Can anyone tell me the Eye Relief on the supplied SW 10mm?

[Eastridge]

Hi,

I'm looking at getting a c. 15mm EP and would like the 15mm TV possl but concerned re ER as I observe with glasses. I know that I can view comfortably thru the 10mm EP supplied with our Skyliner Dob, I also have read that although it's badged 'super possl' it's really another design - archromatic I think.

Anyway, I am thinking that if I know what the ER on that is I can use it as my 'baseline' and that may extend the range of EP's I can choose from.

Can anyone help please?

Thanks.

[Capricorn]

The standard eyepiece is a plossl and as a fair guide the eye relief on a plossl is something like 2/3 of the focal length of the eyepiece, 60-65%. So longer ones eg 15mm give more eye relief.

On your 10mm it will be in the region of 6 to 6.5mm, the 15 will be around 10mm eye relief.

One point to remember is that this is not from the top surface of the top lens, it will be defined as from the optical centre of the lens and that could be 1 or 1.5mm below the top face. So the reality is you get about 5mm from the top surface of the lens, for that one.

[Astro Imp]

The standard eyepiece is a plossl and as a fair guide the eye relief on a plossl is something like 2/3 of the focal length of the eyepiece, 60-65%. So longer ones eg 15mm give more eye relief.

On your 10mm it will be in the region of 6 to 6.5mm, the 15 will be around 10mm eye relief.

One point to remember is that this is not from the top surface of the top lens, it will be defined as from the optical centre of the lens and that could be 1 or 1.5mm below the top face. So the reality is you get about 5mm from the top surface of the lens, for that one.

I think the standard 10mm SW eyepiece gives considerably more eye relief than 5/6mm. I observe with glasses and like the OP do not have a problem with this eyepiece but would doubt this would be possible with only 5/6mm. Unfortunately I cannot answer the OP question.

[Alfian]

Hi

I found the standard 10mm EP that I had with my celestron tiring to use because of the lack of eyerelief and bought a 9mm SWA to replace because of the increased eye relief. My 15mm Vixen NPL plossl is much better and has a quoted eye relief of 11mm which I can comfortably use with my specs.

Incidentally the Vixen 10mm plossl (which will not be so dissimilar from most standard plossls)) is given as 6.5mm. (FLO)

[Eastridge]

Thanks guys. If it's 5/6mm I'm stunned (but in a good way) as that's way below what people seem to say you can use with glasses, but that would be great.

As we seem to have disagreement between 2 very experienced Advanced Members can anyone else chip in with a definitive answer?

[Moonshane]

Plossls usually have eye relief of 0.8x the focal length so I'd expect it to be 8mm.

[rwilkey]

The sw ep is not a plossl but an MA (modified acromat) so don't judge the eye relief by the plossl design, the eye relief on the 'super MA' is actually better, I can only guess from my experience, I think it is around 10-12mm, so if you look for something with this type of eye relief I think you will find what you want. BST Explorers/StarGuiders have this amount of eye relief as a matter of interest, so something like these, esp. with a wide eye lens I am sure you would find suitable.

[Moonshane]

ahh, sorry I thought it was a plossl. I can confirm the eye relief is more on the 15mm TV plossl as I have used both.

[Eastridge]

Thanks Robin - that makes sense. I had discounted the BST e/SG until now but it would save us a lot of money if I can use those & if TV possls in the mid range are suitable as well that's a bonus.

[rwilkey]

The TV Plossl's certainly have acceptable, if not good eye relief in the mid range and more than adequate eye relief in the lower powrs (32mm TV) for example. The TV Plossl's are also incredibly sharp and give exceptional contrast.

[Naemeth]

I've used a rule of thumb of 16mm for eye relief, but like you, I can actually use the standard SW 10mm, not that I like to though, it's not a particularly nice eyepiece, and doesn't barlow very well. I think the reason why the eye relief on the 10mm MA is usable is because the image is formed so close to the lens you look through, and 8mm or so would easily be enough for glasses.

Using the 16mm ER rule of thumb, the shortest FL Plössl you can use is a 20mm .