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Can someone explain the ortho e/p please,and it's difference from the Plossl,advantages if any,possible uses.Ta.:eek:

Orthoscopics are a 3-element design with limited eye relief, but exceptional sharpness all the way to the edge of the field. They were developed for use with longer refractors and excel in planetary and lunar work.

The Plossl design (developed by Georg Plossl in the 1860's) is a 4-element design that offers better eye relief and a much wider field of view, usually 55-60 degrees, compared to 40-45 degrees for an ortho (A 25% improvement in the field of view!). Plossls were very uncommon until about 15 years ago, when the design made a resurgance due to marketing work by Meade and Parks Optical in the States, who re-popularized the design in the late 80's and early '90's.

The price has come way down with Asian manufacture and mass marketing. Many scopes come with basic Plossls, and although some snear at these 'bargain' eyepieces; they are vastly better than the Kellner and Ramsden designs that were standard fare with starter scopes in the 80's and earlier.

Hope that helps,

Dan

Edited by Ad Astra
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Thanks Prof;now I know the 5mm ortho I bought on a whim is just a curiosity....still,like Alice,I believe in being curious.Unlike her,I never got out of the rabbit hole.Nice to hear from you again.

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hi Dave

I use Baader Genuine Orthos (which I find preferable to the volcano top types) and find orthos easier to use than the equivalent plossl. I think the reason for this is that they have very slightly longer eye relief by ratio than plossls (I think usually x0.85 focal length rather than x0.8 as in plossls) but mainly because when observing I prefer to see the field stop and the ortho has a somewhat narrower field.

For planetary / lunar / double star observing I feel there's nothing better (or at least nothing I have tried is better) as long as you can take the short eye relief and preferably have some sort of basic tracking mount in RA/azimuth at least. I was also surprised that due to their really good light transmission and contrast, they are excellent on all targets including galaxies and planetary nebulae as long as the target fits in the FOV.

Personally for £70 approx I feel BGOs are a true bargain. Don't get me wrong, I also have a Nagler and an Ethos but for their designed purpose I feel orthos cannot be beaten. I certainly observe planets and the moon with orthos rather than my Ethos.

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Orthoscopics are a 3-element design with limited eye relief, but exceptional sharpness all the way to the edge of the field. They were developed for use with longer refractors and excel in planetary and lunar work.

The Plossl design (developed by Georg Plossl in the 1860's) is a 4-element design that offers better eye relief and a much wider field of view, usually 55-60 degrees, compared to 40-45 degrees for an ortho

That's just plain wrong. Both orthos and plossls are 4 element designs with very similar eye relief (in fact the standard Abbe ortho eye relief is 85% of the focal length, whereas the standard plossl is 80%).

Both ortho and plossl designs work well at f/6 and acceptably at f/5 but neither works well in scopes faster than that.

A standard 4 element plossl will not have an acceptable field of view exceeding 50 degrees. There are modified designs ("super plossl") which have extra elements - 5 or 6 elements in 3 groups - and do have a larger field of view.

The standard Plossl has more tendency to "ghost images" caused by internal reflections than the ortho design. Super Plossls can be even worse; the extra surfaces reduce transmission and tend to reduce image contrast (the effect is far worse in this respect than the small central obstruction in reflectors).

Additionally, for long term use, note that the eye facing lens in an ortho (the one which tends to get wiped more than anything else in the optical train) is made of scratch-resistant hard crown glass, whereas the eye facing lens in the plossl design is soft, easily scratched flint glass.

Monocentrics are to be preferred for planetary work in driven scopes, when their "drinking straw" field of view is not an issue. Plossls of good quality are fine for general use but a good quality ortho like the Baader Genuine Ortho is superior for planetary, lunar and solar work, the higher contrast trumping the slightly wider field of view.

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I do not find sufficient difference between the view produced by a plossl and an Abbe orthoscopic to warrant duplication of focal lengths. Both types are fine for concentrating on a small object. This is for my 9mm and 12.7mm Antares orthos (similar BGO) and 20mm and 30mm Vixen NPL plossls ((and using a Vixen barlow to get focal lengths close). However, at 59 years old my sight is unlikely to be as good as it was and I use spectacles due to astigmatism so my opinion may not be worth that much...

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