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merlin100

Sky-Watcher 28mm 2" Long Eye Relief - Thoughts?

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I see that FLO is selling a Sky-Watcher 28mm 2" Long Eye Relief for £29. Would this be suitable for my Sky-Watcher Skyliner 200P as a wide angle lens?

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Posted (edited)
1 hour ago, John said:

That eyepiece has a 56 degree apparent field of view according to the spec.

It's not going to show any more sky than a decent 1.25 inch 32mm plossl such as the Revelation does, a touch less in fact.

https://www.telescopehouse.com/revelation-32-0mm-plossl-eyepiece-1-25.html

 

 

Thanks John, I wasn't sure. I thought it might make a reasonable first 2 inch lens...   I note in the description elsewhere, that it's a Kellner. I know they're not great in the scheme of things, but being a 2 inch - I didn't think it really mattered.🤔

Edited by merlin100

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You get the advantages of the 2 inch barrel format when the field stop (the internal ring defining the edges of the view) is larger than it can be within the confines of the 1.25" eyepiece barrel.

To use a larger field stop though the optical design needs to be relatively well corrected across the wide field that is created. The Kellner design is limited in this respect which is why they have not pushed the field stop diameter out as far as it can go within the 2 inch format.

In the 28mm focal length you can get an 80+ degree field of view if a large field stop is used but an optical design that can provide a reasonably well corrected image (ie: stars continue to look star shaped) over that view is required and that costs money to design and implement so such eyepieces cost more for us to buy.

The scope that the eyepiece is going to be used in also affects the degree to which the quality of the view is maintained across the field of view. "Fast" scopes (eg: F/4, F/5, F/6) are more demanding on eyepiece design than "slower" ones (eg: F/8, F/10 etc).

 

 

 

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18 minutes ago, John said:

You get the advantages of the 2 inch barrel format when the field stop (the internal ring defining the edges of the view) is larger than it can be within the confines of the 1.25" eyepiece barrel.

To use a larger field stop though the optical design needs to be relatively well corrected across the wide field that is created. The Kellner design is limited in this respect which is why they have not pushed the field stop diameter out as far as it can go within the 2 inch format.

In the 28mm focal length you can get an 80+ degree field of view if a large field stop is used but an optical design that can provide a reasonably well corrected image (ie: stars continue to look star shaped) over that view is required and that costs money to design and implement so such eyepieces cost more for us to buy.

The scope that the eyepiece is going to be used in also affects the degree to which the quality of the view is maintained across the field of view. "Fast" scopes (eg: F/4, F/5, F/6) are more demanding on eyepiece design than "slower" ones (eg: F/8, F/10 etc).

 

 

 

Thanks once again, John.  I think I'll scrub this one off my list...

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A good introductory 2 inch eyepiece would be the Skywatcher Panaview 32mm. You can pick those up for around £50 on the used market.

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Posted (edited)

I have one. (paid £7.50GBP from a seller on UKAB&S). Having the twist-up eyegauard is why I wanted it... and find out what the hype is about with 2" - not to bad for an entry level 2" e/p... though there are better ones out there.

Edited by Philip R
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