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cantab

Celestron Omni Plossl 6mm - eye relief problem

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Tonight was the first clear night since I received my 6mm Celestron Omni Plossl, and I tried it out in the Heritage 76. I was immediately disappointed, the eye relief is appalling. I knew it wouldn't be generous, I did not expect it to be worse than my SR4! I'm unable to see the whole field even with the eyecup down and my eyelashes jammed into the eyepiece. (By contrast with the SR4 I can see the full, admittedly narrow, field with my eyelashes only just brushing the EP.)

The views of Jupiter and M35 were fine, and the build quality clearly a step up from the various stock EPs I have, but the eye relief makes it unusable.

Is this normal for a 6mm Plossl in general or the Omni Plossl in particular, or is it possible I've got a lemon? I did buy it from Curry's (maybe there's where I went wrong), so I can assume it won't have been checked at all before it was sent.

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If the ER is 5mm, then it's among the most generous one among the plössls, usually plössls has 0.7x to 0.75x of focal length, i.e.4.2mm to 4.5mm for a 6mm plössl.

For inexpensive short focal eyepieces with good performance, HR planetary (TMB clones) have many different focal length with comfortable 16mm ER.

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Yes that's pretty typical -  anything under about a 10 plossl starts to get very tight.  That's why BST and TMB planetary etc are very popular at this focal length.

andrew

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For a plossl the figure appears to be 0.7xFocal length. So for a 6mm that means just over 4mm.

Add in that the measurement is not from the top of the lens but from the optical centre then a bit less.

TV stop doing plossl's at 8mm and I suspect that the eye relief is a factor in that decision.

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I bought the same ep (along with the 15mm equivalent) as my first upgrades from the stock SW 10mm and 25mm. I was VERY disappointed. Like you I found the eye-relief hopeless. I have never used it again so it was a waste of money.

I should have heeded the advice on this forum and paid more for a better eye-piece. I have since bought the 8mm BST Starguider and the WO SPL 6mm to cover that range ... And there is no comparison; they are SO much better.

The moral for me is that I should have taken advice and spent a bit more to get a more usable result.

It's also a pity as the Omni plossls are nicely made. But the 6mm spoilt my evening's viewing. The 15mm wasn't bad though.

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I had another look at the 6mm Omni Plossl, and noticed the eye lens is heavily recessed, must be a good 2 or 3 mm below the top of the eyepiece. None of my other eyepieces have the lenses anything like that.

That would certainly account for the viewing problems, but if it's meant to be that way then I find it a shocking design decision by Celestron.

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I have a vintage Meade MC 6 mm ortho eyepiece which is supposed to be good, but it is unusable for me for the very same reason: I can hardly see all the field by putting my eye ball almost inside the eyepiece, so I don't use it any more. The modern planetary eyepieces are much better in this regard (although perhaps not in sharpness, I don't know)

Javier

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I dislike Plossls below about 20mm due to eye relief (I wear glasses). I got rid of the (good quality) 10mm Plossl that I bought with the scope fairly quickly, and replaced it with a Vixen LV 9mm (20mm eye relief). By contrast, I only replaced the 26mm Plossl that came with the scope when I got the 22mm T4 Nagler. The same holds for Orthos. The 5mm I used to have was my least-used EP in my old F/8 scope, not due to lack of quality, but simply due to eye relief. The 25mm I had was my most liked EP in the same scope. My kids have a 6.3mm Plossl and my eldest now wears glasses so simply cannot use it, and even my youngest (without glasses) found it uncomfortable. I got a 4mm planetary EP with 58 deg FOV and 16mm eye relief, and they are well pleased.

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In the end I wrote off my loss and got a 6mm BST from skies_unlimited. Just had it out in the Heritage and much, much better. Jupiter looked fine, even got a hint of a third band, and M35 was spectacular. (And I spent too long looking at M35 so the clouds rolled in before I got round to turning to the Orion Nebula.)

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I have a yet unused gso 6 but can see the problem being very much the same. But I also have a ts optics 12 which is miles better so I think a barlowed 12 will be the next best option. I will barlow the 6 as well and try it as a 3mm and see how that performs.

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