10 replies to this topic

[rlw1138]

I wanted to reduce the FoV of an eyepiece (to help 'hide' a too-bright nearby object), so I added a ring of matte black cardstock to the field stop. It cut the FoV from about 60* down to maybe less than 40*

 

Worked like a charm, but it got me thinking (uh-oh!!) -- did I just make my Plossl "orthoscopic" (or at least, more orthoscopic)?

 

Or is there more to it than that?

 

 

[daniel_h]

-- did I just make my Plossl "orthoscopic" (or at least, more orthoscopic)?

 

Or is there more to it than that?

yep

 

nope

[Astrojensen]

yep

 

nope

Wrong. There is more to it than that (but most Plössls are orthoscopic to begin with).

 

 

Clear skies!
Thomas, Denmark

Edited by Astrojensen, 28 November 2020 - 03:20 PM.

[vtornado]

An abbe ortho has a different optical design than a plossl.   Typically for a given focal length the ortho has a slightly longer focal length then the

plossl.  The plossl has a field of view of roughly 50 degrees and an abbe ortho has a field of view 42 degrees.

 

I have a bunch of abbe orthos and plossls  If the quality of polish, shaping, centering, coating is held constant, the two design yield about the

same quality of view for planetary viewing.   I only deal with the cheap stuff.  I don't know about top shelf varieties.

[Thomas_M44]

For textbook (though not *absolutely* perfect) orthoscopy, simply select any well-made Abbe orthoscopic.

 

IMO, probably best to just let a Plossl remain a Plossl 

Edited by Thomas_M44, 28 November 2020 - 09:10 PM.

[Starman1]

Wrong. There is more to it than that (but most Plössls are orthoscopic to begin with).

 

 

Clear skies!
Thomas, Denmark

Not quite.  In comparing a lot of 40-50° eyepieces last summer in daylight use, I noticed that nearly every one had some pincushion near the edge--straight lines curved inward slightly.

It wasn't noticeable at night, but you could definitely see it when a telephone pole drifted across the field.

It was slight, but it wasn't zero in any one I tested and that included some Abbe orthos.

So not one of them was truly "orthoscopic", wherein zero RD is present.

[Jon Isaacs]

I wanted to reduce the FoV of an eyepiece (to help 'hide' a too-bright nearby object), so I added a ring of matte black cardstock to the field stop. It cut the FoV from about 60* down to maybe less than 40*

 

Not many Plossls have an 60 degree AFoV... Most are right around 50 degrees if measured.

 

Jon

[Thomas_M44]

I have to say, of all the various eyepiece types, I find there's just something uniquely aesthetically pleasant about the extremely well-corrected 40-degree AFOV field of a good Abbe orthoscopic.

 

Not just at high magnifications. I really like the un-Barlowed view through my 25mm and 18mm Abbe orthos in conjunction with my 600mm FL refractor. Comfortable eye-relief, extremely well-behaved exit-pupil,  super-clean FOV...

[BillP]

Not quite.  In comparing a lot of 40-50° eyepieces last summer in daylight use, I noticed that nearly every one had some pincushion near the edge--straight lines curved inward slightly.

It wasn't noticeable at night, but you could definitely see it when a telephone pole drifted across the field.

It was slight, but it wasn't zero in any one I tested and that included some Abbe orthos.

So not one of them was truly "orthoscopic", wherein zero RD is present.

My tests showed similar, all Plossls had RD, and more RD than Abbes.  Also, all Abbes had a little so not completely absent.  I do have two Plossls though that have only slight RD like the Abbes, they are old Meades in 4mm and 6mm but they are branded as "Or".  They are also quite sharp and contrasty little buggers too - love them!

[MartinPond]

I have a B&L  10-mm "Orthoscopic"  where the field very

   much looks the part.....no 'hula-dancing' at all in the distant tree trunks.

It is a 1,2  (Konig type 1,  "reverse Kellner").   It is a 47-deg afov.

Contrast is very slightly shy of the mark, though.

 

 

"Orthoscopic" is a term that is not exclusive to the  Abbe, for sure.

An ES 52-degree complies with that flatness better than an Abbe...it seems.

 

The OP says....does cropping a Plossl view down to 40 degrees afov

 make it 'more orthoscopic'.   Yes...it must, necessarily.

   By a lack of physical warpage, it certainly does.

 Mathematically, quantitatively, cutting-the-core-out-of-the-watermellon, yes. 

   It is just 40 degrees.

 

The RKE (1,2)  is the only EP to back it up,

   by specifying the geometric errors.   That is a 43 degree field.

 

The question for me has always been....contrast.

 

The Abbe design is especially good at shedding stray light.

A Plossl choked to 40 degrees would do well, but you might

   need a thinner blackened  iris  than card stock. Or edge-tapered.

Edited by MartinPond, 29 November 2020 - 03:37 PM.

[BillP]

Those really excellent 4mm and 6mm Meade "Or" eyepieces that I found, which are Symmetrical Plossls and not Abbes, have AFOV of around 45 degrees.

Edited by BillP, 29 November 2020 - 08:24 PM.