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michael.h.f.wilkinson

Comparison of distortion between Morpheus, Nagler, Delos, XW, and LVW

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In response to a discussion on my new Morpheus 14mm, I have just compared the degree of rectilinear distortion on a range of my EPs. I set up the 80mm F/6, and aimed at a wooden pergola. I inserted each EP, focused, and moved the scope so one of the uprights moved from one side to the other in the FOV. First up was the Baader Morpheus. That showed more pincushion distortion than I expected, but less than that of the Nagler 12T4 I tested afterwards. The Delos 8mm showed roughly the same distortion as the Morpheus (or a bit more, hard to tell). The XW10 was astonishingly different, in that it showed just a tiny amount of pincushion distortion. The XW7 was similar. I checked out the other Type 4 Naglers and they were similar to the 12mm. Finally, I checked the LVW 42mm, which showed distortion that was similar to or slightly worse than the Naglers.

Mileage may vary on others from these series, of course

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It's an interesting test isn't it ?. I did something similar (I used a nearby telephone wire) when I was testing the Vixen SSW's which also show a fair amount of pincushion distortion. I'd expect to see it in an 82 degree eyepiece of course but I was more surprised to see some astigmatism and a touch of CA towards the edges of those as well as I commented in the review.

The Ethos shows pincushion as well but the extent was less than I was expecting there.

I agree that the XW's, at least those below 14mm, seem remarkably free of edge distortion but manage to present sharp star images to the edges of their fields of view despite this.

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Excellent Michael, these test's can be very revealing. The results may show why some love the XW's, possibly without knowing exactly why, for some of the reasons. Pincushion can help with some other stuff I think and an EP with "just enough" is an asset for solar/lunar/planetary.

Interestingly most of my orthos will show distortion...

The Baader Morpheus sounds like a really nice all round eyepiece. I wish we had some form of standardized contrast test.... my guess is that the Morpheus would be next in line to the Ethos on many DSO.

Thanks again, Gerry

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Thanks Michael, very interesting results. :smiley:

My calculation showed slight barrel distortion of 14mm Morpheus, all the 3 T4 Naglers, and a even less barrel distoriton Nikon SW 17.5mm, 14mm and 7mm. Looking at straight telepost, the 17.5mm showed just barely visible barrel distortion in the very edge, while the other two just as straight as in the middle, despite the exact amount of barrel distortion by calculation, maybe the exit pupil size can affect perceived distortion to some extend?

AMD in the Nikons can be seen by looking at checker paper, where straight lines kept to be straight line everywhere, but the squares in the middle becomes rectangulars to sides(up, down, left and right), and become smaller and smaller squares in 45°/135°/225°/315° directions.

Looking at night sky, the NIkon's are astigmatism free in 120ED, Vega is pin sharp across the FOV.

Pincushion distortion s inMaxvision 20mm and 24mm are just as calculated

This means calculated results may not always conform actual measurement, probably partly depending on exactness of focal length and FOV provided

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What did you use to calculate the distortion? FOV vs field stop diameter? There is definitely no barrel distortion in the Morpheus, but quite clear pincushion distortion

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I use the source code Chris Lord used here:

http://www.brayebrookobservatory.org/BrayObsWebSite/HOMEPAGE/THE_EVER_WIDER_AFOV.html

His original graph looks like this

post-30887-0-39194000-1446024742_thumb.p

Here's my enlarge graph for easier measument:

post-30887-0-10684500-1446025231_thumb.p

The red E=0 is distortion free line, curves about E=0 line showing pincushion distortion, and curves below showing barrel distortion.

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That does assume that the field stop and FOV figures are accurate. This can be problematic, especially in the case of EPs with Smyth lenses. You can only get the effective field stop diameter.

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The Morpheus 6.5 mm also has pincushion distortion. I suppose it is more or less exactly the same as that of the 14 mm.

