The Big LVW 42mm debate.

[Franklin]

Well maybe not, but I have read in various threads of disagreement on the AFOV of the Vixen 2" LVW 42mm eyepiece and would just like to put my side of the debate up for anyone who may be curious. 

Earlier runs of this eyepiece were marked with 72deg and later models 65deg. I have one of the older 72deg samples and I have also just got hold of a new NLVW 30mm which is 65deg and I have a few 1.25" LVWs which are 65deg as well.

The AFOV of the LVW42mm is clearly greater than all the other 65deg eyepieces I have and is more akin to the AFOV of the WO 40mm Swan (72deg) that I used to own.

So whether it was a misprint or not the LVW 42mm has an AFOV of 72deg. Having said that, whether it is worth looking at the extra 7deg in the outer field opens up a whole new debate because the Vixen LVW 42mm does suffer towards the field edge somewhat.

[Mr Spock]

I covered the 42mm LVW in depth here 

To quote that thread:

"Firstly, the aforementioned LVW 42mm. Some say 72° on the barrel, some say 65°. So which is it? The answer is 'yes'! OK, let's clarify. When you look through it, it definitely looks 72° - between a 65° LVW and an 82° Nagler visually - , but, when you measure it, it's only 65°. Confusing isn't it - read on. The problem is the eyepiece has huge amounts of distortion. The image at the edge is much larger than the image in the centre.
It's only actually 42mm in the centre... So, if you average out the magnification, it weighs in as a 38mm 72° eyepiece. To me that makes much more sense. It's not a 42mm 72° eyepiece, nor is it 65°!"

[Don Pensack]

Here is the spec sheet from Vixen Japan:

 

[Louis D]

I wonder if the Lacerta ED 40mm (TMB Paragon) is better, worse, or the same corrected as the 42mm Vixen LVW? 🤨

[Mr Spock]

But that's not right - that's the reason for the investigation. You only have to look through one to see it isn't 65°...

 

[Louis D]

Probably 65° eAFOV.  My Meade 5000 SWA, Pentax XW-R, and Lacerta ED 40mm's are all 65°/66° eAFOVs despite having 69°, 70°, and 65° AFOVs, respectively.  If the Vixen LVW has a field stop of around 46mm +/- 0.3mm, that's the most likely explanation.

[Mr Spock]

Unless you look though one, you won't understand what is being said 

In my eyepiece spreadsheet, which lists my scopes, eyepiece focal lengths and fields of view, I have the 42mm in as 38mm as per my findings in the article above. This gives in my 102mm 3.83° and 305mm 1.80°, both of which can be confirmed with a star drift test 

As I said in the article, it is definitely 72°, but has magnification distortion. So it is only 42mm in the centre... So it is neither a 42mm 72° nor a 42mm 65° eyepiece - it's a 38mm 72° eyepiece. If only Vixen had put that on the barrel instead of the two other measurements it would have avoided all this confusion and discussion.

Whatever, it gives fabulous views in both my scopes and I'm happy with it.

[michael.h.f.wilkinson]

9 hours ago, Louis D said:

I wonder if the Lacerta ED 40mm (TMB Paragon) is better, worse, or the same corrected as the 42mm Vixen LVW? 🤨

I had the TMB Paragon 40mm and that listed a "true" AFOV of 68 deg (as measured from the effective field stop, i.e. how much of the sky do you actually see), and an "apparent" AFOV of 69 deg (the angle of the image you see looking through the scope). This means the Paragon has negligible distortion and can as such be named orthoscopic (as it said on the barrel). I sold  the EP because it got little use after I got the Nagler 31 mm T5, but I must say I regretted the sale, and got the Vixen LVW 42mm when one came up secondhand. The LVW has a true AFOV of 65 deg, and an apparent AFOV of 72, showing much more pincushion distortion. Great EP, nonetheless.

