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40mm Plossl

Kain

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Kain

Kain

Posted
Posted

I have been offered a brand new 40mm plossl for £8 off a mate who bought it then realised he had already got one lol.

Are they worth it?

It will probably be more useful on the 10inch rather than the 4.5..

What do you think?

Kain

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Albireo380

Albireo380

Posted
Posted

I guess that the quality of the Eyepiece will depend on who made it. Is there a name on it, or is no one owning up to it?

Having said that, for £8 - does it really matter? Buy it, give it a try and if it is no good, then it has only cost you the equivalent of 3 pints of beer :)

Tom

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Ambermile

Ambermile

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Having said that, for £8 - does it really matter? Buy it, give it a try and if it is no good, then it has only cost you the equivalent of 3 pints of beer :)

Tom

Or eight cans of Grolsch :) :occasion5:

Get it - you're not going to lose and it's only the higher-powered EP's you have to watch out for in faster scopes.

Arthur

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John

John

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Posted

Hi Kain,

Sounds like it may be an Antares plossl eyepiece in which case it's probably good - Antares stuff is usually very competatively priced but works well.

John,

North Somerset

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ric_capucho

ric_capucho

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There's something about the plossl design that means that afov of a 1.25" plossl starts to decrease inherently above 32mm. We've never offered plossls, so I never got to the bottom of it, but it's there.

Check out the Celestron/Meade/Tele Vue plossls, and you'll see that the 'standard' afov of about 50deg for the plossl range suddenly drops down to 43deg for the 40mm version.

I tested against the Televue eyepiece calculator (on their website) and came up with the following: For a 200mm f/10 SCT, the 40mm gives yer 50x and 0.8deg tfov, while the 32mm gives yer 62.5x and 0.8deg tfov. With a f/6.8 80mm refractor, yer get 13.8x and 2.8deg tfov with the 40mm, and 17.2x and 2.8deg tfov with the 32mm. Erm, the tfov's are the same, so yer better off with the slightly higher magnification of the 32mm 'cos it gives yer a darker background.

Of course, a cut price 40mm is also worth having, so if it's a half-decent brand, and it's been looked after, then bite his hand off.

Ric

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John

John

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I think it's because the limiting factor to the field of view of an eyepiece is the diameter of the field stop, which is in turn limited by the internal diameter of the eyepiece barrel. So the widest field stop a 31.7mm eyepiece can have is about 28mm (basically the diameter of the inside of the barrel) which with a 40mm focal length equates to around 43 degrees FoV. This is also why 2 inch eyepeices can have wider fields of view.

Vixen used to make a 45mm plossl in the 31.7mm fitting but the FoV was limited to around 36 degrees - that really was like looking down a narrow tube.

Someone please correct me if I've got this wrong.

John,

North Somerset

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John

John

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Posted

Gaz,

Yes you did - and you did it more concisely ! - I think it was Churchill who once wrote "..sorry for the long letter, I did not have time to write a short one...".

Regardless of all that - £8 for a 40mm plossl has got to be worth a shout.

John,

North Somerset

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