High magnification Ortho's, how easy to use?

[barbusg60]

Guys,

I love my 12.5mm Volcano top Ortho and would like to get some more.

How comfortable/easy to use are the smaller sizes like the 4/5/6mm ones as the lens look tiny in the pics!

They are to go in a Heritage 130P to give me some high magnification.

Thanks in advance,

Rob.

[faulksy]

i use the 9mm bgo and find it great no problems at all, better being seated though so you can keep your melon still. dont think you will get better views

[John]

I've used orthos down to 4mm I found them tight but OK. The volcano top design helps as it raises the eye lens up towards your eye but the eye lens below 7mm is getting very small and can be a challenge to find with your eye in the dark. With the 4mm your eye needs to get to around 3mm away from the top lens which is rather "intimate" !

The views are great though

If you wear glasses when observing I'd forget anything less than the 12.5mm though.

[JamesF]

Agree with John. I love my BGOs, but you almost have to climb inside the 5mm.

James

[Mr Moff]

I use BGO's down to 5mm and they are great. The 7mm is my favourite of the bunch.

You are looking through a tight hole but the views more than make up for it.

[JamesF]

At the other end of the scale, if I'm DSO hunting and find my 32mm or 25mm eyepieces don't really do the business on smaller objects, I quite often pull out the 18mm BGO because it's so nice to use.

James

[John]

I owned a 5mm TMB Supermoncentric for a while. It's eyelens and FoV are even tighter than an ortho as the picture below shows. The views were superb but it was hard work at times !:

[JamesF]

Blimey. You'd need a magnifying glass just to find the lens. There must be a point at which someone says "You know chaps, I think we might have overcooked this one a bit".

James

[John]

Blimey. You'd need a magnifying glass just to find the lens. There must be a point at which someone says "You know chaps, I think we might have overcooked this one a bit".

James

I tend to agree James. The TMB Supermono's were made by a Zeiss sub-contractor and they do take minimal glass eyepieces to an extreme possibly only exceeded by these "ball" eyepieces that some are using and discussing on the Cloudnights forum.

I'm now pretty content with the Pentax XW in my 5mm "niche"

[alan potts]

I owned a 5mm TMB Supermoncentric for a while. It's eyelens and FoV are even tighter than an ortho as the picture below shows. The views were superb but it was hard work at times !:

John,

I am fast becoming of the opinion if it's Ep's then your the man. I continue to read posts abour Ortho's. Years ago I found I had problems with my eye lashes with the model I had, I have long forgotten which. What advise would you give to someone what to try one with this problem. I am at the moment using Radians for high end Magnifiction.

Alan.

[michael.h.f.wilkinson]

I had a 4 or 5mm Circle-T Ortho (0.965" fitting), with my 6"F/8 Newtonian which I absolutely hated for its tight eye relief. Even without my glasses I just hated it. The image was sharp but it was a real pain to see it. The 25mm was superb, and very easy to use. In those days (1978) long eye relief at short focal lengths was not available at prices remotely near the budget of a 16 year old. I much prefer the image through my Pentax XF 8.5 these days (unfortunately they only have an 8.5 and a 12 in that range). The XF 8.5 beats a Radian 8mm by a good margin in terms of sharpness, transmission, and colour neutrality.

[E621Keith]

I have a 5mm BGO and a 5mm Circle V (0.965). Both have very tight eye relief.

I also own a 2.5mm XO which is surprisingly comfortable to use. The XO is actually a barlowed plossl so it has longer eye relief than its focal length suggests.

[barbusg60]

Thanks Guys,

I was thinking of going for a 4mm & 5mm & 6mm but I may stick to my original plan of a 7mm & 9mm with a Barlow.

Cheers,

Rob.

[Moonshane]

I have the 9mm and 7mm 9 and in fact the 12.5mm) BGOs and really like them. They do what they say on the tin and provide sharp, contrasty views but with pretty short eye relief and narrow field. I have the 6-3mm Nagler zoom for higher powers and think this is more comfortable than would be a 6mm or shorter BGO.

[nightfisher]

Much as i adore the circleT ortho`s i dont see me going below the 7mm that i have, but i have a 5mm bst for high power work when and if its needed

[Mr Spock]

I find the 9mm and 7mm I have easy to use.

I do have a 4mm 0.965" lying around somewhere. That is like looking through a pinhole pressed against your retina...

[John]

I have a 5mm BGO and a 5mm Circle V (0.965). Both have very tight eye relief.

I also own a 2.5mm XO which is surprisingly comfortable to use. The XO is actually a barlowed plossl so it has longer eye relief than its focal length suggests.

The odd thing is that the Pentax XO 5mm has a stated ER of 3.6mm (about what you would expect a 5mm ortho to have) but the 2.5mm has 3.9mm. It makes me think that the 2.5mm is the same optics as the 5mm but with an added barlow / multiplier / smyth lens set to get the shorter focal length and a by product of slightly lengthened eye relief (as barlows tend to do).

