

The formula is:

TFOV = AFOV/ Mag

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Comparing the views from eyepieces of the same focal-length but different FOV is like looking at an object through different size portholes: The object is the same size but you see more of the surrounding area through the larger portholes.

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I have a Moonfish Widescan 32mm / 70 Deg. It's "not bad" in an ST102 / F5 refractor. It does have it's limitations e.g. a fair bit of astigmatism? Mine has a slightly "dud" coating too <sigh> but this doesn't "effect the optical performance" as they say. But, having tried a couple of £60 eyepieces, with an F5 scope, I suspect such things are inevitable, unless you spend quite a bit more money. On an positive note the very flaws of such eyepeices are (kindof?) instructive!

At the rsik of repetition (I might learn something): With a 32mm 70 Deg 2" Eyepiece, whatever the scope, if it fits (2"), you cannot go too far wrong? With focal lengths greater than 32mm, you might e.g. (as the saying goes) "loose light". That said, this ARTICLE (by Nagler) may put this notion into context. More likely, the sky background will be a less dark with a 40mm. You may also start to "see" your secondary mirror too. Whatever... a 40mm / 70 Deg is about the max field for the 2" eyepiece. Some 50mm eyepecies might squeeze just a little bit more, but are perhaps more specialist, i.e. useful where the "framing" of a particular object is required.

So you're left with "something" in the range 30-40mm. If you have funds, it's not unreasonable to buy a typical 32mm / 70 Deg eyepiece of this type and price. Many of them are generic and, labeling apart, probably originate from the same (Chinese) factory anyway. Now, if my finances return to normaility, I'd be looking at upgrading to a (S/H) 35mm Pan or (maybe) TMB paragon. Beyond that (pricewise) the sky is the limit...

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