[Louis D]

3 hours ago, michael.h.f.wilkinson said:

I had the TMB Paragon 40mm and that listed a "true" AFOV of 68 deg (as measured from the effective field stop, i.e. how much of the sky do you actually see), and an "apparent" AFOV of 69 deg (the angle of the image you see looking through the scope). This means the Paragon has negligible distortion and can as such be named orthoscopic (as it said on the barrel).

The Lacerta ED 40mm also claims to have a 69° AFOV while in fact it has a 65° AFOV and a 66° eAFOV.  It's focal length in the center is 40.1mm and 31.1mm radially at the edge by my calculations.  Once again, like the 35mm Baader Scopos Extreme (70° claimed AFOV, 66° measured AFOV, 68° eAFOV), and 26mm Meade MWA (100° claimed AFOV, 83° measured AFOV, 90° eAFOV), the 40mm Lacerta ED (TMB Paragon) is targeting the AFOV of its benchmarked competition (35mm Panoptic for the Baader, 25mm ES-100 for the Meade, and 41mm Panoptic/40mm Pentax XW/etc. for the Lacerta/TMB).

It's quite easy to see below that the Baader is not 70° and the Lacerta is not 69° despite claims on either their barrels or in their marketing literature:

The Baader has a center focal length of 34.8mm and a radial edge magnification of 28.8mm, so only a 17% difference versus 22.4% for the Lacerta.  Thus, even more "orthoscopic".

The 40mm Meade Series 5000 Plossl is also low distortion at a 21.2% difference (40.4mm vs 31.8mm).  The edge does get pretty blurry at f/6, but the moon looks quite naturally round across the field in it.

A bit more magnified, you can see that there is some magnification distortion in the Lacerta at the edge, so it's not perfectly orthoscopic.  It's less than the "competition", though.

[Don Pensack]

It should be noted that angular magnification distortion and rectilinear distortion do not have exactly the same effects.

Angular magnification distortion expands or shrinks the image in all directions.  Think of it as expanding or compressing the area in the field.

The effect increases with apparent field width.

Rectilinear distortion (pincushion, barrel) is a radial distortion and expands (pincushion) or shrinks (barrel) the image along radial but not circumferential lines.

The effect increases with apparent field width.

The image of a circle with AMD will become somewhat egg shaped as the image nears the edge, with the edge either being smaller (usual) or larger than the circle closer to center.

The image of a circle with RD will become oval radially (pincushion, due to expansion) or circumferentially (barrel, due to compression).

 

Describing the image at the edge as a function of magnification, or a change in focal length, doesn't really describe what's going on.  It implies the image is larger or smaller, but doesn't really describe the distortion in the image

that takes place.

 

We cannot escape distortion--the wider the apparent field, the more there is.  The designer does have the ability to determine what the expression of distortion is, however, i.e. the type of edge distortion we see, but not whether

we see it or not.

Here is a chart showing the two forms of distortion in the optical image with apparent field.

 

 

[Louis D]

Are the types of fisheye mappings applicable to eyepiece distortion?

 

[Don Pensack]

Rectilinear distortion illustrated:

http://m43photo.blogspot.com/2013/01/geometric-distortion-correction.html

You might find this illuminating:

https://www.cloudynights.com/topic/420879-what-does-amd-look-like/

That was back in 2013, where I was just exploring the effects of distortion on telescope images.

Star images in focus do not seem to be affected, which is why we have eyepieces with barrel distortion and pincushion distortion existing in the market side by side.

One thing I would note, however, is that lunar viewing in an undriven scope is better in eyepieces of 65° and narrower because of edge of field distortion.

And quick panning of the sky, as in a comet seeking use, would be better with reduced RD and reduced AMD, or a combination of both.

Rolling ball (globe) distortion is obnoxious in that case, as is serious RD of either sign.

In such a case, it would also seem that a tight control of astigmatism would be beneficial.

 

It makes it easy to understand why planet observers like orthoscopic eyepieces (narrow field with near-zero distortion) if the scopes don't track.

Planet images would stay a constant shape.

Edited August 14, 2022 by Don Pensack