Fantastic eyepieces by all accounts - possibly the finest of their type available to buy these days (I'm assuming you just can't buy the Zeiss Ortho's today).

[E621Keith]

The odd thing is that the Pentax XO 5mm has a stated ER of 3.6mm (about what you would expect a 5mm ortho to have) but the 2.5mm has 3.9mm. It makes me think that the 2.5mm is the same optics as the 5mm but with an added barlow / multiplier / smyth lens set to get the shorter focal length and a by product of slightly lengthened eye relief (as barlows tend to do).

Fantastic eyepieces by all accounts - possibly the finest of their type available to buy these days (I'm assuming you just can't buy the Zeiss Ortho's today).

I found this diagram of the XO. Both 5mm and 2.5mm has a negative element assembly in the front. The rear lens assembly resembled a plossl, which would have even shorter eye relief without the negative. The spacing of the negative is different which may account for the different magnification factor in additional to one being a doublet and the other a singlet.

http://pentaxplus.jp/archives/tech/xo-xw/51.html

[astrofox]

Given all that is said on eyerelief I still use my 5mm BGO as the views are sharp crisp and punchy

[John]

I found this diagram of the XO. Both 5mm and 2.5mm has a negative element assembly in the front. The rear lens assembly resembled a plossl, which would have even shorter eye relief without the negative. The spacing of the negative is different which may account for the different magnification factor in additional to one being a doublet and the other a singlet.

http://pentaxplus.jp...h/xo-xw/51.html

Huge credit to Pentax to produce 5 and 6 element eyepieces that rival the Zeiss orthos and TMB Supermonocentrics. They really are too modest about their eyepiece lines I reckon.

[alan potts]

I have seen excellent reviews on these two on a American site somewhere, but they are very expensive eyepieces.,

Alan.

[tingting44]

i was thinking of getting a BGO 5mm but thinking twice now as i have really long eye lashes , its soo hard trying to pick my 1st ep to buy

[michael.h.f.wilkinson]

It's the number of groups that is key to contrast, much more than the number of elements. Glass-glass interfaces produce very little internal reflection, because the index of refraction changes only very little at the interface. Glass-air surfaces are the killer, producing about 5-6% reflection per interface. The Pentax XO have three groups, so 6 glass-air interfaces. Before the days of multi-coating this would produce a lot of glare due to internal reflection (and 30% or thereabouts transmission loss to boot). A monocentric without coating loses just 10%. When single coating is applied (1% per surface) these figures become 6% and 2% respectively, which is much more tolerable. Pentax is renowned for its multi-coatings (0.1% light loss per interface). Once coatings are that good, light loss within the glass starts to dominate, and more surfaces lead to better control of aberrations over a large field.

Some might say that the XO is not a true orthoscopic, but note that what we call an orthoscopic is more properly known as an Abbe orthoscopic. This refers to the distortion-free character needed for astrometry. Other EPs have been refered to as orthoscopics.

[KaStern_Former_Member]

Guys,

I love my 12.5mm Volcano top Ortho and would like to get some more.

How comfortable/easy to use are the smaller sizes like the 4/5/6mm ones as the lens look tiny in the pics!

They are to go in a Heritage 130P to give me some high magnification.

Thanks in advance,

Rob.

Hello Rob,

as a rule of thumb you can asume that the eye relief of an Abbe orthoscopic will be about 3/4 of it`s focal length.

Tis is comfortable at an 25mm eyepiece, works well for me with my 18mm and 12,5mm Abbes,

starts being uncomfortable with my 9mm, is uncomfortable with my 6mm and annoying with my (sold) 4mm Abbe.

Since you want to use it with an f/5 telescope I recommend to use a longer fl Abbe in addition with a good barlow.

Edge performance in fast scopes is much better with the barlowed Abbe and according to my tests viewing Jupiter

and Moon on axis performance is better too.

This is not so astonishing as it seems at first. An Abbe Orthoscopic was made to be used in slow scopes.

It works extremely good in f/15 scopes, but it shows spherical aberration when used in a fast f/6 scope like mine.

Greetings, Karsten

[E621Keith]

Huge credit to Pentax to produce 5 and 6 element eyepieces that rival the Zeiss orthos and TMB Supermonocentrics. They really are too modest about their eyepiece lines I reckon.

Indeed, I don't think I've ever seen an official Pentax ad in any astronomy magazine, not even in the ad section in the back. Even Canon has more on ads in astronomy magazines advertising their 20Da, 60Da, and IS bino.

I read 'orthoscopic' is a definition of the optical performance rather than an optical design. An orthoscopic needs to be free of rectilinear and angular magnification distortion out to 40deg with a f5 light cone. There is also a requirement to suppress coma and astigmatism. Any eyepieces that meet these definitions are 'orthoscopic'. Abbe is one of the first designs to meet this definition. I've little doubt XO also meet this requirement, so it is a true orthoscopic.

http://www.astrosurf.com/re/evolution_of_eyepieces.pdf