For both I calculate 15.1% rectilinear distortion (from the afov) and -1.5% angular magnification distortion (from the afov, focal length and field stop). The RD is pincushion, the AMD barrel. The tiny AMD corrects some of the RD, but I  don't think you can just add the two to get the resultant pincushion.

For the Pentax 14 mm I get RD = 12.8% and AMD = -2.9%. Compare to the Morheus this is less pincushion from the RD and more barrel from the AMD. This may explain why you see so little pincushion in the Pentax.

Baader says they aimed for very little angular magnification distortion in the design of the Morpheus. For astronomy that is probably the best choice.

The Pentax might be a better choice for terrestrial observations.

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The Morpheus 6.5 mm also has pincushion distortion. I suppose it is more or less exactly the same as that of the 14 mm.

For both I calculate 15.1% rectilinear distortion (from the afov) and -1.5% angular magnification distortion (from the afov, focal length and field stop). The RD is pincushion, the AMD barrel. The tiny AMD corrects some of the RD, but I  don't think you can just add the two to get the resultant pincushion.

For the Pentax 14 mm I get RD = 12.8% and AMD = -2.9%. Compare to the Morheus this is less pincushion from the RD and more barrel from the AMD. This may explain why you see so little pincushion in the Pentax.

Baader says they aimed for very little angular magnification distortion in the design of the Morpheus. For astronomy that is probably the best choice.

The Pentax might be a better choice for terrestrial observations.

That was what they were originally aimed at, but they are cracking astronomical EPs.

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Both Morpheus and modern TeleVue eyepieces are designed for low AMD, so the similar performance is unsurprising.  With Pentax eyepieces being designed for terrestrial viewing, low RD would be expected.  This page for TV gives a good reference:http://www.televue.com/engine/TV3b_page.asp?return=Advice&id=113#.VjHiFJf1G1E

Not unexpected at all. What surprised me the most was the comparatively large rectilinear distortion of the LVW 42mm.

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This is one of my turnoffs about most TeleVue eyepieces, may my head be struck from my shoulders for such sacrilegious comments... :p

Unless you get one of the 'good ones' (those magic number combinations in the enormous Nagler range, for example) which are usually highly-sought after and mostly unobtainable, you will always be putting up with distortion of some kind and as I love to pan across starfields it just screams in my eye all night long. Even the glorious Ethos with that unbelievable viewport does this to some extent, so unless I have a DSO I want to examine in a particular way I barely use it.... I suspect it'll go on AB&S soon.

Now no widefield eyepiece can be truly 'flat', but the Pentax XWs offer the most unobtrusive distortions out of all the wide-AFoV ranges that I've used, I'd recommend them to anybody.

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This is one of my turnoffs about most TeleVue eyepieces, may my head be struck from my shoulders for such sacrilegious comments... :p

For visual this is surely user preference, with experts (Nagler, TM Back and others) favouring low AMD, while some observers hate the rolling ball effect it produces in very wide angle eyepieces.  The effect  is very dependent on AFOV; minimal, 10% difference, up to 60°, 20% up to about 80° and severe by 100°, 36%.  A summary of calculations appears below:

AFOV Half Radians Tan %tage difference

40 20 0.35 0.36 4.3%

50 25 0.44 0.47 6.9%

60 30 0.52 0.58 10.3%

70 35 0.61 0.70 14.6%

76 38 0.66 0.78 17.8%

82 41 0.72 0.87 21.5%

100 50 0.87 1.19 36.6%

Edited by cs1cjc
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Right, Chris, that's the geometric distortions of eyepieces.  You can trade one distortion(RD or AMD) to the other, but you can't get the total distortions smaller than that.

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Right, Chris, that's the geometric distortions of eyepieces.  You can trade one distortion(RD or AMD) to the other, but you can't get the total distortions smaller than that.

Well you could in theory go halfway between the two, but I am not sure that anybody sells eyepieces like that...

Edited by cs1cjc

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Well you could in theory go halfway between the two, but I am not sure that anybody sells eyepieces like that...

Tuneable eyepiece ?, now there's an idea :evil:

Select the abberation levels of your choice ....

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