Celestron 6mm Omni EP

[Spacecake2]

Hi, 

I'm just wanting some advice on the eye relief and the quality of the 6mm Omni ep. I have a Nextar 127slt MAK. Can someone please tell me if I should buy it?

Thank you,

Spacecake 

Edited October 7, 2020 by Spacecake2

[Cosmic Geoff]

I would not. The eye relief will be small and it is too powerful for the telescope.  The quality will be OK, and anyway a Mak with its long focal ratio is not demanding of eyepieces. An 8mm eyepiece would be more suitable.

[Spacecake2]

Thank you! 🙂

[MrFreeze]

The 6mm Omni is about the limit of useable magnification on a 127 MAK. Max magnification is normally given as twice the objective diameter, so 2x127 = 254. A 6mm eyepiece will give a magnification of 1500/6 = 250 on your scope. Whether the seeing conditions will allow that may be more problematic. The eye relief on the 6mm Omni is 5mm, so no use if you wear spectacles, but probably OK if you don't, although a little tight. 

On the other hand, they are dirt cheap if bought from China, and the quality is excellent - they are aluminium bodies with fully multi-coated glass lenses. I have the 4mm, 32mm and 40mm Omnis and they are all good, although the 4mm is too much for the MAK.

https://www.aliexpress.com/item/32754506897.html?spm=2114.12010612.8148356.33.41e07ff7cjx4SK

Rats - that's another impulse buy added to my cart.

David

[chiltonstar]

With my 127 Mak, a 6mm EP is indeed near to the limit, as MrFreeze says! Needed though for Saturn for example to really bring out the ring detail. You will only be able to use it when the seeing is excellent or better. I had an old Plossl 6mm but bought a (much better) 6mm Baader Ortho. Beware though, the eye relief is small and you will find yourself cleaning the outer surface of the lens to remove eyelash grease.

Chris

[MrFreeze]

It is odd that there are so few 6mm eyepieces available - a very usefull focal length. I was viewing mars night before last, using my Hyperflex zoom and wanted a bit more manification, so tried a 5mm BST Starguider, and it was much worse - I could have done with a 6mm then.

There is a 6mm TMB planetary eyepiece available, which has 16mm eye relief, but I haven't tried any of these yet. (I have an 8mm in the post from China - couldn't resist at £19 from the hilariously named 'Safety Burglar Store')

David

[Neil H]

I have the 6mm TMB dont use it a lot but is really good on a good night 

[Louis D]

8 hours ago, MrFreeze said:

It is odd that there are so few 6mm eyepieces available - a very usefull focal length.

There's the 6mm TV Ethos and Delos, Vixen SLV and NPL, Tak Abbe Ortho and Starbase Ortho, Orion LHD and Edge-On Planetary, Antares Speers-Waler Series 4, Astro-Tech Hi-Grade Plossl, Fujiyama (KK) HD Orthoscopic, Burgess Optical Ultra Mono, Vernonscope Brandon, Baader Classic Ortho, Altair Lightwave LER, Smart Astronomy Sterling Plossl, many 58 degree TMB-type Planetaries, many 66 degree Expanse-type wide fields, and many 50 to 52 degree Plossls.  So, agreed, there's not much to choose from at 6mm. 😉

[John]

I find 6mm an extremely useful focal length but it's probably not as useful in a 127mm mak with a focal length of 1500mm.

Mine is a 6mm Ethos.

[RT65CB-SWL]

I have and use the following 6mm e/p's... 

• 2x Ortho's (one is a 0.965" + 1.25" adapter... the other is a 1.25" Circle-T).
• 1x TV Radian. 
• 1x 'generic' UWA.

...and used in my Celestron C6/SCT-xlt & 'remodded' ETX105, 6mm is pushing to the max.
Of all of them, the TV Radian can just about cope under a steady night sky and eye placement.

[Franklin]

Omni Plossls are ok in the longer focal lengths, wouldn't go below 12mm. I had an omni 4mm once and it was terrible, I wouldn't even sell it on so it got binned. If you need a reasonable 6mm the SW planetary is good. Longish eye relief and a 58 degree AFOV, not bad for around £35-£40. I prefer to use a 2x barlow with my 12mm BST for the same mag as a 6mm.

 

[MrFreeze]

This morning I received my 6mm Omni eyepiece that I bought from the link further up the thread. I must say that it's a really nice eyepiece, and was a bargain at only £10. The eye relief is short, but not really that bad, and it is very sharp and crisp. It has a narrow FOV as you would expect from a short FL plossl. In general it performs very similarly to a 6mm Circle-T ortho that I'd forgotten I had, but the ortho has a slight edge in clarity. Then again the ortho cost nearly 4 times a much, and that was second-hand.

I'd also forgotten about two 6mm UWA 'gold line' eyepieces tucked away in the same box, which seem to perform much better than I remember - they can be fussy about eye position and 'kidney bean' far too easily. The FOV is huge though compared to both the ortho and the Omni. I recently also got a TMB Planetary II clone 6mm, and I must say it was awful. Maybe I got a bad one, but it's easily the worst eyepiece I have ever used. It gives a very 'soft' image as if the lens has been smeared with vaseline - it hasn't - I checked. It has a large FOV, similar to the gold line eyepieces, but unfortunately without the clarity.

David